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Old 28th May 2007, 11:34 PM   #1
star882 is offline star882  United States
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Default True-RMS?

Exactly how important is True-RMS? Is there an easy test to find out if my meter has that feature? (I can return it and get one with True-RMS if it doesn't.)
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Old 29th May 2007, 01:21 AM   #2
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The question comes down to whether you'll ever need to know the RMS voltage of a non-sine wave. That might be the case if you wanted to know the power dissipation of a resistor with some odd waveform across it. My guess is most people would rarely need it. The meter I use the most isn't true RMS, and very rarely do I ever say, "oops, gotta get the true RMS for this measurement". If your meter is true RMS, it will likely be prominent in the advertising. You can tell by measuring a 2.828V peak to peak square wave at 100Hz or so. An averaging meter will read about 1.52 volts, whereas a true RMS meter will give the correct value of 1.414 volts. Or proportion the numbers to anything convenient.
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Old 29th May 2007, 03:35 AM   #3
star882 is offline star882  United States
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I don't think my meter is True-RMS, then. If I replace it, should I still look for at least a CAT III rating? (I've heard that CAT III is important when working with power supplies with large primary side capacitors.) Or should I go for a CAT IV just to be prepared for future work, such as working on a hybrid car or large UPS system? (I'm not sure if a hybrid car (the main motor circuit of one, that is) would be CAT III or CAT IV. And UPSes rated for CAT IV are the ones intended for large datacenters, right? Not sure what I would be working on in the future, though!)
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Old 29th May 2007, 06:03 AM   #4
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No idea about CAT ratings, I'm more of a dog person. If you need a certain rating for commercial work, definitely get it. The current crop of Fluke handhelds are really nice and have the ratings. You can get more features for less money with the imports, but I consider them somewhat disposable. Don't forget that some meters have built in computer interfaces, so consider if you need that as well. Personally, I'd use whatever I had until there was some good reason to replace it. I'm still using my Fluke 77 as my general purpose meter- it's probably at least 10-15 years old and has gone through maybe 3 batteries and 4-5 fuses when I forget to switch the leads from current to voltage
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Old 29th May 2007, 07:05 AM   #5
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If your meter has true RMS capability, it will be written in big letters on the front panel.

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Old 30th May 2007, 06:50 PM   #6
star882 is offline star882  United States
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Quote:
Originally posted by Conrad Hoffman
No idea about CAT ratings, I'm more of a dog person. If you need a certain rating for commercial work, definitely get it. The current crop of Fluke handhelds are really nice and have the ratings. You can get more features for less money with the imports, but I consider them somewhat disposable. Don't forget that some meters have built in computer interfaces, so consider if you need that as well. Personally, I'd use whatever I had until there was some good reason to replace it. I'm still using my Fluke 77 as my general purpose meter- it's probably at least 10-15 years old and has gone through maybe 3 batteries and 4-5 fuses when I forget to switch the leads from current to voltage
Based on what I work on, I'll need one rated for at least CAT III. (However, if hybrid cars are CAT IV, I would like to be prepared instead of having to upgrade again at that point.)
All the multimeters with USB are expensive, so I think I'll make a simple ADC box for my PC when the need comes. I think I'll use optical S/PDIF to get the isolation. S/PDIF can go up to 192kHz at 2 channels (not sure what frequency with more than 2 channels) so I think it will work fine if I DC-couple the ADC chip.
I think I'll get a separate current probe. Is the $80 TPI at Fry's any good? http://shop3.outpost.com/%7BatBzJ2-3...H:MAIN_RSLT_PG (it has to be compatible with both AC and DC)
For the meter itself, I have decided on one of these:
http://shop3.outpost.com/%7BatBzJ2-3...H:MAIN_RSLT_PG
http://shop3.outpost.com/%7BatBzJ2-3...H:MAIN_RSLT_PG
http://shop3.outpost.com/%7BatBzJ2-3...H:MAIN_RSLT_PG
Any suggestions?
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Old 30th May 2007, 11:41 PM   #7
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I'm not really up on the latest and greatest, but have used TPI probes in the past. They were fine for an inexpensive probe. As for the meters, the Triplett seems good for the money. IMO, the Fluke is near the bottom of their line, and I'd want more features and a higher voltage rating. Ok, I'm an equipment snob, and would try to start a bit higher in price.
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Old 31st May 2007, 01:10 AM   #8
Magura is offline Magura  Denmark
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The fluke 114 is fine for the wast majority of what you would ever stumble on that dosn't require a scope anyway. I have one of those myself, and have so far been able to measure anything needed for audio electronics, electro-mechanical work and basic trouble shooting.
An LCR meter is a nice addition though.

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Old 1st June 2007, 03:35 AM   #9
star882 is offline star882  United States
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I found a good deal on a Mastech MS8226 at a local electronics parts store. It has True-RMS, CAT III rating, temperature measurement, and isolated RS232. I still need a current probe, though. Any suggestions for that?
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