[deacon]

Hi, I am trying to calibrate a digital hygrometer (HTC-1) to figure out humidity levels in our home. The setup is a plastic sandwich box containing the hygrometer and a bottle cap with damp salt for a saturated solution. With table salt, it stabilizes at a relative humidity of 83% at 18.5°C which is quite high when the RH of NaCl should be 75%. The other compound I tried was household caustic soda which gives a RH of 10% at 19.1°C
From figures I have found online the RH for NaOH should be 8.9% at 20°C, 7.6% 30°C, 6.3 at 40°C,
If that is linear, my reading at 19.1°C should be 9.02% instead of 10%

I have a few questions from that.

Is the RH measurement affect by impurities in the salts used?
The table salt contains sodium ferrocyanide as an anticaking agent.
The caustic soda is labelled 98% w/w Sodium Hydroxide but I wonder how much of it is actually Na₂CO₃.

If the figures from the salts are accurate, is my hygrometer more accurate at lower RHs, or is it out by about 10% for both?

Are there any other common household compounds I could play with bread soda, cream of tartar, borax and what are their RHs?

[billnotgatez]

http://en.wikipedia.org/wiki/Hygrometer

sling Psychrometer

have you thought about using dew point differences
wet bulb vs. dry bulb

just curious

[deacon]

http://en.wikipedia.org/wiki/Hygrometer

sling Psychrometer

have you thought about using dew point differences
wet bulb vs. dry bulb

just curious

Hi Bill, Thanks. I had wondered about wet and dry bulb thermometers but they looked less accurate than saturated salt. http://en.wikipedia.org/wiki/Psychrometrics talks about errors of up to 15% which is worse than the ±5%~±8% quoted for the digital hygrometer. Even if you do it properly, the wiki page you quoted says it can still be out by 2-5%. Saturated salts should give precise calibration points so I can see how far out the digital hygrometer is.

[Arkcon]

Even if you do it properly, the wiki page you quoted says it can still be out by 2-5%. Saturated salts should give precise calibration points so I can see how far out the digital hygrometer is.

Well then, using household table salt is definitely not going to give you accurate (precise means something different, look them both up please) results.  Certainly the other salts, like anti-caking reagents are going to affect the solution, all the more becasue they're specifically chosen to absorb water preferentially.  If you want accuracy better than 2%, you'll have to work very carefully, with laboratory grade reagents, assuming your instrument is even designed to be that accurate, or precise.