Maintaining the microclimate at a given level is important not only for some production processes, but also for the comfortable living of people. One of the key parts for this are humidity and temperature sensors. This measuring equipment has several models, which differ in size, cost, used physical principle and measurement accuracy.
Content
- Theoretical information
- Features of different designs
- Capacitive type
- Optical sensor
- Resistive and electronic hygrometer
- Thermistor variety
Theoretical information
Air humidity characterizes the amount of water vapor in it at a specific temperature. There is absolute and relative humidity. The first shows the amount of water in the form of steam, located in a unit volume of gas, and is measured in grams per cubic meter. Each gas at a certain temperature has its own absolute humidity, upon reaching which the process of liquid condensation begins.
Relative humidity indicates the extent to which the current humidity state has reached an absolute value. It is measured as a percentage and also depends on the temperature of the gas. At a relative humidity of 100% at a specific temperature, the “dew point” is reached - the process of moisture separation from the gas begins in the form of small droplets. The higher its temperature, the greater the absolute humidity. Therefore, many domestic and industrial climate control systems are equipped with a combined temperature and humidity sensor.
Features of different designs
There are five of the most popular types of air moisture sensors. Namely:
- capacitive;
- optical;
- resistive;
- electronic;
- thermistor.
Each type has its own advantages and disadvantages. The choice of an air humidity sensor for installation in a specific device is based on the balance of advantages and disadvantages of a particular model. Manufacturers choose the best option in terms of reliability, measurement accuracy and cost.
Capacitive type
The simplest version of such a measuring device is an air condenser. When the amount of water vapor in the gas changes, the properties of the gas itself change, including the dielectric constant. Measurement of the latter with subsequent data processing gives representation of the saturation of air with water vapor.
A more complex modification differs in that instead of air, a special dielectric is placed between the capacitor plates, sensitive to the amount of moisture in the air. Such a sensor is able to measure the moisture content of not only gases, but also small solid objects.
A more perfect and accurate version of the capacitive sensor consists of two comb electrodes placed on a substrate. They replace the capacitor plates,
Among the advantages of capacitive sensors are the following:
- simplicity of the device;
- low cost;
- reliability of work.
But there are also disadvantages:
- low measurement accuracy when the value approaches the lower threshold;
- the dependence of the readings on the purity of the medium (small dusty particles in the gas have a significant effect on the dielectric constant);
- the need for power supply to the sensor.
Optical sensor
This type is the most accurate, but also the most expensive. It is used, as a rule, in laboratory devices and in production with technological processes sensitive to water vapor. In household appliances, the use of an optical sensor is not practical.
The principle of operation is as follows: a beam of light is fed to the mirror, which, after reflection, is captured by the photosensor, the gradual cooling of the mirror surface leads to the onset of the process condensation of water vapor and scattering of the reflected light beam, fixation and processing of the "dew point" and the current state of temperature and pressure gives information about the degree of moisture air.
The optical type is both a humidity and temperature sensor.
Resistive and electronic hygrometer
These sensors are simple in design, but their measurement accuracy is the lowest. The principle of operation of a resistive element is as follows: a material with low resistance and high sensitivity to moisture is placed between the two electrodes (usually it is aluminum oxide), the total resistance is measured, the data is processed to calculate the relative humidity.
The principle of operation of the electronic type is based on the property of the electrolyte applied to the electrical insulating layer to change its concentration when the amount of water vapor in the air changes. This type of humidity level sensor is small in size, which allows it to be installed in small-sized devices. At the same time, electronic hygrometers have sufficient measurement accuracy for household appliances and reliability. Another advantage is the ability to measure air humidity regardless of the ambient temperature.
Thermistor variety
A thermistor is a non-linear electronic element in which the resistance changes to a large extent with a slight change in temperature. In the sensor, one thermistor is built into a dry air sealed chamber and the other is in direct contact with the environment. Moisture vapors, when they hit the second thermistor, partially condense and evaporate. This changes its temperature and resistance. Comparing the resistance of the two thermistors gives an idea of the degree of humidity in the air.
