Tuesday 3 June 2025: 09:03. Seeing as the house build is now deprioritised due to disemployment, and lack of regular income means no mortgage possible and therefore I no longer have to please the banks for the mortgage application, I must admit I’ve gone a little mad on the spending.

(Yes I know most people laid off go on a spending binge as a psychological coping mechanism, but I swear I’m different – I’m finally free of the Eye of Bank Sauron questioning every ‘unusual’ spend of my money that I personally worked for and earned, so yes I let loose a bunch of pent up spending on things I’ve wanted for a while)

As mentioned a few posts ago as priority item number two for my period of unemployment, I’m going to be spending a lot more time with my children this summer than they have ever had before. We have five days in Amsterdam and another five days in London together coming up. For the nine weeks of summer remaining, when the weather is good I expect to be taking them on day trips somewhere in Ireland (which is after all a tourism superpower), or somewhere local. We fortunately live in the middle of some of the best hiking country in the world, but most of it hasn’t been doable due to short legs not being able to hike far. I believe I have solved that problem and several other problems simultaneously by purchasing a cargo e-bike which can carry two children with me!

Choosing a bicycle

Henry happened to take Clara’s bike for a spin a few weeks ago, from which it became obvious to all that Clara’s bike is the right size for him and therefore Clara now needs a new bike. It’s kinda painful to admit that Clara’s new bike is probably Megan’s adult bike. My how she has grown!

Which means Megan now needs a new bike, and luckily four years have elapsed since we last bought a bike under the Irish government’s Cycle to Work scheme which at current rates allows an employer to purchase a bike for its employees up to a limit of €1,250 for a push bike, €1,500 for an electric bike, and €3,000 for a cargo bike. The employer may purchase this bike up to those limits not as a benefit in kind and without inducing tax of any kind, and therefore it comes out of pre-tax money which is 38.5% of post-tax money i.e. that €3,000 bike is equal to €1,155 in post-tax money. It is therefore one of the most efficient things a micro business in Ireland can spend money upon.

I’ll set aside how immoral taxation at a 61.5% rate is by definition to focus on ‘what can you get today for under the Irish government limits?’. Four years ago a Trek FX3 Equipped was bought, and four years before that a Trek FX2 was bought under that scheme. Both are fine bikes, and we learned that having the manufacturer equip the bike with luggage racks and stands etc was a far superior bike than aftermarket equipment and the Irish Revenue scheme doesn’t care what is bundled with the bike so long as it’s under the price limit and any additionally purchased items not in the bike order are on Revenue’s approved list of permitted additional purchases for bikes under the scheme. We could go for another Trek FX3 grade of bike and stay under the limit. Or we might try something else.

Choosing an electric bicycle

I decided to take a punt on something else: a cargo e-bike, which might complement our existing bikes none of which are going anywhere (and why buy a duplicate of what you already have?). Unfortunately cargo e-bikes which are anyway reasonably specced are far beyond €3,000 in the current Celtic Tiger frothiness. So I was constrained to direct to consumer Chinese vendors the biggest of which in Europe are ADO, Engwe and Fiido.

There are at least a dozen such direct to consumer Chinese vendors operating in Europe. And many more again under the likes of Halfords ‘own label’ marks etc. Personally speaking, I’d tend towards the Western marks (even though they’re made in China) as the quality tends to be a touch above. However, in battery and electric motors, the Chinese undoubtedly lead the world. Those BYD batteries I bought for the future house are second none in my opinion – world leading. The inverter is not quite as good as European inverters, but given it’s half the price and it’s definitely at least 80% as good, I’m sold. I suspect the same applies to e-bikes even if it does not to push bikes. Yes the non-EV bits will be inferior, but if half the price and if the EV bits are superior, that works for me.

ADO and Engwe appear to be strongest on folding e-bikes – most Deliveroo riders choose an Engwe, a few choose an ADO, and I would assume they would not if they weren’t good value for money. ADO’s bikes cost a bit more than either Engwe or Fiido, but they’re a bit better made, and usually a bit lighter through better design rather than simply throwing more metal at solving problems. In particular ADO appears to make more use of extruded aluminium, whereas the others weld their aluminium. Extruded aluminium doesn’t suffer from weak points at the welds, so you can make the frame lighter and thinner without risking fracture. Engwe and ADO can use mid mounted instead of rear mounted motors. Engwe tends to be the first to market with new technology or refinements. Fiido and then ADO tend to follow after.

All these general statements apply to the general bike offerings, and not necessary to cargo bikes, which are ones designed to carry luggage. Of the cargo e-bikes that ADO, Engwe and Fiido offer – and do bear in mind the current models have been on the market about eighteen months in total after their vendors entered the direct to European consumer business no more than five years ago – ADO’s cargo e-bike clearly was not competitive in terms of price to spec, and the rebadged own brand labels suffered from a lack of online reviews as to their quality. This left the Engwe LE20 cargo e-bike for €1,599 inc VAT delivered and the Fiido T2 Longtail (2025 edition) cargo e-bike for €1,499 inc VAT delivered. Both suffer from extensive ‘aggressive brand management’ typical of Chinese direct to European consumer businesses whereby paid shills and trolls are employed to ‘manage’ the brand identity in a way offputting to most Europeans because it is rather too forced down the throat. To be honest, they’d do better in Europe by letting the product speak for itself, and not trying so hard with the public image which makes it look like they’re trying to hide something. Still, excessive execution on the brand management side can mask actual value – nobody today would say that BYD cars aren’t competitive with any other, and that’s partially because BYD learned to be less publicly insecure about how good their stuff really is.

Having decided that a direct to consumer cargo e-bike was definitely the proposed buy, I read everything I could online about the Fiido T2 Longtail and the Engwe LE20. Most Youtube reviews on these were fairly obviously incentivised, and most Reddit and other social media and forum posts were also questionable. It really didn’t help that conventional bicycle comparison reviews refused to include cargo e-bikes from these direct to consumer vendors, so you’d have this comparison review of twelve cargo e-bikes from all the major Western brands and not a single Chinese brand in there. Luckily, with enough foot leather expended, you can eventually find individual comments and mini-reviews by people who are cycling the major Western brands AND a Engwe and/or Fiido. If after researching their account I felt they were an actual genuine true person not being paid to write what they were writing, their opinion was ranked very highly relative to others.

