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True RMS

What is True RMS on my meter and will I need it for troubleshooting HVAC controls?

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 17,155
    True RMS

    should be a measure of the average voltage or current, over time (usually a very short time -- one cycle!), being measured -- as opposed to peak voltage, which would be the highest voltage.  It's really only of concern with alternating current or rectified alternating current without capacitors, filters, chokes and sundry other bits used to smooth out the voltage or current.  In alternating current, for example, the true average voltage or current is zero -- since half the time it's positive and half negative.  Not a very useful result...



    Do you need it to troubleshoot HVAC equipment?  I doubt it, so long as you meter has some way to measure AC at all (and I've yet to see a multimeter which didn't).



    It's actually not all that simple to build a circuit which will measure true RMS.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 15,064
    fairly necessary

    if troubleshooting modern HVAC controls and equipment. I know the output from the triac relay on many solar controls cannot be read accurately with a non RMS meter. Worth the extra bucks I think.



    A good RMS explanation here from Extech





    http://www.youtube.com/watch?v=XUdRW0XgYQs
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,217
    Fluke

    Out of all the multimeters I have used, Fluke brought home the bacon on many different levels.



    Harvey
  • Jamie Hall
    Jamie Hall Member Posts: 17,155
    True enough, Hot Rod

    and thank you.  I was mired in the sine waves of the twentieth century!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SWEI
    SWEI Member Posts: 7,356
    Speaking of Extech

    I just have to put in a plug for their 380942 mini clamp meter.  I love my Fluke for higher power stuff, but the small jaw and high resolution of the Extech makes it perfect for a lot of troubleshooting tasks.