True RMS

Servicewiz_3

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

Jamie Hall

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.

hot_rod

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

Harvey Ramer

Fluke

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



Harvey

Jamie Hall

True enough, Hot Rod

and thank you.  I was mired in the sine waves of the twentieth century!

SWEI

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.