8.15.4 Why Do Two Instruments Sometimes Give Different Readings?
Different readings do not automatically mean that one instrument is faulty. Before comparing results, the assessor must confirm that the instruments measured the same quantity, frequency range, location, time, source condition, detector response, and averaging basis.
Why Do Two Instruments Sometimes Give Different Readings?
Every measuring system has a defined response and an associated uncertainty. In a spatially or temporally variable RF environment, even two suitable instruments may not display identical values.
Confirm the Same Quantity and Configuration
An electric-field probe, magnetic-field probe, power-density system, and current probe do not measure interchangeable quantities. A broadband meter and a frequency-selective system may also include different parts of the spectrum. Probe orientation, isotropic summation, cables, accessories, and selected measurement range must be comparable.
Probe and Instrument Responses Differ
Instruments have different frequency responses, sensitivities, dynamic ranges, isotropic errors, linearity, and response times. A probe may under-respond outside its specified band, while overload can create an unreliable result even when the display appears plausible.
Settings and Signal Behavior Matter
Resolution bandwidth, detector type, sample interval, averaging time, hold function, and software processing can change the reported value. Pulsed, intermittent, traffic-dependent, or beamformed signals make timing particularly important.
Position and Source Variability Matter
Moving a probe by a small distance can change the reading in a nonuniform or multipath field. Measurements made sequentially may also capture different source states. A controlled comparison uses the same position, support arrangement, operator separation, and operating condition as far as practicable.
Calibration and Uncertainty
Calibration links an instrument's indications to reference standards and characterizes its response under stated conditions. It does not reveal an unknowable exact value. Calibration corrections and the relevant components of measurement uncertainty must be applied or included as required by the method.
Compare Results Correctly
Results should be compared with their uncertainty intervals and the expected variability of the source and field. A small difference may be entirely consistent; a material unexplained difference calls for checks of settings, calibration status, overload, location, and source conditions before either result is accepted.
Summary
Two instruments can differ because their measurands, responses, settings, timing, positions, and uncertainties differ. A valid comparison controls those factors and investigates any difference that is significant for the intended decision.
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