The AM Forum

I am trying to come up with an accurate way of measuring my AC line voltage because I seem to have readings that do not correlate.

I measure with two good quality DVM's and a scope. All of these devices seem to correlate within a few percent.  The scope has a built-in DVM. I calculate RMS from the P-P reading. ex. (320/2)*.707= 113.12

I imagine I can trust those measurements however all of my smart UPS's seem to give much higher readings. Also a simple AC panel meter reads higher. The DVM's and scope might read 115, while the panel meter reads 120 and the UPS's read 124.

I guess I need to drag out my Simpson 260!

I wonder what the power company uses to measure line voltage.

Doug,

I bought a Simpson 260, the first piece of electronic gear I ever owned, for a graduation present to myself in 1950. The a-c voltage accuracy was 3% then, probably closer to 5% today, given its copper oxide rectifier and component ageing. At half-scale on the ridiculous Simpson  25 - 100 - 250 range selections the typical (which means nothing) accuracy could be as bad as 10%.

Your accurate peak reading converted to RMS is probably the best you can do, and should be pretty close, given waveform purity presently maintained. The UPS measurement is suspect because of waveform distortion and lack of knowledege about the metering circuit.

The power company probably has a $5000 certified expanded scale chart recorder "somewhere" in the system, but then you don't know where you are on the power line or what its regulation is at any time of the day.

Next big hamfest pick up a Weston mirror-scale meter with an 8" scale in a nice mahogony box for $25 tops. They knew something about measurement.

Bob - NE

I use a HP DVM which I believe to be accurate.  I back this up with a number of large lab type AC voltmeters, mostly Westons and others.  All my meters read within a volt of each other.  That's the best I've been able to do.

I just bought a 30 dollar DVM from HD and it reads AC as good as my HP.

Using a scope is not likely to be more accurate than a good meter.  The Simpson 260 (I have about 5 of this type meter) is probably the least accurate meter to use.

Fred

I just use a DMM to see what the line v. is (it is always at least 121 v. here usually 122 or more) and get a buck transformer or variac to lower it to 110 to 115.  The gear doesn't care as long as it is not up there much over 115.  If it is summer and the line is 125 I put the transformer on 115 so it will still be at or over 110 when it goes back down.  If it's 120 I put the transformer at 110 so it will still be under 115 when the line goes up.  I don't think this has to be a precision thing.