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

[Lux, Jim]

On 11/29/22 4:45 AM, Poul-Henning Kamp via time-nuts wrote:
>> So, now I'm about to design and build a new system that has better long
>> terms stability.
> PT100 or PT1K are fine and stable for temperature.
>
> We have no technology which can make long term stable precision
> measurements of humidity and pressure:  You will need periodic
> calibration no matter what kit you get.
>
> "Long" calibration periods are two years, and if you can avoid
> condensation, you can get down to 0.5%RH drift over that period.
>
> The big player in this space is Vaisala.com and they are not cheap.
>
> Calibrating RH and pressure sensors are essentially just highschool
> physics experiments, and if that's your hobby: Go for it.
>
> You dont need to do it /all/ from scratch, see for instance:
>
> 	https://store.vaisala.com/en/products/HMK15
>

One can measure humidity a variety of ways - one way is a dewpoint 
sensor - a peltier device on a mirror and you sense the temperature at 
which the mirror stops being specular. If the air is run through a 
filter, then you don't need to clean it so often.

One can also measure humidity by measuring the loss or propagation speed 
at microwave frequencies.

Measuring pressure or density is a bit trickier. I would think that the 
standard piezoresistive aneroid sensors (sealed vacuum behind a strain 
gage, typically micromachined silicon, but could also be on silica) are 
long term stable. These sensors often have a significant tempco.