Calibrating a Hygrometer

[thegoalie22]

People on here are always saying that many stick on hygrometers are wrong. This is probably true, but there is a way to correct this. To calibrate a hygrometer you need:
1-2 teaspoons of IODIZED(table) salt- cannot be sea salt
A freezer bag
A bottle cap, about the size of a vitamin water cap
Water
And the hygrometer
First, put the salt in the cap and add water so that it is moist, NOT waterlogged. This will probably be about 1/4-1/2 teaspoon.
Second, put the cap and the hygrometer in the freezer bag. They should not be touching and no salty water should leave the cap. Seal up the bag very well. There should be absolutely no airflow.
Third, put this basically anywhere at room temperature for about 12 hours. When you come back, it should say 75 percent humidity. If it does not, adjust the small screw on the back so that it does say 75 percent. These gauges are always off linearly, so once corrected,they will stay that way. Not all hygrometers have an adjustable screw, so check to see if it does before you start. It should be on the back.

[2kdime]

A good accurate test that I have used before!

[blackcrystal22]

I'd rather just buy a hygrometer/thermometer digital combo for 5$ more at Walmart.
More accurate too.

[thegoalie22]

I know that, but I am just saying if you have a crappy one already. And actually as far as humidity measuring there is not difference. That is like my whole point.

[2kdime]

What this test does is the salt water provides 75% humidity in the bag VERY consistently. You then measure what your "device" reads and calibrate it from there whether you can turn a dial or just know in your head to add or subtract.

I'd rather just buy a hygrometer/thermometer digital combo for 5$ more at Walmart.
More accurate too.

[m00kfu]

Good to know, I've got a few Accurites that I don't use any more because the hygrometer isn't accurate.

[nixer]

anymore i dont think many of them have adjustment screws.

[2kdime]

They dont, your right.

But if you want to use an Acurite like Chris said that you fear isn't accurate, you do this test.

When I used to do this, I'd calibrate them and then put a plus or minus on the front of the Acurite with a Sharpie the % of difference it was from 75%. So if I peeked in a cage and it said, 60% and then had a +5, i'd know it was really 65%.

anymore i dont think many of them have adjustment screws.