How to Calibrate a Hygrometer

A hygrometer is a weather instrument employed to measure relative humidity. Relative humidity is a measure of the amount of water the air is holding with respect to the maximum quantity of water the air can hold, at a given temperature. A hygrometer can be analog or digital; digital units are more precise and authentic than analog units.

The calibration of hygrometer involves a transfer of water in measured mass increments using calibrated microlitre syringes, into an evacuated vessel of known volume. By adjusting the mass increments to particular preset values, the dew point or frost point in the vessel can be selected for any value between -40 and 20°C.

The total pressure in the vessel can be raised to the desired level by using ultra-pure dry nitrogen, which can be more than the saturation vapor pressure at the dew or frost point. The dew or frost point remains invariant throughout the entire pressure span.

• A hygrometer
• ½ cup table salt
• Nearly ¼ cup water
• Coffee cup
• Large resealable freezer bag

♦ Put the salt in the coffee cup and add water. Stir for a while, so that the water saturates completely. Though, it won't dissolve entirely, it will produce a wet sand-like consistency.

♦ Place this mixture in the resealable plastic bag, along with the hygrometer, and seal the bag. Make sure that the mixture is not in direct contact with the hygrometer. Then, carefully place the cup in the resealable plastic bag.

♦ Keep this bag aside at room temperature for about 8-12 hours. The place should not be windy, away from direct sunlight, and heating or cooling vents. The temperature should be fairly constant.

♦ After being kept in the bag for 8-12 hours, measure the reading. It is advisable to read it while it is still in the bag, because if the outside air is dry, the reading will immediately drop once the hygrometer is out of the bag.

♦ The relative humidity in the sealed bag should be 75%. If the reading is other than 75%, it should be changed to 75%. For non-adjustable hygrometers, make a note of the reading and then add or subtract from the true reading to get the correct humidity level. For example, if your hygrometer reads 72% then add 3% to it, and to all future readings you take.

Before taking the readings, allow the hygrometer to stabilize for 2 hours, as the variations in it may take some time to register correctly on the display. Due to its delicate design, hygrometers are more prone to incorrect readings, and hence, it is necessary to calibrate them at least once a year to measure relative humidity correctly.