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My Boveda packet is out of date, and I wasn’t able to find what I considered a GOOD guide to salt testing a hygrometer anywhere online.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!

[Prospector]

Just a suggestion. Allow your test to run longer than 8 hours - say 36 or 48 hours - and take readings periodically throughout the full time. This would show that the humidity has indeed stabilized and is not still on the rise. This would be critical to know in the case that your water-to-salt ratio is too high.

[Mark C]

Originally Posted by Prospector

Just a suggestion. Allow your test to run longer than 8 hours - say 36 or 48 hours - and take readings periodically throughout the full time. This would show that the humidity has indeed stabilized and is not still on the rise. This would be critical to know in the case that your water-to-salt ratio is too high.

That's a good idea, definitelly will try that in the future. For this test, I actually saw the 75 after much less than 8 hrs, but let it stew a little longer just to be sure. I've got plenty of time this month, so I'll add that to the list, thanks!

[Prospector]

This will be a very interesting exercise and I'm looking forward to hearing your results. I've had many of the same questions as you but not had the time to pursue them with an experiment such as this.

Thank you for undertaking it, and keep up the good work

[madurolover]

Originally Posted by Prospector

Just a suggestion. Allow your test to run longer than 8 hours - say 36 or 48 hours - and take readings periodically throughout the full time. This would show that the humidity has indeed stabilized and is not still on the rise. This would be critical to know in the case that your water-to-salt ratio is too high.

I always test at least 36 hours.

[Jimbo14]

I may get flammed for this but I think salt testing is NOT an accurate way to testing a hygrometer. I have done so many salt tests its a joke.

I once did a salt test for over 70 hours and at NOT one point in that period did the humidity stablise. Some times in that time it was 75% and sometimes more. It kept fluctating depending on the ambient tempreture.

Buy some heartfelt beads or xikar crystals and you dont even need a hyrgometer.

Salt tests are bullcrap.

Now time for me to get flammed....

[Wharf Rat]

Originally Posted by Jimbo14

I may get flammed for this but I think salt testing is NOT an accurate way to testing a hygrometer. I have done so many salt tests its a joke.

I once did a salt test for over 70 hours and at NOT one point in that period did the humidity stablise. Some times in that time it was 75% and sometimes more. It kept fluctating depending on the ambient tempreture.

Buy some heartfelt beads or xikar crystals and you dont even need a hyrgometer.

Salt tests are bullcrap.

Now time for me to get flammed....

Not a flame, but salt tests are a standard laboratory way to maintain RH. For example, here's a link to the RH generated by different salts on the Omega web page, who sell laboratory and process control instruments.

http://www.omega.com/temperature/z/pdf/z103.pdf

Table salt is sodium chloride. You can see the very weak dependence of RH on temperature.

Bob

[Mark C]

Originally Posted by Jimbo14

I may get flammed for this but I think salt testing is NOT an accurate way to testing a hygrometer. I have done so many salt tests its a joke.

I once did a salt test for over 70 hours and at NOT one point in that period did the humidity stablise. Some times in that time it was 75% and sometimes more. It kept fluctating depending on the ambient tempreture.

Buy some heartfelt beads or xikar crystals and you dont even need a hyrgometer.

Salt tests are bullcrap.

Now time for me to get flammed....

Also not a flame, but you're wrong. You must have screwed up your salt tests somehow, or have a crappy hygrometer, but the science is solid and the test is proven. If you got bad results, examine your procedures and your equipment. Like the above post said, this test has very little temperature dependence, so you can rule out that factor. I used to have the same opinion you did, that's why I set out to write a more detailed test procedure than what I've read previously.

I've had one hygrometer sitting in a salt test container for about 5 days now, every time I look at it, it's reading a steady 79% (non-adjustable, so 79%=75%).

Beads are great, but I like to verify things are working as they should. Trust but verify

[Mark C]

Lot of salt test questions lately... Bump for test instructions!

[SmokinDuck]

I came across this and decided to check my hygro again because these directions are different from what I had found previously.

According to your directions, you say 2 parts salt and 1 parts water, a 2 to 1 ratio.

But yet you use 1/2 tablespoon of salt and 3/4 teaspoon of water.

Shouldn't it be if you use 1/2 tablespoon of salt you would need 1 and 1/2 teaspoon of water for a 2 to 1 ratio. If 1 tbsp = 3 tsp then 1/2 tbsp = 1 & 1/2 tsp.

Wouldn't it be better to use 2 tbsp of salt and 1 tbsp of water? 2 to 1 ratio, right?

I'm not a math wiz but followed your directions for testing my hygro and am coming up with a very high rh reading vs other instructions on how to test a hygro.

On everything I have found on how to test the rh on a hygro, each one is different on how the "slurry" of salt should be and how much water to add to the salt. I tested my hygro weeks ago following different directions and it read much lower than it does now by following these directions.

Can anyone that has a lot of experience with this give some specific measurements on how much salt and water to use.

What he says might be the correct way to do this but I keep coming up with really high rh readings on my hygro by following these directions and I just would really like some clarification. If this is correct, then my hygro is way off and will make note of it.

If I'm wrong someone please correct me.

Thanks.

[Shervin]

I just completed a salt test on 3 seperate hygro's and what I've read thus far conflicts just as much. I've been told that a 2:1 ratio is fine and to use a dropper to saturate the salt to the point where it is not slurry but damp......bought a calibration test just to be sure...yea I'm anal about my PH and what lol!

[Mark C]

Originally Posted by SmokinDuck

According to your directions, you say 2 parts salt and 1 parts water, a 2 to 1 ratio.

But yet you use 1/2 tablespoon of salt and 3/4 teaspoon of water.

Shouldn't it be if you use 1/2 tablespoon of salt you would need 1 and 1/2 teaspoon of water for a 2 to 1 ratio. If 1 tbsp = 3 tsp then 1/2 tbsp = 1 & 1/2 tsp.

You're half right. 1 T = 3 t, and thus 1/2 T = 1.5 t.

Your mistake is that you missed the 2:1 ratio. 2 parts salt = 1/2 T = 1.5 t, so 1 part water is half that amount, 3/4 t.

You could use 2 T salt and 1 T water, in fact I found one site that said 1 cup of salt, 1/2 c water. I used a smaller amount to make sure it all fit into my shotglass.

I suspect that too much water will iincrease the RH, but I haven't tested that yet. Which numbers did you use in your test?

[SmokinDuck]

Yup, missed the connection between 1 & 1/2, or 1.5 = 3/4. I can be a little dense sometimes.

Either way, I did use 1/2 tbsp of salt and 3/4 tsp of water and it was way too watery which is probably why it was reading a very high rh. I also mixed up 2 tbsp and 1 tbsp, for the 2 to 1 ratio just to see and that also was very watery but didn't use it to test.

I think maybe I'll just order a couple of those calibration packs.

[Wigg]

Originally Posted by Wharf Rat

Not a flame, but salt tests are a standard laboratory way to maintain RH. For example, here's a link to the RH generated by different salts on the Omega web page, who sell laboratory and process control instruments.

http://www.omega.com/temperature/z/pdf/z103.pdf

Table salt is sodium chloride. You can see the very weak dependence of RH on temperature.

Bob

Nice touch with the charts Bob! I LIKE it...