Maintaining Consistent Temperatures with High and Low Temperature Constant Temperature Chambers

I. Working Principle of Each Component of High and Low Temperature Constant Temperature Chamber
The cooling system of high and low temperature test box is one of the important parts of the comprehensive high and low temperature chamber. Generally, the cooling modes of high and low temperature test box are mechanical cooling and auxiliary liquid nitrogen cooling. Mechanical refrigeration adopts steam compression refrigeration, which is mainly composed of compressor, condenser, throttling mechanism and evaporator. The cooling system of the high and low temperature test box produced by the instrument consists of two parts, referred to as the high temperature part and the low temperature part, each of which is a relatively independent cooling system. In the high temperature part, the refrigerant evaporates and absorbs the heat from the refrigerant in the low temperature part to vaporize; In the low temperature part, the refrigerant evaporates to absorb the heat from the cooled object (the air in the test box) to obtain the cooling capacity. The high temperature part and the low temperature part are connected by an evaporator condenser, which is both the condenser of the high temperature part and the condenser of the low temperature part. The refrigerant of the high temperature part of the high and low temperature test box adopts medium temperature refrigerant, while the low temperature part adopts low temperature refrigerant, and the temperature inside the box can reach -60-150 ℃.

The heating system of high and low temperature test box is relatively simple compared with refrigeration system. It is mainly composed of high-power resistance wire. Because the test box requires a large heating rate, the heating system of the test box has a larger power, and the heater is also set on the bottom of the test box. The control system is the core of the comprehensive test box, which determines the important index such as heating rate and accuracy of the high and low temperature chamber. Nowadays, the controllers of the test boxes are mostly PID control, and a few are combined with PID and fuzzy control. The sensor system of high and low temperature test box is mainly temperature and humidity sensor. Platinum electric group and thermocouple are more commonly used for temperature sensors. There are two methods for measuring humidity: dry-wet ball thermometer method and solid state electronic sensor direct measurement method. Due to the low accuracy of dry-wet ball method, the solid state sensor is gradually used to replace the dry-wet ball to measure the humidity. The air circulation system is usually composed of centrifugal fan and motor to drive it.

LISUN Temperature Humidity Chamber | Thermal Chamber | Climate Chamber is according to IEC60068-2-1 (GB/T2423.1) and IEC60068-2-2 (GB/T2423.2), IEC60068-2-78, IEC60598-1, GB/T2423.17, GB/T5170.2, GB/T5170.5. The climate chamber is used to test the CFL/LED which meets IES LM-80-08, electricity products, automative cars, home appications and mobiles.

II. Precautions for Using High and Low Temperature Constant Temperature Chamber

1. In order to ensure the normal progress of the test, the appropriate test equipment should be selected according to the different conditions of the test product, and a reasonable proportion should be maintained between the test sample and the effective volume of the test box. For the test sample with heat generation, its volume should not be more than one tenth of the effective volume of the test box. For the test sample without heat generation, its volume should not be more than one fifth of the effective volume of the test box. For example, a 21-color TV can meet the requirements when doing temperature storage test with a cubic test box, but when it is working under power, it can not meet the requirements, and a larger test box should be changed because the TV needs to radiate heat when working.

2. Preparation before the use of high and low temperature test box. When doing high and low temperature test box environment test, it is necessary to be familiar with the performance of the test sample, test conditions/test procedures and test technology, to be familiar with the technical performance of the test equipment used, to understand the structure of the equipment, especially to be familiar with the operation and performance of the controller. Carefully read the operation and use manual of the test equipment in order to avoid the abnormal operation of the test equipment caused by operation errors and the damage of the test sample, and the incorrect test data.

3. Correct placement of test samples. The position of the test samples in the high and low temperature test box should be more than 10 cm away from the box wall. As far as possible, multiple samples should be placed in the same plane. The samples should not block the air outlet and return air outlet, and a certain distance should also be left for the temperature and humidity sensor to ensure the accuracy of the test temperature.

4. Use of wet and hot test box. There are certain requirements for the wet gauze cloth (wet paper) used for the wet and hot test box. Not all cloth can be replaced. Because the reading of relative humidity is related to the distance from temperature and humidity, it is strictly related to the local atmospheric pressure, wind speed at the time. The indicated value of wet bulb temperature is related to the water intake and surface evaporation of the gauze cloth. These are closely related to the quality of the cloth. Special “wet cloth” woven from flax should be used for gauze cloth to avoid incorrect wet bulb temperature (humidity). In addition, the cloth length of the wet cloth should be 100mm, tightly wound around the sensor probe, and the probe should be 20-30mm away from the humidity water cup, and the cloth should be immersed in the water cup to ensure the accuracy of the equipment control and humidity.

5. Correct addition of medium test in high and low temperature test box. For example, there are certain requirements for the water used in the wet and hot test box. The resistance of the test box water should not be lower than 500 ohms. Generally, the resistance of tap water is 10-100 ohms, distilled water is 100-10000 ohms, and deionized water is 10000-100000 ohms. Therefore, distilled water or deionized water should be used for wet and hot tests, and it must be fresh. Because water can be contaminated by carbon dioxide and dust after contacting with air, its resistance will decrease with time. There is an economical and convenient purified water on the market, and its resistance is equivalent to distilled water.

III. Points for Selection of High and Low Temperature Constant Temperature Chamber
When selecting high and low temperature constant temperature chamber, users should meet the temperature conditions stipulated by the test requirements, no matter whether they are selecting high and low temperature test box or other test equipment. To ensure the temperature uniformity in the test box, forced air circulation or non-forced air circulation can be selected according to the heat dissipation of the sample. The heating or cooling system of the high and low temperature test box should have no effect on the sample. It is convenient to have relevant sample racks to place samples, and the mechanical properties of the sample racks will not change due to high and low temperature changes. The high and low temperature test box should have protective measures, such as observation window and lighting, power phase-off, over-temperature protection, various alarm devices, etc.

According to the customer’s requirements, whether there is remote monitoring function or not. When conducting cyclic test, the test box must be equipped with automatic counter, indicator light and recording device, as well as automatic shutdown and other instruments, and must have good record and display function. According to the sample temperature, there are two measurement methods, upper air and lower air sensor temperature, the temperature and humidity control sensor position and control mode in the high and low temperature test box can be selected, and the appropriate equipment should be selected according to the customer’s product test requirements.

Lisun Instruments Limited was found by LISUN GROUP in 2003. LISUN quality system has been strictly certified by ISO9001:2015. As a CIE Membership, LISUN products are designed based on CIE, IEC and other international or national standards. All products passed CE certificate and authenticated by the third party lab.

Our main products are Goniophotometer, Integrating Sphere, Spectroradiometer, Surge Generator, ESD Simulator Guns, EMI Receiver, EMC Test Equipment, Electrical Safety Tester, Environmental Chamber, Temperature Chamber, Climate Chamber, Thermal Chamber, Salt Spray Test, Dust Test Chamber, Waterproof Test, RoHS Test (EDXRF), Glow Wire Test and Needle Flame Test.

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