Definition and functional positioning
High and low temperature test chamber is a closed environmental test equipment that can accurately control and realize unidirectional changes (high or low temperature) or alternating changes (high and low temperature cycles) of the space temperature in the chamber according to preset procedures. Its core function is to simulate various extreme temperature conditions or rapid temperature changes that products may encounter during storage, transportation and use. As a cornerstone equipment in the field of environmental reliability testing, the high and low temperature test chamber is used to expose design defects, verify material resistance, evaluate the failure rate of electronic components, and determine the final environmental adaptability level of the product by applying controllable temperature stress to the test product, providing key data support for product quality identification and improvement.
Principle
The working principle of the high and low temperature test chamber is based on the energy transfer and refrigeration cycle in thermodynamics, and the temperature in the chamber is accurately controlled through a closed-loop control system. The process involves the synergy of the two core functions of heating and cooling, as well as the assistance of the air circulation system.
The heating function is mainly achieved through resistive heaters, usually using nickel-chromium alloy heating wire or PTC heating elements. When the temperature needs to be raised, the controller outputs a pulse width modulation signal through the solid-state relay according to the deviation between the set temperature and the measured temperature, and adjusts the power-on time ratio of the heater, so as to control the heating power. The required heating power can be estimated based on the thermal equilibrium equation:
Pheat = (c·m·ΔT)/t + Ploss
where Pheatis the required heating power, c is the specific heat capacity of the air, m is the air quality in the box, ΔT is the target temperature difference, t is the expected heating time, PlossIt is the heat lost through the box wall. A certain margin is usually considered when designing to cope with different load conditions.
The refrigeration function is the technical core of the high and low temperature test chamber, and the vapor compression refrigeration cycle is generally adopted. When the target temperature limit is high (e.g. above -20°C), a single-stage compression refrigeration system can meet the demand. However, for lower temperatures (such as -40°C to -70°C), due to the large compression ratio of single-stage compression, the exhaust temperature will be too high and the cooling efficiency will be drastically reduced, and a laminated refrigeration system must be used. The system is coupled by two independent refrigeration cycles: the high temperature stage uses medium temperature refrigeration working fluid (such as R404A) and the low temperature stage uses low temperature working fluid (such as R23), and the two cycles exchange heat through an evaporative condenser, so that the low temperature stage can obtain a lower evaporation temperature, so as to achieve a chamber temperature of -70°C or even lower. The regulation of the refrigeration system is usually achieved by controlling the start and stop of the compressor or by regulating the cooling volume through a hot gas bypass valve.
The air circulation system consists of a centrifugal fan and an air duct that forces the air flow inside the chamber to accelerate heat exchange and ensure temperature uniformity in the workspace. The design of the air flow organization directly affects the distribution of the temperature field, and a well-designed air duct can maintain the wind speed in the working area at 0.5~2.0 m/s, avoiding local eddy currents and temperature dead zones.
As the nerve center of the equipment, the control system is the nerve center of the equipment, and the modern high and low temperature test chamber generally adopts programmable logic controller (PLC) or special microcomputer system, combined with PID (proportional-integral-differentiation) control algorithm to achieve precise temperature adjustment. The controller dynamically adjusts the heating and cooling outputs based on real-time feedback from the temperature sensor (usually a Class A Pt100 platinum resistor) to stabilize the temperature around the set value. For alternating testing, the control system can automatically operate according to a preset temperature-time curve and record complete test process data.
Measurement method
Whether the performance of the high and low temperature test chamber meets the standard requirements needs to be verified by standardized measurement and measurement methods. According to the national standard GB/T 10592 and related verification procedures, measurements are usually carried out under no-load or specified load conditions.
Measurement and calculation of temperature deviations: No less than 9 test points (including eight vertex and center points) are arranged according to specifications in the workspace. After the temperature of the equipment stabilizes, the temperature values of all points are recorded at regular intervals (e.g. 2 minutes) for at least 30 minutes. Temperature deviation refers to the difference between the measured maximum temperature, minimum temperature and nominal temperature of each test point in a steady state. This indicator reflects the degree to which the actual operating temperature of the equipment conforms to the set temperature. The general standard requires the temperature deviation to be within ±2°C.
