Surely in your home, you have heard the term humidity since it is quite common and we assure you that it is always related to water. In fact, the humidity is due to the water molecules. But how do we gauge it? The humidity can be measured by humidity sensors.

In simple terms, the definition of a humidity sensor can be as follows. It is a device that helps you measure the level of liquid or relative humidity in a specific area.

The term humidity which is the physical magnitude that makes these sensors have their reason to exist and, therefore, should be a section that we must study to better understand the operation of these sensors.

What is humidity?

Humidity is the phenomenon that is present in a predetermined area when the water molecules make contact with each other, it causes a reaction to what we call moisture and can be found mainly in two states: solid or gaseous.

Humidity and temperature sensors use this property since relative humidity, unlike absolute humidity, tends to take both temperature and pressure into account. Therefore, sensors manufactured for relative humidity measure both humidity and humidity. air temperature

Where are humidity sensors used?

It is very common to use these sensors with regulators to maintain humidity in some particular places. They are also quite used in heating, ventilation and air conditioning systems, and even in production systems where humidity control is required.

Another great use of these sensors is usually to measure moisture in materials such as cereals, rice, plastic granules, coal, cement, coating masses, biomass, etc.

How do humidity sensors work?

The operation of the humidity sensor can be summarized as follows when the sensor comes into contact with the environment you want to measure the humidity, the sensor takes a sample and transforms it into electrical signals, which are usually sent to devices control, such as microcontrollers, PLC, etc.

And how is it doing this transformation? Well, that is a bit more complicated since it requires a slightly more technical explanation, since in this process, a semiconductor material is required which determines the humidity and even medium temperature values ​​with very good precision.

What types of humidity sensors exist and how are they classified?

Interesting question, right? We will summarize that information in the best possible way.

We can classify these types of sensors according to the material and / or the principle used to make the measurements, for example:

Capacitive humidity sensors

Capacitive sensors for measuring humidity are very interesting and if you are not related to the capacitive term I will mention what it is about quickly.

What is the capacitive effect?

The capacitive effect in electronics is a phenomenon in which a device or material has the capacity to store charge.

When a material has capacitive properties, it means that it has the ability to store electric charge and prevents conduction, these types of components are known as capacitors. Capacitors that use a material known as the dielectric, increase capacitance --a property that indicates how much charge it can store.

Now, when the load is large enough this dielectric is defeated by the electric current and the component behaves like a conductor.

Thermal conductivity humidity sensors

This type of sensor falls within the classification of resistive, has the ability to measure the absolute humidity of both air and water vapor.

Operation of humidity sensors with thermal conductivity

Humidity sensors based on thermal conductivity for its operational use have negative temperature coefficients and are connected in a bridge-type circuit, where one of the thermistors is hermetically sealed inside a chamber filled with dry nitrogen and while the other is exposed to the environment through small ventilation. And at the moment the circuit is turned on, the resistance of the two thermistors is calculated and the difference between the two is directly proportional to the absolute Humidity and ready you have sensed the humidity.

What is a thermistor?

A thermistor is a type of electrical resistance whose value varies depending on the temperature. In other words, it is said that a thermistor is a type of sensor that is used to measure changes in temperature detected through resistance changes according to the cold or heat detected.

There are two fundamental types of thermistors:

1. Negative Temperature Coefficient or NTC

2. Positive Coefficient Temperature (PTC)

As they are small in size, they make the response to temperature changes quickly, achieving greater sensitivity to temperature changes compared to other transducers.

Humidity and temperature sensors

I think it is not so necessary to delve into very complex technical aspects to understand that humidity sensors with thermal conductivity react to temperature changes, as we saw a moment ago in the explanation of thermistors. It only remains to choose which one you need, one with a positive or negative temperature coefficient.

Advantages of moisture sensors with thermal conductivity

1. It is highly efficient in high corrosion and high-temperature environments.

2. Their manufacturing makes them extremely durable

3. It has a higher resolution than other sensors.

Application of humidity sensors

The most important aspects of these sensors that have many applications in our daily life and we go step by step analyzing these magnificent components.

If they are exposed to gases with thermal properties that nitrogen cannot withstand, moisture measurement usually gives erroneous data.

The most important applications can be summarized in:

1. Drying ovens

2. Clothes dryers

3. In food dehydrating machines

4. Pharmaceutical plants

Conclusion

When it is required to choose some humidity sensor it is important to consider the following points and thus have the correct sensor.

1. Sensor accuracy depends on the scale and magnitudes you want to measure

2. Initial calibration and preventive maintenance

3. Sensor size, you can not always use a generic and more in cars you must use the one marked by the manufacturer

4. Sensor cost and replacement cost, don't get a surprise when you want to replace

5. Output Repeatability

6. The complexity of the circuit

7. Pollution resistance

8. Sensor reliability

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