Moisture Sensor | Air Humidity Measurement Sensor

A hygrometer is necessary for measuring relevant indicators, not only in everyday life but also in agriculture and industry (for example, to measure soil moisture or residual moisture in wood during the drying process).

Humidity Sensors

In everyday life, an air humidity control sensor ensures microclimate control; in enterprises, it ensures the accuracy of technological processes and the safety of equipment; in agriculture, it assesses soil quality and fertility. Of course, the setup of a room sensor differs from an industrial one. Moreover, the measurement method itself differs. To draw conclusions or configure equipment for joint operation, it is important to understand exactly what quantity humidity is measured by. There are several options here:

Absolute value, in grams per cubic meter;
Relative value, in RH units;
As a percentage of the mass of the samples studied, if referring to solids or materials;
In parts of water per 1,000,000 parts of sample weight or ppm.

Absolute humidity or moisture capacity can vary from 0 to 100% (i.e., up to full saturation, theoretically). Most household hygrometers measure this value.

Principle of Operation of the Air Humidity Measurement Sensor

There are 5 types of hygrometers, differing by their operating principle:

Capacitive. These are simple models representing capacitors with air as the dielectric. The dielectric constant of air is directly related to humidity, and when humidity changes, the capacitance of the air capacitor changes as well. There are also models with a dielectric in the air gap: they perform better than "just air" ones. Such devices can already measure water content in solids (allowing measurement of the humidity of the sample placed between the capacitor plates if it exceeds 0.5%). Thin-film hygrometers with comb electrodes instead of plates also belong to this category. They also include temperature sensors that provide compensation.
Resistive. Structurally, these humidity sensors consist of two electrodes on a substrate, with a low-resistance material applied over the electrodes (the resistance value changes significantly depending on humidity). Aluminum oxide, which absorbs moisture well from the environment, is often used as the coating. Resistive sensors measure the current flowing and are inexpensive.
Thermistor or psychrometric. These devices consist of a pair of identical thermistors (nonlinear electronic components whose resistance strongly depends on temperature). They work as follows: one thermistor is placed in a sealed chamber filled with dry air, the other in a chamber with holes through which air for measurement passes. The thermistors are connected in a bridge circuit: if the output voltage is zero, the humidity in the chambers is the same; if not, the difference in humidity between the chambers can be measured according to the output voltage value.
Optical, also called condensation hygrometers. This is the most accurate type of device, based on the physical concept of the "dew point." The temperature at which condensation forms on the material surface is determined. Depending on the dew point temperature, the ambient humidity is measured. In the simplest case, such designs consist of an LED illuminating a mirrored surface, after which the light beam changes direction and hits a photodetector. The mirror is heated or cooled by a high-precision temperature controller (thermoelectric cooler), and the temperature is recorded by a corresponding sensor at the moment condensation forms. It is important that the mirror is clean: in condensed water droplets, light rays refract, and the current in the photodetector circuit decreases.
Electronic. The main operating principle of this device is measuring the concentration of the electrolyte coating the electrically insulating material. A concentrated lithium chloride solution, highly sensitive to humidity changes, is often used. Electronic hygrometers are often equipped with a thermometer, allowing highly accurate measurements. Electronic hygrometers are also used for soil moisture measurements, consisting of two electrodes immersed in the soil. Humidity is measured depending on the soil's conductivity level.

Types and Varieties of Air Humidity Measurement Sensors

When choosing a specific sensor type based on its operating principle, the main factors to consider are:

Which humidity quantity needs to be measured – relative or absolute;
Where the humidity will be measured – in air, soil, or material sample;
Whether hysteresis matters, the required measurement accuracy, and the measurement range.

Thus, optical sensors are considered the most accurate but also the most expensive. Capacitive sensors are often used in household appliances and industrial equipment. Their key advantage is resistance to high temperatures and chemical vapors. Resistive detectors, commonly used in everyday life, operate with a relatively short response time, from 10 to 30 seconds. They can operate in a temperature range from -40 to +100 degrees but are sensitive to chemical and oil vapors. Electronic sensors are good because, thanks to computer calibration, they work with high accuracy.

All these models have advantages and disadvantages, as well as factors affecting measurement accuracy.

Application of Air Humidity Sensors

In industrial conditions, hygrometers measuring relative humidity are more commonly used to determine the relative humidity of soils, materials, or rooms. They are equipped with built-in signal converters and easily integrate into the corresponding measurement system. These devices may also have a built-in temperature sensor to conduct comprehensive microclimate control and establish a real connection between temperature and humidity levels.

Several types of sensors are most accessible for measuring relative air humidity: psychrometric, aspirational, capacitive, and resistive. Let's consider each type of sensor in more detail.

Capacitive and resistive type sensors are often used in office climate control systems, where humidity levels can vary from 30 to 70%.

For agro-industrial complexes (greenhouses, mushroom farms, vegetable storage facilities), such models are not suitable, as in conditions of high humidity and possible condensation, they malfunction and can show values with an error of up to 6%. In this case, the use of psychrometric sensors is recommended.

If measurements are made in areas with airflow, it is worth using an aspirational sensor, that is, a psychrometric sensor supplemented with a fan. Due to the operation of the electric fan on the wet thermometer, a regulated airflow is created. When measuring high relative air humidity, such a device gives an error of 1%, no more.

Overall, the scope of application of air humidity sensors is very wide and includes:

Maintaining the microclimate within specified limits in production equipped with humidity-sensitive electronic devices;
Monitoring humidity levels in office premises and at home;
In the housing and communal services sector – in boiler rooms and water treatment plants to prevent condensation formation;
Periodic monitoring helps prevent the appearance of mold and mildew on building walls or in warehouses.

Connection Diagram of the Air Humidity Sensor, Its Setup and Installation

In most cases, such sensors are mounted on a solid surface. The housing can be fixed to the wall with screws (it is solid, durable, and made of fireproof plastic). Inside the hygrometer housing is a terminal block with contacts used for connection (the scheme provided by the manufacturer is used).

Connection is made by cable through a cable gland, with the corresponding nut tightened to the stop to maintain the housing's tightness (in most models it corresponds to the protection class against external influences IP65). A shielded cable can also be used if it is assumed that the device will operate in an area with a high level of electromagnetic interference. Setup and calibration are performed after connection under "working" conditions.

Humidity Meter

At "Izmerkon" company, you can purchase humidity sensors, temperature and humidity converters with relay outputs, with digital interfaces, with external probes, as well as WEB sensors. There are hygrometer models with Wi-Fi connection capable of transmitting data via the internet.

Humidity Sensor — T1110

Output: 4-20 mA

Relative humidity measurement accuracy: 5 to 95%
Operating temperature range: -30 to +80°C