Temperature

Typical temperature tests are conducted to evaluate short term effects of temperature on:
Material safety and integrity Equipment performance
Typical temperature tests are not considered practical for evaluating time-dependent degradation (aging) where synergetic effects may be involved. Long term effects are evaluated using accelerated aging tests where the material characteristics are taken into consideration.
Relative humidity (RH) control during temperature tests is normally not necessary. Only in special cases, where extremely low humidity may have a significant effect on the materiel, should humidity be considered. High humidity conditions are a different test.
Short Term Effects
Parts bind.
Lubricants change characteristics.
Materials change in dimension.
Packing, gaskets, seals, bearings and shafts become distorted and possibly fail.
Gaskets display permanent set.
Closure and sealing strips deteriorate.
Resistors change values.
Electronic circuits lose stability.
Transformers and electromechanical components overheat.
Operating/release margins of relays and magnetic or thermally activated devices change.
Shortened operating lifetime.
High pressures created within sealed cases
Discoloration, cracking or crazing of organic materials.
Outgassing of composite materials.
General Temperature Test Considerations
When defining a test consider:
The purpose of the device or material.
The exposure circumstances.
How to determine whether the purpose of the device or material has been met.
If both storage and operational conditions need to be considered.
Storage Test Purpose
The storage test is used to investigate the effect of high or low temperatures on materiel or equipment during storage. The test item is exposed to high and low temperatures that may be encountered during normal storage followed by a performance test at the intended high and low operating temperatures.
The number of cycles for the storage test is generally set at a minimum of seven to coincide with the one percent frequency of occurrence of the hours of extreme temperatures during the most severe month in an average year at the most severe location. When considering extended storage, critical materials, or materials determined to be very sensitive to high or low temperature, the number of cycles should be increased to assure the design requirements are met.
Operation Test Purpose
The operation test procedure is used to investigate the effect of high and low temperatures on the materiel or equipment performance while it is operating. For establishing the test item operating procedure, use the following guidelines.
Include the various operating modes.
Include a range of input power conditions.
Introduce the cooling media that normally would be applied during service. Use cooling inlet temperatures and flow rates that represent both typical and worst-case conditions.
Typically there are two ways to perform operational tests, either with cyclic or constant temperature tests.
Cyclic Conditions
Determine the test duration based on the number of cycles required to satisfy the design requirements. Generally, the minimum number of cycles for operational exposure is three. This is normally sufficient for the test item to reach its maximum or minimum response temperature. When consistent temperature response is difficult to obtain, testing is frequently extended to a maximum of seven cycles. During cyclic temperature testing, test item thermal stabilization may not occur.
During cyclic conditions the test item, at each temperature extreme, may be operated either continuously or periodically.
Constant Temperature
Expose the test item to the temperature condition until thermal stabilization occurs. Stabilization is determined by monitoring the test item temperature with thermocouples. Thermal stabilization is generally defined as a change in specimen temperature of less than 2 C in a one hour period. Once thermal stabilization is achieved, the temperature should be maintained for at least two hours.
Determining Test Temperatures
When selecting test temperatures identify the appropriate conditions for the areas in which the materiel will be operated and stored including:
  The indirect effects of solar radiation.
  Heating caused by other equipment
Test Item Configuration
The test item configuration is based on the anticipated configuration(s) of the materiel or equipment for storage and operation. As a minimum, consider the following configurations:
Storage Operating
In a shipping/storage container or transit case Restrained
Protected under canopy, enclosed, etc. Openings covered or unobstructed
Unprotected Modified with kits for special applications
Stacked or palletized Forced or natural cooling
Test Apparatus
Temperature ChamberThe test apparatus consists of a chamber or cabinet together with auxiliary instrumentation capable of maintaining and monitoring the conditions of temperature (and humidity, where required) throughout an envelope of air surrounding the test item as well as the test item temperature itself. As a general rule the rate of temperature change. should not exceed 3 C per minute to prevent thermal shock.
  Temperature Conversion Chart


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