How Do Humidity Sensors & Probes Work?

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Humidity probes detect changes that alter the temperature or electrical currents in the air. There are three main types of sensors: capacitive, resistive, and thermal conductivity depending on what industry or application you will be measuring. 

Humidity is how much water air contains. The presence of water in the air can affect human health and many applications and industries. Water vapor in the air also affects physical, chemical, and biological processes, which is why it is important to monitor humidity levels with a humidity sensor and probe. 

Humidity probes are used everywhere. Ever wondered how an AC unit keeps the air dry while it is still humid outside? Well, that is because a humidity sensor inside cleans the air for you. So, let’s take a look at how humidity probes can do that. 

How Do Humidity Probes Work?

Humidity probes detect changes that alter the temperature or electrical current in the air. Most humidity probes rely on two electrical conductors (electrodes) with a non-conductive polymer film between the electrodes to create an electrical field. The moisture from the surrounding air is collected on the film, which causes a change in the voltage level between the two electrodes. This change is then converted into a digital measurement for you to read the relative humidity, taking the air temperature into account.  

There are three different types of humidity sensors:

  • Capacitive 
  • Resistive 
  • Thermal conductivity 

Capacitive

Capacitive humidity sensors are the most commonly used  humidity probes. They get their name from their “capacitor”; the capacitor is made of two electrode layers with a dielectric material between them. The dielectric material can absorb moisture from the surrounding air, the technical name for this is hygroscopic. The dielectric used in capacitive probes is a polymer film, (for example metal oxide) which usually has a dielectric constant between 2 and 15. 

When the dielectric material absorbs water vapor, the dielectric constant increases. This then increases the capacitance (ability to collect and store energy) of the probe. The relative humidity of the air, the moisture inside the dielectric material, and the capacitance all have a direct relationship. Changes in the dielectric constant are directly proportional to relative humidity, therefore, when the capacitance is measured, the relative humidity can be recorded. 

They are among the most popular humidity sensors, as capacitive humidity probes provide users with stable readings over long usage and can detect a wide relative humidity range (0-100%). 

Capacitive Humidity Sensor

Resistive

Resistive humidity sensors (or electrical conductivity sensors) measure the atoms’ electrical impedance by utilizing ions in salts. When humidity changes, so do the resistance of the electrodes, this is what establishes relative humidity. 

There is a hygroscopic conductive layer inside the probe that contains a set of comb-like electrodes laid out in an interlocked pattern. This pattern increases the contact area between the electrodes and the conductive layer.  As more water vapor is absorbed, a decrease in resistivity is recorded as the conductivity of the material increases. 

These humidity probes are low cost, small, and are highly interchangeable as they have no calibration standards. If you do use a resistive humidity sensor, it is important to note that they are relatively sensitive to contaminants and output readings can fluctuate if you are using water-soluble products.

Resistive Humidity Sensor

Thermal Conductivity

Thermal conductivity humidity sensors are used to measure absolute humidity. One probe measures the ambient air and the other is hermetically sealed (airtight) in dry nitrogen. The difference in resistance of the two probes will be directly proportional to the absolute humidity.

They are often used for measuring humidity in high-temperature settings or corrosive environments, are very durable, and provide users with higher resolution than other humidity probes. When using this sensor, avoid exposure to a gas that has a different thermal property than nitrogen, as it can affect the humidity reading.

Thermal Conductivity Humidity Sensor

Applications That Use Humidity Probes

Humidity probes are used to measure and control humidity and moisture conditions in many industrial, commercial, and consumer applications. 

  • In HVAC (heating, ventilation, and air conditioning) systems, humidity probes are widely used in homes and the workplace. The probes are used to enhance temperature monitoring to make environments more comfortable and energy-efficient. 
  • Humidity probes are also used in newborn incubators and other neonatal ICU facilities in medical applications
  • Weather stations rely on humidity probes to track environmental conditions and help create forecasts. 
  • Humidity probes are used to control cabin ventilation and are used to prevent windshields from fogging in automobiles
  • In the food processing industry, humidity probes are used to monitor and regulate complex processes such as brewery techniques and within the milk and butcher industry. 

Selecting A Humidity Probe

It is extremely important to measure humidity, and selecting one that is right for you can sometimes be overwhelming. Before selecting a humidity probe, there are a few things you must consider:

  • Accuracy & reliability of the probe
  • Calibration requirements
  • The size of the probe
  • How much it will cost and the cost for replacement parts
  • Output reliability
  • Circuit complexity
  • Resistance to contamination

Whether you want to measure humidity levels inside your home, workspace, or even outside, Atlas Scientific’s  humidity probe provides you with accurate readings that can be used in countless applications, plus, it is 100% operational out of the box – calibration is unnecessary!

Summary

Humidity can affect human health and many applications and industries

Most humidity probes rely on two electrical electrodes with a non-conductive polymer film between the electrodes which creates an electrical field. The moisture from the surrounding air is collected on the film, causing a change in the voltage level between them. 

The three types commonly used are: Capacitive, resistive, and thermal conductivity. 

If you would like to learn more about humidity, or what other temperature testing kits we have to offer, do not hesitate to contact our world-class team at Atlas Scientific.

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