Humidity Sensor Types

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There are three primary humidity sensor types: capacitive, resistive, and thermal conductive sensors. The first two are designed to sense relative humidity (RH), and thermal conductivity sensors are used to detect absolute humidity (AH).

A humidity sensor is one of the most important devices used in a wide range of applications to measure and monitor humidity levels. 

The working principle of a humidity sensor is simple, it detects changes that alter electrical currents or temperature in the surrounding air. There are three main types of humidity sensors: capacitive, resistive, and thermal. 

Humidity Sensor Basics

Before we dive into the different humidity sensor types, it is important to understand some basic terms. 

Humidity: Humidity is the amount of water in the surrounding air, which can soon become uncomfortable for mankind if it increases dramatically, therefore environments must be monitored. 

Humidity is also an essential factor to measure when using electronics or when operating other sensitive equipment to ensure that the humidity of the environment is suitable for such devices.

Absolute Humidity (AH): Absolute humidity is the ratio of the mass of water vapor to the volume of air, expressed in grams per cubic meter. Absolute humidity can be calculated from relative humidity, air temperature, or measured directly. These systems use two thermistors in a bridge configuration.

Relative Humidity (RH): Relative humidity is the amount of moisture in the air compared to the moisture level at the same temperature and pressure needed for saturation, expressed as a percentage. Most humidity sensors work using the RH principle because of the high accuracy, dependability, and low cost.

Dew Point: The dew point (expressed in °C or °F) is the temperature and pressure to which air must be cooled to become saturated with water vapor, assuming that the water content and the air pressure are constant. 

Capacitive Humidity Sensors

It is estimated that 75% of humidity sensors follow the capacitive technique. These humidity sensor types rely on electrical capacitance to provide the user with a humidity value.

Capacitive relative humidity (RH) sensors consist of two metal electrode layers between a dielectric (non-conductive) material, typically a polymer film with a dielectric constant of around 2-15. The dielectric film inside the capacitive humidity sensor attracts and absorbs moisture from the surrounding air. Once the moisture contacts the electrodes, a voltage change occurs.

When moisture is absent, the capacitance is determined by the geometry of the capacitor and the dielectric constant (relative permittivity) of the dielectric material; As the humidity changes in the surrounding air, the dielectric polymer absorbs and releases water vapor, therefore the electrical capacitance of the sensor changes. 

To put it into perspective, the dielectric constant of water vapor at room temperature measures 80, which is much larger than the dielectric material. When the dielectric material absorbs water vapor from the surrounding environment, an increase in the dielectric constant occurs, increasing the capacitance of the humidity sensor. 

In capacitive humidity sensors, there is a direct relationship between the RH of the surrounding air, the amount of moisture in the dielectric material, and the capacitance (dielectric constant) of the humidity sensor. The change in the dielectric constant is directly proportional to the RH, therefore, by measuring the dielectric constant, the RH can be calculated. 

Pros Of Capacitive Humidity Sensors

  • They can detect a wide measurement range of relative humidity
  • The output voltage is almost linear – can measure relative humidity from 0% to 100%
  • They provide the user with stable results over a long usage period
  • Low cost
  • Requires little maintenance 

Cons Of Capacitive Humidity Sensors

  • Distance limitations between the signaling circuit and the sensor
  • Loss of relative accuracy below 5% relative humidity

Capacitive Humidity Sensors Uses

  • HVAC systems
  • Automobiles
  • Food processing systems
  • Printers 
  • Weather stations 
  • Refrigerators
  • Ovens
  • Laundry machines

Resistive Humidity Sensors

Resistive humidity sensors, also known as electrical conductivity sensors, measure the change in resistivity between two electrodes inside a humidity probe (connected to the sensor) to establish relative humidity. 

They have a similar principle to capacitive sensors; an electrical change is measured, producing an RH value. However, resistive humidity sensors use a moisture-absorbing (hygroscopic) material, so their operation principle is slightly different.

