TRue RMS meters

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brother said:
what does 'true RMS meter mean?' Does RMS stand for 'root mean square'.

This is something ive never asked but always wondered. I know Fluke makes the 'true RMS' meters for reading current.
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RMS meters are more accurate in their readings compared to the actual measurement. All other meters are averaging. Close, but not very accurate. If you can afford it, get an RMS meter. It will serve you better in the future.

And yes, you are correct in "root mean square."
 
480sparky said:
RMS meters are more accurate in their readings compared to the actual measurement. All other meters are averaging. Close, but not very accurate. If you can afford it, get an RMS meter. It will serve you better in the future.

And yes, you are correct in "root mean square."
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Thanks for the info, i was always told the rms meters are more acurrate, but im curious as to how are more accurate. Ive used 'average' current meters for taking current readings and they seem to be fine.
 
How can you know it works 'fine'? Just because it gives you a number near what you are expecting?

With harmonics, electronics, and such, RMS meters will give you a more accurate measurement. With true sine-wave waveforms, averaging meters are just fine.

Once you start into HID lighting, computers, networks, dimming systems, VFDs, RMS meters will be a better investment.
 
080615-2116 EST

What is the significance of an RMS reading?

Start with DC. Voltage times current is defined as power and this is the rate of use of energy. Energy dissipated in a load causes heating. If the load is a constant resistance and the voltage is constant, then Power = V * I, where V and I are average and constant.

The instantaneous power is p = v * i. The average power would be the integral of v*i over a time period of interest divide by that time interval for either or both v and i varying. This does not require that the waveforms are identical and of the same frequency and/or phase.

In an AC circuit the instantaneous power is varying thru a cycle. For a resistive load and sine wave excitation it is proportional to sin t * sin t = 1/2( 1 - cos 2*t ). Thus, it has a DC componet with a superimposed double frequency component. When averaged over an integral number of half cycles of the line frequency the result is a constant.

Root mean square means you square an instantaneous value, average this over N cycles and take the square root of that average.

The RMS value of a waveform ( current or voltage ) applied to a resistance produces the same heating effect as a DC value of the same numeric value.

When you do the intergation and other calculations on a sine wave you get the result that the RMS value = the peak of the sine wave divide by the square root of 2 or a multiplier of approximately 0.707. If you calculate the average value of the full wave rectified sine wave it is approximately 0.636 of the peak value.

A Simpson 260 or 270 meter measures the average value of the input AC voltage, but the meter is calibrated to read the RMS value of a sine wave. If the wave shape is different than a sine wave, then there may be an error in the reading relative to the actual RMS value of that waveform.

A true RMS meter gives a fairly good approximation to the actual RMS value for many waveforms if the waveform is not too peaked relative to the average value.

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Seems to me, in my observation, that the Fluke 87 is the true RMS meter that most electricians seem favor. If your observations are different, post it here. I sorta like to stay up on what most people are using, tools-wise.
 
080615-2200 EST

Marc:

I use an 87, actually one I gave my son, and did a test on it at one time with a short rectangular pulse. It was was fairly good.

Since it has a capacitor at the input you can not read the RMS value of an AC component superimposed on a DC component because it removes the DC component.

Whereas a two coil meter movement gives you the combined result of the AC and DC.

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BEWARE of relying too much on those "true" RMS measurements in certain circumstances.

Case in point: Recently we were troubleshooting some magnetic contacts that hold fire stop doors open at an auditorium. There were some doors that would not stay open on the magnets --- they seemed to be a bit weak.

Taking voltage measurements with a Fluke true RMS meter showed something around 50+ volts or so -- way more than the 24 Volts that was supposed to be present.

Using a different meter showed 26 Volts, just about right for such an installation.

Something in the power supply transformer or other circuitry was giving off those false readings using the so-called "true" RMS values.

So, before you take everything apart, use sound judgement and check things with more than one meter, if necessary.

FWIW, we replaced those "weak" magnets with new ones, and have not had any more problems with them.
 
gar said:
Marc:

I use an 87, actually one I gave my son, and did a test on it at one time with a short rectangular pulse. It was was fairly good.
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I guess I alternate the Fluke 87 with a TPI ScopeMeter 440 if I'm looking for some glitch in particular. They're both around the same price. The Fluke is sturdier, but the TPI has a scope.
 
kbsparky said:
BEWARE of relying too much on those "true" RMS measurements in certain circumstances.

Case in point: Recently we were troubleshooting some magnetic contacts that hold fire stop doors open at an auditorium. There were some doors that would not stay open on the magnets --- they seemed to be a bit weak.

Taking voltage measurements with a Fluke true RMS meter showed something around 50+ volts or so -- way more than the 24 Volts that was supposed to be present.

Using a different meter showed 26 Volts, just about right for such an installation.

Something in the power supply transformer or other circuitry was giving off those false readings using the so-called "true" RMS values.

So, before you take everything apart, use sound judgement and check things with more than one meter, if necessary.

FWIW, we replaced those "weak" magnets with new ones, and have not had any more problems with them.
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Possibly you had the Fluke on DC?
 
