Do Elements (Slugs or Plug-ins) Determine the Measurement Accuracy?

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Absolutely! The element couples a portion of the RF signal and converts it to a DC signal that can be displayed on the meter. 

Most elements have an accuracy of +/-5% of full scale but there are a few elements that are +/-8% so always double check the specification.

Example

If you are using a 5W element and the accuracy is +/- 5%, then the accuracy of your measurement will be:  Power Reading (W) +/-0.25 W

If you are using a 100W element with +/- 5% accuracy, the measurement can be Power Reading (W) +/- 5 W and be in specification.

For more information, please refer to this article: Elements, Slugs or Plug-ins; How Do They Work?