EMF safety: broadband probe correction factors
It’s not uncommon that we get questions about how a meter, like the Narda FieldMan, reads and applies probe calibration / correction factors.
All probes designed for professional use should be supplied with calibration data which shows how probe performance changes with frequency. Like many other devices it’s common for mid-band performance to differ from that at the band edges.
If the calibration factors are stored in the probe e.g. Narda NBM & FieldMan series and the meter has the requisite functionality the meter can see and apply that factor. The problem is it’s not an automatic process.
Because the sensors in the probe provide no frequency information the meter is unable to automatically apply a correction factor. The frequency of interest must be a manual input to the meter.
In many instances it’s just as easy to manually apply a correction factor to your recorded values i.e. don’t rely on the meter at all, look at the calibration data and use the appropriate multiplier. In any case (excuse the pun) it’s good practice to keep calibration data with the equipment, this way equipment users have clear visibility of how probe performance varies with frequency, it just adds confidence and understanding to the measurement process.
But it’s not the end of the world if a correction factor isn’t applied. Applying a correction factor is just a means of reducing measurement uncertainty. If no factor is applied all it means is the measurement has more uncertainty associated with it – the actual amount can be derived from the given probe specification.
Uncertainty and how you deal with it becomes more important when recorded values are close to a given limit e.g. CEMFAW action levels. However if levels are tiny compared to maximum exposure levels then it’s really not necessary to worry about reducing uncertainty.
The important thing is to state in your measurement report if you have or have not applied correction factors and list your uncertainty budget accordingly. It sounds dull but listing how you’ve dealt with uncertainty is a requirement of measurement standards and in some instances can be a regulatory requirement, whatever it’s good practice and indicates a level of competence.
Information on how to make EMF measurements, reporting, uncertainty, calibration factors and much more is included in our EMF measurement training course.