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I have a Fluke 8846A and I want to make a frequency measurement with an error less than 1 mHz. I'm having trouble interpreting the manual as to calculate the error of a 120V 60Hz measurement.

Manual: https://www.instrumart.com/assets/Fluke-8845A-8846A-Manual.pdf

On page 1-12, the 100V, 10Hz-20kHz, 1 year (its calibrated every year) is states the error is 0.06% of the measurement + 0.03% of the range.

Say I measure 60Hz exactly, does that mean my error is (60 Hz * 0.0006 + 0.0003 * (20 kHz - 10 Hz)) = 6.033Hz? That seems too high of an error though.

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You're looking at the wrong figures. The frequencies in that table refer to the flatness (frequency response) of the instrument, instead of frequency measurements. You may want to use the figures on page 1-20.

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As stated in the manual, errors are expressed as a percentage of the reading, not of the scale.

As pointed out by Ambiorix, the table at the top of page 1-20 applies to frequency readings.

If you've just had the unit calibrated (within 24 hours), the error is less than .006%, or 3.6 mHz. If it's been less than 90 days, error is less than .01%, or 6 mHz.

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