Hygrometer to Measure Humidity

What is Humidity?

Humidity is the amount of water vapor in the air. Absolute humidity is the actual amount of water vapor in a specified volume of air. Relative humidity is the ratio of moisture in the air as compared to the maximum amount of moisture the air can hold, which varies depending on the air temperature. Hotter air, for example, can hold more moisture.

Humidity affects the way we perceive a given temperature – in other words, at the same temperature, higher humidity will generally make us feel warmer, and lower humidity will make us feel cooler. This is because our bodies’ cooling mechanism is perspiration, and when the air holds more moisture, the dampness on our skin can’t evaporate as easily.

Knowing the relative humidity indoors is important because humidity affects comfort, health, and the condition of household possessions. When indoor humidity is too high (above 55%), people tend to consider the room to be hot and stuffy even though the temperature may be at a level that is normally comfortable. When the indoor humidity drops below 25%, the dry air also reduces comfort. Keeping humidity in the optimal range indoors helps to control bacteria, viruses, fungi, mold, allergies, and asthma. Humidity monitoring provides a way to check how well humidifiers, dehumidifiers, and air conditioners are working. Proper humidity control can also help equipment performance and reduce energy consumption.

Outdoor humidity also makes a difference in what a particular temperature feels like. It is more difficult for our bodies to handle hot weather when the humidity is high. While unfortunately humidity control is impossible outdoors, knowing the relative humidity level can help us to understand how warm or cool the air will feel at a given temperature. Using a humidity monitoring device outdoors can also alert you to rain, snow, or reduced visibility. When the relative humidity is 100%, the air becomes saturated, meaning it can’t hold any more moisture at that temperature. If it cools down even a little, the air will release water in the form of dew on the ground, fog or mist in the air, or precipitation from the clouds.

How Can You Measure Humidity?

A device to monitor humidity is called a hygrometer. Hygrometers may be designed for indoor or outdoor humidity monitoring use or both. Remember that the hygrometer is reading indoor humidity if mounted inside your house, and it can be vastly different than outdoor humidity, as reported by the National Weather Service.

Analog Spiral Hygrometers use a moisture-sensitive material that is attached to a coil spring. The spring controls a needle on an easy-to-read circular dial. Typically the hygrometer is dial marked to indicate optimal and problematic humidity levels. Between 40 to 70% relative humidity the accuracy is ± 5% accuracy. They can be slow to reflect abrupt changes in humidity, sometimes taking up to 30 minutes to reach an accurate reading. Should you need to calibrate your analog hygrometer, it is important to follow the included instructions carefully.

Digital Hygrometers determine the relative humidity by using a sensor to monitor an electric current that is affected by moisture levels. Basic digital hygrometers have an LCD display that shows temperature and relative humidity, and they may include additional information such as optimal comfort ranges and daily highs and lows. They generally measure to within ± 8%. Digital humidity devices do not require manual re-calibration; some allow manual calibration for fine tuning.

How To Calibrate a Hygrometer?

Towel Method Calibration

Dampen a towel (not dripping wet) and wrap the hygrometer in the towel for 30 to 45 minutes. Then unwrap it and quickly read the humidity. If your hygrometer is perfectly calibrated (few are) it will be reading exactly 100% humidity. Most likely, it will be reading somewhere between 80 and 90%. At this point, whatever the hygrometer reads, you can either set the needle to exactly 100% immediately after the test, or if you're a little lazier, make a mental note of how far over or under the actual humidity is from the reading from your hygrometer.

Salt Method Calibration

When salt (NaCl) and water (H2O), are in a saturated solution at equilibrium, the resultant humidity is 75%. This gives a fantastic reference point to calibrate a hygrometer and an easy way to determine if your hygrometer is accurate.

For this procedure you need:

• Ziploc Bag
• Screw-on plastic bottle cap (or similar small container)
• Regular Table Salt
• Water

Follow the steps below:

1. Place the salt in the bottle cap (or other small container)
2. Dampen the salt with water. Do not put so much in that the salt gets "sloppy". You want a damp pile of salt in the bottle cap.
3. Place both the hygrometer and the bottle cap full of damp salt in the Ziploc bag and seal it well. (It is important not to let air in or out while the test is going on.)
4. Keep it like this for 8 hours.
5. After 8 hours in the damp salt environment, the actual humidity inside the bag will be 75%.
6. The hygrometer should also read 75%. If not, you will then know exactly how far off your hygrometer is.
7. If the hygrometer has a calibration screw to adjust it, you can set the hygrometer to 75% immediately after the test.

A hygrometer should be tested and re-calibrated every 6 months or so to be sure of the accuracy.