Humidity Sensor Questions and Answers

Simply put, humidity sensors measure the amount of water vapor present in the air. There are different types of humidity sensors, though. 

A relative humidity sensor measures how much moisture is in the air compared to the maximum amount the air can hold at a given temperature. It’s expressed as a percentage (%). For example, 50% relative humidity means the air is holding half of the moisture it could at that temperature.

An absolute humidity sensor, on the other hand, measures the actual amount of water vapor in the air, regardless of temperature. It’s usually expressed in grams of water per cubic meter of air (g/m³).

At a basic level, humidity sensors have three core functions: 

Sensing moisture by using materials that change properties when exposed to water vapor.

Converting the change into an electrical signal. 

Providing a reading by processing the electrical signal and translating it into a humidity reading, typically expressed as a percentage.

Beyond the relative and absolute humidity distinction, humidity sensors are categorized into three types based on their measurement methods. 

A capacitive humidity sensor measures relative humidity. It uses a hygroscopic dielectric material between two conductive plates. When the dielectric material absorbs water vapor, it alters the dielectric constant and the sensor’s capacitance (the ability to collect and store an electrical charge), which is measured and converted into a relative humidity reading. These sensors are known for high accuracy over a wide range (0-100%).

A resistive humidity sensor (also known as an electrical conductivity sensor) measures relative humidity using the electrical impedance of ions within salts. To do this, it employs a hygroscopic conductive layer of interlocked comb-like electrodes to absorb water vapor. As moisture levels rise, the layer absorbs more water vapor, resulting in decreased resistivity and increased conductivity, and this variation is measured to determine the relative humidity reading. These sensors are less accurate than capacitive sensors due to the absence of calibration standards and are often used in applications where cost is an overriding concern. 

A thermal conductivity humidity sensor uses two probes to measure ambient humidity. One probe is exposed to ambient air and the other is hermetically sealed in dry nitrogen. As humidity changes, the thermal conductivity of the surrounding air changes, affecting the resistance between the two probes. This temperature difference is measured and converted into a relative humidity reading. These sensors are known for their durability in corrosive environments and are used in high-temperature environments when highly precise moisture control is critical.

Humidity sensors are used in a wide variety of industries, including: 

Pharmaceuticals: Monitoring storage conditions for critical assets, such as drugs and vaccines.
Hospitals and Healthcare: Maintaining appropriate conditions for medical equipment and patient care areas.
Life Science Research: Ensuring conditions are vital for researchers and developers to prevent inaccurate results and incomplete research.
Third-Party Logistics: Ensuring the highest product quality and safety throughout the supply chain.
Food and Beverage: Ensuring proper storage and processing conditions.
Manufacturing: Controlling processes that are sensitive to humidity.
Aerospace: Monitoring environmental conditions for equipment and materials.

To choose the right humidity sensor, ask the following questions:  

Do I need to monitor relative humidity or ambient humidity? This is important because selecting the wrong type will provide you with irrelevant data for your specific application. 

How accurate of a sensor do I need for my application? Choosing a sensor with insufficient accuracy can lead to costly errors or compromised product quality. Ensure the sensor covers the humidity range you need and provides the accuracy required for your application. 

How quickly do I need the sensor to respond to changes in humidity? Response time is critical in applications where rapid humidity fluctuations occur because any delay in detection or corrective action can damage products or processes. 

Will the sensor be placed in environments with extreme temperatures, pressure or near contaminants or corrosive material? Neglecting these considerations can result in premature sensor failure or inaccurate readings, and possibly costly replacement or process disruption. If you are operating in one of these environments, choose a sensor that can withstand these factors. 

How easily can the sensor integrate into our existing systems? Make sure to check the sensor’s compatibility with other devices and software. 

Correctly calibrated humidity sensors are essential for ensuring accuracy so you can remain compliant and protect your assets. For all intents and purposes, only individuals who are trained in National Institute of Science and Technology (NIST) standards and specialized equipment should conduct calibrations. The steps include: 

1. Placing the sensor and a standard in a confined, stable environment. 
2. Running comparison tests and recording how much the sensor deviated from the standard.
3. Making zero-point and span adjustments to ensure accuracy. 
4. Performing a final calibration reading.  

Dickson offers A2LA and NIST-traceable calibration services to ensure sensor accuracy. To speed up the process, Dickson also offers Replaceable Sensors™. Instead of doing the calibration yourself or sending your device in for service, you can simply order a pre-calibrated sensor by mail, pop the old one out, and plug the new one in to complete recalibration.

Learn which Dickson products are compatible with Dickson Replaceable Sensors™.

Over time, humidity sensors can lose their pinpoint accuracy due to natural wear and tear. For the most part, you should recalibrate your sensors at least once a year. However, you may need to calibrate more or less frequently depending on your specific application, environment, sensor quality, and risk assessment. For more specific guidance, consult your sensor’s manufacturer or a calibration specialist.

Click here to learn more about Dickson’s calibration services. 

Can I install a replaceable sensor like this myself, or do I need professional assistance?
If you have Dickson data loggers or chart recorders, it only takes a few seconds to replace an old sensor with a Dickson Replaceable Sensor™. 

Dickson humidity sensors are made to perform at a higher level of accuracy compared to typical sensors. 

Many common devices exhibit accuracy ranges of ±3% to ±5% relative humidity. A basic Dickson humidity sensor is calibrated for an accuracy of ±2.0% relative humidity within a 5 to 95% relative humidity range. 

This level of accuracy is often required for industrial and laboratory settings, where slight deviations in humidity can have significant consequences.

Over time, the components inside a humidity sensor can degrade due to constant exposure to moisture, extreme temperatures and potential contaminants. This degradation causes the sensors to “drift” from their original calibration and produce inaccurate readings. 

Where the sensor is installed can also have a significant impact. Air flow, proximity to heat sources and other factors can alter readings. 

Dust, chemical vapors and other airborne particles can accumulate on the sensor’s surface, interfering with its ability to accurately measure humidity. 

How often you calibrate a humidity sensor can also play a role in sensor accuracy. 

Yes! Humidity sensors require careful storage and handling. They should be kept in clean, dry environments within specified temperature and humidity ranges and be protected from contaminants and excessive vibrations or sudden impacts. Additionally, it’s crucial to avoid direct handling of the sensing element and to store sensors with their calibration data to maintain accuracy.

There are currently six Dickson Data Loggers that are compatible with Dickson’s humidity sensors: 

Cobalt X Data Logger
DicksonOne Touchscreen Data Logger
Cobalt XS Data Logger
DicksonOne Display Logger
Compact USB-Enabled Data Logger
Display USB-Enabled Data Logger

Depending on your needs, the options for loggers range from USB-enabled loggers to our DicksonOne, LoRaWAN, Ethernet and Wi-Fi connected loggers.