How Do Humidity Sensors Work? | Smart Monitoring Technology

Think about the last time you noticed hot and muggy air in a commercial space. Maybe someone was boiling water in a restaurant kitchen, or left the windows open during a heavy thunderstorm. Both situations raise indoor humidity levels. If humidity gets too high, it can spur mold growth and trigger respiratory issues — two things that can harm occupant health and building structure. 

Luckily, there's a way to track indoor humidity levels without guesswork: using a humidity sensor. But how do humidity sensors work? Essentially, they measure water vapor in the air by identifying shifts in electrical capacitance or resistance. Humidity sensors are a must-have for any building owner who wants to safeguard indoor air quality (IAQ) in their space, whether it's an office, apartment building, or industrial complex. 

In this guide, we'll cover how humidity sensors work, their common types, and how to choose the right one for your commercial building.

The Science of Sensing: Relative vs. Absolute Humidity

Sensors evaluate moisture in the air using two primary measurements: relative vs absolute humidity. 

Relative Humidity

Relative humidity is the most common humidity measurement. It evaluates humidity based on the maximum amount of water that air can carry at a certain temperature. The result is a percentage, such as 80% humidity. Higher percentages mean that humidity is elevated, while lower percentages indicate drier air.

As temperatures climb, air can hold greater amounts of water. For example, at 104° Fahrenheit (40° Celsius), air can hold up to 25 g/m3 of water. But at 50° Fahrenheit (10° Celsius), maximum water capacity drops slightly below 5 g/m3. When outdoor humidity levels meet the 100% threshold, the result is precipitation. 

Humidity's impact on IAQ is substantial. In an indoor environment, high relative humidity means an increased risk of mold growth, condensation, and rust. This is bad news for building assets and occupants, which is why you'll want to keep a close eye on relative humidity levels in your building. According to the U.S. Environmental Protection Agency, the optimal indoor relative humidity ranges from 30% to 50%. 

Absolute Humidity

Absolute humidity measures the exact amount of water in the air. The result is a simple, numerical output, such as 5 g/m3. But to the casual observer, that number doesn't mean much. Unless you're carrying a graph in your back pocket that compares maximum moisture levels with temperature, you won't know whether 5 g/m3 means high or low humidity. There's no context behind the numbers.

That's why most indoor sensors use the relative humidity measurement. It breaks down humidity levels into a percentage that's easy to understand. 

Capacitive Sensors: The Gold Standard For Range And Stability

Humidity sensors fall into three categories: capacitive, resistive, and thermal. Of the three types, capacitive humidity sensors are used most often. That's because they can detect humidity ranges from 0 to 100%, and they do so consistently.

A capacitive humidity sensor consists of a capacitor and a polymer dielectric film. The capacitor includes two electrodes, with the polymer dielectric film inserted between them. 

When water vapor forms in the air, the polymer dielectric film absorbs it. This causes the dielectric constant of the film to increase, and the result is transmitted through the electrodes. Sensor data is used to translate the result into a relative humidity percentage, giving you insight into an area's humidity levels.

Resistive And Thermal Conductivity: Alternative Sensing Methods

Other probes used to measure humidity include resistive and thermal sensors. While they do the job, they aren't commonly used to measure humidity in commercial building environments.

Resistive Humidity Sensors

A resistive humidity sensor has two electrodes, similar to a capacitive sensor. However, instead of using a special film to collect water vapor, it relies on hygroscopic materials such as ionized salts. These materials are placed between the electrodes. When the salts absorb water vapor, the resistance of the electrodes falls, signaling an increase in humidity.

Resistive humidity sensors have a few problems. They're highly sensitive to other pollutants, which can skew results. They're also susceptible to equipment degradation, leading to a shorter lifespan and reduced accuracy.

Thermal Humidity Sensors

A thermal sensor uses two separate probes to evaluate humidity. One probe evaluates ambient air, and the other contains dry nitrogen, which is kept in an airtight container. Changes in the measurements between the two sensors calculate absolute humidity.

You'll find thermal humidity sensors in industrial settings, such as data centers and pharmaceutical labs. These areas often require humidity to be kept at specific levels.

Thermal humidity sensors only evaluate absolute humidity. They don't measure relative humidity. And while they're considered durable, they are sensitive to gases that have different properties from nitrogen. For these reasons, they're not commonly used to measure indoor air quality in commercial building settings.

Why Accuracy Depends On Integrated Temperature Monitoring

When you check your building's humidity, you'll want to see its relative humidity. The measurement tells you whether your property is within optimal levels. However, since relative humidity measurements are dependent on temperature, it's critical to have accurate temperature readings. You can accomplish this through integrated temperature monitoring.

Integrated temperature monitoring combines humidity and temperature sensors for improved accuracy. A temperature sensor uses various materials to measure changes in resistance or voltage. These changes determine the current temperature of the environment.

Relative humidity sensors integrate data from the temperature sensor to determine relative humidity. For example, say the temperature is 68° Fahrenheit (20° Celsius), and the humidity sensor detects 5 g/m3 of water content in the air. Those two data points are used to calculate the relative humidity of the space.

Without a reliable temperature sensor, there's a risk of inaccurate relative humidity readings. 

Choosing The Right Sensor For Your Facility Goals

Tracking the humidity levels of your building can help you identify small issues before they become big problems. However, for the best results, you need a reliable system that includes humidity and temperature sensors. 

While resistive humidity sensors are low-cost, they're not very durable, and accuracy can vary. These types of sensors are best for small appliances and smart home devices. For large spaces and commercial buildings, capacitive relative humidity sensors are your best bet.

Attune is the market leader in fully customizable, commercial indoor air quality systems. We use high-quality capacitive sensors to evaluate humidity levels and transfer data in real-time to our user-friendly platform. With Attune, you can track the key metrics that matter and take action when problems arise. To learn more about our indoor air quality solutions, schedule a demo today.