I mention all this to explain what comes next, and before that I ought to list specifications:

Bike	Price	Battery	Weight	Claimed torque	Brakes	Tyres	Gears	Notes
Engwe LE20 (2025 edition)	€1,349 after discounts	922 Wh	36.8 kg	100 Nm mid drive	180 mm hydraulic	20 inch by 3 inch	7 speed Shimano Tourney	has turn signals
Fiido T2 Longtail (2025 edition)	€1,384 after discounts	998 Wh	39.5 kg	55 Nm rear drive	230 mm hydraulic	20 inch by 4 inch	7 speed Shimano Tourney	can be unlocked from EU regulations

(the discounts from the list prices above can be had by collecting youtube review referral discounts, which obviously proves that the review is incentivised. Still, it can save you a few hundred euro)

I’ve emboldened the outstanding feature of each bike, and underlined the weakest. To be clear, both of these 2025 models have torque sensor pedals not the cadence sensor pedals typical of 2024 and earlier models in this price range. Both are EU regulations compliant in having a motor constrained to 250 watts and a maximum speed of 25 kph, which their 2024 models were not (and indeed which famously resulted in some Engwe bikes in the UK being very publicly destroyed by the government). Both have Shimano front suspension forks to soak up the crappy roads typical in Ireland and the UK.

And, despite being more expensive, heavier and having an apparently lower spec, I plumped for the Fiido. Here it is after completing assembly, and then with Julia on the back sitting in the ‘child cage’ optional accessory which also doubles as a goods carrying basket if you fit the bag fabric:

So why did I choose the Fiido T2 Longtail instead of the Engwe LE20?

To be clear, either is a great cargo e-bike in the sub €1,500 price range (and also to be clear, neither is as good as a €3,000 or especially a €4,000 cargo bike). But from very extensive cataloging of what I reckoned were likely trustworthy user feedback and reviews, the Fiido beat out the Engwe despite the clearly worse specifications.

I should caveat that claim by the reviews last year were tainted somewhat by the Engwe being then a few hundred euro more expensive, which is no longer the case now it’s cheaper. Everybody agrees the mid vs rear drive is superior, especially as the Engwe has an especially torquey motor and gears can be applied to turn that motor into any ratio of drive you want which means far less slowdowns on steep hills. The Engwe is lighter, the Engwe has turn signals which reviews of the Fiido T2 often mention is lacking. One probably ought to have chosen the Engwe given the higher spec and lower price.

However I decided to follow the evidence first: put bluntly, there were more positive reviews that I trust of the Fiido than the Engwe. Many mentioned that the Engwe’s seat was especially awful, whereas the Fiido’s seat was just bad. I became convinced after my research that if you’re going to go fat tyre, then go fat tyre all in and therefore the four inch fat tyres beat the three inch fat tyres. And, finally, the braking: everybody, absolutely everybody, agrees that the braking on the Fiido is by far and away amongst the best braking on a bike anywhere. The brakes are branded models, and they normally fit that brand and model on motorcycles. And yes you can absolutely tell that – I went from 50 kph to full stop while going down a hill in under three metres. The Fiido has phenomenal braking power between its brakes and its motorcycle style fat tyres which grip the road far better than thinner tyres. Better than my Trek bikes, which weigh maybe 18 kg versus the ~40 kg of the Fiido. The Fiido I am quite sure would stop the quickest from speed of all my bikes. And that, when I expect to have two of my children on the back of this bike, really does matter to me.

Yes I know most bicycle enthusiasts hate the fat tyres common on Chinese branded bikes. They add weight and road noise, reduce manoeuvrability and rate of turning at speed, and have more rolling resistance which means they’re harder to pedal and reduce battery range. For some reason – I assume market demand – Chinese branded bikes fit bigger batteries and more powerful motors i.e. adding even more weight whilst still keeping the fat tyres. Me personally – I’ve ridden motorcycles and mopeds before, so I’m more used to fat tyres. I know they’re better on loose stone, and far better on sand than bicycle tyres. As mentioned last paragraph, all that extra grip makes for superb braking but it also means you don’t feel unsafe at 50-60 kph like I do on my Trek bikes. I think fat tyres are just a different cycle experience, they have their pros as well as their cons. Their biggest con in my book is the harder pedalling, but for an e-bike it’s rare you actually have to push the pedals by much. Similarly, the added weight matters much less when it’s the motor pushing the bike around rather than me.

The other cause of my choosing the Fiido is that you can still disable the EU imposed power restrictions via an engineering debug menu, whereas Engwe due to the very public crushing of their bicycles on social media has completely removed that option. I’ll be very blunt here: I can personally testify that the Fiido with EU constraints removed is a very different bike to EU constraints imposed which is the default. The unshackled bike is a monster, oodles of torque and power. Is there a 15% gradient hill ahead? I’ll power up that at 20 kph with an extra child onboard. I’m currently personally a little heavy at 84 kg, so on the flat alone unshackled bike reaches about 40 - 42 kph. But even unshackled it remains shackled to a max power assist below 50 kph which is a shame because it’s never uncomfortable nor is there ever feelings of unsafety at 60 kph which I can reach going down a steep hill. I wish they’d allow unshackled power assist to 60 kph … but okay, this is a sub €1,500 e-bike, and we’re already into illegalness in the EU if you go faster than 25 kph on a public road.

Having compared EU legal vs what the bike can actually do … the EU regulations are plain dumb and stupid. If I have a licence to drive a car and I have driven the past decade without even a parking ticket never mind a speeding ticket, why should my bike which is perfectly safe at 50 kph be artificially limited to 25 kph? Why can I drive a car at 50 kph but not a bike – which is far less lethal – also at 50 kph?

There is an argument that cars shouldn’t be allowed to drive at 50 kph in urban areas either, and should be constrained to 30 kph. If drones can be constrained based on location, and so can cars, why can’t e-bikes? The app on your phone knows your location and speed limits. It could impose geolocated 30 kph or 50 kph limits same as for cars for people with valid driving licences. Then, if you’re out on rural roads, you can open it up.

I think the EU could do far better here. Absolutely insist on bikes having better braking power than cars as a start. For those who validate their driving licence with the app, I’d raise the maximum speed to 30 kph as 25 kph is just a touch too slow, and I’d make those speed restrictions geolocated same as for drones and cars.

I expect nobody able to do something about it will be reading this so it won’t matter. But it just seems a missed opportunity to me. With the range and carrying capacity a cargo e-bike has, it’s a viable alternative to a car for short runs like to a shop. We should be encouraging that, not discouraging it.

The negatives of the Fiido T2 (2025) longtail e-bike

I feel that I haven’t been fair. I’ve been all positives and I haven’t mentioned any negatives about my choice of the Fiido bike, and it wouldn’t be fair if I did not. I should point out beforehand that everything I say here refers to the 2025 edition of this bike. Later editions will probably be better, and the 2024 and earlier editions are definitely worse.

Our Trek bikes have mid-range bike gear, so their gear changing isn’t the worst but isn’t the best either. Similarly for the brakes – our FX2 in particular has the old fashioned rim-based brakes, whereas our FX3 has hydraulic disc brakes. If I am honest, I am unconvinced that the FX3 is worth 2x the cost of the FX2 which was the purchase price difference. Yes the FX3 is better in many areas, but nowhere near 2x better overall. In this, I think push bikes are like whiskey – after a certain point, 2x the cost is at best +20% better. I mention this now, because it will make sense later.

