Translating AC voltage readings between old and new multimeters.

[Synthtech]

Can I ask a question about the deviation in AC volt readings between different generations of multimeters?

I am about to embark on the checking and calibration of an AVO vacuum tube tester. The calibration procedure specifies the use of an AVO Electronic Testmeter and an AVO Model 7 or Model 8 Voltmeter.

I have neither of those early vacuum tube meters that use an analogue mechanical movement and I will have to instead use a Keysight U1282A or a Hioki DT4282.

I don't know what principle the early analog voltmeters use to measure and display AC voltages, I assume that it is not true RMS but I will only be dealing with sine waves during this calibration. If I use my True RMS multimeter to measure the AC values in the machine I am uncertain as to whether it will display these values as the early analogue volt meter would have and therefore If I adjust the machine to the service manual values using a True RMS meter I might be setting it to the wrong calibration values.

Can anyone enlighten me on whether there will be a significant difference in the displayed AC voltage reading between an analogue volt meter and a digital True RMS meter when measuring a simple sine wave?

Thank you!

[TheDefpom]

Another aspect to consider is the input impedance, older meters likely don’t have 10MOhm inputs.

[The Electrician]

Can anyone enlighten me on whether there will be a significant difference in the displayed AC voltage reading between an analogue volt meter and a digital True RMS meter when measuring a simple sine wave?
Thank you!

If the sine wave you're using is a pure sine wave, distortion free, there will be no difference in the readings you get with analog or digital meters.  This assumes the meters you use are in proper working order and properly calibrated.

The sine wave of voltage from the grid is not such a sine wave, so don't use it for your reference.

[Specmaster]

Digital meters will very likely read higher because they will impose a far higher impedance to the circuit under test. Seeing as the instructions for the valve tester specifically state using an Avo model 7 or 8 than that what is the best instrument to use. Seeing that the Avo meters are a 20,000 opv and digital meters are typically 10,000,000 opv devices, you can see the problem. Avo meters are a meter that uses 50uA to achieve FSD (full-scale deflection), so if you don't have a AVO, do you have any analogue meter, most are 20,000 opv but might not be 50uA FSD, but in reality will fairly close and therefore close enough for your purposes I feel.

[Synthtech]

Thanks guys, I might see if I can find a 20k Ohm per volt analogue meter and compare the readings with the digital meter and see if I can get am idea of the amount of difference between the readings and what the trend is.

[bdunham7]

If you are measuring sine waves or close to it, a modern TRMS meter would be close enough that I wouldn't bother finding an actual averaging (or analog) meter.  You didn't say what voltages are involved, but if they are low voltage you do need to keep in mind that the AVO 7 and 8 are 1000ohms/volt on the AC ranges.  Whether you need a parallel resistor is an open question.  Modern meters may be 1M or 10M (don't assume they're all 10M on AC--look it up) so on the 1000V range the AVO would have the same input impedance as many bench meters.  Also, I doubt that there is any expectation that this measurement will be any more accurate than 3%, so I wouldn't overthink it. 

Avo meters are a 20,000 opv and digital meters are typically 10,000,000 opv

1000 Ω/V on AC ranges, many DMMs are 1M (not per volt) on AC ranges, some are 10M--mostly handhelds.

[TimFox]

All passive analog AC voltmeters known to me are "average responding", which means they display an RMS-calibrated value derived by multiplying the measured mean absolute-value of the waveform by the appropriate factor for a pure sine wave.  This works fine for sine waves with low harmonic distortion, but needs correction for other waveforms, such as pulse trains or square waves, or for noise measurements.  As stated above, the voltmeter resistance on AC is relatively low, and is quoted in "ohms per volt", where the voltage is the full-scale voltage for that range.

[Synthtech]

If you are measuring sine waves or close to it, a modern TRMS meter would be close enough that I wouldn't bother finding an actual averaging (or analog) meter.  You didn't say what voltages are involved, but if they are low voltage you do need to keep in mind that the AVO 7 and 8 are 1000ohms/volt on the AC ranges.  Whether you need a parallel resistor is an open question.  Modern meters may be 1M or 10M (don't assume they're all 10M on AC--look it up) so on the 1000V range the AVO would have the same input impedance as many bench meters.  Also, I doubt that there is any expectation that this measurement will be any more accurate than 3%, so I wouldn't overthink it. 

Avo meters are a 20,000 opv and digital meters are typically 10,000,000 opv

Thank you, I am perhaps worrying too much. It's good to at least think it through! I will be measuring up to 200V.