Measurement and calculation of temperature uniformity: Using the data collected in the above stabilization stage, the difference between the highest and lowest measured temperatures of all test points in each record is calculated, and the average of these differences is taken as the temperature uniformity. It characterizes the consistency of temperature in the spatial distribution within the workspace. Uniformity is an important indicator for assessing the core technology of equipment, and the standard usually requires ≤ 2°C.
Measurement and calculation of temperature fluctuation: refers to the maximum deviation of the temperature of the center point of the working space with time in the steady state. It is usually expressed as half the difference between the maximum and minimum temperature recorded by the center point in 30 minutes, and is called "±". This indicator reflects the stability and anti-interference ability of the equipment control system, and generally requires ≤± 0.5°C.
Measurement of temperature rise and fall rate: For alternating test chambers, it is necessary to measure their temperature change ability within a specified time. When measuring, the time required to rise from the lower limit of temperature to the upper limit (or vice versa) is usually recorded, and the average rate of the whole process is calculated; or measure the slope of temperature change over a specific linear interval. It should be noted that the temperature rise and fall rate is divided into "no-load" and "full-load", and the heat capacity of the sample during actual testing will significantly affect the rate performance.
Key factors that affect performance
The actual operating performance of the high and low temperature test chamber is restricted by a variety of factors, which are directly related to the accuracy and reproducibility of the test results.
Load characteristics are one of the important influencing factors. The volume, quantity, and heat capacity of the test sample all affect the thermal equilibration process in the chamber. When the proportion of the total volume of the sample to the effective volume of the working room is too high, the temperature uniformity in the box will decrease significantly. Practice shows that for non-heating samples, the total volume should not exceed one-fifth of the volume of the chamber; For heated samples for energized work, due to their own heat production, the volume ratio should be stricter, usually no more than one-tenth. The placement of samples is also critical, any surface should be at least 10 cm away from the box wall, and the air outlet and return air outlet should not be blocked to ensure smooth airflow.
The sealing performance of the box has a significant impact on the energy consumption, stability and low temperature performance of the equipment. If there is a defect in the sealing structure, it will lead to external air infiltration, on the one hand, it will increase the load on the refrigeration or heating system, resulting in an increase in energy consumption; On the other hand, the infiltrated hot and humid air may condense into frost on the inner wall of the box or evaporator under low temperature conditions, which not only affects the heat exchange efficiency, but also causes ice blockage in severe cases, making the equipment unable to reach the set low temperature. In addition, the uncontrollable airflow introduced by poor sealing will interfere with the stability of the temperature field, resulting in increased temperature fluctuations, which directly affects the confidence of the test data.
Environmental conditions cannot be ignored either. The temperature and ventilation of the equipment installation environment directly affect the condensation efficiency of the refrigeration system. If the laboratory is poorly ventilated and the ambient temperature is too high, it will lead to an increase in condensation pressure, a decrease in cooling capacity, and even a high-voltage protection shutdown. It is usually required to leave sufficient heat dissipation and maintenance space around the equipment, and the ambient temperature is controlled within the range of 5~35°C.
The accuracy of the measurement and control system directly determines the performance of the equipment. The accuracy of the temperature sensor, the aging drift, and the appropriateness of the controller's algorithm parameters will affect the quality of temperature control. Regular calibration of sensors and optimization of PID parameters are necessary means to maintain equipment performance.
Typical application areas
The application of high and low temperature test chambers runs through all stages of product development, quality inspection and failure analysis, covering a wide range of industries.
In the electrical and electronic industry, this equipment is used to evaluate the reliability of components, printed circuit boards, and complete machines under conditions of high-temperature storage, low-temperature start-up, and temperature cycling. For example, according to GB/T 2423.1 and GB/T 2423.2 standards, low- or high-temperature tests are conducted on products to test their working ability and structural integrity at extreme temperatures. For automotive-grade chips, they need to follow the AEC-Q100 standard and conduct more stringent temperature cycling, high-temperature working life and other tests to verify their long-term stability in the automobile engine compartment or cold environment.
