How do I calibrate my hygrometer? (Humidity Gauge)
Analog hygrometers are manufactured with a factory tolerance of +/- 5% points of humidity (and sometimes more) through the normal range of 40-80%. Although humidity gauge is pre-set at the factory, its calibration may be off within this range for various reasons.
A simple, easy to do at home means of calibrating hygrometers uses standard salt and a sealed container. Table salt, when used properly, will maintain an exact 75% humidity in a perfectly sealed environment. So, to do this, you'll need three items. First, obtain an airtight container such as a see-through tub with a tight fitting snap-on lid (the kind usually found in most kitchens). Alternately, a heavy-duty zip-lock bag will do. Second, you'll need about a teaspoon of salt. Third, any small, shallow open container is needed (to hold the salt in) such as a screw cap from a plastic bottle.
Place the salt in its small container and add a few drops of water to moisten it. Don't dissolve it yet. With just a few drops of water, you'll get what you need, which is damp salt. Next, carefully place the salt in its small container, along with your hygrometer, into the airtight container. Please do not get any moistened salt on the gauge. Also, check to see that the starting point (current reading) is anywhere in the 40% to 80% range.
Seal the container tightly. Note: if you have any doubt as to whether or not it is a perfectly sealed environment, double-it-up by putting it in a second container, or into a second baggie. Do not try to remove the remaining air trapped inside. Now, wait for at least 5 or 6 hours until the environment has stabilized. Do not open the container. Read the gauge's humidity % level. It should be exactly 75%. If it is not, note the deviation as being the amount your hygrometer is out of calibration. If for example, it reads 68%, the gauge is 7% low. If on the other hand it reads 80%, the gauge is 5% high.
Now, remove the gauge from the container and locate a very small jeweler’s flathead screwdriver. If it fits through the small hole in the center of the rear plate, it should work well. Put it in that hole, and turn it slowly while watching the dial on the front. If your gauge was low by 4%, turn the screwdriver clockwise and make the dial rest 4% points higher than it did previously. Conversely, if your gauge was high by 6%, turn the screwdriver counter-clockwise and make the dial rest 6% points lower than it did previously. That's it. Your gauge is now calibrated and you should feel very comfortable with its accuracy from this point on.
:::Courtesy of http://www.shroomery.org :::