In our living room we have a hygrometer (a device that measures relative humidity) and a thermometer in a frame my father made.
When I was playing a wooden clarinet regularly I decided I needed a hygrometer for my clarinet case, so I bought one.
When I took up ukulele I decided I needed a hygrometer for my case; I no longer have the wooden clarinet, but contrary to what you might expect the hygrometer I bought for that doesn’t fit well in the uke case. So I bought a smaller one.
Problem: They disagreed. So I performed a salt test.
This involves putting salt in a vessel (I used a soda bottle cap), adding water (it should be distilled, but I used tap), and putting that and the hygrometers in a closed container (zip bag) for a few hours. Assuming the room humidity is less than 75%, the salt solution should raise the humidity in the bag to 75%. When I did this, two of the hygrometers (the small, instrument-case size ones) read 85% and 78%; they do not have an adjustment screw. The third hygrometer does, so I set it to 75%. So do I just subtract 10% and 3% on the other two?
Well, no! After settling back down to room humidity the adjustable hygrometer read 43%. So I’d expect the other two to read 53% and 46%. Instead they read 29% and 32%!
Okay, said I, I’m going to buy a good electronic hygrometer, and I ordered a Caliber III from Stewart MacDonald. They quote a customer: “These are the finest instruments for measuring temp/humidity we have ever used.” Gotta be good, right?
Well, I got one, and it was a bit odd. For one thing, no adjustment. It’s calibrated at the factory and that’s that. Second, the instructions warn you not to try checking the calibration. “DO NOT do the salt test.” They don’t say exactly why, although they hint it could damage the precision electronics. Uh huh.
For a third thing, it read way lower than the other three hygrometers.
So at this point the experimentalist in me really kicked in. I went and bought two thermometers — the glass, alcohol filled kind — amusingly, the ones I found also had hygrometers on them. I got a piece of fabric and set up a wet bulb psychrometer. I put the other hygrometers next to it:
After wetting the fabric and running the fan a few minutes, the dry/wet temperatures stabilized at about 71F and 60F. From the table in the above link, that’s around 52% RH. And the hygrometers read:
about 47% (adjustable), 38% and 40% (non adjustable), 43% and 45% (on the thermometers)… and 28% (the electronic one).
Okay, the wet bulb setup was a little rough and ready, but it can’t be that far off. (To get 28% at 71F the wet bulb would have to read 53F.) Conclusion: The “finest instrument for measuring temp/humidity we have ever used” is junk… and the ideal case hygrometer still eludes me.