What Is A Humidity Sensor?


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A humidity sensor detects changes in temperature or electrical currents in the air, typically using a humidity probe connected to a receiving device. There are three types of humidity sensors: relative humidity (RH) sensors, absolute humidity (AH) sensors, and thermal humidity sensors. 

If you have ever grown produce in a greenhouse, or your house is prone to mold, then you will be familiar with the importance of measuring humidity levels. For many of us, high humidity usually means it’s time to unbox the air conditioner and fans to keep you and your house cool. This time of the year is also when your product could potentially spoil in your greenhouse or on your farm. 

To better understand humidity and how it is measured in a wide range of settings, let’s take a look at what a humidity sensor is, and why we use them. 

What Are Humidity Sensors?

Humidity sensors will become your best friend when it comes to monitoring the humidity levels in your home and/or work setting. A humidity sensor is an electronic device that measures the humidity of an environment, corresponding to an electrical signal produced by a humidity probe.

Humidity sensors come in different sizes and functionality. For example, some are handheld, while others are integrated into large systems. Depending on what industry or application you are working with, will determine which humidity sensor is best for your needs. 

Humidity sensors can be split into three groups: 

  • Relative Humidity (RH) Sensors 
  • Absolute Humidity (AH) Sensors 
  • Thermal Humidity Sensors

Relative Humidity Sensors

The working principle of relative humidity (RH) sensor involves comparing the current humidity level at a given temperature, to the maximum level of humidity of air possible at the same temperature. Therefore, the temperature must also be measured with a temperature probe, when using an RH sensor. 

Relative humidity measurements are given as a percentage (%).  

Relative humidity sensors are separated into capacitive and resistance humidity sensors.

Capacitive humidity sensors use two electrodes to monitor the electrical charge (capacitance) between a thin metal inside the humidity probe. The change in the humidity is directly proportional to whether the metal’s capacitance increases or decreases. If a change in voltage is detected from an increase in humidity, the signal is sent to the receiver (usually an embedded computer) for processing. 

Capacitive humidity sensors are one of the most popular humidity sensors, as they provide users with stable readings over long usage and can detect a wide relative humidity range (0-100%).

Resistive humidity sensors (electrical conductivity sensors) operate differently from capacitive sensors. They 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. 

Absolute Humidity Sensors

Absolute humidity (AH) sensors do not require the temperature to be measured, as they calculate humidity without a reference to temperature. They work by measuring the amount of water vapor in the air per unit volume. Measurements are expressed as mass per unit volume, usually grams per cubic meter (g / m3).

The higher the AH, the more water vapor in the air, the lower the AH, the less water vapor is present in the air. 

Thermal Humidity Sensors

Thermal humidity sensors are commonly used to measure absolute humidity. They also differ from RH sensors, as they use two humidity probes instead of one. This is because thermal humidity sensors require one probe to measure dry nitrogen and the other to measure the surrounding air (exposed probe).

The difference in thermal conductivity is measured when humidity is collected on the exposed probe and received by the sensor. From there the AH is calculated. 

What Is A Humidity Sensor’s Dew Point? 

You may have come across the term “dew point” when measuring humidity levels. The dew point is the temperature at which air must be cooled to achieve 100% RH or full saturation.

If the air is cooled past that temperature, the water vapor will escape from the atmosphere, producing fog or rain. The greater the dew point, the more water vapor is in the air, and therefore, the more humid the surrounding air.

Humidity Sensor Applications

Humidity sensors are used in many applications and industries including cold storage management, making environments comfortable in buildings, and preventing industrial failures, such as leakage. 

Cold storage management measures humidity in temperature-sensitive environments like food, beverages, and pharmaceuticals. Humidity sensors not only check the status of the equipment, but also ensure increased product safety, and improve environmental performance by reducing waste.

Smart buildings and facilities are now using humidity sensors to improve the comfort of occupants. HVAC (heating, ventilation, and air conditioning) systems are used to control the moisture in different rooms and offices, offering a more comfortable, healthy, and productive working environment. Humidity sensors are also used for reducing energy consumption. 

Humidity sensors also have great use in museums and galleries, where paintings and sculptures must be kept at certain humidity levels. Humidity sensors are used to prevent damage and lower insurance costs. 

On a larger scale, humidity sensors are used to identify issues before they get out of hand in real-time and long-term tracking of plant transpiration. Humidity sensors are also applied to warn industrial workers of potential leakage, maximizing efficiency. 

Other applications include monitoring temperature levels inside greenhouses, controlling humidity levels when growing cannabis inside a grow room, and in weather stations to track environmental conditions and help create forecasts. 

How To Select A Good Humidity Sensor?

As the price and effectiveness of different humidity sensors differ, selecting one that is right for you can be challenging. When selecting a humidity sensor, there are some judgment considerations:

  • Accuracy: Humidity sensors have a standard curve, typically using a 9-point system, which ensures accuracy. 
  • Linearity: This interprets the BFSL’s (best fit straight line) distinctive value and the measured output voltage value, transformed to relative humidity. 
  • Reliability: The humidity sensor must be able to provide accurate readings. 

At Atlas Scientific, the EZO Humidity Circuit is designed to be embedded with your electronics, reading the humidity of your enclosure.

The EZO Embedded Humidity Probe has a weather-resistant housing that allows for multiple mounting options, great for monitoring the humidity levels inside a home, workspace, or even in your backyard.

Summing Up, Humidity Sensors

Humidity sensors play a critical role in many applications to control and measure humidity levels so that, if needed, changes can be made. Humidity sensors include relative humidity (RH) sensors (capacitive and resistance sensors), absolute humidity sensors (AH), and thermal humidity sensors.

If you have any questions about humidity, or what humidity probe 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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