Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Traceability in testing, case study - moisture resistance

A part of the testing of safety of household and similar electrical appliances (in the scope of LVD) is also testing of moisture resistance related with insulation resistance and electric strength. For this reason the appliance is put into a climatic chamber, which has to provide a required relative humidity at certain temperature. Temperature is usually not a critical parameter but the relative humidity is. Especially because the required relative humidity is very high, usually 93%±2%. The requirement of standard is very strict and it is hard to meet the requirement, especially in older climatic chambers with pshychrometric regulation. Another difficulty represents the standard description of achieving such high humidity with the help of different salts (Na2SO4, KNO3), which is practically impossible. The largest problem represents the gradients of relative humidity inside a chamber (homogeneity). Therefore careful evaluation of climatic chambers in terms of absolute values, stability and homogeneity is important. It is also important whether a chamber is empty or full with load. The standard on evaluation of climatic chambers does not exist. The problem was analysed and as a part of the task a procedure on evaluation of climatic chambers was elaborated taking into account metrological parameters to support the testing requirements.

On the basis of our evaluation procedure, and a draft of the standard for evaluation of climatic chambers we have developed an automated system for evaluation of climatic chambers. The system is portable so it can be used for on site measurements of climatic chambers as well as for the laboratory work. The system can be used with thermocouples as well as with platinum resistance thermometers. The uncertainty of the automated system satisfies all requirements. The final result of the system is a measurement report from which one can see the traceability scheme of measurements as well as positions inside a chamber's with lower or higher temperature than at a reference position. The system can also be used for determining thermal gradients and fluctuations inside the laboratory or any other larger space (warehouses, cold storage houses, etc.). In the case that the system is used for evaluation of larger objects, the uncertainty due to the thermometers cable length shall be taken into account.

The results of measurements in the climatic chamber, which is used for LVD testing in LMK, showed some deviations from set values. With the use of calibrated thermometers and humidity sensors during testing the climatic chamber could be set to such values that the requirements in the standards for LVD testing are met.

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University of Ljubljana
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