The Fiido definitely has cost reduction elements about it. It is good that they chose anything Shimano at all given how bad some of the bottom range Chinese derailleurs are. But that said the Shimano Tourney … it really is bargain basement compared to the also Shimano derailleurs on the FX2 never mind the fancy Shimano derailleur on the FX3. The chain audibly grinds as it goes, and the gear changes are noticeably less clean than on the Trek bikes. Yes Shimano Tourney gets the job done, but you’d be slightly wincing as it executes. There is too much grinding metal.

I didn’t think much of T2’s front axle – it’ll be fine, but the engineering isn’t what you’d get on our Treks. The chain, gears and bearings on the Fiido noticeably grind when they turn in a way like on a very cheap bike, and not the fluid and relative silence you get on our Trek bikes. There is a very obvious difference in quality there. I would infer that the wear and tear will be higher on the Fiido, though they do supply replacement parts on their website at acceptable prices. My point is you might get ten years from a chain on a Trek bike at the rate we cycle, and I doubt you’d get five years on a chain from a Fiido T2 bike given the audible wear being put on it. All that said, right now from Fiido’s website a replacement chain costs €24 for the T2. Assuming Fiido remain in business and selling that chain, that seems acceptable considering an OEM Shimano chain for the Trek bikes currently costs €36 and the T2’s chain is 50% longer.

The Fiido’s four inch wide tyres soak up a fair amount of road micro-shittiness which is very pleasing compared to the thin road tyres of the Trek bikes which by definition shakes your bones to their very core every single cycle especially at speed. But the tread on the tyre is noticeably thin which means the tyres will need replacing sooner. I feel this is avoidable for very little extra cost. There is also noticeably more road noise from these fat tyres, and between the drone of the electric motor and these tyres it’s a much noisier experience cycling than with the Trek bikes. In the cities this won’t matter, but out in the countryside with zero noise other than nature … well, the extra noise is unfortunate.

Cheap bikes tend to weigh more – my daughter’s smaller bike is noticeably heavier than our Trek bikes due to steel vs aluminium frames. The Fiido has an aluminium frame, but it has a whole ton load of frame like the old British made Raleigh bikes used to. On the one hand, you will never, ever, break those frames no matter what you do which was the case for those British made Raleigh bikes and I’m fairly sure is the case for the Fiido T2 Longtail frame. On the other hand, you’re lugging around many unnecessary kilograms of weight. The electric motor mitigates this somewhat, but consider the same motor on a 15 kg frame – assuming a 80 kg rider, it would be ~20% faster and more responsive. I can see where Fiido are coming from – a few years ago they had bike frames cracking for which they had to do a recall, which undoubtedly led them to be extra conservative about frames. Similarly, Engwe had all those bikes crushed for not being EU regs compliant, so they’ve gone overboard on being EU regs compliant to their overall detriment.

Finally, it’s a small thing but the torque sensor on the 2025 Fiido T2 longtail which is apparently made by ‘Mivice’ isn’t great in my opinion. To be extremely clear, I would choose this torque sensor any day and at any time over the cadence sensors in the bike models preceding. But there is no avoiding that this particular torque sensor ‘Mivice S200’ is not a great example thereof. Its torque detection range appears to have at most five levels over a narrow range. As a result, when at the threshold, the bike unhelpfully oscillates between power levels in a rough sort of way as you pedal. It generates a rough graduation of power delivery e.g. if you are cycling behind traffic held up by a tractor, you need to have a speed matching the traffic. But the Mivice S200 will choose from your pedalling a speed which oscillates between too slow and too fast for the traffic. This causes a fast-slow oscillation, so your bike rides ‘jumpy’. I mean, it works right, you don’t run into the car in front and you don’t hold up the car behind. But it is hardly smooth.

The same lack of graduated range problem also affects the throttle which has a distinctly non linear application of power to how much you push it down. This also causes jumpiness, because the bike lurches forward causing your finger to retract cutting the power, so now you fall forwards again and the power comes back full on.

We all love how torquey this bike is, but sometimes you just want smoothness instead. Engwe have solved this in their 2025 range using a ‘boost button’ such that the default is a smoothed, averaged torque which takes a while to adjust to pedal pressure. If you want it to respond immediately to pedal torque, you hold down that boost button to turn off the low pass filter the bike is running on the torque sensor. This is one way of solving the problem, however I have to think that this is 100% software stuff, it’s possible that Fiido could fix this ride oscillation problem in a future firmware update, and I do hope that they do so.

I should stress that these negatives are all small niggles. There is absolutely nothing showstopper in here. To be honest, the bike is great, it is comfortable to ride, after 125 km on the clock the battery seems to last plenty of distance, and it has plenty of power and torque. Yes it could be better, but like with my solar inverter, it’s less than half the price of Western marks and not far from as good. Western marks are maybe 20% better at double the cost. This is a very good bike for the money.

Where e-bikes will probably go next

Considering that four years from now I may be able to buy another bike under the Irish Cycle to Work scheme, it is worth reflecting on where e-bikes might be four years from now.

The most likely thing will be improvements to that torque sensor – within four years, I’d expect an unobtrusive match of pedalling to velocity without any roughness oscillating between power levels. This seems extremely doable within four years, especially as torque sensors only entered this price range in 2025. I think future budget torque sensors will be more graduated over a longer range, and thus more fluid and less jumpy. I think firmwares will be better by then at figuring out what you’re doing and the most pleasing response. Or, boost buttons could become standard.

The next most likely thing will be battery capacity. Right now for ~€1,400 inc VAT I can get one kilowatt hour of battery capacity. I would be surprised if in four years from now you can’t get two kilowatt hours for the same money given how battery capacity per euro has been trending.

The third most likely thing will be reduction in weight for the same money. That is something Western manufacturers have tended to focus upon once they maximise every other cost-benefit adjusted feature from their perspective. I would be surprised if Chinese manufacturers don’t follow the same path, and it does have advantages in terms of handling and efficiency.

The fourth thing is suspension for the rear as well as the front. As much as those low pressure fat tyres soak up the micro-shittiness of the roads, they can’t do much for the potholes and the sometimes very rough road surface. Then the front suspension works overtime, and the whole bike still shakes which is neither good for it nor is particularly comfortable for me. If the rear wheel had similar suspension to the front, that would be great.

Engwe’s late 2025 new models I see all their heavy bike models will now come with dual suspension. That should improve the ride on Irish roads quite considerably. If Engwe are shipping dual suspension this year, Fiido surely will next year to keep up.

