The working principle of the five systems of the high and low temperature alternating test box is composed of refrigeration system, heating system, control system, air circulation system, and sensor system, etc. The above systems are divided into electrical and mechanical refrigeration. The working principle and working process of several main systems are briefly described below.

1. Sensor system: The sensors of the high and low temperature alternating test chamber are mainly temperature and humidity sensors. Platinum groups and thermocouples are used in temperature sensors. There are two methods for measuring humidity: dry and wet bulb thermometers and solid state electronic sensors for direct measurement. Due to the low measurement accuracy of the dry and wet ball method, the current test chamber is gradually replacing the wet and dry balls with solid-state sensors for humidity measurement.

2. Air circulation system: The air circulation system generally consists of a centrifugal fan and a motor that drives its operation.

3. Refrigeration system: The refrigeration system is one of the important parts of the comprehensive high and low temperature test chamber. Generally speaking, the refrigeration modes of high and low temperature alternating test chambers are mechanical refrigeration and auxiliary liquid nitrogen refrigeration. Mechanical refrigeration uses vapor compression refrigeration. They are mainly composed of compressor, condenser, throttling mechanism and evaporator. The temperature of the low temperature should reach -55 °C, and single-stage refrigeration is difficult to meet the requirements. Therefore, the cooling method of the test box generally adopts cascade cooling. The refrigeration system of the high and low temperature test chamber produced by Eurocopter consists of two parts, called the high temperature part and the low temperature part, each of which is a relatively independent refrigeration system. The evaporation of the refrigerant in the high temperature portion absorbs heat from the refrigerant in the low temperature portion and vaporizes; the evaporation of the low temperature portion of the refrigerant absorbs heat from the object to be cooled (air in the test chamber) to obtain the amount of cold. The high temperature portion and the low temperature portion are connected by an evaporating condenser which is both a condenser at a high temperature portion and a condenser at a low temperature portion. The high temperature part of the high and low temperature test chamber uses medium temperature refrigerant, and the low temperature part of the refrigerant uses low temperature refrigerant, and the temperature inside the box can reach -70-150 °C.

4. Heating system: The heating system of the high and low temperature alternating test chamber is relatively simple compared to the refrigeration system. It mainly consists of high-power resistance wire. Because the test chamber requires a large heating rate, the heating system of the test box has a relatively large power, and a heater is also provided on the bottom plate of the test box.

5. Control system: The control system is the core of the comprehensive test chamber, which determines the important parameters such as the heating rate and accuracy of the high and low temperature test chamber. Most of the controllers in the test chamber now use PID control, and a small number of control methods that combine PID and fuzzy control. Since the control system is basically in the category of software, and this part is in use, there is generally no problem.