IEC 60068-2-38:2009 pdf download – Environmental testing – Part 2-38: Tests – Test Z/AD: Composite temperature/humidity cyclic test
Similarly, the breathing effect will be more apparent on specimens which contain relatively large air-filled or gas-filled voids, but again, the severity of the test will depend to some extent on the thermal characteristics of the specimens.
3.2 Application of the test For the reasons given above, it is recommended that this test procedure be limited to component type specimens when the construction of the specimens suggests a “breathing” type of damp heat test combined with icing and where the thermal characteristics are compatible with the rates of change of temperature, etc. of test Z/AD. For solid type specimens, e.g. plastic encapsulated, where there may be small hairline cracks or porous material, the absorption or diffusion mechanisms will predominate and a steady damp heat such as test C of IEC 60068-2-78 is preferred for investigating these effects. For larger specimens such as equipment or when it is essential for components to ensure thermal stability during the various phases of the cycle, test Db of IEC 60068-2-30 should be employed, although due to the reduced number of cycles in a given period, the degree of acceleration may not be as fast. In this case, test Db should normally form part of a sequence such as that defined in IEC 60068-1 . As in other damp heat tests, a polarizing voltage or electrical loading may be applied to the specimens. In the case of electrical loading, the loading should be such that the temperature rise of the specimens does not unduly affect the chamber conditions. From the above, test Z/AD should not be considered to be interchangeable with, or an alternative to, either steady-state or other cyclic damp heat tests, but the choice of test procedure should be made with due regard for the physical and thermal characteristics of the test specimens and the types of failure mechanisms which are significant for each particular case.
4 Description of test chamber The exposure to moisture, followed by cold, can either be performed in one chamber or in two separate chambers.
4.1 Chamber for exposure to moisture The chamber for the exposure to moisture shall be so constructed that:
a) The temperature can be varied between 25 °C ± 2 K and 65 °C ± 2 K in a period of between 1 ,5 h and 2,5 h for both rising and falling temperatures.
b) The relative humidity can be maintained at (93 ± 3) % during the periods of constant or rising temperature and between 80 % RH and 96 % RH during the falling temperature periods.
c) The conditions prevailing at any point in the working space are uniform and are as similar as possible to those prevailing in the immediate vicinity of suitably located temperature- and humidity-sensing devices. The air in the chamber shall therefore be continuously stirred at a rate necessary to maintain the specified conditions of temperature and humidity.
d) The specimens under test shall not be subjected to radiant heat from the chamber conditioning processes.
e) Water used for the maintenance of chamber humidity shall have a resistivity of not less than 500 Ωm.
f) Condensed water shall be continuously drained from the chamber and not used again unless it has been repurified.
Precautions shall be taken to ensure that no condensed water from the walls and roof of the test chamber can fall on the specimens.
4.2 Chamber for exposure to cold The chamber for exposure to cold shall be so constructed that
a) the temperature can be maintained at –1 0 °C ± 2 K,
b) the conditions prevailing at any point in the working space are uniform and are as similar as possible to those prevailing in the immediate vicinity of suitably located temperature- sensing devices, The air in the chamber shall therefore be continuously moving. Care shall be taken that the thermal capacity of the specimen under test does not appreciably influence conditions within the chamber. The humidity chamber may be used for exposure to cold, in which case it shall meet the requirements of 4.1 and, in addition, shall be so constructed that 1 ) the temperature can be lowered from 25 °C ± 2 K to –1 0 °C ± 2 K in a period of not more than 30 min, 2) the specimen can be held at a temperature of –1 0 °C ± 2 K for a period of 3 h, 3) the temperature can be raised from –1 0 °C ± 2 K to 25 °C ± 2 K in a period of not more than 90 min.
