We have all walked into a room and immediately felt that it was “stuffy” or “close.” Even if the thermostat says the temperature is fine, the air feels heavy. That invisible factor is humidity. While we often associate it with weather forecasts and frizzy hair, humidity is actually one of the most critical environmental factors in our built environment.
It dictates the comfort of office workers, the safety of art in a museum, the efficiency of a server room, and the health of tenants in social housing. Yet, unlike temperature, which is easy to feel, gauging exactly how much water is in the air is difficult for humans to do accurately.
This is why the Humidity Sensor (or hygrometer) is a cornerstone of the Internet of Things (IoT). Modern devices, such as the Elsys ERS CO2 Lite, combine humidity monitoring with other metrics like temperature and CO2 to provide a complete picture of indoor air quality. But how does a small electronic chip actually “count” water molecules in the air?
At Concept13, we deploy thousands of these sensors to protect buildings and people. This guide explains the science behind humidity detection and why it matters for your business.
The Quick Answer
A humidity sensor is an electronic device that measures the amount of water vapour in the air. The most common type used in modern IoT devices is a Capacitive Humidity Sensor. It works by trapping water molecules between two electrical plates. Water has a unique electrical property (dielectric constant) that changes how much energy those plates can store. The sensor measures this change in electrical capacity and converts it into a digital percentage reading, known as Relative Humidity (RH). This data is then transmitted via networks like LoRaWAN to give facility managers a real time view of environmental conditions.
Understanding the Basics: What are we Measuring?
Before diving into the mechanics of the sensor, we must clarify what “humidity” actually means in this context.
Relative Humidity (RH)
Most sensors, including the Elsys range, measure Relative Humidity. This is expressed as a percentage (0% to 100%).
Think of the air as a sponge.
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0% RH: The sponge is bone dry.
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100% RH: The sponge is completely saturated and cannot hold another drop. If you add more water, it drips (this is rain or condensation).
Crucially, the size of the “sponge” changes with temperature. Hot air expands, so it acts like a bigger sponge that can hold more water. Cold air contracts, acting like a smaller sponge. This is why relative humidity is “relative” to the temperature. A room at 20°C with 50% RH holds far more actual water than a fridge at 4°C with 50% RH.
Dew Point
Some advanced systems use this RH data to calculate the Dew Point. This is the specific temperature at which the “sponge” becomes full (100% RH) and the water vapour turns back into liquid water. This metric is vital for predicting condensation on cold windows or walls, a key factor in mould prevention strategies for landlords.
The Science: How the Sensor Works
In the past, humidity was measured using horse hair, which expands when wet and contracts when dry. Today, we use Micro-Electro-Mechanical Systems (MEMS). These are microscopic components etched onto silicon chips.
There are two main ways these chips detect moisture: Capacitive and Resistive.
1. Capacitive Sensors (The Industry Standard)
This is the technology found in high-quality devices like the Elsys ERS CO2 Lite.
The Mechanism: Imagine a tiny electrical capacitor. A capacitor is essentially two metal plates separated by an insulating material (a dielectric). In a humidity sensor, that insulating material is a special polymer that acts like a sponge. It absorbs water vapour from the surrounding air.
The Detection: Water has a very high “dielectric constant” (it affects electric fields strongly). As the polymer absorbs water molecules, its ability to store an electrical charge changes. The sensor constantly sends a small voltage signal to the plates and measures the capacitance.
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More Water in Air: Polymer gets wetter > Capacitance changes significantly.
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Less Water in Air: Polymer dries out > Capacitance returns to baseline.
Why it is preferred: Capacitive sensors are incredibly accurate, react quickly to changes, and are stable over long periods. This makes them perfect for battery-powered LoRaWAN devices that need to run for years without maintenance.
2. Resistive Sensors
These work on a simpler principle: electricity flows better through wet things.
The sensor consists of two electrodes connected by a conductive polymer or salt. As the material absorbs water, its electrical resistance drops (it conducts electricity better). The sensor measures the resistance to calculate humidity.
Why it is less common: While cheaper to make, resistive sensors are generally less accurate and more sensitive to chemical contamination than their capacitive cousins. For professional building monitoring, capacitive is the standard.
From Chip to Cloud: The Role of LoRaWAN
A sensor that knows the humidity is useless if it keeps that information to itself. In a smart building, we need that data on a dashboard.
Inside a device like the Elsys ERS CO2 Lite, the capacitive sensor is paired with a microprocessor. This processor takes the raw electrical reading and uses a mathematical formula to convert it into a human-readable number (e.g., “45% RH”).
This data packet is then transmitted using LoRaWAN.
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Low Power: Unlike Wi-Fi, which is “always on,” the LoRaWAN radio wakes up, shouts the humidity reading in a fraction of a second, and goes back to sleep. This allows the device to monitor your building for up to 10 years on a couple of batteries.
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Penetration: The signal can travel through walls and floors, meaning you can place a sensor in a basement, archive room or a bathroom without worrying about Wi-Fi dead zones.
Why Combine Humidity with CO2?
You might wonder why the product we recommend, the Elsys ERS CO2 Lite, measures humidity and Carbon Dioxide (CO2). Why not just measure humidity alone?
The answer is that environmental factors are linked.
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Damp and Mould: High humidity can cause mould, but it is often a result of poor ventilation. CO2 is the best metric for measuring ventilation. By examining both, you can identify the root cause. Is the room damp because of a leak (High Humidity / Low CO2) or because people are breathing in a sealed room (High Humidity / High CO2)?
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Comfort: Humans feel temperature differently depending on humidity. A hot, humid room feels oppressive. A comprehensive sensor allows you to calculate the “thermal comfort” index, ensuring your staff or students are working in optimal conditions.
Real-World Applications: Why Measure Humidity?
Why should a business invest in this technology? The applications go far beyond simple comfort.
1. Health and Virus Transmission
There is a “Goldilocks zone” for humidity, typically between 40% and 60%.
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Too Dry (<40%): Viruses like flu and COVID-19 travel further and survive longer in dry air. It also dries out our sinuses, making us more susceptible to infection.
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Too Wet (>60%): Dust mites and mould thrive, triggering asthma and allergies. By monitoring levels with an Elsys sensor, you can ensure your building stays in the safe zone. The Chartered Institution of Building Services Engineers (CIBSE) provides extensive guidance on these healthy ranges.
2. Protecting the Building Fabric
High humidity is the primary cause of mould growth in social housing and rented accommodation. Mould damages the building structure, destroys decoration, and poses a severe health risk to tenants. Automated sensors allow landlords to identify “at-risk” properties remotely before the mould even appears, enabling proactive maintenance.
3. Server Rooms and Archives
In data centres, low humidity generates static electricity, which can fry expensive servers. High humidity causes condensation on motherboards. Similarly, in archives and museums, humidity fluctuations destroy paper and canvas. Continuous monitoring is the only way to ensure these valuable assets.
Conclusion
A humidity sensor is far more than a tool for predicting rain. In the context of the built environment, it is a critical instrument for health, safety, and asset protection.
By utilising the precision of capacitive sensing technology and the reach of LoRaWAN, devices like the Elsys ERS CO2 Lite provide the invisible intelligence needed to run a modern building. Whether you are preventing mould in social housing or ensuring productivity in an office, understanding and managing humidity is the key to a healthier, smarter environment.