In the field of materials science and chemical engineering, high and low temperature test chambers are used to study the changes of physical properties of materials with temperature, such as the low-temperature brittleness transformation of metal materials, the thermal aging and dimensional stability of polymer materials, and the attenuation of interlayer bond strength of composite materials after thermal cycling. These tests provide a basis for material application selection and process improvement.
Automotive and rail applications include testing the volatility of interior components at high temperatures, the discharge performance of battery packs in low temperature environments, the ability of rubber seals to maintain tightness under alternating hot and cold conditions, and the functional stability of electronic units such as vehicle controllers under various temperature conditions.
The aerospace and military sectors place higher demands on equipment with extreme temperature capabilities and rapid change rates. It is used to simulate high-altitude and low-temperature environments, aerodynamic heating effects on the surface of aircraft, and the storage and operation adaptability of weapons and equipment in extremely cold areas. The test results are directly related to the reliability of the equipment and the success rate of the mission.
In the new energy and photovoltaic industry, it is used to evaluate the ability of photovoltaic modules to withstand day and night temperature differences, seasonal changes, as well as the capacity retention rate and safety performance of lithium-ion batteries in the process of high-temperature storage and low-temperature discharge.
Selection decision elements
As a high-value fixed asset for long-term operation, the selection of high and low temperature test chambers is a prudent decision that requires comprehensive consideration of current needs and future development.
Volume determination is the first step in selection. The required studio volume is calculated based on the maximum form factor, quantity, and placement of the sample being tested. The general principle is that the total sample volume should not exceed one-third of the volume of the chamber to ensure a smooth air circulation path and avoid deterioration of temperature uniformity due to excessive loading. At the same time, the bearing capacity of the sample holder should also be considered, and the load capacity of a single layer should usually not be less than 50 kg. In addition, the size and handling path of the laboratory door should be evaluated in advance to ensure that the equipment is in place smoothly.
The temperature range is determined to cover the most severe conditions required for testing, taking into account appropriate margins. For low temperature requirements, it is necessary to clarify whether it needs to reach -40°C, -70°C or lower, which directly determines whether the refrigeration method is single-stage compression or overlapping refrigeration. For high-temperature requirements, it is necessary to confirm the maximum operating temperature and pay attention to the stability and thermal insulation performance of long-term operation at this temperature point. It is recommended that the temperature range be 10%~15% wider than the actual operating temperature to meet the potential needs of future standard upgrades or product iterations.
The consideration of performance parameters should pay attention to core indicators such as temperature deviation, uniformity, fluctuation, and temperature rise and fall rate. For general thermostatic testing, uniformity and fluctuation are key; For alternating and temperature shock tests, the temperature rise rate and the ability to follow the set curve are more important. It is necessary to carefully check whether the parameters provided by the manufacturer are measured under no-load or full load conditions, and confirm whether they meet the requirements of relevant national standards (such as GB/T 10592 and GB/T 2423 series).
Structural and material considerations involve liner material, insulation thickness, and viewing window configuration. The inner liner is generally made of SUS304 stainless steel to ensure corrosion resistance and easy cleaning. The insulation layer is usually polyurethane rigid foam, and its thickness and density determine the insulation performance and energy consumption of the equipment. The observation window should be multi-layer hollow tempered glass and equipped with an electric heating anti-frost device to facilitate real-time observation of the sample status during low-temperature testing.
The evaluation of control and data systems is critical. Modern devices should support programmatic control with the ability to edit multiple temperature profiles. The control system should have data logging, storage, and export capabilities (e.g., via USB interface or network) to meet data traceability and compliance requirements. The safety protection system needs to be perfected, including over-temperature protection, compressor overload protection, leakage protection, phase loss protection, etc., to form multiple safety redundancy.
When making a final decision, it is also necessary to evaluate the supplier's technical strength, manufacturing process, quality assurance system and after-sales service network coverage. The purchase cost of equipment should not be the only determining factor, but should be combined with the total cost of ownership (TCO) of the whole life cycle, and the best cost-effective solution should be selected based on the energy consumption, maintenance costs, potential downtime losses, etc. of the equipment.