"T i m" wrote in message
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This means that it must be accurately measurable so when a 32 psi tyre
drops it's pressure to 24 psi (23?), it should raise the alarm.

I've found that it triggers the alarm at a much smaller reduction than 25%.
Our Honda developed a very slow puncture and when I checked the tyres, one
was about 0.2 bar (3 psi) lower than it should have been.

The instructions for calibrating cycle speedos require you to measure
the *loaded* rolling circumference of the wheel bearing the speed
sensor (when it would be easier just to put a tape round the wheel).

How much does the radius of a bike tyre (at the point of contact) decrease
when you sit on the bike. It's difficult to tell when I'm the one sitting on
the bike so I don't get a side-on view. I think my speedo actually
recommends measuring the (unloaded) circumference by marking a point on the
tyre that is in contact with the ground and rolling the wheel along the
ground until the point is next in contact. But I agree that *if possible*
you should try to measure the radius under load and *assume* that the whole
tyre is that radius.

I've heard is suggested that there is *significant* error between a brand
new tyre and one with a worn tread, though I'd have thought that it was
negligible.

https://www.halfordsautocentres.com/...pth-and-safety
says that a new tyre has about 8 mm tread. If you use it until the tread is
2 mm, and assuming the same pressure in both cases, then the radius has
reduced by 6 mm in a total radius of 635 (for my car's 215/65/15 tyres) so
about 1%. I wonder how much the effective radius varies for an
under-inflated tyre, assuming the trigger level for a sensor is 25% loss of
pressure.

Any sensor has to be able to distinguish between expected change in radius
due to tyre wear and unexpected due to loss of pressure.