[time-nuts] Re: Long term stable environmental sensors

[ed breya]

A simple and expedient solution is to just get a fresh BME280 (or maybe 
a bunch of them for spares). Temperature measurement devices can have a 
very long service life, but absolute pressure and RH devices degrade due 
to reference leakage and contamination, respectively.

Small and relatively low cost consumer grade parts may work fine for a 
while, but should probably be viewed as consumables in the system. So, 
for less than a hundred bucks, you could have say a dozen BME280s 
stashed away, and figure on changing them like light bulbs every three 
years or so, if necessary. If your original one failed prematurely for 
some reason, then they could be actually much more reliable, and you may 
never have to change another, but they'll be there just in case.

One way to get very long life and stability in absolute pressure is to 
use industrial grade transducers that have a fairly large reference 
volume, are mostly high grade metals (typically SS and Inconel), truly 
hermetic and welded shut, and even gettered, to get down to very stable 
vacuum. That all is partly why they're so expensive.

For RH, I agree with Bob and Jim that a chilled-mirror dew point 
measurement device is best and longest-lasting, due to the ability to 
clean and maintain the working parts. And, you can indeed roll your own 
without too much grief.

Ed