Humidistat and Hygrometer

A humidistat is an instrument used to measure and control humidity within an air conditioned space, be it;

  • your room,

  • your greenhouse, where you keep your beloved plants,

  • or any space of your choice

It is also known as a hygrostat. You might be comfortable with using the name humidistat, or your might prefer to refer this humidity instrument as a hygrostat.

Either way, it directs towards the same instrument.

This equipment is used either on its own, in an air conditioning equipment, or integrated with thermostat – to achieve the required comfort level.

Humidistat or hygrometer is used in conjunction with fans, humidifiers, and dehumidifiers by itself.

You can use it in conjunction with thermostat in central air conditioning units for better comfort control.

However, this central air conditioning unit should have humidifier installed.

A humidistat controls the humidity through;

  • varying the speed of (or switching on and off) fan or blower,

  • activating or deactivating valves of a humidifier,

  • switching on, or off a dehumidifier,

  • varying opening of dampers


Measurement units:

A hygrostat will measure the relative humidity value.

In other words, the control and display, of a humidistat will be shown in terms of percentage (%).

Click here for more information about relative humidity and air properties.


Working principles and construction of a common hygrostat:

The theory of humidity measurement in air is not complicated.

Try to cut a piece of orange into four. Place one inside your refrigerator, and the other into water.

The rest is for your consumption.

Over time, you’ll notice that the piece inside water will remain in size, and the other shrunk.

This implies that water molecule from the “humid” surface of an orange, migrated into a dry refrigerator atmosphere.

This principle is used in common hygrometers. However, we can’t simply use any material.

Material used have to be sensitive (i.e. able to absorb and expel water vapour) to moisture changes in air.

As such, human hair, or silk is used as moisture sensing elements in a humidistat.

This moisture absorbent must be long and thin, such that small changes in humidity may be detected. In other words, it has to be sensitive to humidity changes in air.

The sensing element will be in tension if humidity setting is higher than ambient humidity, and will slacken in lower humidity setting.

If the sensing element is in tension, it will activate a sensitive switch to either;

  • activate the humidifier (if you live in dry climate and using a humidifier) or,

  • switch off the dehumidifier (if you’re in damper climate and using a dehumidifier)

Let’s say that you live in a dry climate and you’re using a humidifier;

  1. you’ll naturally set the humidity according to your comfort level

  2. this humidity setting will be higher than your ambient relative humidity – and therefore keeps the sensing element in tension

  3. humidifier will be switched on as a result

  4. eventually, the humidifier will switch off as the sensing element slackens and deactivates the switch


Other common humidistat or hygrometer principles:

Here, I’ll explain briefly for you on some variations in working fundamentals of a humidistat;

  • phosphorus pentoxide is also used in sensing humidity in air, as it is very sensitive to humidity changes.

    Air will be passed through the element, and the weight of the element will be continuously weighed.

    The reading will be translated into relative humidity reading

  • electrical resistance due to presence of water, is used as a method to detect humidity in air.

    Change in resistance will be used to interpret change in relative humidity

  • some humidistat use the principle of water condensation to determine the relative humidity of air.

    A polished plate will be subjected to low temperature. As such, water will condense on the plate.

    A light sensor will keep sending signal to the plate, and receive the returning signal.

    Formation of water on the plate will be detected, as the signal is slightly deflected. The time required for condensation to happen will also be recorded.

    Finally, the light signal, and time data will be used to determine relative humidity of air


Variations in hygrometer design:

These are the types of hygrometer or humidistat design, you can find from manufacturers;

  • the older, yet reliable mechanical hygrometers,

  • the later, yet very popular digital humidistat, and,

  • (you’ll love this one) pre-wired humidistat.

    A pre-wired unit works in the same principle as other humidistat. The significant difference lies in its simplicity to use.

    You just have to plug it to power source, and plug your electric heaters, humidifier, fan, or dehumidifier.

    However, it is not usable for air conditioners or heat pumps, as start-stop of an air conditioner during operation must be controlled internally, by its circuit, and not through the power plug

  • you could also integrate it with outdoor humidity sensor, but personally, I don’t feel that it is required

  • similar to thermostats, some hygrometers can be controlled remotely, through phone. Cool stuff





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