The correct selection of a high and low temperature test chamber is crucial for ensuring accurate and reliable environmental testing. When placing a product—whether it's a component, subassembly, or complete system—inside the test chamber, it's essential to ensure that the surrounding environment meets the specified test conditions. To achieve this, there are specific guidelines regarding the size of the test chamber relative to the dimensions of the product being tested.
Firstly, the volume of the product (W × D × H) should not exceed 20% to 35% of the effective working space of the test chamber. For heat-generating products, it's recommended to limit this to no more than 10%. This helps maintain proper airflow and prevents overheating or uneven temperature distribution. Additionally, the windward area of the product should not occupy more than 35% to 50% of the cross-sectional area of the test chamber. This ensures smooth air circulation and avoids localized hot or cold spots.
Another important consideration is the distance between the outer surface of the product and the walls of the test chamber. It should be at least 100–150 mm. This allows for sufficient airflow around the product and reduces the risk of thermal interference from the chamber walls. In summary, the working chamber volume of the test chamber should be at least 3 to 5 times the external volume of the product being tested.
Why is this important? If the product takes up too much space, it can restrict airflow, leading to increased flow velocity. This can cause inconsistent heat transfer between the product and the surrounding air, which may not accurately reflect real-world conditions. The standard typically limits the airflow speed in the test chamber to no more than 1.7 m/s. At no load, the average wind speed is usually around 0.6–0.8 m/s. However, if the space and area ratios are exceeded, the wind speed could increase by 50% to 100%, reaching up to 1.7 m/s. Exceeding these limits can compromise the validity of the test results.
Moreover, the accuracy of environmental parameters in the test chamber is measured under no-load conditions. Once a product is placed inside, its presence can affect the uniformity of temperature and airflow. The larger the product, the greater the impact. Studies have shown that temperature differences between the windward and leeward sides of the product can reach 3–8°C, and in some cases, even more than 10°C. Therefore, following the above guidelines is essential to ensure consistent and reliable test results.
Another factor to consider is the temperature gradient near the chamber walls. The air close to the walls is usually 2–3°C different from the center of the chamber. At extreme temperatures, this difference can increase to 5°C. Since the wall temperature differs from the air temperature nearby, it’s important to avoid placing the product within 100–150 mm of the chamber walls to prevent thermal distortion.
When selecting the control method for a high and low temperature test chamber, you have two main options: constant temperature chambers and alternating temperature chambers. Most standard test chambers are constant temperature models, which maintain a set target temperature automatically. These are suitable for tests that require stable conditions over time. On the other hand, alternating chambers can cycle through multiple temperature settings according to a pre-programmed schedule. They are ideal for simulating real-world temperature fluctuations but come with higher costs due to the need for advanced control systems and faster heating/cooling capabilities. The price of an alternating chamber is typically about 20% higher than that of a constant chamber. Therefore, choosing the right type depends on the specific needs of your testing process.
Regarding the temperature range, most foreign test chambers cover -73°C to +177°C, while domestic models often range from -70°C to +150°C. These ranges are generally sufficient for most military and civilian applications. Unless there's a specific requirement, such as testing in proximity to a heat source, it’s not advisable to increase the upper temperature limit unnecessarily. Higher temperatures can reduce internal airflow uniformity, shrink the available working space, and increase the cost of insulation and sealing materials.
The temperature change rate is another critical parameter. Standard chambers typically take 90–120 minutes to cool from ambient to the lowest temperature. Alternating chambers, however, can adjust temperature at a rate of 1°C/min or faster, depending on the model. Rapid temperature change chambers can achieve rates as high as 15–30°C/min in certain ranges. Different standards define the temperature change rate in various ways—some use the average rate over the entire process, while others specify a linear rate every 5 minutes. For precise testing, it's best to choose equipment that provides both metrics.
Wind speed is also important. According to standards, the airflow in the chamber should be less than 1.7 m/s. While lower speeds are preferable for minimizing heat exchange effects, some high-speed chambers may require higher wind speeds to maintain uniformity, especially in rapid temperature change or multi-factor environments. Wind speeds in such chambers can range from 2 to 3 m/s.
Temperature uniformity is key to simulating real-world conditions. The General Rules for Environmental Test Methods for Military Equipment (GJB150.1-86) specify that the temperature near the test sample must be within ±2°C of the test temperature, with a maximum variation of 1°C/m or 2.2°C overall when the sample is not operating.
Lastly, temperature fluctuations are relatively easy to control. Most modern test chambers can maintain fluctuations within ±0.3°C, ensuring stable and repeatable test conditions.
By carefully considering all these factors, you can select the most appropriate high and low temperature test chamber for your application, ensuring accurate, reliable, and meaningful test results.
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