Two good suggestions already. I have all of the documentation / manuals. It isn't showing the low battery indicator, but the age and run time are certainly maxed out. It just uses a CR 2032 battery, which I have those on hand; her thermometers use the same battery. I'll put in a new one tomorrow. It sounds odd that only the average speed would be affected by a low battery, but who knows? The battery should be replaced anyway, so well worth a shot.
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A second magnet sounds like a really good idea. I'll have to think it through - see what I might be missing or not accounting for. But a second pickup would double the amount of time she has to complete a full revolution and that would be plenty. I'm using small rare earth magnets because they are a lot easier to use for this application. I have a couple more here, so that should work. I'll have to offset placement left / right, rather than top / bottom, but that shouldn't be a problem. I can't have a magnet too near the sensor position because if she hops off the wheel it will swing back and forth (having a natural 'top' and 'bottom' because it isn't trued). I have to avoid having a magnet within that sensing zone or she would rack up a lot of unearned revolutions.
With two magnets per wheel revolution, I think that I would just need to halve the data. For example, average speed would be reading as double what it really would be.
With a second pickup, the computer would also think that the speed was much higher, so if reliability at very low speeds is an issue, that could eliminate that issue at the same time. I will think this through further and probably try it out tomorrow. She has put on 1 hour, 5 minutes tonight and it is showing 0.00 average speed, with a distance of 1.65 miles (max speed 4.75 mph).
Her old wheel was 12" > 958 mm. Her new 18" wheel, after deducting for the track added to it is 1,400 mm. I could change the wheel size and do it that way with an additional pickup/magnet, but just halving the data is probably the easiest thing. I was having to do that before anyway because the 1009 won't accept a wheel size smaller than 1,000 mm, so I doubled the size and have always halved the data, which we record in a spreadsheet.