How to Calibrate 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. The hygrometer can be analog or digital in nature. Digital hygrometers are more precise and authentic than the analog ones. Whether it's a digital or analog hygrometer, to preserve and age cigars, the most important point is to keep the humidity level between 68% to 72% inside a humidor. To adjust and monitor the humidity level, the reading on the hygrometer should be accurate.

The calibration of hygrometer involves a transfer of water in measured mass increments by 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.

How to Calibrate a Hygrometer

To calibrate a hygrometer, the things required are:

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

Steps to be followed:

1. 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.
2. 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.
3. 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.
4. After being kept in the bag for 8-12 hours, measure the reading of the hygrometer. 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.
5. The relative humidity in the sealed bag should be 75 percent. If the reading is faulty, the adjustable hygrometers can be changed to 75 percent but for a non-adjustable hygrometer, 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 percent then add 3 percent to it.

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, it is more prone to incorrect readings and hence, it is necessary to calibrate the hygrometer at least once a year to measure the relative humidity correctly.