Shock absorbers are heavy, and any bike with them will always be heavy. Fitting to both wheels you’ll never see the weight below 30 kg without using exotic materials. A 40 kg bike isn’t going to be much heavier with suspension added to the back, and given the improvement to the rider I’d welcome Fiido catching up to Engwe on that.

How the Fiido T2 longtail rides

I’ve only done around 125 km at the time of writing on this bike, so I can say nothing about longevity nor reliability other than nothing has broken yet over some very rough road at speed with a heavy bike.

Here are two routes from my usual cycling route roster with the push bike. The first is that you cycle out to one end of Analeentha greenway, do the 5 km of the greenway which is off road, then cycle back. The second is that you cycle pretty much entirely up hill to behind the nearby quarry, where you get some excellent views, and then it’s downhill from there.

I’ve ordered these as 1A, 1B and 2A, 2B where A is on my Trek FX3 push bike, and B is on the Fiido:

    

For the first route, on the push bike it took me 1 hr 43 min though in years past before I got fat I could do it in 1 hr 27 min if I remember rightly. On the e-bike, it is a mere 48 minutes. As the average velocity shows, I did a touch above 25 kph average which would appear to be illegal – but in fact that’s because I absolutely hammered the greenway which is off road and downhill. You might notice my peak heart rate for the Fiido ride is very considerably higher than for the push bike ride – that was adrenalin. That greenway is covered in loose stone, it currently has lots of foliage blocking your view around corners, and there was quite a lot of ‘oh shit brake’ followed by ‘max throttle’. This does get the blood pumping!

(I felt bad to do the Analeentha greenway so quickly. It is very pretty. On the push bike, I normally just leave it roll slowly downhill under gravity and I look at the wonderful sights along the way)

For the second route, on the push bike it took me 1 hr 15 min and on the e-bike 34 minutes. Average speed again looks a touch over 25 kph despite me being always on public roads – this is because the bike powers up the hills at 25 kph, the legal limit, and goes rather faster than that downhill under gravity.

I reckon I did about 38 km on these two trips since I charged the battery, so I put it back on the charger and according to the watt meter, it consumed 578 Wh at the socket to bring the battery to full. Assuming ~15% of that was wasted to heat, the battery might have been about half full, despite that the bike showed two out of five bars of charge (40%). I read online that the bike does have a limp home mode and it reserves the last 20% percent of battery capacity for limp home, but I don’t know if that is included in the meter shown. If the meter does represent the whole capacity of the battery, I reckon it uses about 13.2 watt hours per km. Interestingly, the very best EVs in real world testing (not what their manufacturer claims) currently use almost exactly ten times that at 140 Wh/km, and the best of affordable EVs are currently above 150 Wh/km. E-bikes are a far more efficient electric transport.

Having done those two small tests, I then went for the gold standard test:

1. Cycle from Dromahane to Bweeng.
2. Go up Bweenduff mountain, emerging in Lahern Cross.
3. Cycle from Lahern Cross to the base of Mount Hilary.
4. Go up Mount Hilary, emerging near Banteer.
5. Reach the site in Banteer, take a short break, then cycle from Banteer back to Dromahane via the quickest way possible.

If nothing goes wrong, that is a round trip of 50 km, which leaves a little safety margin.

Well, I can tell you things did go wrong. Here is the route I actually did which I had to reconstruct because the GPS route capture didn’t work due to me fat fingering the recorder:

Due to me getting lost on Bweengduff and accidentally heading south, the bike thinks I rode 48 km to reach the site (it slightly undercounts according to the GPS recorder, but isn’t far off). I didn’t fat finger the recorder for the trip home, and that was 19 km making a total of 67 km, with one mountain ascent rather than two. The watt meter says the charger consumed 894 Wh to restore the battery to full, so maybe 775 Wh was consumed by that trip which is 11.6 Wh per km. Given the single > 300 metre ascent, I think that’s pretty good going – though during the ascent, the motor overheating warning appeared and it reduced torque considerably, though not enough to stop the bike.

The bike drops quickly from five bars of charge to four, and takes longer to drop to three, then two. I reached one bar about half way through the cycle back home. If 775 Wh was all that was actually consumed, the battery still had more than 20% in the tank and hence I never saw limp home mode, which would match it happily powering me up the steep hill just before home without complaint. Fiido claim that the bike does under 10 Wh/km on mostly flat if it’s 27 C, and given the 15 C temperature and the > 300 metre ascent, that range claim actually seems plausible.

The gold standard test had me going both up and down steep gravel tracks. I didn’t disable the EU speed restrictions at any point during this test to preserve battery longevity, however I certainly got up to 60 kph going down hill on gravel just from gravity alone.

Yes I did just say I did 60 kph on gravel. That would be nearly guaranteed death on the Trek bikes. On those fat tyres, it was surprisingly safe feeling, even going around corners and braking. When I braked at the end when it reached a main road, the bike stopped in plenty of time despite the speed and the gravel flying everywhere.

The Fiido T2 isn’t supposed to be used as a gravel bike. But it’s a damn fine gravel bike. I had a lot of fun on it on Bweengduff, and I would be surprised if I don’t take that bike up a gravel pathed mountain again soon before the kids exit school.

What’s next?

As my company may not earn money again for a while, we ought to exercise in full our options for utilising the Cycle to Work scheme by Megan also getting an e-bike. She wants a folding one she can easily get up the stairs at her work, so it probably ought to be not too heavy. Here are the current mid 2025 options for direct to consumer Chinese folding e-bikes, with bold for the best feature and underline for the weakest feature:

Bike	Price	Battery	Weight	Claimed torque	Suspension	Tyres	Gears	Notes
ADO Air 20 Pro	€1,499 after discounts	346 Wh	18 kg	40 Nm rear two gear drive	Front wheels only	20 inch by 2 inch	None	appears to surcharge delivery to Ireland and not anywhere else in Europe, including Britain
Engwe L20 3.0 (2025 edition)	€1468	346 Wh	33 kg	75 Nm rear drive	Both front and rear	20 inch by 3 inch	7 speed Shimano Tourney
Engwe P20 (2025 edition)	€999	346 Wh	18.5 kg (excludes rear rack)	42 Nm rear drive	None	20 inch by 2 inch	None
Fiido D11 (2025 edition)	€1,098	418 Wh	22 kg	40 Nm rear drive	None	20 inch by 2 inch	7 speed Shimano Tourney	launches in July. Are the mudguards bundled?
Fiido X (2025 edition)	€1,398 after discounts	418 Wh	22 kg	40 Nm rear drive	None	20 inch by 2 inch	7 speed Shimano Tourney	has annoying battery lock; rear rack obscures rear light; front light doesn't point with the steering; locking mechanism isn't as good at the D11's; D11 is a good bit cheaper for otherwise identical spec

Apart from the Engwe L20, these are all 36v systems rather than the 48v system in my cargo bike, so you get a good bit less torque. Apart from the weight, that Engwe L20 3.0 looks amazing – had I not specifically needed a cargo bike to fit two children I’d have gone for that, albeit I would miss the inability to remove the EU power restrictions. I wouldn’t want to lift 33 kg up any stairs though, but for getting it into the boot of a car it would be acceptable and I suspect they’ll sell loads of those for that price and featureset. I mean, if it can fit into the boot of a car, even at that weight you could totally take it for off road riding up and down mountains etc.