[Specmaster]

Yes, I'm sorry, I gave you OPV figures for DC but your measuring AC, in this case the sadly, much the same scenario exists, the AVO will have an OPV figure of just 1,000 and most digital meters will be somewhere in the scope of 1,000,000 to 10,000,000 OPV and where and what you are actually measuring will have a huge differance on your readings with your meter. This is a big gotcha with newbies fooled into think that they have discovered a fault when they haven't and they substitute a resistor in order to get the correct meter reading and when they remove the meter, all kinds of other problems can set in because of either under or over voltage being applied to the circuit in question.

Perhaps the bet example I can give you is the following extract from a Heathkit VTVM (vacuum tube volt meter) (or a modern DMM) and analogue meter, this is why decent service charts, manuals will normally specify the test equipment used to measure voltages, and so it makes sense if at all possible to use the same type of meter as mentioned.

Consider the example given in the extract of a valve that requires a 50V plate voltage derived from a 100V supply rail, then it needs a dropper resistance being inserted in the circuit. If the circuit voltage was measured with the AVO then in the calculation given in the section between page 24 and 25, "Using the VTVM" you will see that the AVO would indicate a reading of 14.3V but if the VTVM or a DMM used, then you see a reading far higher, in the case of the VTVM it would be 49V and slightly less with a meter having input impedance of 10M and far less if the impedance was 1M.

The AVO Electronic Testmeter you mentioned, also has the same input impedance as the VTVM by Heathkit used in the example above, so for voltages measured in the service manual where this instrument has been specifically mentioned, a modern DMM could be used but expect to see lower readings as already mentioned.

This why it essential when working older equipment to check what the equipment used in providing the readings, actually was and trying your best to replicate the same set-up, otherwise you will have problems.

[Synthtech]

Yes, I'm sorry, I gave you OPV figures for DC but your measuring AC, in this case the sadly, much the same scenario exists, the AVO will have an OPV figure of just 1,000 and most digital meters will be somewhere in the scope of 1,000,000 to 10,000,000 OPV and where and what you are actually measuring will have a huge differance on your readings with your meter. This is a big gotcha with newbies fooled into think that they have discovered a fault when they haven't and they substitute a resistor in order to get the correct meter reading and when they remove the meter, all kinds of other problems can set in because of either under or over voltage being applied to the circuit in question.

Perhaps the bet example I can give you is the following extract from a Heathkit VTVM (vacuum tube volt meter) (or a modern DMM) and analogue meter, this is why decent service charts, manuals will normally specify the test equipment used to measure voltages, and so it makes sense if at all possible to use the same type of meter as mentioned.

Consider the example given in the extract of a valve that requires a 50V plate voltage derived from a 100V supply rail, then it needs a dropper resistance being inserted in the circuit. If the circuit voltage was measured with the AVO then in the calculation given in the section between page 24 and 25, "Using the VTVM" you will see that the AVO would indicate a reading of 14.3V but if the VTVM or a DMM used, then you see a reading far higher, in the case of the VTVM it would be 49V and slightly less with a meter having input impedance of 10M and far less if the impedance was 1M.

The AVO Electronic Testmeter you mentioned, also has the same input impedance as the VTVM by Heathkit used in the example above, so for voltages measured in the service manual where this instrument has been specifically mentioned, a modern DMM could be used but expect to see lower readings as already mentioned.

This why it essential when working older equipment to check what the equipment used in providing the readings, actually was and trying your best to replicate the same set-up, otherwise you will have problems.

Thank you for your reply, it gives me a lot to go on, it seems that in the end I will have a lot to take into account depending on which readings I am taking so that.i don’t end up un-calibrating the AVO even more trying to calibrate it!

[Specmaster]

Exactly that, if you were just measuring the open circuit voltage of say a transformer primary then what ever type of meter you used, it wouldn't really matter. But once you're dealing with a voltage other than the full voltage, as for example in the illustration I gave in the other post, then if you want to achieve the same results as the designer, then you must replicate precisely what they did and that does not mean that you need to match their test equipment brand and model number, but you need to match their characteristics and capabilities electrically.

[Synthtech]

Thank you, that makes sense. We have the two AVO meters that the tube tester service manual being shipped to us, it will be fascinating to also use a digital meter to compare readings with and log the results!

[Synthtech]

To add to the pudding a lot of the AC voltage on this device to be calibrated also have A DC offset, sometimes over 100V.