Ref. Credit: ASTM D5346-1999 :
The air circulation within a forced-convection oven should be considered when loading specimens into the oven chamber. Forced-convection ovens are designed to provide uniform air temperature throughout the test chamber. This uniformity of air temperature is achieved by using baffles, or different sizes of holes, or both at the air inlet and outlet to and from the chamber.
The air flow within an oven is almost laminar at the air inlet and outlet and near the walls of the test chamber. The air is turbulent within the center of the chamber.
It has been noted that specimens positioned in the laminar air flow regions age faster than those in the turbulent region. The reason for this phenomenon is not understood, but it has been observed that the leading edges of flat specimens are a degree or two hotter than the trailing edge. There also may be a tendency for plasticizers to be stripped more rapidly from specimens in the laminar flow regions because of higher air velocity across the surface of the specimens.
For maximum reproducibility of test results, specimens should be placed in the turbulent air flow region. In an empty oven chamber, the turbulent flow region occupies the inner half of the chamber volume. This region increases somewhat with loading, depending upon specimen geometry and positioning. In any case, placement of specimens near the inlet or outlet ports should be avoided, because of the laminar air flow.
The temperature variation in a loaded oven may differ significantly from that measured in an empty oven. Therefore, the recommendation is to use a natural air change oven where air change can be directly measured and to make tests on a loaded oven, particularly with heavy loading, should be observed.
- UL 44
- UL 94
- UL 1581
- UL 2556