If one does want to lift it up the stairs, it looks like the ADO is the first pick and probably the Fiido D11 is the second pick. The ADO’s two gear motor, carbon belt drive and front suspension within an 18 kg package is impressive, even though the battery is a touch on the low side (but plenty for Megan’s commute). I’ve sent an enquiry to see what can be done about that Irish delivery surcharge.

I hope you enjoyed this unexpectedly long post about purchasing an e-bike. I fully expect to use both my push bike and e-bike going forth. They’re actually orthogonal use cases, despite the apparent similarities. After that 67 km trip I was quite, quite sore and it felt like I’d done a similar time, if not distance, on the push bike. So even with the motor assist, they are a fair workout. I’ll be making use of that this summer if the weather permits me.

#bicycle

Sunday 1 June 2025: 21:12. Last post when I said what my future posts will be I forget to mention this post which is the annual update to my periodic comparison of storage bytes per inflation adjusted dollar for magnetic hard drives, flash SSDs, and Intel Optane XPoint devices (you can find all the past posts here), which I have done every June since 2012:

Raw data: http://www.nedprod.com/studystuff/SSDsVsHardDrives.xlsx

You will note that I have stopped collecting prices for Optane drives – historical inventory for them has been sold, and you can’t really get them new anymore. Nor is there any close substitute. I’ll keep the historical data on the graph for now in case somebody does launch new storage with affordably priced non-volatile memory based storage. It’s a real shame how Optane went, had Intel not messed it up so badly, it should have replaced flash for storage.

This year is a normal looking year – spinning rust storage has become quite a bit cheaper, while flash storage has become quite a bit more expensive. It is, however, anything but a normal year because this year’s prices include the effects of the 10% import tariff currently being applied within the United States.

I ummed and awed about whether to try removing the effects of those taxes to keep this year’s figures comparable to previous years. But then this graph is in inflation adjusted US dollars, so we are by definition heavily tied into US economic specifics in any case. And they now have an import tariff making electronics more expensive. So I decided to not attempt to adjust anything. This graph is for the US economy and everything about that economy in each year.

It turns out SSDs are almost exactly 9% more expensive than last year. If you include inflation, it is 7.5% in real terms. Not quite the 10% tariff tax, but undoubtedly a fair price increase.

Hard drives, on the other hand are 28% cheaper including the tariff and excluding inflation. If you include those, they’re more like 40% cheaper than last year, which is getting towards half price. That’s an enormous price decrease in a single year, and most of it I think can be explained by a huge slump in demand for hard drives.

This time two years ago I predicted a recession would cause storage prices to tumble. Here looks like that recession, but so far it hasn’t appeared in the wider US economy, though it has in the wider European and Asian economies.

As regular readers here will remember, I recently picked up a factory recertified 28Tb hard drive recently for a dedicated AI inferencing machine for the site’s security cameras. I got that drive delivered for €400. That might seem a lot, but minus sales taxes (23%) and delivery (maybe €30) that 28Tb enterprise hard drive cost about €300. That same drive cost nearer a grand after taxes this time last year. It’s madness just how much hard drive prices have fallen in a single year. I can’t remember anything like it in recent memory.

#ssdsvsharddrives

Friday 30 May 2025: 20:52. I am now officially unemployed for the first time in many many years! My last full time day with Category Labs was today. As described last post, I shall be on retainer with them until Autumn, which equals a summer of freedom for the first time in a very long while.

During the past month that I’ve known a period of unemployment was coming, I was very busy planning what I’d be doing, getting my affairs and finances in order, and wrapping things up at work so there would be a good hand off. I was very much looking forward to being free of this two metre squared space I have been tethered to for so many years now. However, now unemployment is actually here … I must admit feeling a certain bittersweetness.

At the time, I worked very hard to get into Monad, more than perhaps any startup I’ve ever campaigned to get hired into. I was very keen on the proposition (a better implemented Ethereum), I thought the founders were the right mix to make it happen and therefore a good bet, and I wasn’t unaware that my participation in this project if successful would slightly change the future of all mankind, so my work there would have genuine true real long lived impact.

I had hoped to be contributing until mainnet launch, but after the Series A round the founders decided to localise my part of the workforce into New York and they now had the means to do so. I interviewed my replacements, and I was well aware that I was likely putting myself out of a job at the time. They’re the people which will take this to the end.

As I’ve been part of half a dozen startups by now and therefore more experienced than most startup founders, I’ve often said to startup founders that the people you need to bring you from tiny to small are often not the people you need to bring you from small to big. In my previous roles in startups, I tended to be retained across that transition as many others were let go. This time round, it was not the case, I ended up being one of those surplus to requirements. I am grateful that they kept me on as long as they did, and that they have retained me until the Autumn to hopefully see this thing through from the coalface.

I wish Monad/Category Labs the very best of luck with their mainnet launch, and my genuine gratitude to their founders for taking a chance on me – who was their first international remote hire of many since. I hope I was useful as I pitched to them at the time that I would be.

---

For the past eighteen months there has been headcount reduction in my industry, and in the past six months that rate has been increasing. When I go on LinkedIn right now, I see many high end devs I know personally who are unemployed, and they are not finding new work at whatever pay rates they’ve been willing to currently accept. A few weeks ago I began noticing lay offs even in AI, which until now has been insulated from the general tech industry recession.

I certainly think another year of tech industry downsizing remains. They over-hired during and after the pandemic, and the revenues just aren’t there to support that much workforce at those pandemic elevated pay rates. They therefore will keep downsizing until average pay levels get low enough that it makes business sense. We have a while to go on that I think.

Tech tends to precede the wider economy, so tech gets a recession first and the wider economy follows after. This has been the longest delay between tech recession and everybody else recession by far in my lifetime, but only from the perspective of the Anglo-Saxon countries. In Central Europe, the everybody else recession followed the tech recession within the traditional delay. What’s different this time is that the everybody else recession has not begun yet in the US, and therefore not in Ireland (much of whose economy is US multinationals).

I think much of why has to do with the US running such a massive deficit – they are borrowing at wartime levels to juice their economy and have been doing so for so many years now that the situation has become detached from normal economic fundamentals. Historically that will turn, eventually, into an especially deep economic collapse along the lines of Argentina which was once one of the richest countries in the world.

