Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: jjasilli on February 02, 2011, 05:03:00 pm
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For those of us with bench DMM's, and continuing in the spirit of endless regression. . . My meter has not had its annual checkup. And, I can see it's a bit out of calibration. The manual calls for some exotic, not to mention ghastly expensive, test equipment to run the calibration tests. And who's to say the test equipment is calibrated !?!
Is there a down & dirty, inexpensive, short-cut method to calibration?
EDIT: What about this?: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=280622409285&fromMakeTrack=true&ssPageName=VIP:watchlink:top:en
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And who's to say the test equipment is calibrated !?!
N.I.S.T. (http://www.nist.gov/index.html)
Is there a down & dirty, inexpensive, short-cut method to calibration?
Yes,,,, if you have access to similar test equipment that has a current NIST cal sticker. You just compare readings of your meter (or whatever) to readings obtained by the calibrated meter (etc.). It's called verification rather than calibration.
I verify all my test eq with NIST calibrated equipment at work. This gives a pretty good level of confidence in my 'uncalibrated' test eq. If you have another meter that you feel good about, just compare readings with the Fluke. Or maybe you know someone.
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I don't think I have access to calibrated equipment. Probably the most important measure for accuracy is mV across a bias sense resistor. The precision 5VDC device I found on eBay might work. But in my meter, the 200mV DC range is controlled by a different trim pot than the 2V DC and higher settings. I guess I could divide the known 5VDC by 100 with precision resistors. Then set the trim pot so the meter reads 50mA. Is that worthwhile?
EDIT: I also see that "a 5 V voltage source in series with a 4.7 kilohm resistor will provide an approximately constant current of 1 mA (±5%) to a load resistance in the range of 50 to 450 ohm." (Wikipedia: current source)
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Probably the most important measure for accuracy is mV across a bias sense resistor.
The importance of the accuracy of that check is waaaaay overated. I check it too, but I'm much more interested in bias readings that are close together rather than knowing it's exactly 31.5ma. Even a $10 meter can tell you the readings are the same.
Anything you can do to build confidence in the accuarcy of your equipment is worthwhile. I'm a tool junky. That goes for test equipment too. And I love to have stuff that works well and is accurate. I'm lucky to have a job where I can 'test' my test equipment. And I'm pleased that my Fluke 87 is just as accurate as the Fluke 87 at work, even though its not really 'calibrated'. OTOH, my Triplett VTVM is not within mfg specs on any AC RMS voltage range, and I can't make it right with the cal pots, but I'm OK with just knowing that.
Your Fluke 8000 is way more meter than is required for guitar amp work, even if it is only +10% accurate.
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OK, I'll rest easy. I'm sure my meter is close enough for Blues! :grin: AND, I've got 2 more 8000a's on the way from eBay. Thanks for all your help and insights!
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Steve brings up a good point.
Accuracy vs. repeatability.
what is important is that it reads the same every time on a fixed value.
I cannot begin to count the number of times that I have been reminded that +-20% is good work in the thermionic world.
You could make you own "standard".an old computer PS, a 12V light bulb from a car for a load and a 1ohm resistor to measure across.
Hook your meter across the resistor take your reading and write it down. Verify it is fact in the ball park with a different meter. then whenever you want check your meter on it, if it has varied check your standard with the same different meter. if it too has varied its your standard, if it hasn't its you're Fluke.
OMG
JJ's gonna start hoarding antique calibration equipment now! :laugh:
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> My meter has not had its annual checkup.
Annual Checkup??
If you use it all the time, often on the same gear, you'll know if it gets real sick.
Sluckey does Government Work and Life Critical so they like his gear to be checked regularly.
_THE_ standard calibration for VTVMs is a _fresh_ carbon-zinc C-cell or D-cell. Historically these were 1.56V. That's what HeathKit tole you to calibrate your VTVM kit with. That's what Dynaco tole you to cross-check the bias in their big-amps with (in fact the bias resistor was 15.6 ohms, so when bias-tap equaled fresh D-cell, you had 100mA spot-on).
The volt is the basic measure in most multi-meters. Current is sensed with low-value resistors, precision appropriate for what the meter cost, often wire-wound. Wire-wounds either work or they have been burnt-out.
> I also see that "a 5 V voltage source in series with a 4.7 kilohm resistor
That's just basic Ohm's Law, shouldn't need Wikipedia. 9V battery, 1K resistor, and ammeter sure better read 9V/1,000= 0.009A= 9mA.
Resistance can now be calibrated with a bargain-pak of 1% resistors.
AC checks are tougher. It helps a lot to have a DC oscilloscope. With a 'scope you can compare any handy AC source (such as 3V heater-tap) to that 1.56V D-cell.
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Accuracy vs. repeatability. OK! I'm checking a particuklar 9V battery and a particular resisitor every day for 5 days, with my Fluke and a small Radio Shack DMM. The metetrs give very close readings to one another; and ea one gave the identical reading to itself for two days now.
That's just basic Ohm's Law, shouldn't need Wikipedia. 9V battery, 1K resistor, and ammeter sure better read 9V/1,000= 0.009A= 9mA.
:book1:
Resistance can now be calibrated with a bargain-pak of 1% resistors. Got it! Thanks again!
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FWIW I did tests over 5 days. Used 2 DMM's: Fluke 8000a; and a $20 RadioShack handheld. Items tested: a specific 9v battery; a specific 620R power resistor. Both DMM's were run on battery power. Results:
Fluke Radio Shack Fluke Radio Shack
1. 618R 615R 9.40V 9.38V
-9.38 -9.37
2. 618 615 9.40 9.37
-9.38 -9.37
3. 617 614 9.39 9.37
-9.38 -9.37
4. 617 614 9.39 9.37
-9.38 -9.37
5. 617 614 9.39 9.37
-9.38 -9.37
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I don't know how meaningful that check is, but one thing stands out... Your Fluke is at least as good as your RS! :grin:
I think you're good to go.
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:laugh:
I've gotten into vintage test equipment lately. My heart would like to collect it; but my head says there's not much room left on my bench! Meanwhile I've discovered a treasure trove of vintage reference material and manuals on-line.
Well, I'm not into football, so I'm gonna sign of and actually play some guitar.