Inside a resistive humidity sensor, the hygroscopic conductive layer* acts as a polymer humidity sensing film, which contains comb-like electrodes. The electrodes typically come from noble metals like gold, silver, or platinum, and are arranged in interdigitated patterns to increase the contact surface area between the electrodes and the hygroscopic layer. The output voltage has an inverse exponential relationship to RH. As more water vapor is absorbed, the resistivity decreases due to an increase in the non-metallic conductivity material’s conductivity

Modern resistive humidity sensors contain a ceramic substance, which provides the sensor with extra protection. This is particularly important to shield the sensing elements from contaminants.

*The hygroscopic layer is typically a conductive polymer, salt, or treated substrate.

Pros Of Resistive Humidity Sensors

  • Low cost 
  • They have a small footprint
  • As they have no calibration standards, they are highly interchangeable 
  • The distance between the humidity sensor and the signal circuit can be large (high surface-volume ratio), therefore they can be used for remote monitoring applications and measuring humidity changes in environments with up to 90% RH at room temperature

Cons Of Resistive Humidity Sensors

  • They are sensitive to contaminants, hence the introduction of ceramic substances for added protection
  • If using with water-soluble products, the output readings can shift
  • They are not optimal for measuring RH values below 5%

Resistive Humidity Sensors Uses

  • Industrial, domestic, or residential and commercial applications

Thermal Conductivity Humidity Sensors

Thermal conductivity sensors have different operating principles compared to capacitive and resistive humidity sensors. These types of sensors measure the absolute humidity (AH) of the surrounding air/environment by calculating the difference between thermal conductivity in dry air vs humid air. 

A thermal conductivity humidity sensor consists of two matched negative temperature coefficient (NTC) thermistor elements, suspended by thin wires, in a bridge circuit. One thermistor is located in an exposed chamber via several ventilation holes, exposing it to the surrounding environment. The second is hermetically encapsulated in dry nitrogen, and located in a different section within the humidity sensor. 

An electrical circuit passes a current between the two thermistors, resulting in the thermistors self-heating; resistive heating increases the sensor’s temperature. When one of the thermistors is exposed to humid air, the conductivity changes. The difference in resistance between the two thermistors (bridge circuit) is directly proportional to absolute humidity. 

Pros Of Thermal Conductivity Humidity Sensors

  • They are very resistant 
  • They can be used in high-temperature environments
  • They are suitable for high-corrosive environments
  • They provide the user with a greater resolution at higher temperatures than capacitive and resistive humidity sensors 

Cons Of Thermal Conductivity Humidity Sensors

  • Exposure to gases with thermal properties different from nitrogen can affect humidity readings 

Thermal Conductivity Humidity Sensors Uses

  • Pharmaceutical applications
  • Ovens 
  • Drying kilns
  • Drying machines
  • Food dehydration methods

Humidity Sensor Considerations

When deciding which humidity sensor will suit your testing needs, you should also consider the following points. 

  • Accuracy
  • Repeatability
  • Interchangeability
  • Long-term stability
  • Ability to recover from condensation
  • Resistance to chemical and physical contaminants
  • Size
  • Packaging
  • Cost-effectiveness

Atlas Scientific Humidity Sensor Types

Atlas Scientific offers:

This humidity sensor type works on a wide measuring range (0-100%) with the ability to display air temperature, absolute humidity, as well as the dew point. 

The accuracy of humidity sensors in measuring humidity is +/- 2% within a range of 0-100%, and the response time is 1 reading per second (UART mode) or 1 reading per 300 milliseconds (I2C mode). 

Summary 

Humidity sensors are a useful tool to measure the amount of water in the surrounding air. There are three primary types of humidity sensors: capacitive, resistive, and thermal conductivity humidity sensors.

Capacitive and resistive humidity sensors are designed to sense relative humidity (RH), and thermal conductivity sensors are used to detect absolute humidity (AH). Although they follow the same principle,   capacitive humidity sensors are typically more accurate and stable, while producing repeatable results.

If you have any questions about humidity, or what humidity sensor will best suit your testing needs, do not hesitate to contact our world-class team at Atlas Scientific.

Humidity Circuits & Probes

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