I’ve already talked about all that here before, and I don’t have much new to say about it. At some point the debt will need to be paid. Between now and then, it’s entirely possible that the US can carry on borrowing 8-10% of GDP per year for several more years to come. So detachment from economic fundamentals may continue for a while yet, indeed in the Anglo-Saxon countries the tech industry may exit recession into some sort of very weak recovery before the general economy enters recession.

The Irish economy may contain an unusual proportion relative to its size of the US economy, but it is more strongly tied to the European economy than the US economy overall. There are big market shifts happening in Europe. We’re about to build an absolute ton load of weapons all of which will require lots and lots of system engineers like me. The sixth generation European strike fighter is very noticeably completely independent from the US. It likely will be developed by a consortium of the major European powers and Japan and Australia (Canada currently wants to join after US threats upon its sovereignty, but I don’t think that will survive long term after the current US administration exits – after all, such a project has a half century long time horizon). Like the competing US, Chinese and Russian sixth generation strike fighters, it will be more an nVidia CUDA based supercomputer than airplane. As the programming languages well supported by CUDA is very limited (and does not currently include Rust), that could turn into stable long term employment for people like me who specialise in C, C++ and Python.

As this is Europe, pay rates will be a fraction of those elsewhere. But stable – or any – income is better than none. So I think Europe may be an exception to the wider pan-tech industry malaise if you’re a systems engineer. Ireland not being in NATO nor having any security clearances will mean Irish citizens won’t be allowed to work on anything secret, so we’ll likely be given dual use technologies like drones, resilient communications, and perhaps core stack stuff like deterministic runloops and work dispatchers. Anything secret will be reserved for citizens of the participating countries. I guess we’ll find out.

I’ve found myself pondering what do I want in my next employment? I do genuinely actually find AI appealing, and no I’m not lying nor faking that sentiment. I think AI will not solve 95% of the things everybody is trying to use it to solve. But that other 5% … I think it could be transformative to my personal productivity, and possibly to enough others to matter overall to an economy wide picture.

However, I must be realistic: I am forty-seven years old, and almost all of AI does not make direct use of the low level skillsets I specialise in. I am therefore unlikely to be a good fit for getting into AI at my age and skillset. Also – to be blunt – I think the AI balloon is beginning to deflate in any case anyway, and then it’ll go through a period of being out of fashion until it comes back into fashion again. Such is the tech economy.

Having pondered on this for the past month … why on earth hasn’t IoT become an expanding bubble yet? I, like many others, feel that IoT ought to be an exponential growth opportunity like AI has just been at some point. But we never seem to reach that point. Rather, IoT seems to just constantly keep incrementally creeping forwards and inwards subtly and never experiencing a ‘break out’ moment as it infests ever more parts of the global economy and human existence in general. Maybe those are its dynamics, who knows?

I guess what I’m really asking here is ‘where will the tech industry go next?’ i.e. what inflating balloon do I need to hitch myself onto before it rises? I have been relatively successful and/or fortunate in predicting/happening to be right place right time so far. I need to repeat that magic. I will admit that at the time of writing, apart from European weapons systems, I can’t think of anything likely to be the ‘next big thing’. Here’s hoping some clarity arrives on me in the next few months.

What’s next?

One of my long time unfinished projects has been a dedicated AI inferencing security camera PC. I combined a fifteen year old case with a ten year old PC guts and a ten year old second hand enterprise nVidia AI inferencing card plus a 28 Tb recertified hard drive to build a top of line security camera monitoring and recording box. This box has been built, debugged, and tested. I just need to write it up. There will surely be a post about that here soon.

Believe it or not sixteen months after I shook hands with my replacement builder a first draft of construction detail has turned up. Yes of course it would appear now when I’ve lost my job. In any case, there is now a reasonable expectation that a final quote could be issued some time in the next six months. I almost certainly cannot take a decision on build vs no build until the Autumn, but if we could get that final quote before then … that would be great as there would be actual forward progress at very long last. There will surely be a post here about the first draft of construction detail etc at some point.

The differential pressure sensors I mentioned some posts ago have turned up and those will need testing. I have also acquired an anemometer so I can finally say what air flow is passing for given readings of those barometric sensors. That will likely also result in a post here.

Finally, the next post will be all about cargo e-bikes, as I already have written that post due to buying and testing a cargo e-bike. All will be explained in that post!

#unemployment

Saturday 17 May 2025: 22:36. It’s been a month since my last post about my house build and there is big news. No, not of a ‘yay they’re going to start building my house next week variety’, but rather of a ‘I probably will have to hit pause on building this house’ variety unfortunately.

At the end of last month I was given notice of early contract termination from my current client, so I am currently working out my final month of full time with them. Without a regular income, no mortgage provider will lend. And without a mortgage, I cannot complete this house build. Indeed, as self employed people get no unemployment welfare benefits in Ireland, we’ll be living entirely off savings aka house building fund. So it would be imprudent to begin building a house in any case until more money is found.

As much as this sucks, this is not a terrible time for this to have happened. Had we been just before first fix drawdown, we would have been in deep doo doo because we’d be out of cash to proceed the build AND out of cash to stay alive, and the banks here are known to not issue further drawdowns until you restore regular income. That would be the single worst time possible to lose regular income, and losing it now is very far from the worst time possible.

All that said, it is yet another delay one actually caused by me this time and I’m mindful that planning permission will expire in 2027. We need to get everything to wall plate level by then, or we are truly hosed.

My field of work has been in recession for about eighteen months now. It has been a constant story of headcount reductions throughout my industry with very little hiring especially for fully remote workers like me. I do not expect to gain new employment at an income level sufficient to get the mortgage I need (Ireland caps maximum mortgage to 3.5x annual income) in the next twelve months given the state of things. So there will be a period of unemployment – normally a welcome thing for me, especially given I have been working for eight years straight without a break which is unusual. But in the context of building a house, it isn’t helpful.

Given the near term unlikeliness of finding new fully remote work sufficiently well paid, I have come to an arrangement with my current client that I should remain available to them at short notice until Autumn which means I am not available for new full time employment until then. This guarantees that I will have the summer to clear a large number of backlogs which have built up during the past eight years of working without breaks. This post will be listing that priority queue of stuff to get done now I finally have the time to get them done.

1. Lose weight

Sitting in front of a computer all day long tends to add body weight especially as you age. I had been 65 kg or so before I went to St. Andrews, and I was 72 kg on exit due to too many fine dinners (I mean that quite literally – I came into some inheritance in my final year there, and much of it went on some very excellent restaurants with intelligent and interesting female companions, a very worthwhile use of that money). Working for BlackBerry in Canada took me to my heaviest ever, 86 kg and I found it hard to shift on return to Ireland in 2013. That led to blood pressure related health issues, so I tried all the usual things such as diets, more exercise etc all to no avail over the next few years.

