1. High and low temperature constant temperature test chamber>Working principle of each component
1. Refrigeration system: refrigeration The system is one of the important parts of the comprehensive high and low temperature test chamber. Generally speaking, the refrigeration methods of high and low temperature test chambers are mechanical refrigeration and auxiliary liquid nitrogen refrigeration. Mechanical refrigeration adopts vapor compression refrigeration, which are mainly composed of compressors, condensers, throttling mechanisms and evaporators. The refrigeration system of the high and low temperature test chamber produced by the instrument consists of two parts, called the high temperature part and the low temperature part, each part is a relatively independent refrigeration system. The evaporation of the refrigerant in the high temperature part absorbs the heat from the refrigerant in the low temperature part to vaporize; the evaporation of the refrigerant in the low temperature part absorbs heat from the cooled object (air in the test chamber) to obtain cooling capacity. The high temperature part and the low temperature part are connected by an evaporative condenser, which is not only the condenser of the high temperature part, but also the condenser of the low temperature part. The high-temperature part of the high-low temperature test chamber uses a medium-temperature refrigerant, and the low-temperature part uses a low-temperature refrigerant, and the temperature in the chamber can reach -60-150 °C.
2. Heating system: Compared with the refrigeration system, the heating system of the high and low temperature test chamber is relatively simple. It is mainly composed of high-power resistance wires. Since the temperature rise rate required by the test chamber is relatively large, the power of the heating system of the test chamber is relatively large, and a heater is also installed on the bottom plate of the test chamber.
3. Control system: The control system is the core of the comprehensive test chamber, which determines the heating rate, precision and other important indicators of the high and low temperature test chamber. At present, most of the controllers of test chambers use PID control, and a small part of them use the combination of PID and fuzzy control. Since the control system basically belongs to the category of software, and this part generally does not have problems during use.
4. Sensor system: The sensors of the high and low temperature test chamber are mainly temperature and humidity sensors. Platinum electric groups and thermocouples are widely used in temperature sensors. There are two methods of measuring humidity: wet and dry bulb thermometer method and solid-state electronic sensor direct measurement method. Due to the low measurement accuracy of the dry-wet bulb method, the current test chamber is gradually replacing the dry-wet bulb with a solid-state sensor for humidity measurement.
5. Air circulation system: The air circulation system generally consists of a centrifugal fan and a motor that drives it.
2. Precautions for using high and low temperature constant temperature test chamber>
2 1. Preparation before use of the high and low temperature test chamber. When doing the environmental test of the high and low temperature test chamber, you must be familiar with the required test sample performance, test conditions/test procedures and test techniques, and be familiar with the technical performance of the test equipment used. It is necessary to understand the structure of the equipment, especially the operation and performance of the controller. Carefully read the operation manual of the test equipment to avoid damage to the test samples due to abnormal operation of the test equipment due to operational errors. The test data is incorrect.
3. Place the test sample correctly. The test sample in the high and low temperature test box should be placed more than 10cm away from the box wall, and multiple samples should be placed on the same plane as much as possible. The sample should not be placed Block the air outlet and return air outlet, and leave a certain distance for the temperature and humidity sensor. To ensure the correct test temperature.
4. Use of damp heat test chamber. Wet ball gauze (wet ball paper) is used for damp heat test chamber There are certain requirements, not any gauze can be substituted, because the relative humidity reading is the difference between the root distance and the temperature and humidity, strictly speaking, it is also related to the local atmospheric pressure and wind speed at that time. The wet bulb temperature indication is related to the amount of water absorbed by the gauze, It is related to the surface evaporation. These are directly related to the quality of the gauze. The wet ball gauze is made of special wet ball gauze woven from flax. In order not to cause the wet bulb thermometer (humidity) to be incorrect, the placement of the wet ball gauze requires gauze Length: 100mm, tightly wound the sensor probe, the probe is 20-30mm away from the humidity cup, and the gauze is immersed in the cup to ensure the correctness of equipment control and humidity.
5. Correctly add the medium environment in the high and low temperature test chamber test Test. If there are certain requirements for the water used in the damp heat test chamber, the resistivity of the water used in the test chamber shall not be lower than 500 ohms, the resistivity of tap water is generally 10-100 ohms, the resistivity of distilled water is 100-10000 ohms, and the resistivity of deionized water is 10000-100000 ohms. Use distilled water or deionized water for damp heat test, and must use fresh water, because water is easily polluted by carbon dioxide and dust after contact with air, and water has the property of dissolving many substances., the resistivity will decrease. There is a kind of pure water on the market that is more economical and convenient, and its resistivity is equivalent to that of distilled water.
Third, the selection points of high and low temperature constant temperature test chamber>
1. Whether the user is choosing a high and low temperature test chamber or other test equipment, they should meet the test requirements Specified temperature conditions;
2. To ensure the temperature uniformity in the test area of the test chamber, you can choose whether to use forced air circulation or no forced air circulation according to the heat dissipation of the sample;
3. High The heating or cooling system device of the low temperature test chamber should have no effect on the sample;
4. The test chamber should be convenient to have a relevant sample rack to place the sample, and the sample rack will not change its mechanical properties due to high and low temperature changes;
5. The high and low temperature test chamber should have protective measures. For example: there are observation windows and lighting, power supply phase failure, over-temperature protection, and various alarm devices;
6. Whether there is a remote monitoring function according to customer requirements;
7. The test box is in circulation The implementation of the test must require the installation of automatic counters, indicator lights and recording equipment, as well as automatic shutdown and other instrumentation devices, and requires good recording and display functions; Two measurement methods, the temperature and humidity control sensor position and control method in the high and low temperature test chamber can be selected, and the appropriate equipment should be selected according to the customer's product test requirements.