What is a Hygrometer?
Why Would I Need One?

A hygrometer is a device for measuring relative humidity. These devices are often called humidity sensors, or relative humidity indicators, which makes their purpose easier to understand. They are often confused with hydrometers, for rather obvious reasons. A hydrometer is used to measure the specific gravity of a liquid; that is its density compared with that of water.

Relative humidity is expressed as a percentage. If we think of a bucket half full of water; it has a relative "fullness" of 50%. Air with 50% relative humidity contains half of the amount it is able to hold, it is "half full".

Unlike the bucket, air can hold more water when it is warmer. The amount it can hold roughly doubles for every 20°F/10°C increase in temperature.

In the home relative humidity is important because it affects our comfort and health. It also may affect the fabric of our homes and their contents if it is not controlled within certain upper and lower limits. We cannot know whether we are managing relative humidity effectively unless we can measure it, and this is the purpose of a hygrometer.

For your comfort relative humidity must be maintained within a range of 30% to 70% but at the higher levels within that range other risks remain. Mold growth and dust mites will flourish in a more humid environment and maintaining relative humidity slightly below 50% is necessary to prevent this. By using a humidity sensor we can monitor the level of relative humidity in our homes and be confident that the measures taken to control it are effective.

If you have taken all the obvious steps, preventing water entering the home, ensuring adequate ventilation and so on, it may be necessary to increase or reduce humidity by using a humidifier or dehumidifier. Even after you have obtained one of these appliances your humidity sensor will still prove useful.

Humidifiers and dehumidifiers usually have humidity sensors installed inside them. When the humidistat is set on a dehumidifier, for example, it is the humidity sensor which signals that relative humidity has risen past the programmed level and signals the appliance to turn on the compressor and start removing moisture.

If you already have a dehumidifier you will have noticed that the fan runs continuously, or runs for a few minutes every hour, even when the compressor is not working. This is because the sensor needs a flow of air from the room to test for humidity in order to detect when to send that "get back to work" message to the compressor.

You may be wondering why, if the appliance has a humidity sensor, should you bother to keep your hygrometer and monitor the reading? Measuring relative humidity accurately is a tricky business and even the most expensive and sophisticated humidity sensors suffer from a small margin of error.

Dehumidifier manufacturers operate in a market place and controlling costs is essential if prices are to remain competitive. Put simply, a less expensive model is likely to have a less expensive humidity sensor and the margin of error will probably be greater than that of the costlier sensor used in an up-market unit.

With this in mind it is a wise precaution to invest in a hygrometer. Electronic models are probably the better choice and a decent quality device can be bought for around $30. How do you use it? In effect you use the hygrometer to check, and if necessary adjust, the humidity sensor in the dehumidifier.

It works like this. Run your appliance with the humidistat set at 50% RH. When the compressor has switched off you will know that the unit "thinks" it has lowered relative humidity to 50%. Now check your hygrometer. If it shows 50% all well and good. If it shows 55% you know that you need to adjust the humidistat to 45% to achieve the result you want.

If this sounds like strange advice, just do the math. Choice one; buy a $300 dehumidifier in the expectation that it will (or may) have a more precise humidity sensor or, Choice 2; buy a $200 model, plus a $30 hygrometer, and be assured that relative humidity in the house is being controlled within a range that will protect both your family and your home.

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