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Constant temperature and humidity machine
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Constant temperature and humidity machine is composed of refrigeration system, heating system, control system, humidity system, air supply circulation system, and sensor system, which is divided into two major aspects of electrical and mechanical refrigeration.The working principle and working process of several main systems are described below.

1. Refrigeration system: the refrigeration system is one of the key parts of constant temperature and humidity machine.Generally speaking, the refrigeration mode is mechanical refrigeration and auxiliary liquid nitrogen refrigeration, mechanical refrigeration using steam compression refrigeration, they are mainly composed of compressor, condenser, throttle mechanism and evaporator.If the low temperature of our test should reach -55℃, single stage refrigeration is difficult to meet the requirements, so the refrigeration method of constant temperature and humidity machine generally adopts cascade refrigeration.The refrigeration system of constant temperature and humidity machine is composed of two parts, respectively called the high temperature part and the low temperature part, each part is a relatively independent refrigeration system.In the high temperature part, the refrigerant evaporates by absorbing heat from the low temperature part.Evaporation of the cryogenic part of the refrigerant absorbs heat from the object being cooled (the air in the testing machine) to obtain the cooling capacity.The high temperature part and the low temperature part are connected by an evaporative condenser, which is both the condenser of the high temperature part and the condenser of the low temperature part.

2. Heating system: heating system is relatively simple compared with refrigeration system.It is mainly composed of high-power resistance wire. Due to the large temperature rise rate required by the test, the power of the heating system is relatively large, and there is a heater in the bottom plate of the testing machine.

3. Control system: the control system is the core of the comprehensive test box, which determines the temperature rise rate, accuracy and other important indicators of the testing machine.Most of the controllers of the testing machine use PID control, but also a small number of PID and fuzzy control combined control.

4. Humidity system: the temperature system is divided into two subsystems: humidification and dehumidification.

In general, steam humidification method is adopted, that is, low-pressure steam is directly injected into the test space for humidification.This method has high humidification ability, high speed and sensitive humidification control.

There are two ways of dehumidification: mechanical refrigeration dehumidification and dry dehumidification.The dehumidification principle of mechanical refrigeration is to cool the air below the dew point temperature, so that more than saturation moisture content condensation precipitation, so as to reduce the humidity.Dryer dehumidification is to use air pump to extract the air in the test box, and to inject dry air, at the same time, the wet air into the recyclable drying for drying, drying and then into the test box, so repeated cycle for dehumidification.

5. Sensor system: sensors are mainly temperature and humidity sensors.Platinum group and thermocouple are widely used in temperature sensor.There are two methods for measuring humidity: dry and wet bulb thermometer and solid state electronic sensor direct measurement.Due to the low measurement accuracy of wet and dry bulb method, the current constant temperature and humidity box is gradually replacing wet and dry bulb with solid sensor to measure humidity.

Application: high and low temperature testing of consumer electronics products.

6. Supply air circulation system: the air circulation system is generally composed of centrifugal fan and motor to drive its operation.It provides circulation of air in the testing machine.

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