I had to take a year off earning after the birth of my son in 2016 as my wife needed me to be home. This gave me the opportunity to properly focus on losing weight which now succeeded given I had the time to do it, going from 86 kg to 81 kg by Feb 2017, then to 78 kg by April 2017, and then to 74 kg by Sept 2017. I got it as low as 73 kg, but I never reached the 72 kg I had leaving St. Andrews. In any case, all my blood pressure related issues disappeared, and I was much better off generally at the lower weight e.g. climbing stairs no longer causes you to puff.

I did a good job of keeping the weight off after that not rising above 76 kg until covid, when it piled on again not helped by being laid up in bed by covid for four months. I have failed to shift that weight since 2022, and I’ve been between 80 and 83 kg for the last three years. Blood pressure related issues have been oscillating in the background reminding me it’s not wise to be this heavy. I need to get back towards 75 kg if I am to remain healthy long term.

Now I have the free time to properly commit to it, I intend to lose as much weight as I’ll be allowed to. It greatly helps if I can (a) sleep as much as my body needs so I’m not tired (b) not have to keep sugar levels up throughout the day so I can concentrate and (c) I have the time to get in far more hours of daily exercise instead of the quick daily 4000 steps I might grab around the park if the weather permits when I am working full time.

Given I’m a good bit older than last time, 73 kg is probably unrealistic as my metabolism will have slowed. But we’ll see how it goes. I’ve got another two weeks of paid work left, then I intend to work hard on weight loss.

2. Do stuff with the kids I couldn’t normally do

I’ll be on morning childcare this summer as usual, but normally I can’t go too far or do too much as I’d need to be back home to start work. Well, not for this summer!

They’ve got camps throughout the summer for varying weeks, so there is actually only one week where I have all three at once and for that I think I’ll take them to London during the weekdays. I’ve often said that we ought to. Now I can. So we shall.

There is only one other week where I have the elder two only so I think I’ll take them to Amsterdam that week. We can do older stuff that Julia is too young for which interests the elder two. We might also meet up with their Belgian godparents. Why not!

(The reason ‘why Amsterdam and London?’ is because those two are the only two with affordable direct flights from Cork airport. There are direct flights elsewhere, but they are not cheap. Actually, I lie – there is also a cheap direct flight to Milan, and I keep meaning to go but I’ve never been)

Those will be the standout weeks, but I expect now that I don’t have to be home by a deadline that we’ll be taking a lot more day trips and seeing how the day goes. Ireland is a tourism superpower, very few places on earth have the consistent density of tourist stuff that Ireland does. We’ve not exhausted what’s within an hour’s drive from us in my lifetime, despite our best efforts these past ten years. Ireland is very, very, very good at tourist stuff. If I can even drive ninety minutes away, it opens up a bonanza of more stuff to see and to do. I intend to make use of our very fortunate country of residence.

3. Move ISO standards committees

This June’s WG21 standards meeting will be my last. I gave it seven years of my life during which I achieved absolutely nothing. Not one of my proposals got anywhere. As I’ve mentioned elsewhere, I think that more on the C++ standards committee than on me. They have become very detached from reality recently, and it doesn’t help how much industry funding for C++ has simultaneously shrivelled either.

I’ll be taking my family with me to that June meeting, which is in Sofia, capital of Bulgaria. I’ll say goodbye to all the many excellent people I got to know during my years of service there. I’ll be moving on to the WG14 standards committee whose next meeting is in Brno, Czechia in August. There I expect to present three major feature proposal papers. Let’s hope I am far more effective at the C standards committee than I ever was at the C++ standards committee.

Now I’m free of work contracts restraining my time away from work, I intend to resume presenting and teaching at conferences. I expect to submit a talk proposal for ACCU in Bristol for April 2026. We’ll see how it goes.

4. Clear project backlogs

I only have nine weeks of summer left after all the above! But I very much intend to clear all the partially or nearly finished prototypes and projects for the house.

The highest priority of those is the 3D services layout plan. I’ve got ventilation and plumbing done. I have the wiring for one bedroom done. TBH, it’s very tedious work, I keep finding excuses to not work on it. But getting it done would save very significant time later on, so it’s highly value adding work. I just need to make myself get it done. Once it is done – and especially because it’ll be an accomplishment to have gotten it done – I don’t doubt there will be a post here about it as a show and tell with pictures.

There are several other projects I didn’t fully finish – the blind automation was not fully debugged, as an example. Yes it works, but it had quirks. Those need to be debugged and fixed. Ideally a blind should be put into long term testing too, make sure it’s truly bug free over months of testing. If unemployment continues to stretch out, I can start work on ESPHome assemblies e.g. the automation for the thermal store, or the automation for the sauna or the underfloor heating. There is plenty of work I can do now so I don’t have to do it after the frames are up.

Anything else?

There is a temptation to try more, do more, begin more now I have the freedom to do it. But I must be realistic – I will have to probably return to full time work in the Autumn. Starting anything new – as attractive as that is – doesn’t make sense as a result.

Maybe by the Autumn my builder might actually want to build me a house by then more than eighteen months since we shook hands on it? Who knows. If so, everything will hang on the quote he gives me. If I can afford to get that house watertight and still leave a sufficient cushion for us to live off in case my unemployment stretches on, I’ll probably go for it. If his quote would make that cushion too thin, I’ll just have to hit pause for now and hope sufficient new income turns up soon to reenable the mortgage application.

Things could be better. But things could be very considerably worse too. I cannot complain.

#house

Friday 25 April 2025: 20:47. Back to posts about my house build! A few weekends ago I was tidying stuff up and I found the project for testing barometric pressure sensors with the ventilation boost fan. The BMP390 pressure sensor had arrived from Aliexpress months ago, and I’d even gotten round to soldering pins onto it. I just hadn’t gotten around to completing the testing begun last October.

It was very little remaining effort, so I have now done so. To quickly recap, a proper air velocity sensor costs €55 inc VAT delivered each, so I had been wondering if I could misuse a cheaper sensor to estimate approximate air flow instead. The obvious candidate is a pressure sensor, as static pressure roughly correlates with air velocity squared. To that end, in that last post, I sellotaped a cheap BMP280 temperature + pressure + humidity sensor onto the boost fan, and took some measurements for various fan speeds.

I found that 50% and 100% forward fan speed turned into a -5 and -22 Pa pressure difference, and that 50% and 100% backwards fan speed turned into a +12 and +16 Pa pressure difference. As perhaps already obvious, the problem was the amount of noise in the readings – the BME280 is only specced for a +/- 12 Pa relative accuracy, and I found the sensor saw a 6 Pa steady state pressure reading difference just by leaving it for a few minutes – all within spec, but not useful for the air speeds I want to measure.

