Calibration of Hygrometers for Vivaria Use

[SludgeMunkey]

(Stolen from my post over at FrogForum)

Calibration of Hygrometers for Vivaria Use

Many reptile and amphibian keepers use hygrometers, both digital and analog, to help maintain a healthy environment for their pets. A hygrometer is a gauge type device to measure relative humidity in the ambient atmosphere. (In English that means it tells you how much water is in the air, sort of...) What many hobbyists do not realize is that these devices should be calibrated regularly or the readings become highly inaccurate over time. This can lead to all sorts of issues, from mold out breaks to dead, desiccated pets. I do it every six months or so.

<DISCLAIMER> This is not intended as a laboratory guide. This method is perfectly suitable for amphibian keepers. Also note that this should be done at ambient room temperature at 68-70F or roughly 20C)


Materials Needed:

Air tight plastic container such as a small zipper seal sandwich bag or Tupperware type container. Try to use a container as small as possible for better results.

Plastic bottle top from a two liter soda-pop bottle. (20oz and 1 liter tops are the same size as well!)

Table Salt. Yes, regular plain old fine grain table salt. Rock salt and sea salt work, but generally have larger grain size making it hard to work with.

Hygrometer(s)

Measuring Teaspoon. For folks in the US any measuring spoon of this size will work. Do not try to use an actual tea spoon as the volume is a bit larger. For the rest of the world that bothered to get with the times and go metric (bear with me here, I don't cook in metric!) use roughly 5.7g of salt. (By volume I believe this is about 5mL , but I have no idea what a metric measuring spoon is graduated with!)

Eye dropper or pipette or anything else that allows you to control measure small amounts of liquids. (I use a graduated plastic syringe intended for dispensing wood glue...)


You should only calibrate one hygrometer at a time using this method.

Now that you have your shopping list, the rest is easy. Take the bottle cap and measure in one US Teaspoon of plain old table salt. Carefully add regular tap water a few drops at a time until the salt is saturated. The salt should be a thick, wet paste. All the water should be absorbed by the salt. If you have free standing water in the cap, you added too much water! (I am very lazy so I add water, then soak up the extra with a quick dab from a dry paper towel.)

Next, place the salt paste cap and your hygrometer in the air tight container and seal it up. Now go clean your enclosures, do water changes, and feed your critters. Watch a few videos on the web. take your pug for a walk. Anything to burn up a few hours. At the bare minimum yo need to let it sit for two hours. Longer is better. (I let mine sit for 12 hours.)


After you have let your hygrometer sit, check the reading. At 70F(21.1C) the hygrometer should read 75% humidity. If it does not, follow the manufacturer's instructions and push buttons so it does. If you are using an analog Hygrometer, there is an adjustment screw on the back to set the indicator. If your hygrometer cannot be set for what ever reason, replace it with one that can.


Your hygrometer is now ready to go back into the enclosure. I use the salt for my hot pretzel snacks.


This procedure can be found all over the web. There are all sorts of versions of it. I will not bother to provide references as I learned it originally from the Boy Scouts years ago while earning my Environmental Science and Weather merit badges.

 

[Molch]

After you have let your hygrometer sit, check the reading. At 70F(21.1C) the hygrometer should read 75% humidity. If it does not, follow the manufacturer's instructions and push buttons so it does.

...but wait! This should only be true if the volume of the bag/container is also a constant. However, people will be using different size containers for this: in a larger container, hence larger air volume, the moisture reading will be less because the air will be less saturated. Also, the air humidity of the surrounding air will be a factor as well. If you're surrounded by air that has 80% humidity, you'll not likely get 75% in the bag.....

 

[SludgeMunkey]

Agreed!

Why it works, I have no clue. I guess I should have included that you need to use the same container each time. Me, I use a standard ziploc sandwich bag.

 

[Molch]

but...but...then it works for you, but the next person, unless they use the exact same container, will get a different result. So...your 75% may be someone else's 65%, because that person used a larger bag.
so...by telling people to set their hygrometer to 75% regardless of the size of bag they used, you may actually get them to set it to the wrong value.

one way to test this is to use the same hygrometer and test it repeatedly in different size containers. If you get 75% every time, then the bag size makes no difference.

 

[SludgeMunkey]

Good call. It will take a few days, but I will put it to the test.


But since you put me to work on the subject, I found some more interesting data. Turns out the type of salt makes a difference.(Still learning why, me.)


SALT PUBLISHED RH AT 25°C

LITHIUM BROMIDE 6.37%
LITHIUM CHLORIDE 11.30%
POTASSIUM ACETATE 22.51%
MAGNESIUM CHLORIDE 32.80%
POTASSIUM CARBONATE 43.16%
MAGNESIUM NITRATE 52.89%
SODIUM BROMIDE 57.57%
POTASSIUM IODIDE 68.86%
SODIUM CHLORIDE 75.30%
POTASSIUM CHLORIDE 84.34%
POTASSIUM SULFATE 97.30%

That means read the ingredients on your table salt. Hate to say it, but not all table salts are just salt anymore.

 

[Molch]

makes sense on the salt - lithium is hygrophilic and will suck moisture out of the air. It's an old chemistry experiment: put a bit of lithium chloride in a jar, go to lunch, and when you come back the stuff is actually wet, because it bound water from the air.

I wonder if it would be even better to use just plain water without the salt to eliminate that variable?

 

[SludgeMunkey]

Could be. I had to raid the university bookstore today , so between that and the web I should have better data after a few hours more reading. The experimentation will take a week or so though, just to get a better matrix to work with.

 

[Molch]

you the man, dude

I look forward to the results.

 

[SludgeMunkey]

After a weeks(or so) worth of experimenting I have some updates.


1. Use only a "standard" size zipper lock sandwich bag for fastest results. You get a definitive result at the 36 hour mark.

1a. Time does not seem to affect results after the 36 hour mark. Once you hit that time with the sandwich baggie, the variation is less than 1.8% for the next 24, and even less for longer.

2.The bigger the container, the longer you must let it sit sealed up. Anything bigger than a 1 quart bag and you get exponentially skewed results.The math is crazy calculating for different larger sizes, so for simplicity sake, just use the sandwich size!

 

[Cliff]

A better way may be to wrap the hygrometer in a wet towel for 24 hours and then set it to 100 humidity if it doesn't say so already?