Hi all, I have a question for you about moving iron meters. I'll explain the background first:
I recently did a check of the accuracy of the analog AC voltmeter in my Hickock 539A tube tester up against an accurate digital voltmeter. The result was that the analog meter (a moving iron instrument with 130VAC full-scale) has a 2V offset, but almost no sensitivity error: If I subtract the 2 volts from whatever reading across the scale, all readings are within ±0.8% from 20V and up, and most are a lot closer. For an instrument of this type, I believe this is almost as good as one can hope to get it. I should note that the pointer was set to perfect mechanical zero before the measurements, so the offset is not due to incorrect setting.
My quick and easy fix to this was to simply move the mechanical zero to compensate for the offset. The instrument then indicates correctly in the range of interest, but it's somewhat disturbing to see the pointer off zero when the tester is powered off. And some who borrow it might adjust it to zero in their best intentions, thinking they corrected an error.
So the question is: Is there a way to correct this inherent offset error? If any of you can enlighten me on what would be required to do this, it would be most welcome.
I should add that I'm operating the meter from 50Hz and not 60Hz as is was designed for. I don't know moving iron instruments well enough to say how much of a difference this might make, nor if it would create an offset or a sensitivity error. But from the measurements it seems the sensitivity is not affected.
I recently did a check of the accuracy of the analog AC voltmeter in my Hickock 539A tube tester up against an accurate digital voltmeter. The result was that the analog meter (a moving iron instrument with 130VAC full-scale) has a 2V offset, but almost no sensitivity error: If I subtract the 2 volts from whatever reading across the scale, all readings are within ±0.8% from 20V and up, and most are a lot closer. For an instrument of this type, I believe this is almost as good as one can hope to get it. I should note that the pointer was set to perfect mechanical zero before the measurements, so the offset is not due to incorrect setting.
My quick and easy fix to this was to simply move the mechanical zero to compensate for the offset. The instrument then indicates correctly in the range of interest, but it's somewhat disturbing to see the pointer off zero when the tester is powered off. And some who borrow it might adjust it to zero in their best intentions, thinking they corrected an error.
So the question is: Is there a way to correct this inherent offset error? If any of you can enlighten me on what would be required to do this, it would be most welcome.
I should add that I'm operating the meter from 50Hz and not 60Hz as is was designed for. I don't know moving iron instruments well enough to say how much of a difference this might make, nor if it would create an offset or a sensitivity error. But from the measurements it seems the sensitivity is not affected.
It is unlikely you can compensate a power-OFF error with a resistor.(?)
It *may* be possible to tweak the power-off zero with a permanent magnet, though these meters are obviously balanced for minimal effect from external fields. (Are you sure that 18 inch woofer magnet isn't under your bench?<G> I had to maintain strict magnetic segregation in my shop.)
I would think about putting a bold mark
It *may* be possible to tweak the power-off zero with a permanent magnet, though these meters are obviously balanced for minimal effect from external fields. (Are you sure that 18 inch woofer magnet isn't under your bench?<G> I had to maintain strict magnetic segregation in my shop.)
I would think about putting a bold mark
If setting it off zero results in a dead on reading in the area of interest, that means the not-on-zero at rest position is pretty much just an aesthetic complaint.
MAACO points out this is a 1% error, surely close enough. And consider that accuracy specs for analog meters is percentage of full scale, I'd say the solution to your "problem" is to buy a new meter with greater accuracy.
If my 5% 100k resistor reads 95k, would I add a 5k resistor? Or would I just buy a 1% 100k?
MAACO points out this is a 1% error, surely close enough. And consider that accuracy specs for analog meters is percentage of full scale, I'd say the solution to your "problem" is to buy a new meter with greater accuracy.
If my 5% 100k resistor reads 95k, would I add a 5k resistor? Or would I just buy a 1% 100k?
1 % accuracy on a moving iron meter is good and it does read both 50Hz/60Hz AC RMS accurately unlike a moving coil meter which needs additional components.
Older MI meters used a non linear scale but nowadays the iron parts are modified to be used with a linear scale.
Some moving iron meters I owned when off didn't register zero but indicated sub zero but on power up displayed the correct indication , unlike MC meters MI meters will register non sinusoidal waveform accurately.
Older MI meters used a non linear scale but nowadays the iron parts are modified to be used with a linear scale.
Some moving iron meters I owned when off didn't register zero but indicated sub zero but on power up displayed the correct indication , unlike MC meters MI meters will register non sinusoidal waveform accurately.
Good question Bansuri while working for BT in the UK a digital meter was useless for testing long lines due to its high impedance and registering every bit of induced/radiated RF interference.
Many cheap digital meters are only able to display a very low frequency AC RMS voltage anywhere near accurately.
Thats why it pays to buy a quality product which ACCURATELY measures a wide bandwidth.
Also how pure is the sine wave you are checking with. Harmonics will produce different results with different meters due to bandwidth and how the RMS is derived. The only accurate way to check is with a low (.1% or better) distortion sine wave. Typical AC power can be 3% or more. Also the distorted waveform can have different effects for the same RMS voltage on the DC out of a rectifier. I would use an average indicating rectified meter for AC going into a DC supply since it will give a better prediction of the DC out of the supply. TRMS can be misleading in that application.
A MI meter measures RMS direct no rectifiers required that's what the OP has installed in his Hickock .
What does the moving iron voltmeter measure in a circuit, average voltage or RMS voltage? - Electrical Engineering Stack Exchange
I agree the mains electricity in the USA/UK is far from "pure " even worse nowadays due to all the modern gadgets injecting distortion /noise into the supply.
What does the moving iron voltmeter measure in a circuit, average voltage or RMS voltage? - Electrical Engineering Stack Exchange
I agree the mains electricity in the USA/UK is far from "pure " even worse nowadays due to all the modern gadgets injecting distortion /noise into the supply.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.