Products > Test Equipment
Fluke 8840A/8842A and AF Differences?
J-R:
I still couldn't see Fluke printing 1000V MAX on the front of all the 8840A/AF units, where the AC board is not optional, for military customers. So that got me thinking more about this.
I went through the True RMS theory of operation on page 5-19 of the 8840A manual. The block diagram shows how the input is scaled based on the selected range. In all cases, the output from the TRMS board is always 0-2V to the track/hold circuit:
- 200mV, r1: divide by 5, multiply by 5, multiply by 10 = 2V
- 2V, r2: divide by 5, multiply by 5 = 2V
- 20V, r3: divide by 5, multiply by 0.5 = 2V
- 200V, r4: divide by 500, multiply by 5 = 2V
- 2000V, r5: divide by 500, multiply by 0.5 = 2V
It also states that the input is clamped by MOVs to 1560V. So 1000V RMS (1414V) should be no problem, even for the 8840A and 8842A. Of course different firmware would be needed, and I suspect some of the input protection is different on the 8840A/AF since 1560V might be a bit close to 1414V.
With regard to the calibration discussion, I don't see how you could rely on anything without a somewhat recent calibration (self or otherwise). That is the entire point, provide evidence that it works correctly so you CAN rely on it. In the context of this thread, the OP is not going to have the necessary equipment to calibrate it himself, so that means paying for it.
alm:
--- Quote from: J-R on June 26, 2023, 08:33:02 am ---With regard to the calibration discussion, I don't see how you could rely on anything without a somewhat recent calibration (self or otherwise). That is the entire point, provide evidence that it works correctly so you CAN rely on it. In the context of this thread, the OP is not going to have the necessary equipment to calibrate it himself, so that means paying for it.
--- End quote ---
If you need to prove to a third party that what you measured is correct, sure. Like in any safety critical industry.
But for a home lab, I disagree. In my experience this class of meters will usually be pretty close to its 1y spec, or way off. So if I could spot check on a few values, and those values are correct, I'd feel pretty confident. I say that as a person who in the past compared used 4.5-7.5 digit DMMs as best I could with improvised signal sources to look for inconsistencies. And the only deviation from 1y specs I could measure was a meter which had compents in the ADC circuit replaced, a meter which was just always reading zero for ACV, and some differences in high frequency ACV.
How often do you do a measurement where absolute accuracy down to say 50 ppm matters? For me the high resolution DMMs are more about relative measurements, like the change of a resistor with temperature, rather than absolute values.
J-R:
Come on, you guys know better than to keep pushing back on the calibration stuff. It's fine if you don't want it or need it, but to keep making these claims that just checking a few things makes you "confident" is just 100% false. Don't use those words around others because you're just plain wrong, sorry to say. Go ahead and say "good enough for my purposes" or something similar instead. And it's also fine if you have the equipment to check everything, as do I, but most others do not and its a big step to get there.
These specs are not published by Fluke for just for the fun of it, the devices have their limits and need to be calibrated regularly if you want to rely on it. There are no magical low-drift virginal vintage ones out there that you can just check a few data points and then assume everything is fine.
The 1 year rating on these units is atrocious if taken at face value, especially the extra low range on the 8842A. The last two digits mean nothing. I wouldn't have it around if it only met spec. As an example, the 1 year rating for 2V would be 8 counts. The 8842A I self calibrated/adjusted almost 2 years ago still bests the 24 hour spec. That is one of the big advantages of these older units, but it needs a somewhat recent calibration (SELF or otherwise).
Also, it is rare to find these with intact front and rear calibration stickers. The back end can be taken off and the calibration switch manipulated without disturbing the front sticker. So you don't know if there is some random function/range that is off because the previous owner was tinkering with it. A professional calibration will typically have a lot more data points than just the published performance test points.
bdunham7:
--- Quote from: J-R on June 27, 2023, 03:39:30 am ---There are no magical low-drift virginal vintage ones out there that you can just check a few data points and then assume everything is fine.
--- End quote ---
Out of a dozen units or so I have worked on I have never found one to be out of spec if it powers up, reads nominally correctly and passes a self test. The only time I have changed any calibration constants has been when I replaced, added or repaired the AC boards. Has your experience been any different?
--- Quote ---The 1 year rating on these units is atrocious if taken at face value, especially the extra low range on the 8842A. The last two digits mean nothing. I wouldn't have it around if it only met spec. As an example, the 1 year rating for 2V would be 8 counts. The 8842A I self calibrated/adjusted almost 2 years ago still bests the 24 hour spec. That is one of the big advantages of these older units, but it needs a somewhat recent calibration (SELF or otherwise).
--- End quote ---
Well that is an interesting perspective. Aside from the zero/offset issue in the low ranges of the 8842A, these meters have better specifications than any other 5.5-digit meter that I'm aware of and they are much, much better than modern 5.5-digit models. So if you 'wouldn't have them around if they just met spec', what would you have? Of course they were expensive and 30 years ago they cost as much as a decent 6.5-digit DMM does today, so they are sort of in their own category. I have 8842A models that look like a truck ran over them and they meet their 24-hour specs, 2 counts or better on the 2V/20V ranges.
--- Quote ---So you don't know if there is some random function/range that is off because the previous owner was tinkering with it. A professional calibration will typically have a lot more data points than just the published performance test points.
--- End quote ---
That's true, I suppose, if you only have one meter and no accurate voltage standards and no idea where the meter came from. But if you have only one standard and a few parts, say 5V or 10V and some resistors, you can easily check all the ranges by comparing them with each other. The nightmare scenario is if someone has attempted the ADC linearity calibration and not done it properly--that would be difficult to recover from without a calibrator. This is one of the reasons that I would prefer a unit of unknown origin over one where the seller says "gee, I just calibrated it!". Checked calibration=good, adjusted=no bueno, IMO. Unless there is a convincing explanation.
As far as your offset/zero complaints go, this just goes with the territory when you have that extra gain with a meter that doesn't use autozero. It takes a few extra steps to get accurate readings at very low levels, but it is perfectly manageable. There's a reason that the autorange doesn't go down into those levels.
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