Humidity sensors are used to measure the humidity level in an area. Their working principle is based on the principle of capillary action. When water evaporates, it leaves behind the mineral salts that are dissolved in the water. The salts left behind cause hygroscopic materials (that is, materials that absorb water) to swell up and become lighter than they were before.
What is Humidity Sensors?
A humidity sensor is a device that detects the amount of water vapor present in the surrounding atmosphere. They are used to measure relative humidity, which is defined as the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature. There are many ways one can create a humidity sensor, but what they all have in common is that they are able to measure what the humidity is at a given time.
Humidity sensors are often found on mobile devices such as smartphones, tablets, and laptops and include everything from capacitive humidity sensors to pyroelectric humidity sensors. They are often used to save battery life by reducing screen brightness when the light is not necessary, but can also be used for other things like automatic dehumidification in a room or adjusting the heating/AC according to what’s needed.
Humidity sensors are also commonly found in manufacturing equipment such as paint-curing ovens which need very specific humidity levels to work properly.
The use of humidity sensors ranges from what is commonly seen on consumer devices, to what can be found in very specific industries where accurate measurements are required. It’s the accuracy that lies behind these uses that makes them so important for what they can do.
Humidity sensors are used for what they are capable of measuring and the possibilities of what can be done with those measurements.
Type of Humidity Sensors
Humidity sensors come in many different types and styles depending on what they are used for, but all work on the basic principle of detecting what is going on around them and reporting it back.
When it comes to humidity sensors, you will find that there are several different types. Some of these types include electrochemical, resistive temperature devices (RTD), capacitive and polymer-based sensors.
Electrochemical Type Humidity Sensors
The electrochemical type of humidity sensor is often referred to as a ‘wet’ sensor. This is because they are used in applications where the humidity level can be measured through condensation. The best example is alcohol sensors, which use an electrolytic cell to detect any percentage of alcohol present based on the associated resistance change or voltage drop across the sensor.
RTD Type Humidity Sensors
The RTD type of humidity sensor is also known as a resistive temperature device. These are often referred to as ‘dry’ sensors due to the fact that they do not rely on condensation for their measurements. They use a thin polymer film, which changes its electrical resistance depending on the surrounding air’s humidity level.
Capacitive Type Humidity Sensors
These type of humidity sensors work on a principle known as capacitance. These sensors will create an electric field and measure the dielectric constant between two electrodes to determine the level of humidity present. These types of humidity sensors are often used in air conditioning applications that demand high accuracy.
Polymer Type Humidity Sensors
In contrast to RTD and capacitive types, polymer-type humidity sensors rely on the fact that they contain polymeric electrolytes. These electrolytes change their conformation as humidity levels rise and lower. This changes the conductivity of the sensor material, allowing it to produce a signal which can be used to determine any present humidity.
There are a lot more different types of humidity sensors, but the main thing to remember is that they vary depending on what humidity levels they can measure.
How do Humidity Sensors Work?
Humidity sensors are very important to modern technology because they allow devices to adjust what they do according to what’s going on in the environment, whether that means adjusting what apps run and how much power is used by default, or saving battery life and looking for weather updates to decide what activities can be done outside
Humidity sensors are often used in workplaces to monitor the humidity levels of certain areas. There are several different types of humidity sensors, with most relying on an electrical conductivity-based measurement. When water is present, it can act as a solvent making it easier for ions to flow through a solution.
Probe Humidity Sensor:
This type of humidity sensor is based on the electrical conductivity changes as a result of the presence of water in the air. They are relatively low-cost and can be made from off-the-shelf components to mimic any desired humidity range.
The probe humidity sensor consists of two electrodes made from nickel-plated brass. The electrodes are placed inside a protective tube, to help prevent the propagation of contaminants into the cell body. These electrodes are then inserted into the sensor package, where they can be exposed directly to the surrounding air or submerged in water.
As the humidity rises, so too does the conductivity of the solution between the two electrodes. This increased conductivity is measured by a microprocessor inside the sensor. The resistance between these two electrodes increases exponentially with increasing concentration of water vapour.
To compensate for any changes in the resistance value, the sensor has to be calibrated with a known humidity level. The microprocessor inside the sensor performs this calibration by sending a signal between both electrodes. This known signal is sent through the solution and back again to determine its conductivity using Ohm’s law.
The circuit inside the sensor can be adjusted to mimic any desired humidity range, which makes it ideal for many different laboratory setups. But it’s not just water vapour that affects conductivity – other contaminants in the air can have a similar effect.
Most probe humidity sensors are best used in scenarios where contaminants are kept to a minimum.
Capacitive Humidity Sensor:
This type of humidity sensor is based on the capacitance or electric charge storage of an object. A capacitor is made up of two conductive surfaces separated by insulation – this insulation allows for the storage of electric charge.
The capacitance of a capacitor is affected by the dielectric constant present between its plates. Capacitive humidity sensors rely on this concept of measuring the changes of an electric field as a result of water vapour in the air.
A typical capacitive humidity sensor consists of a metal electrode and a polymer dielectric material.
The conductive surface of the electrode is covered with a film layer consisting of resistors made from silica-based material, making them basically an array of resistors. The capacitance of this capacitor varies as different concentrations of water vapour are introduced to the surrounding air.
The capacitance of this sensor can be mathematically modeled as a function of frequency and atmospheric pressure, allowing accurate readings to be taken over any desired range of humidity levels.
This type of sensor is best suited to work in clean and dry conditions, due to the need for a sensor surface that can provide a high degree of purity.
Resistance Temperature Sensor (RTD):
A resistance temperature detector (RTD) is essentially a type of resistor which has been carefully designed with an accurate resistance-to-temperature relationship.
RTDs are commonly used in industry to measure the temperature of a hot surface, such as the combustion chamber of a gas turbine engine. The resistance value of these devices can be heavily affected by any gases that might be around – this makes them unsuitable for use in humidity sensing.
A humidity sensor using an RTD will work in much the same way as a resistance bridge. A fixed voltage is applied between two leads, and the resulting current is monitored through these leads. The result of this process can be mathematically used to determine the resistance value at that given moment in time.
RTDs don’t require calibration and they’re very accurate – but they tend to be expensive, and can take a very long time to stabilize in humid conditions.
Infrared (IR) Sensor:
An infrared sensor is essentially an array of photocells that respond to a given change in light intensity.
See Video How Do Humidity Sensors Work?
So you’ve learned that humidity sensors measure the concentration of water vapor in the air, and they use a polymer to do so. They also use electrical properties to convert this value into something more human-readable (like percentage relative humidity). Not only that but humidity sensors can be used in many different applications.
That’s it! Thanks for reading my article on how do humidity sensors work. You can find a more in-depth explanation in the original text, and I hope you check that out to learn even more about humidity sensors. Until next time!