Calibrating humidity sensors

[texaspyro]

I posted this in the thread on measuring resistor T.C.,  but on second thought it deserves a thread of its own:

The Vaisala HMK15 manual has a good discussion of using salts to calibrate humidity.   There is a table of how the humidity changes with temperature.

https://www.vaisala.com/sites/default/files/documents/HMK15_User_Guide_in_English.pdf

[Conrad Hoffman]

Salts are a good way to do it. Another useful tool is a chilled mirror dewpoint sensor. Commercial ones are expensive, but it turns out building one is quite easy. It's nothing more than a mirror mounted on a peltier device, with an LED and phototransistor (or diode) to sense when the mirror fogs up. With some feedback, you can hold the mirror just at the dewpoint. If you have the dewpoint and the ambient, you can calculate the RH. Same calculation works for a wet bulb/dry bulb thermometer. I've done all these and can say, like the man with two watches, you're never quite sure what the RH is, since the methods won't agree precisely.

[GEOelectronics]

I knew about and have used the salt method, but the chilled mirror is a new one, thanks.

Aspirated wet/dry bulbs is my old school method of measuring R.H. lots easier and less prone to damage than the sling psychrometer, but those definitely ave their cool factor too.

George Dowell

[texaspyro]

Here's some more tables with more salts listed:

Greenspans extensive list (this is the industry standard reference):
https://nvlpubs.nist.gov/nistpubs/jres/81A/jresv81An1p89_A1b.pdf

Omegas:
https://www.omega.com/temperature/z/pdf/z103.pdf

[texaspyro]

The chilled mirror is good for measuring humidity.   The salt solution method is used to generate a known humidity.

You could use the mirror method in a box with a way to control the humidity inside to make an adjustable humidity reference, but that would be a lot of work...

[cellularmitosis]

THanks for starting a thread texaspyro, I’ve just started down this road myself: https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg1481424/#msg1481424

I’ll post my progress here.

That manual you posted appears to give some directions for preparing the saturated solutions, which is something I’ve been looking for!

[texaspyro]

I have a couple of laboratory humidity sensors that agree with 0.5% (they also agree with what NOAA says when they are outdoors...  I'm around 1 mile from a NOAA weather station).

I also have a couple of USB temp/pressure/humidity sensors that also agree with each other within 0.5%.  I think they use BMP280/BME280/etc sensors.

Buuuutttt...  the two USB sensors disagree with the laboratory sensors around 6-8% (lower)... WTF?    I wonder if the USB sensors are using somebody's library code that has a bug in it?  Hence the need for a humidity reference.

[Conrad Hoffman]

We've used the TE sensors at work for various things ( http://www.te.com/usa-en/product-CAT-HSA0001.html ) and they're handy and accurate. One caution- this type of sensor, and I'm assuming many others are similar, is subject to a permanent shift if exposed to very high humidity for an extended period. High heat probably makes it worse. Thus the need to periodically check with salts or other methods.