True RMS meters?

Discussion in 'General Electronics Chat' started by sdowney717, Mar 17, 2015.

  1. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    Apparently I dont own one?

    I have an
    Elenco ST-3030,
    Craftsman 82028
    Harbor Freight Centech freebie

    I was measuring an MSW inverter AC output and they all read about 95 to 98 vac when inverter is loaded with 1000 watts.
    Can you factor in a number to get an approximate volts?
    The ELenco's box says it is 'true rms', so why do they all read about the same then?
    I emailed Elenco, but did not get a response
     
  2. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    If the instrument is indeed a 'true RMS' indicator the reading should be accurate -- FWIW, in my experience, off-the-shelf power inverters not uncommonly run slightly (Ca 10 to 15 volts) on the 'low side'... Please note that it is imperative that the waveform be monitored during any attempt to adjust the output level!

    Best regards
    HP

    PS -- I concur that the stated reading appears consistent with a 'sinusoidal assumption' applied to a MSW --- Harbor Freight strikes again! :eek:
     
    Last edited: Mar 17, 2015
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
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    most of the dvm's on the market read "true rms" if they didnt what would the meaning be of an ac reading on the 120 volt ac (rmss) line? and all "true rms" meters will read true rms on sine waves. square waves or waves with severe distortion will read wrong, theres no way to predict what without knowing how distorted they are.
     
  4. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Such measurement is most practically achieved through SLTF or 'dissipative' metrics...

    FWIW all the "true-RMS" Speced instruments I've used have, remarkably, proven accurate, despite waveform, at (fundamental) frequencies < 5kHz

    Best regards
    HP
     
    Last edited: Mar 17, 2015
  5. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
    13
    It was both encouraging and discouraging to find all 3 meters read about the same.
    Encouraging that maybe the output voltage of the inverter is acting normal even though all 3 meters read low.

    Discouraging that the Elenco reads it so poorly. Close to what the el cheapo HF meter reads is discouraging.
    Granted the HF meter has a little trouble locking on a steady number, but reconnecting a couple times you get a better reading.

    So far it has run my big microwave, a big vacuum and can start up the fridge, runs fluorescent lights and a portable heater. Although the Microwave seems to run cooler, but I will check some more when hooked to a big battery bank. I pulled out the car battery to test it and it has low amp hours, quickly gets depleted. I used 2 gauge cable 6 foot long fo the test, so then 12 feet of cable.
     
  6. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    At risk of sounding the shill: Just purchase a Fluke (or other quality) true RMS digital multimeter! Accurate, truthfully labeled, test instrumentation pays for itself many times over! :)

    Best regards
    HP
     
    Last edited: Mar 17, 2015
  7. DickCappels

    Moderator

    Aug 21, 2008
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    If the waveform you are measuring is close to being a sine wave, then if you meters are calibrated, they should all read about the same. They should only show significant differences when the waveform deviates substantially from a sine wave.
     
    Roderick Young likes this.
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    it is harder to find a peak reading volt meter than an rms one. have you checked the dc voltage into the inverter right at the terminals of the inverter? a 1000 watt load will be pulling quite a bit from the dc input, possably voltage drop across the input leads causing the low output voltage.
     
  9. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    I have a good DC voltage of 11.5 to 12 vdc
    They are supposed to self regulate the AC output volts regardless of input until shut down within a few percent, I suppose a real heavy load not so well.
    With the at home setup, I was able to draw 80 amps DC for for about 10 minutes before the VDC drops too low and the MSW inverter beeps.
    That battery is not very good.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Most of low cost dvms on the market are not "true RMS" and read the waveform rectified average value with the meter calibrated to read RMS of a sinewave (a .707 / .636 ratio).
    A meter labeled "true RMS" meter has circuitry that actually determines the RMS function of the waveform, independent of the waveshape within its crest-factor (peak to RMS) limits.

    P.S. sdowney717, I see no mention of the Elenco ST-3030 being true RMS in its manual or in its literature. I suspect it's not.
     
    Last edited: Mar 17, 2015
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  11. Roderick Young

    Member

    Feb 22, 2015
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    I have seen instruments on eBay going for $20 or less that supposedly measure just about any passive or active part, with a microcontroller at the heart of them. If they can do that, you would think that someone could use a similar microcontroller to take (say) 1024 samples of a waveform over a 100 mS window, and actually apply the root-mean-square formula to it.
     
  12. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    There are chips out there that will give you the true RMS of any waveform that has been conditioned to its working range.
     
  13. studiot

    AAC Fanatic!

    Nov 9, 2007
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    There is only one way to truly measure RMS and that is by a themometric meter.

    All calculating type meters will be subject to error, depending upon waveform and its stability of many cycles.

    The nearest you can get is the integrating crossed coil mains electricity meter.
    But these are calibrated for energy not RMS voltage.
     
  14. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    I couldn't find any reference on google or wikipedia to the instrument that you've mentioned (themometric meter) would you mind elaborating? Or maybe it's typo?
     
  15. OBW0549

    Well-Known Member

    Mar 2, 2015
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    I think this Linear Technology application note explains it pretty well, in the section "50MHz Bandwidth Thermal RMS→DC Converter" starting on page AN5-4:

    http://cds.linear.com/docs/en/application-note/an05f.pdf
     
  16. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Interesting... the application note states that the technique is good for frequencies above 500 Khz with a 2% error, while the chip I mentioned is good for frequencies below that point, and it has a 1% error.
     
  17. crutschow

    Expert

    Mar 14, 2008
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    Many years ago Hewlett-Packard (Agilent) made a true RMS accurate to high frequencies that used a thermal measurement technique.
    Don't know if they still make it.
     
  18. OBW0549

    Well-Known Member

    Mar 2, 2015
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    Uh... no, it doesn't say that at all.

    It says, "Analog computing methods are limited to use below 500kHz. Above this frequency, accuracy degrades beyond the point of usefulness in instrumentation applications. Additionally, crest factors greater than 10 cause signifi cant reading errors." and later on, regarding the thermal approach, "Accuracy remains within 2% from DC to 50MHz for inputs of 300mV to 10V. Crest factors of 100:1 contribute less than 0.1% additional error and response time to rated accuracy is five seconds."
     
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  19. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Thanks for clarifying. So the most viable way to measure RMS is through accurate temperature measurement then?
    What about extremely low power waveforms? What other methods are there, other than ADC and then integration?
     
  20. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Mine is an old Marconi Instrument meter to Ministry spec.

    I will try to get a photo to post.

    Meanwhile here are some details of the HP one (Those were the days my friend, Oh yes those were the days.)

    calorimetric1.jpg
    calorimetric2.jpg
     
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