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04-06-2011, 09:49 AM #1
Fluke "true RMS"....why does that matter ?
In measuring everything from 3 phase AC voltages up to 600 volts and DC from 1.2 to 90 volts, I've never noticed any negative measurement issues with my Fluke 75 III "non true" RMS multimeter, so why is "true" RMS so important to some folks ?
I ask this as I'm thinking of buying a second Fluke just to have at another location and wondering if there would be any advantage in paying a little more for true RMS. Having capacitance and frequency measurement might be nice too...got those already on a Radio Shack meter but it's a POS really. What say ye ?
04-06-2011, 10:34 AM #2
"True RMS" comes about because of the need to measure waveforms that are not sinusoidal.
Cheap multimeters in ac voltage mode that purport to measure rms voltage actually measure peak voltage and divide by √2 to display "rms" voltage, which is only true if the voltage being measured is actually a sinusoid and symmetric.
That said, unless your ac voltages are SCR chopped or something weird like that I don't see "True RMS" capability being necessary for just checking ac mains voltages.
04-06-2011, 10:38 AM #3
04-06-2011, 11:06 AM #4
Electronics as opposed to electrical. These days the trick is to get a portable o-scope instead of the RMS meters. You get a LOT more capability.
Measuring PWM power, chopped wave forms, chopped VFD currents...things like that.
Generalized analog waveforms.
As mentioned, if all you need is lines or transformer based power supplies (as opposed to switched) you don't need it.
I have an old analog meter with two scales on the AC, one calibrated to read peak, the other at RMS...at 40-90 hz.
04-06-2011, 03:37 PM #5
RMS = heating equivalent
RMS is the heating power of a voltage source.
For example, if you apply voltage to a resistor, it will get just as hot if you apply 100 volts DC or 100 volts RMS AC. That's the definition of RMS.
Where this becomes important is measuring controlled voltage sources, like light dimmers, that only turn on for a fraction of the normal waveform. In this case you need a "true-RMS" meter to determine the heating energy supplied by that source to the load.
In the vast majority of applications you don't need "true" RMS measurement at all.
A REAL RMS-reading meter has a little resistor inside and measures its temperature, converting that value to the equivalent DC voltage.
04-06-2011, 05:22 PM #6
I know from personal experience that the free energy cranks rely on their meters NOT being a true RMS meter to get their abnormally high readings. Some will react to a true RMS Fluke like a vampire does to holy water (My electrons are different to your electrons!).
I once had a basic voltage converter circuit on the bench that measured 140% efficient without true RMS readings and dropped to 70% when measured with a true RMS meter or filtered to DC and measured.
04-06-2011, 05:45 PM #7
Another application for True RMS meters is for measuring waveforms with both an AC and DC component.
04-06-2011, 06:13 PM #8
RMS stands for root mean square and is equal to .707 times the peak to peak voltage. Like others said for most situations it will not matter; checking incoming power to a machine, or the standard 24V or 110V control circuit. I do use a Fluke true rms meter when troubleshooting problems in customers machines. because I often get into complex problems with servo drives, invertors, PLCs etc. At home I have a cheapo $20 Craftsman multimeter that works just fine.
04-06-2011, 07:22 PM #9
It works well for measuring actual voltage from inverter power supplies and VFD drives. I've seen non RMS meters read inverters like those used in cars and semis at 90 VAC or lower and an RMS meter reads it as 110 VAC like it's supposed to be. Which can make you think the inverter is bad when it really isn't
04-06-2011, 07:49 PM #10
04-06-2011, 10:53 PM #11
And only a REAL OLD rms reading meter uses that method to measure true rms . I doubt Fluke will sell you a meter that uses that method today.
04-06-2011, 11:52 PM #12
Fluke 931B - True RMS Differential Voltmeter | Alliance Test Equipment
This uses one of a matched pair of thermocouples to respond to the RMS, or heating value of the wave form. The second thermocouple is an ambient temperature compensating device.
A reason for needing true RMS voltage is when you are really examining power. That would be the case for audio amplifiers, VFD power, stepper motor capacity and torque, power distribution units, setting taps on transformers in non-linear applications.
The "DC" from your car's alternator is far from DC. The charging current is closer to a series of pulses that are proportional to the RPM and number of phases. In fact it can be shown that the current in a car battery is at times alternating pulses as when the alternator doesn't have the capacity to power the load and charge the battery. A true RMS meter would detect that, not that it's an important characteristic. An instructor or student may find it endlessly fascinating.
04-07-2011, 12:00 AM #13
A quick "WHY would you need a true RMS meter" story . . .
Guy sets up his ACDrive with out of the box settings, motor always runs hotter than a two dollar pistol and lacks torque.
I stop by with my meter, set the motor to run at 30Hz, measure the T-Lead voltage on the motor and I read 190VAC . . . hmmm, kinda low.
Find the mag current parameter in the drive and start adjusting it up until the motor T-Leads read 230VAC . . . much better.
Now crank the motor up to 60Hz and it reads 460VAC . . .
Save parameters and go away - the guy calls me a few week later and says his motor runs cool as a cucumber, quieter, smoother, and way more power - a happy camper!
04-07-2011, 06:04 AM #14
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04-07-2011, 09:20 AM #18
Don't know if they've changed the model numbers in the intervening 20-some years since I bought my Fluke 83, but back then the primary difference between an 83 and an 87 was true RMS on the 87, and about 50 bucks. The 83 has capacitance and frequency capability if that's something you need.
04-07-2011, 05:02 PM #19
04-07-2011, 05:12 PM #20
Is it my imagination or does Fluke make too many different meters ? It's mind boggling how many different models they offer, esp if you consider recently discontinued models in the mix. I guess I want a 175 or 177 in order to get capacitance and frequency....but lord knows what else they have that might be better bang for buck...