Working in the electronic calibration field for some years I have found that any mechanical meter movement has it's greatest accuracy at full scale only. the meter movement's accuracy diminishes as you go down in scale. So if a Bird Wattmeter is calibrated for 5% accuracy at full scale then it's accuracy will be reduced as the meter deflection is reduced. That is why for greatest accuracy with a Bird Wattmeter measuring 9 watts (for example) should be using a 10 watt slug for greatest meter deflection.
Digital readouts will always have the accuracy of the measuring circuitry + or - one count of the least segnifigent digit of the instrument or computer readout no matter how much of the selected range scale is used. Having said that you will have the most accuracy when approaching the upper scale range ( example 90 watts on the 100 watt range and not 9 watts on the 100 watt range).
See Below:
73 and Happy New Year
Dave KD1NA
Bird wattmeter's, the spec is 5% OF THE
FULL SCALE READING - and that is ONLY right after calibration. That
specification says that if a BIRD wattmeter has been recently calibrated
and handled carefully afterwards, with a 100 watt slug, it can have an
error of 5 watts - and that applies to *anywhere* on the scale.
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