Question on Fluke 45 AC voltage measurements

[beuford]

I recently purchased a used Fluke 45. It is in excellent physical shape and seems to work fine except for low-level AC voltage measurement. However, I am aware that AC voltage measurement is usually specified with far less precision by meter vendors, so maybe this is normal? I'm hoping someone can confirm or advise.

When in the auto AC range with shorted leads (different lead sets tried with the same results), the AC voltage reads about .240mV. Manually increasing the voltage range increases the reading, but leaves the digits very nearly the same; see photos below as the meter cycles from the 300mV to the 750V range.

Is this expected for AC measurements with the Fluke 45 (or other meters), or is it in need of repair/calibration? Any advise would be greatly appreciated.

Thanks in advance,

Beuford

300mV range:


3V range:


30V range:


300V range:


750V range:

[alm]

There used to be a Keithley document describing this in detail, but it seems to have disappeared when Tektronix ate Keithley. The best reference I found was the first entry in this Fluke FAQ, but it is light on details. Basically, the true RMS converter has an offset of about 240 counts. With shorted inputs, it will read 240 counts. If you read the manual, it will likely specify ACV measurements only down to something like 3% of full scale. So for the 100.000 mV scale, the minimum value it should accurately measure is 3 mV. For an input signal of 3 mV, the reading would be \$\sqrt{(3.000 mV)^2 + (0.240 mV)^2} = 3.010 mV\$, which I imagine is within the accuracy specifications.

[Deckert]

Hi,

Mine basically reads about half of yours, but in the last pic I see you can null out the input reading by pressing REL.

Auto:


300mV scale:


3V scale:


REL:


--deckert

[Performa01]

True RMS converter exhibits some offset which must not be nulled by means of the REL function.

This is absolutely normal for any true RMS meter, except for some of the latest models, which just sample the input at high speed and calculate the RMS voltage in software, like a DSO would do.

As has already been stated, you can measure AC voltages down to a certain value that is guaranteed to be less than a certain percentage of the range, below that threshold the reading won't change anymore.

[beuford]

Thank you for your responses; it makes sense now. Seems like a decent little bench meter for only $150.

Beuford