The BMP390 sensor is specced to have a relative accuracy of +/- 3 Pa, four times better. It costs approximately 10x more than the BMP280 sensor, but that is still only €4 inc VAT delivered. To recap the sensor spec table from that last post:

	Noise	Relative accuracy	Absolute accuracy	Cost incl delivery
BMP280	3 Pa	+/- 12 Pa	+/- 100 Pa	€0.56
BMP390	2 Pa	+/- 3 Pa	+/- 50 Pa	€4
BMP581	0.1 Pa	+/- 6 Pa	+/- 50 Pa	€53

This time I sellotaped both the BME280 and BMP390 sensors to the fan, and I once again ran the fan at 50% and 100% speeds both backwards and forwards, turning it off in between to establish how much drift there might be:

	Difference 100% backwards (percent)	Difference 50% backwards (percent)	Difference stopped to stopped (percent)	Difference 50% forwards (percent)	Difference 100% forwards (percent)
BMP280 Pressure	+38.23 Pa (+0.0384%)	+8.686 Pa (+0.0087%)	+2.469 Pa (+0.0025%)	-2.856 Pa (-0.0029%)	-18.56 Pa (-0.0186%)
BMP390 Pressure	+18.66 Pa (+0.0187%)	+4.2302 Pa (+0.0043%)	+1.036 Pa (+0.001%)	-3.082 Pa (-0.0031%)	-19.73 Pa (-0.0198%)
Sqrt pressure (static pressure ~= square of velocity)	+4.24	+2.02	0	-2.02	-4.24

Both this test and the testing last October found that the BMP280 sensor has a strong bias to air direction. The BMP390 does not. In fact, I’d say that the BMP390 is looking very good so far, only a little drift between stop-forwards-backwards-stop.

How stable is it over time? I took a reading per second for one minute, and over that time I found:

	BMP280 Humidity	BMP280 Temperature	BMP390 Temperature	BMP280 Pressure	BMP390 Pressure
Maximum difference between readings	0.29 %RH	0.036 C	0.038 C	8.057 Pa	3.833 Pa
As percent of mean	0.503%	0.183%	0.182%	0.0081%	0.0038%

From its datasheet, the BMP280 could be up to +/- 12 Pa out per reading and the BMP390 could be up to +/- 3 Pa out per reading as worst cases (we assume that the oversampling we configured removes the noise). Which is, theoretically, 24 Pa and 6 Pa max-min differences respectively. For our one minute of samples, we are 33% and 64% of worst case for the BMP280 and BMP390, so within spec.

The next obvious thing to do was to calculate a 99% confidence interval, but it turns out that (surprisingly) the variance of the values read is not Gaussian i.e. not normally distributed. They are in fact linearly distributed for the BMP390 and have a power law distribution for the BMP280:

That is an unusual distribution. It made me wonder if it’s a random walk of first order difference:

Which it is. It would appear that for any given oversampled reading from the BMP390, the reading will be a linearly distributed random +/- 2 Pa from the true value. That should be easily removed with a basic low pass filter, perhaps with a ten sample history.

I also think I misread the spec sheet – when it said +/- 2 Pa due to noise, I had assumed that the oversampling would filter it out. It would appear that they really did mean +/- 2 Pa.

Remind me why you’re not using a differential pressure sensor again?

Barometric pressure sensors are absolutely the wrong thing to use to detect air flow. What I should use (as the October post pointed out) is a hot wire mass air flow sensor, which I can’t as it is too expensive on power and won’t work in varying temperature air anyway. Failing that, I should use a differential not barometric pressure sensor.

I mentioned in the October post that a differential pressure sensor costs about €45 inc VAT. That was for a CFSensor XGZP6897D with the I2C interface and 3.3v power supply. Much to my surprise, that exact same sensor can now be had for €9.50 inc VAT delivered only six months later (tariffs? Potential slowdown?). That brings it into the realm of possibility, so I ordered some along with solderable breadboards to attach them to – I guess that makes them actually €10.44 inc VAT each delivered, more than twice the cost of the BMP390 sensor.

The XGZP6897D (and note the ’D’ at the end, the ‘A’ variant is considerably cheaper but emits an analogue signal) can be purchased with varying pressure ranges. I chose the smallest available, which measures between -500 Pa and +500 Pa, and for which its datasheet claims an accuracy of +/- 2.5% with a potential +/- 0.5% drift over months. This is +/- 12.5 Pa, which is no better than the BMP280.

I honestly won’t know until testing – I suspect fitting one BMP390 barometric pressure sensor on the exit of the fan will be the ideal bang for the buck solution. The poor absolute accuracy can be solved by stopping the ventilation completely every few hours, and figuring out the absolute pressure offset for each sensor. Thereafter relative accuracy should be quite good, we should be able to reverse the fan to reduce the airflow to within 40% of max fan speed at 12v (which I have no idea what that is, but to my hand in front of it it is a slow breeze, which I think will be ‘good enough’).

In any case, I was going to need a differential pressure sensor anyway to ensure that the MVHR outlet and inlet don’t exceed a 200 Pa difference, which the MVHR documentation says will otherwise cause excess wear and tear. As we adjust boost fans some to increase flow in some places and reduce it in others, the back pressure to the MVHR will need to be kept balanced.

I still think something good enough for cheap to detect air flow is doable here. Worst comes to worst I can just fit the expensive FS3000 sensor and measure the air velocity directly. It is only accurate to 5%, however because static pressure relates to air velocity squared, our pressure sensors will never do well with low air velocities. A proper air flow sensor should be able to get the net air flow much lower.

Finally, now I’ve bought ten of the differential pressure sensors to amortise shipping costs down, I’m finding myself thinking of several other possible use cases. One is to ensure that the house is kept at neither positive nor negative pressure between the inside and the outside. For normal timber frame (and arguably block) houses, you would want a slight negative pressure to prevent indoor humidity penetrating your building fabric when outdoor RH is less than indoor RH, and a slight positive pressure if indoor RH is less than outdoor RH. However, thanks to the superb air tightness of Passive House, one can usually run a constant slight positive pressure safely. I’ll likely run my system to be neutral with a slight bias towards positive pressure, so only very slightly off neutral. Maybe 2.5 Pa, something close to the error bar interval for the sensor.

There are other potential use cases – within a building there is a stack effect from hot air rising above cold air. For every meter of altitude, there is 11.65 Pa of pressure change at 20 C. If the pressure change is more or less than that, it is probably stack effect.

One thing this particular sensor model can’t do is measure water tank depth unfortunately – 0.1 metres of water exerts 1000 Pa of pressure amazingly enough. So these sensors would exceed their range after 5 cm of water!

#house