So, last August, I had the house repainted, which involved taking down the outside air temperature sensor. The plastic 'screen' thingy promptly fell apart
Time for a new one. I found a thingy on eBay, 3D-printed an adapter bracket to fit it in place of the old one, and swapped out the old sensor for a new PRT probe. The junk box provided me with a 4-20mA loop current transmitter, and I finagled the resistors to give me a sensing range that covered the extremes of the temperature range experienced in England.
What with one thing and another, I didn't get round to fitting it until just recently - when the national high temperature record was smashed by, I think 1.6˚C. So I just spent a slack day recalculating the resistors
again, this time covering about -20˚C to 43.6˚C, which should be OK (I sincerely hope).
Incidentally, since I'm using micropower parts instead of the originals in the transmitter, the loop current works linearly down to about 0.75mA, which helps extend the range. Though the PRT linearisation is only good to about 0.1˚C.
The last step: calibrating it all against my Isotech reference thermometer. I used an ice point and a Grant water bath at 35˚C (as low as it will stabilise at - the lab is currently around 26˚C) to get two fixed points. Checks with the resistance box shows that is plenty good enough. I anticipate an overall error of around 0.2 or 0.3˚C, taking everything into consideration, and assuming the Isotech is as good as it was when it was made (in 2004). Given Isotech's reputation, I don't think that is much of a risk. It reads 0.00˚C in an icepoint made with deionised water, and around -0.01˚C if using tap water.
And I
will fit it tomorrow evening! Promise!