Here's how you can check your hygrometer

Kayak Jim

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So I’m about to get my first solid uke (Mainland mahogany concert) and obsessing a bit about maintaining suitable humidity levels here in the usually frozen North. Been reading all the threads about humidifiers and the like. An issue raised, and a valid one, is the questionable accuracy of many hygrometers.

A fundamental way to determine humidity levels is by use of wet and dry bulb thermometers. The difference in temperatures is directly related to the ambient humidity level. This is what I set out to do, and you can do it too. Being a chemical engineer by training, we covered this back in first year oh so many years ago. This obviously won’t work inside a uke case but it can be used to check the reading on your hygrometer. Your hygrometer should be in the same location as you'll be doing this check for enough time to ensure a steady reading.

You need:

-one or preferably two GLASS thermometers with exposed bulbs (reservoir at the bottom of the thermometer)
-small piece of T shirt or similar cotton

Make note of the reading on each of the thermometers (or the one thermometer if you only have one). They should be the same! This is the dry bulb temperature. (mine read 20 deg C)

Wrap the first inch or so of the bulb end of the thermometer with one or two (no more) layers of the cotton. I cut out one of the hems at the neckline of the T shirt so I had a little tube of cloth (this makes it a little easier to secure). Tie it with some thread so the cotton is tight against the bulb.

Put ¼ inch or so of water (about room temp) in a saucer. Lay the end of the cotton wrapped thermometer in the water and let everything sit for half an hour or so. This is the wet bulb thermometer.

Take the wet bulb thermometer and holding it by the top, swing it back and forth by your side for one or two minutes. Then read the temperature it shows. (mine read 14 deg C). and subtract from the dry bulb temperature (mine was 20 – 14 = 6 C deg)

Look up the temperatures on the chart at this website (charts for both deg C and deg F are there)
http://retirees.uwaterloo.ca/~jerry/orchids/wet&dry.html (Just noticed this website also outlines this same process!)

So on the left side of the chart for my example, dry bulb 20 deg C. Along the top to 6 C deg, the chart shows 51% RH.

Now I’ve only got a “decorative” hygrometer and it was showing way off (65% or so). So for “decorative” read “useless”. The weather channel shows outside RH is about 57% and I was reading inside the house. Central heating hasn’t been taxed much with temperatures only around freezing so I think 51% inside the house is believable.

Now when I buy a new hygrometer for my case I can check it out and feel confident in the readings. I’ll repeat the cross check in the bathroom after running the shower to give two separate verifications.

Let me know if any of this doesn’t make sense. Maybe share your results, especially if you live in a drier climate!

Jim B
 
Behold, the Wet Bulb Hygrometer

Indeed, these are very accurate humidity measurers. I remember them from my junior high days in science class.

If you don't want to make one, they're available, and cheap. I don't think they'd be so good as an in-case hygrometer (correct me if..), but they're excellent for checking your electronic hygrometer's accuracy.

An example:

http://www.avogadro-lab-supply.com/item.php?item_id=1000
 
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