Importance of true RMS to what we do?

[Phil_S]

I've got the bug again to see if I can get a used Fluke for a great price. In some respects, it boils down to whether the meter is True RMS or not.

There are many good explanations of True RMS to be found. I'm not asking what it is. What I'm asking is whether it's important to have that feature or not on the meter, for the sorts of things we generally do. In general, the only ACV I'm measuring is the PT primary and PT secondary(ies). Consider, I've got another meter that will do true RMS, but, as you can guess, it's not a Fluke.

Models without TRMS (AFAIK) include 27, 73/77 series, 8025B.

Or, falling off the wagon again, is BK Precision really not as good as Fluke? Bargains to be had on older TRMS meters:
http://www.ebay.com/itm/300610331978?ss ... 1423.l2649
http://www.ebay.com/itm/400264250295?ss ... 1423.l2649

Opinions please?

[jaysg]

I think if you were really being anal about how much clean power is being delivered to the load, then you'd put a scope and a true RMS meter on the load and read the meter instead of guestimating from the scope. If you wanted full distortion power, it would work for that too. Otherwise, it's just a nice-to-have feature.

[Phil_S]

I think if you were really being anal about how much clean power is being delivered to the load, then you'd put a scope and a true RMS meter on the load and read the meter instead of guestimating from the scope. If you wanted full distortion power, it would work for that too. Otherwise, it's just a nice-to-have feature.

Yeah, that's what I'm looking for. LOL, for the one time I'm probably going to do that between now and when they put me in a pine box, I can use the other meter that has TRMS. My interest is at an amateur level and primarily in low power amps. Generally speaking, low is low and I have never contemplated actually measuring "how low."

I had a Triplett 9045 that I really liked, but did something silly to hose it. I replaced it with an Amprobe, which seems to be an OK meter, but I just don't like the "feel " of it. It feels cheap. I want a Fluke!

Thanks.
Phil

[tubelectron]

Hello,

The True RMS feature is useful if your intention to measure audio signals, not 50/60Hz VAC.

If it's about audio, you need a TRMS meter which has a significative bandwidth : the FL87 III is 50Hz - 20KHz and the 87 V is 20Hz (I think, or at least 50Hz for sure) - 20KHz. The FL79 is only 50Hz - 1KHz.

I checked both with a calibrated bench audio laboratory DM : they were consistent at 2% between 20Hz and 100KHz... Not so bad compared to the initial specs... That's Fluke... Not Wavetek !

If you don't need to measure audio signal, then the TRMS feature with a correct bandwidth has no real interest.

A+!

[husky]

I find the Fluke 87 to be money well spent for Audio. Plus you can record peak. I've had mine since 91 and with minor cleaning it still works fine.

[tubelectron]

I find the Fluke 87 to be money well spent for Audio. Plus you can record peak. I've had mine since 91 and with minor cleaning it still works fine.

For sure ! I forgot to mention these compementary but useful features.

A+!

[diagrammatiks]

most of the ac readings you need to do on a tube amp are better handled by a vacuum tube volt-o-meter.

[Gaz]

Just got an NOS 87 III on Craigslist for $135. Early Xmas for myself! Now I can be one of the guys in the Fluke threads who says, 'just get the Fluke and be done with it.'

[husky]

most of the ac readings you need to do on a tube amp are better handled by a vacuum tube volt-o-meter.

Why do you feel that?

[LOUDthud]

I picked up an Eico VTVM a few weeks back for $14.50 . It included one of those big high voltage probes. They thought it was broken because the ohms didn't seem to work. It just needed a new D cell.

[tubeswell]

Never used a true RMS meter; prefer to measure peak-to-peak and divide by the square root of 2. In any event, as peak-to-peak is only really measurable with a scope if you have a perfect sine wave injected (and even that begs assumptions - like how good is the signal injector's calibration, and how good is the scope's calibration etc), so that even that is academic.

[husky]

Never used a true RMS meter; prefer to measure peak-to-peak and divide by the square root of 2. In any event, as peak-to-peak is only really measurable with a scope if you have a perfect sine wave injected (and even that begs assumptions - like how good is the signal injector's calibration, and how good is the scope's calibration etc), so that even that is academic.

Yeah but what meters do Peak to Peak that don't already do True RMS? Unless you are talking a scope. The 87 has a peak measurement which is pretty accurate to get Peak to Peak. I also have an Extech but I use that more for ACR and stuff like that. I still say an 87 is a life time tool that is well worth it