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Environmental Test Chambers: Temperature Change Rate



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Temperature requirements for environmental test chambers are generally moving in the direction of faster and faster change rates. For example, test requirements for 10-degrees Celsius/min, 150-degrees Celsius or 20-degrees Celsius/min rates are not uncommon today. Faster change rates mean a reduction of total test time. Faster rates also allow for more-realistic modeling for products in the real world reacting to myriad thermal-stress-based situations. In test chambers, it's important to realize that rate of temperature change (of the test specimen) is not the same as the rate-of-change in air temperature.



Every test-chamber maker has their own, unique airflow volume inside chambers they offer. The design of the refrigeration system of a particular chamber is what determines its airflow volume. For example, a common airflow in reach-in chambers is 100 feet/minute through the work space. This airflow rate, or velocity, is fine for steady-state and -temperature-cycling tests. But, with this airflow, temperature of the test specimen will lag behind air temperature.

Air flow rate across the test specimen must be significantly higher so that its temperature is closer to the chamber air temperature during transitions. Normally, 500 feet/minute or more is what is needed to have the specimen's temperature keep up with the air temperature. Hence, it's paramount that chamber airflow rate are high for thermal-shock testing.

For most temperature and humidity tests, the airflow rate in reach-in chambers is acceptable. When the temperature is increased or decreased, the air expands and contracts. The expansion/contraction of air will cause positive or negative air pressure fluctuations in a typical, sealed chamber.

If the air temperature is changed too rapidly (10-30 degrees Celsius/minute), more than 0.25 psi differential may result. It may not seem significant, at first, but over a large surface area of a chamber wall, the force can become large. E.g., a 32 ft2 which has a side wall that's 38 x 50 (1900 inch2), the force on the wall is:
1900 inch2 x 0.25 lbf/inch2 = 475 lbs of force on the walls. This is quite large. Therefore, most chamber designers add some type of pressure-relief vent. This vent will let the chamber "breathe", but is usually closed to minimize outside air from coming in to the chamber while a test is being conducted. During a fast transition, the vent will either draw in outside air to pull the temperature down. Breathing also reduced the force on the chamber structure, notably the walls. It's not unusual or uncommon to the walls deflect in and out during rapid temperature transitions. This is not a concern if the chamber is well-constructed.


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Updated: Tuesday, 2008-08-19 17:52 PST