Page 1 of 3 123 LastLast
Results 1 to 20 of 50
  1. #1
    Milacron's Avatar
    Milacron is offline Diamond
    Join Date
    Dec 2000
    Location
    Coastal Dogpatch, SC, USA
    Posts
    45,403

    Default 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 ?

  2. #2
    kuromaku is offline Plastic
    Join Date
    Jan 2011
    Location
    Northern California, USA
    Posts
    20

    Default

    "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.

  3. #3
    Milacron's Avatar
    Milacron is offline Diamond
    Join Date
    Dec 2000
    Location
    Coastal Dogpatch, SC, USA
    Posts
    45,403

    Default

    Quote Originally Posted by kuromaku View Post
    "True RMS" comes about because of the need to measure waveforms that are not sinusoidal.
    Right, but the point is, WHO actually needs the capability to know AC voltages that precisely...what sort of tech, what sort of business ?

  4. #4
    Miguels244 is online now Diamond
    Join Date
    Mar 2011
    Location
    Denver, CO USA
    Posts
    4,473

    Default

    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.

  5. #5
    Join Date
    Nov 2005
    Location
    Maryland
    Posts
    7,023

    Lightbulb 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.

    - Leigh

  6. #6
    SAG 180's Avatar
    SAG 180 is offline Titanium
    Join Date
    Sep 2007
    Location
    Cairns, Qld, Australia
    Posts
    2,038

    Default

    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.

  7. #7
    BobM3 is offline Aluminum
    Join Date
    Feb 2006
    Location
    Minneapolis
    Posts
    159

    Default

    Another application for True RMS meters is for measuring waveforms with both an AC and DC component.

  8. #8
    misterT's Avatar
    misterT is offline Cast Iron
    Join Date
    Mar 2009
    Location
    Indiana USA
    Posts
    344

    Default

    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.

  9. #9
    JP Machining is offline Hot Rolled
    Join Date
    Jul 2006
    Location
    Wisconsin
    Posts
    549

    Default

    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

  10. #10
    Join Date
    Nov 2005
    Location
    Maryland
    Posts
    7,023

    Lightbulb

    Quote Originally Posted by misterT View Post
    RMS stands for root mean square and is equal to .707 times the peak to peak voltage.
    True only for sine waves of low distortion, in which case you don't need a true-RMS meter in the first place.

    - Leigh

  11. #11
    swarf_rat is offline Titanium
    Join Date
    Feb 2004
    Location
    Napa, CA
    Posts
    2,147

    Default

    Quote Originally Posted by The real Leigh View Post
    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.

    .....

    A REAL RMS-reading meter has a little resistor inside and measures its temperature, converting that value to the equivalent DC voltage.

    - Leigh
    The heating effect is a result of the definition of RMS, not the definition itself (which is the the square root of the average of squares of a waveform).

    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.

  12. #12
    dp
    dp is offline Banned
    Join Date
    Aug 2005
    Location
    Puget Sound, Washington
    Posts
    3,204

    Default

    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.

  13. #13
    motion guru's Avatar
    motion guru is offline Titanium
    Join Date
    Dec 2003
    Location
    Yacolt, WA
    Posts
    3,492

    Default

    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!

  14. #14
    neilho is online now Hot Rolled
    Join Date
    Mar 2006
    Location
    Vershire, Vermont
    Posts
    863

    Default

    Quote Originally Posted by Milacron View Post
    Right, but the point is, WHO actually needs the capability to know AC voltages that precisely...what sort of tech, what sort of business ?
    The true RMS feature is useful for us folks who work with wind and water driven variable speed permanent magnet generators. Wind driven applications can range from 0-200Hz, so if we have a true RMS meter, we don't have to compensate for frequency when measuring voltage.

    Neil

  15. #15
    SAG 180's Avatar
    SAG 180 is offline Titanium
    Join Date
    Sep 2007
    Location
    Cairns, Qld, Australia
    Posts
    2,038

    Default

    Quote Originally Posted by neilho View Post
    The true RMS feature is useful for us folks who work with wind and water driven variable speed permanent magnet generators. Wind driven applications can range from 0-200Hz, so if we have a true RMS meter, we don't have to compensate for frequency when measuring voltage.

    Neil
    Neil, frequency is not the issue but non sinusoidal AC is. I have a cheap Lutron meter that is fine to 8 KHz, it only reads accurately on a sinewave. Lots of home made generators have a peaky waveform with long gaps between peaks that fool a cheaper meter.

  16. #16
    Miguels244 is online now Diamond
    Join Date
    Mar 2011
    Location
    Denver, CO USA
    Posts
    4,473

    Default

    Quote Originally Posted by SAG 180 View Post
    Neil, frequency is not the issue but non sinusoidal AC is. I have a cheap Lutron meter that is fine to 8 KHz, it only reads accurately on a sinewave. Lots of home made generators have a peaky waveform with long gaps between peaks that fool a cheaper meter.
    Meters use a blocking capacitor, this can limits the frequency response of the meter.

  17. #17
    Lakeside53 is online now Stainless
    Join Date
    Sep 2007
    Location
    Woodinville, WA
    Posts
    1,166

    Default

    Quote Originally Posted by SAG 180 View Post
    Neil, frequency is not the issue but non sinusoidal AC is. I have a cheap Lutron meter that is fine to 8 KHz, it only reads accurately on a sinewave. Lots of home made generators have a peaky waveform with long gaps between peaks that fool a cheaper meter.
    yes, but not just "homemeade". Typical small COMMERCIAL home generators also have lousy waveforms (narrow, steep sides, flat'ish top). My so-called 10kw generator is pretty bad, but so are most I've looked at. It measures 238 volts on my cheap meter, 189 on my true rms Fluke. One 12kw generator head made by Gillette had a great waveform and measured pretty good.

  18. #18
    metlmunchr is offline Diamond
    Join Date
    Jul 2004
    Location
    Asheville NC USA
    Posts
    8,334

    Default

    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.

  19. #19
    SAG 180's Avatar
    SAG 180 is offline Titanium
    Join Date
    Sep 2007
    Location
    Cairns, Qld, Australia
    Posts
    2,038

    Default

    Quote Originally Posted by Lakeside53 View Post
    yes, but not just "homemeade". Typical small COMMERCIAL home generators also have lousy waveforms (narrow, steep sides, flat'ish top). My so-called 10kw generator is pretty bad, but so are most I've looked at. It measures 238 volts on my cheap meter, 189 on my true rms Fluke. One 12kw generator head made by Gillette had a great waveform and measured pretty good.

    That squared off waveform indicates magnetic saturation if it's doing that under load. The manufacturer may have skimped on the amount of steel in the magnetic circuit.

  20. #20
    Admin5 is offline Administrator
    Join Date
    Oct 2007
    Location
    Beaufort, SC
    Posts
    819

    Default

    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...

    Milacron

Page 1 of 3 123 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •