Definition
A thermohygrometer is an instrument used to measure air temperature and relative humidity in an environment or specific space. In laboratory and industrial testing, it is a basic and widely used equipment, and its measurement data provides a key basis for environmental control, process monitoring and product quality assessment.
Measurement principle
Temperature measurements are usually based on the pattern that the physical properties of the sensor material change with temperature. Common principles include resistive temperature measurement, which uses metal resistance to change with temperature, and thermocouple effect, which uses the temperature difference between two different metal junctions to generate electric potential. Humidity measurement is mainly for relative humidity, and its principle is mostly based on the changes in electrical properties caused by the adsorption of water molecules by sensitive components. For example, the dielectric constant changes after the polymer film absorbs moisture, which causes the capacitance value to change. Resistive humidity sensors reflect humidity by changing the resistance value after moisture absorption. These physical changes are converted into standard electrical signals by circuits, which are finally displayed through calculations of temperature and humidity values.
Measurement method
According to the measurement method, it can be divided into contact and non-contact measurement. Contact measurement requires the sensor to be in full contact with the air being measured and is suitable for most laboratory and warehouse environments. Non-contact measurements, such as infrared temperature measurement, are only used for surface temperature measurement and are not suitable for humidity detection. In practice, fixed-point long-term monitoring or mobile inspection is usually adopted. For humidity measurement, it is also necessary to pay attention to the balance time, that is, after the sensor is placed in a new environment, it is necessary to wait for its output to stabilize before reading to avoid measurement errors.
Influencing factors
Measurement accuracy is influenced by various factors. Environmental factors include high air velocity that may cause heat dissipation errors in temperature measurements; Corrosive gases can damage the sensitive layer of the humidity sensor. Instrument factors involve long-term stability and drift of the sensor, performance of the temperature compensation circuit, and calibration intervals of the instrument. Operating factors such as placing the instrument in direct sunlight or near a heat source can cause the temperature measurement to be inaccurate; Use in condensation conditions may cause damage to the humidity sensor or abnormal readings. In addition, the representativeness of the measurement location is also crucial, and measurements should be avoided in atypical locations such as wall corners, vents, etc.
Applications
In the laboratory sector, thermohygrometers are used to monitor environmental conditions in chemical and biological laboratories and metrology rooms to ensure the repeatability of experimental results. In industrial production, it is widely used in food storage, electronic device manufacturing, textile, papermaking and other industries to control the temperature and humidity of the production and storage environment to ensure process stability and product quality. In agricultural research, it is used for climate monitoring in greenhouses and grain depots. In the construction field, it is used for HVAC system performance verification and indoor environmental quality assessment. These applications are measured and documented according to the appropriate national or international standards.
Key points of selection
Measurement requirements should be defined first, including temperature and humidity range, accuracy, resolution, and response time. Sensor types, such as the combination of capacitive humidity sensors and platinum resistive temperature sensors can be evaluated for wide range and high accuracy. Stability and long-term drift indicators are critical for situations that require continuous monitoring. The instrument should have the corresponding protection level to adapt to the site environment such as dust and splashing water. Data logging and output functions, such as analog outputs, digital interfaces, or built-in memory, can be selected according to the needs of the data collection system. Finally, it is necessary to confirm whether the instrument has a traceable calibration certificate and plan a regular calibration plan to ensure the reliability of the measurement data.