Accuracy
The accuracy of a meter is defined as a ‘class index'. However, if a meter is classified as Class 1, it does not follow that the error under practical conditions of use will be within 1% of the actual value. It means that the error should not exceed 1% of the actual value under closely specified conditions, and only over a limited part of the measurement range. The permissible error of a meter under working conditions is the sum of the permissible intrinsic error and of the permissible variations due to each of the influence quantities (eg temperature, magnetic field). In practice the actual error is likely to be much smaller than this, because not all the influence quantities are likely to be simultaneously at their most unfavourable values, and some of the variations may cancel each other out.
The main requirements of the Code of Practice with reference to accuracy are:
• Accuracy must be quoted as a percentage of reading.
• Any accuracy defined otherwise must be clearly identified.
• If only a single accuracy is quoted, it must apply to all measured parameters.
Testing
A full test procedure is defined to allow manufacturers to prove compliance with the Code.
It includes:
• General testing procedures, including reference conditions
• Mechanical testing, including shock and vibration
• Climatic testing
• Safety
• Electrical testing.
The new Code offers users the ability to specify a meter that will measure precisely and accurately under practical conditions. It also acts as a reference to which meters can be designed and manufactured – for 50/60Hz, for dc, for 400Hz etc. There are very few meters on the market that could not apply this Code of Practice.
About the author:
Kris Szajdzicki: the managing director of Northern Design.