Have a debate at work about analog meters

studiot

Joined Nov 9, 2007
4,998
There is of course another viewpoint to the question of which (type of) meter should I employ?

That of what are you trying to compare your measurement with.

A great many service sheets specify voltages at test points as measured with a specific meter type. This practice is less common these days.

I remember hearing of a particular incident at Marconi Instruments once where a Ministry inspector stopped the line and refused to accept the output because a particular meter serial abcd1234 was not being employed to as end of line checking. It was pointed out in vain that Marconis (in those days) manufactured the calibration equipment and standards that his cherished meter ( and most others) were calibrated by so could offer far superior test equipment. He insisted on comparing with his 'standard'.
 
Why are you talking about the peak to RMS crest factor which is only pertinent if you have a peak responding meter (which are rare), not an average responding one? :confused:
Because, as I said, my post wasn't intended as a measurement aid. It was a counterexample to this statement from post #7: "RMS (sometimes called effective) is .707 of peak only for a sine wave."

That statement was referring to the ratio of RMS to peak, which is equal to the reciprocal of crest factor. Since that statement was referring to the ratio of RMS to peak, that is what my examples also referred to.
 

Thread Starter

bwd111

Joined Jul 24, 2013
117
An analog meter with no electronics typically reads the average voltage with the readout calibrated to read the value in RMS for a pure sine-wave. Some old RF analog meters detect the peak voltage with a rectifier, also calibrated to read the RMS value for a pure sine-wave.
How about in a ac circuit only. Would that still be average or now is it effective voltage?
 

MrChips

Joined Oct 2, 2009
30,714
There is no definitive answer. There are different types of analog meters.
It depends on the design of the meter and meter movement, damping, ballistics, etc.
 

WBahn

Joined Mar 31, 2012
29,979
What a analog volt and ammeter reads in an ac circuit. Effective average peak to peak voltage
The answer is pretty much whatever you want it to be (within reason). You can point to a meter someplace that reads out that quantity.

For cheap "average-responding" meters, they usually work as follows:

1) Rectify the applied voltage (halfwave if they are really cheap, otherwise fullwave).

2) Apply that voltage to the meter movement through a scaling resistor.

3) The meter movement responds to the average of this modified waveform.

4) Mark the meter scale to read in Volts RMS under the assumption that the applied waveform is a pure AC sinusoid with zero DC component within the meter's bandwidth.

Two things to note:

1) The meter is NOT responding to the average value of the applied voltage. That would be zero for a sinusoid with no DC component.

2) The meter "reads" in effective (i.e., RMS) voltage, but it is correct only if the average value of the rectified input waveform happens to have the same relationship to the effective voltage of the actual input waveform that a pure sinusoid has.
 

debe

Joined Sep 21, 2010
1,389
Not sure if this meter reads RMS or not . But its an RF ammeter using an internal thermocouple. It seem to give a fairly good indication of RF power of a transmitter into a 52ohm load, from HF upto UHF freq. Picked this meter up in a surplus store about 40 yrs ago.
 

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WBahn

Joined Mar 31, 2012
29,979
I would venture that it is all-but guaranteed to read in RMS because it is using the thermocouple to respond to the average power delivered by the waveform and that is what RMS (in this context) is all about.
 
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