June 2010Analytical Instrumentation & Environmental Monitoring
Dietmar Saecker, WIKA
Comments on the new standard for platinum resistance thermometers in industrial applications, DIN EN 60751:2009.
The standard for industrial platinum resistance thermometers, DIN EN 60751, has been re-issued after more than a decade (Edition 2009). The primary reason for this re-issue was that a few things had already been successfully established in practice, but were not yet mentioned in the 1996 standard. The most important examples are the use of thin-film resistors and the so-called 1/3 DIN accuracy class.
Many users were unsure whether they were allowed to use the ‘new’, though non-standardised, technologies at all. The new standard describes these items in detail and defines new realistic limits of use for sensor types and accuracy classes. Unfortunately, however, the new standard loses its usual clarity when it comes to establishing the vibration resistance of thermometers, and opens up a wide range of interpretations.
This trade article provides comments on the most important changes and helps in interpreting the new standard.
Measuring resistor designs
The electrical resistance of a resistance thermometer’s sensor changes with temperature. Since the resistance increases with temperature, this is referred to as PTC (positive temperature coefficient). Pt100 measuring resistors are normally used for industrial applications. In the old version of IEC 60751 (Edition 1996), there is no mention of the design of the sensors used. A long time back, there were only wire-wound measuring resistances. However, during the last decade, thin-film resistors (also referred to as flat-film resistors) have become widely accepted.
Since thin-film measuring resistors were not mentioned in the old standard, many users were of the erroneous opinion that standardised thermometer configurations still only used wire-wound resistors.
Lack of clarity about vibration resistance
In accordance with DIN EN 60751, any resistance thermometer conforming to the standard may be exposed to vibration-induced accelerations of up to 30 m/s², occurring in a frequency range from 10 to 500 Hz. Unfortunately, the new standard’s text is not clear here. The 1996 edition still clearly refers to the specification of ‘peak-to-peak’. This addition is missing in the 2009 edition, since it was revised by the IEC (International Electrotechnical Commission).
This opens up a wide range of interpretations. Most other standards (e.g. IEC 60068 Environmental Tests) discussing vibration tests exclusively use the term amplitude. This means that other measuring instruments, such as manometers, are tested for their stability at a given vibration having given amplitude.
Readers interested in learning more about the limitations of sensor configuration and what the new standard really says with respect to vibration will find the information at: http://instrumentation.co.za/+C13839
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