How to salt test your hygrometer

[Mark C]

My Boveda packet is out of date, and I wasn’t able to find what I considered a GOOD guide to salt testing a hygrometer anywhere online. Everything I read said ‘cap full of water’ or ‘just a few drops’, ‘just enough to wet it’, etc. I kept convincing myself I was screwing up the test because I didn’t have precise instructions, so I figured a few experiments were in order.

Materials:
Airtight container (some suggest Ziploc bags, I used a disposable Tupperware container)
Container for salt (most sites suggest bottle caps, I used a graduated
shotglass to make measurements easier)
2 parts salt

(I used regular table salt, Part II of this ‘how to’ will verify that sea salt and kosher salt work as well)
1 part Distilled Water

Stirrer (toothpick, coffee stirrer, etc)

Procedure:
1) Fill a shotglass with ½ tablespoon of salt.

2) Add ¾ teaspoons of distilled water.
3) Mix the salt and water together, toothpicks work great. You should end up with a sort of salt slurry.

4) Place the hygrometer and shotglass into the airtight container, seal it shut, and wait.

5) After ~8 hrs, your container should have reached 75% RH. Make a note of the reading on your hygrometer and how far off it is from 75%.

6) If your hygrometer is adjusted, make a small adjustment and repeat the test until it does read 75%. If it is not adjustable, I write the difference on the back of the hygrometer.

While this test is somewhat dependent on temperature, the difference in RH is so small it will not be seen with a standard cigar hygrometer. In other words, temperature is not a factor for our purposes.

I have two hygrometers, so I did the test in separate containers, with different amounts of salt. One container had 1 tablespoon of salt, with 1.5 teaspoons of water, while the other container had ½ tablespoon of salt, and ¾ teaspoons of water. Both proved to yield equal results. I let them sit overnight so I’m not sure if one equilibrated faster than the other. I suspect they both were similar. I’m sure there’s a minimum quantity of salt/water, but it looks like I was well above that point during this test.

Follow-on Experiments:
1) Vary the water to salt ratio, trying to find the upper and lower bounds for an effective salt test.
2) Verify that kosher and sea salt work as well as iodized table salt.
3) Find the minimum amount of salt/water necessary for a given container.

If you've got any questions or comments please let me know, especially if I screwed something up along the way!