*For analog or dial type gauges, not digital
written by Rai Ahmes
Relative humidity (RH) is measured as a percentage–absolute and specific humidity are NOT. If it reads in % it’s a relative gauge. RH gauges are fairly inexpensive and commonly available. Gauges measuring absolute or specific humidity are hard to find and expensive; they also read in either g/kg or g/cubic meter (or the non-metric equivalent) not %. “Actual humidity” and “exact humidity” are not measurements.
Dial RH gauges are rarely calibrated properly by the time they reach your home. This is NOT the fault of the manufacturer. It is simply the way it is with this type of hygrometer whether it’s for your tank or for your house. This probably accounts for the wide variation of humidity levels that have been recommended. You can do something about this though 🙂
Calibrating your hygrometer:
First check to see if your hygrometer can be adjusted. Look at the back to see if the small center post has a slot for a small screwdriver. If it does you’ll need to have a screwdriver ready to make any adjustments as soon as you open the jar. If it doesn’t, don’t worry, this procedure will still allow you to use your hygrometer more effectively.
Half-fill a shot-glass or other small dish with table salt and add enough water to wet the salt completely but not dissolve it–it should be like wet sand. Put the shot-glass and hygrometer in a wide-mouth jar, and seal it airtight. After six hours (or more), the humidity in the jar is 75%. The hygrometer should read somewhere around 75%. If it does not, take the screwdriver and turn the screw on the back of the hygrometer until the meter reads 75%.
If your hygrometer is non-adjustable and reads other than 75% in the jar, you must correct all subsequent readings by this difference. Example: if the hygrometer reads 67% in the jar, it is reading 8% low. Thus, it will always read 8% less than the actual RH. Using this example if you wanted the humidity to be 60-70% then the reading on your hygrometer would have to be 52-62%.
Dial RH gauges can be off by 20% or more! They are also usually only accurate to +/- 5% and often take up to 2 hours to finish stabilizing to a new reading if the humidity changes. Please remember that it is simply the nature of the beast and NOT necessarily poor quality or manufacturing. The spring mechanism is delicate and goes through a lot (shipping, etc) before reaching your home.