I don't think so.

I have the Fluke set, with both the true RMS and a clamp-on Amp meter.

The Amp meter gave the accurate reading, and the other one gave the false high reading. Something in that power supply made the RMS value wild.
 
As I understand it the whole point of using a true RMS meter is so that you still get a correct voltage reading even when it is not 60 hz source.

The averaging meters 'assume' the AC source is a perfect 60 hz.
 
080616-0533 EST

kbsparky:

You have to know what your waveform is, and how different meters respond to that waveform to understand the meter reading.

The Fluke 87 may not have been wrong for whatever waveform you were measuring.

What is a "magnetic contact"?
What is "the magnets"?
Why would an overvoltage cause the door to close?

Are these door magnets an electro magnet? Meaning no permanent magnet and just a coil with a current thru it to create the magnetic field.
Is this door magnet excited with filtered DC, a rectifier only, or AC? If it is pure AC, then the coil will require a shading coil to prevent buzz.

Did you ever make a DC voltage measurement of the voltage to "the magnets"?
Did you ever measure the currents, AC and DC, to "the magnets"?
What circuitry exists between the AC source and "the magnets"?
And where was the voltage measured? At the output of a transformer, or what was applied to the megnet's coil?

What average reading meter did you use? If it was Simpson 260 or 270, what was the switch position --- DC, AC, or Output? Note: in the AC position you will get a reading with a steady DC input. To strip the DC component you need to use the Output position. I believe all Flukes in the AC position have an input capacitor, and thus remove the DC component.

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There isn't really any such thing as a "correct" voltage or amperage reading for anything other than DC.

The RMS stands for Root Mean Squared. What that means is that at each instant in time, there is a voltage V. When you square the voltage at that intstant, you get a measure of the power (assuming constant resistance). Now if you add up all the squared voltages over a period of time and divide by the time, you get an average power over that time. To get this back to a voltage, take the square root. Hence the RMS.

Now for DC, the RMS is 1.0 times the peak value - they are the same. If you do the math for a sine wave, you will find the RMS value to be the peak value divided by the square root of 2 (1.414). This is how standard voltmeters work. They assume a sine wave and just show the peak voltage divided by 1.414. A True-RMS meter goes thru the calculation in the previous paragraph.

NOW: Why use RMS?? The answer is that if you know the RMS value of a voltage or current, you can use that to determine the power in the waveform. That's why True-RMS is the correct measurement to use when determining if a wire will overheat. The heat in the wire is related to the RMS value of the voltage waveform because the wire is a pure resistance.

That's a lot to absorb, but I hope it helps.

Mark

Edited: Should be square root of 2 (sorry it's early).

Also, this link has a good description:

http://www.opamp-electronics.com/tutorials/measurements_of_ac_magnitude_2_01_03.htm
 
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080616-0805 EST

busman:

You have a good description, however the constant is the sq-root of 2 not 3 or 1.414. This derives from sin t * sin t = 1/2(1-cos 2t) and when averaged over exactly N cycles the result is 1/2. When you take the sq-root you get sq-root of 1/2 which is 1/sq-root 2 = 1/1.414 = 0.707. Or you can evaluate the integral to get the result.

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080616-0721 EST

iwire:

Relative to post #13.

It is not frequency, but waveform related.

The Fluke or Simpson or any other meter that uses a capacitor in series with the input will produce greater errors as the frequency is lowered. Basically the voltage reading will be lower than the actual voltage. This is gradual because the capacitive reactance increases as the frequency is reduced. Generally filters are defined by their half power point, 0.707 in voltage.

My Fluke 27 has the 0.707 point about 1.1 Hz with a sine wave input. On the high side it is flat to about 40 kHz, then starts to rise, and higher up near 1 mHz is down to near zero. These measurements are assuming the function generator output voltage was about constant. No working scope at home to monitor the voltage.

On my 1948 Simpson 260 in the Ouput position the low frequency half power point is about 550 Hz on the 2.5 V range and 140 Hz on the 10 V range. It will go lower on the higher ranges. This to be expected because the input resistance of the meter changes with the range. The high frequency roll off is about 150 kHz.

On the Fluke 27 when I switch the input from a sine wave to a square wave the reading goes up by a factor of 1.6. This is approximately as expected.

Comparing the 260 with the 27 (both are essentially average measuring on AC). Sq-wave input at 1 kHz. Set 260 in output position and adjusted function generator to produce 10.0 V. The 27 read 10.26 V. Switched to sine wave, assume peak voltage is the same, the 260 reads 6.0 V, and the 27 reads 6.26 V. Theoretically we should see 6.36 V on the sine wave when the sq-wave was set at 10.0 V. A more refined experiment is needed to get an accurate correlation with theory.

On the sq-wave the 260 is constant to about 2 kHz, and the 27 to about 40 kHz.

These are somewhat crude experiments, but give you an idea of what happens with waveform and frequency.

The Fluke 87 is at the shop so is not part of the current experiment.

.
 
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