Problem with calibrating hygrometer

[Bruce T.]

I have 3 inexpensive electronic hygrometers. Two of one brand and one of another. I decided to calibrate them. I put them in a sealed plastic bag with a tablespoon of damp salt. After 3 days the readings are 65%, 65%, 63%. I understand that they can be off calibration but 10% is quite a lot. Also, they are all close in readings. I find it had to believe that 3 different hygrometers are all off calibration by the same 10%.

Any thoughts ?

 

[RacFocus]

Get a Boveda Calibration Kit....probably more accurate than the salt test. You could have too damp salt, or not damp enough. Too many variables in the salt test.

 

[Bruce T.]

I thought the salt test was 100% accurate. The chemical properties of salt result in a stable 75% RH. The amount of water added is not important as long as all the salt is not dissolved. At least that is what I learned in school and on countless hygrometer sites. Am I wrong ?

 

[RacFocus]

You may very well be right, I have just never had much luck with it.

 

[AlohaStyle]

I thought the salt test was 100% accurate. The chemical properties of salt result in a stable 75% RH. The amount of water added is not important as long as all the salt is not dissolved. At least that is what I learned in school and on countless hygrometer sites. Am I wrong ?

Probably true, but amount of water is kinda subjective. Boveda is for sure accurate.

 

[Bruce T.]

Problem solved.

It occurred to me that while a salt solution will hold the RH at 75%, it may not be enough to get the contents of the bag to 75% when my house air is very dry. My house is at about 38% right now. I opened the bag and put a couple of breaths into the bag. This fogged the bag so I knew there was lots of moisture in the air in there now. I closed it up. The hygrometers went to 80% and then settled back to a steady 75% after a few hours. Everything is good.

Bruce.

 

[Mr. McSquirelly]

Good man! May I ask you the brand of hygrometer you just calibrated? I don't understand why people waste their money on Boveda calibration kits. The sodium chloride test (commonly referred to as the salt test) is extremely accurate. Like a good scientist, you figured out the problem and fixed it.

 

[Bruce T.]

The two brands of electronic hygrometers that I was calibrating were Springfield ($12) and Hygoset ($20). They are cheap but they work great.