Products > Test Equipment
Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)
bdunham7:
--- Quote from: SilverSolder on June 02, 2020, 09:43:00 pm ---
Just checked what 1V RMS from the 3325A looked like on the Fluke 8506a thermal RMS voltmeter. No real reason to complain today! (Other than using up my luck for the rest of the year...)
(Attachment Link)
--- End quote ---
OK, so here are eight digits of coincidental exactness. And no, neither was calibrated or adjusted using the other.
bdunham7:
--- Quote from: guenthert on January 26, 2021, 05:11:47 pm --- Doesn't the 8505/6 famously have a 7.5 digit 10V range? That would make 1uV resolution ambiguous.
--- End quote ---
It appears to, when averaging is enabled, but as I observe it in action I think that last digit may have missing codes. IOW, it can't display all possible values. For example, as it drifts around, I see 10.00000 0, 10.00000 1, 10.000004, but never 10.00000 2 or 10.00003. I haven't explored this at all, just a quick observation.
SilverSolder:
--- Quote from: bdunham7 on January 26, 2021, 06:29:17 pm ---
--- Quote from: SilverSolder on June 02, 2020, 09:43:00 pm ---
Just checked what 1V RMS from the 3325A looked like on the Fluke 8506a thermal RMS voltmeter. No real reason to complain today! (Other than using up my luck for the rest of the year...)
(Attachment Link)
--- End quote ---
OK, so here are eight digits of coincidental exactness. And no, neither was calibrated or adjusted using the other.
(Attachment Link)
--- End quote ---
It's not good to use up your luck for the rest of the year already in January! :D
SilverSolder:
--- Quote from: bdunham7 on January 26, 2021, 06:38:01 pm ---
--- Quote from: guenthert on January 26, 2021, 05:11:47 pm --- Doesn't the 8505/6 famously have a 7.5 digit 10V range? That would make 1uV resolution ambiguous.
--- End quote ---
It appears to, when averaging is enabled, but as I observe it in action I think that last digit may have missing codes. IOW, it can't display all possible values. For example, as it drifts around, I see 10.00000 0, 10.00000 1, 10.000004, but never 10.00000 2 or 10.00003. I haven't explored this at all, just a quick observation.
--- End quote ---
I get the full compliment of numbers in averaging mode.
It sounds like the A/D converter may need trimming, if it is missing codes (all the steps are adjustable...)
That is serious project, though. The manual explains it, but you need a ton of precise equipment and it is still an iterative process... (I have never attempted it.)
I'm thinking that today, it ought to be possible to do something over GPIP to automate measuring the A/D performance, but haven't actually tried anything...
Regarding stability in general, I once did a test where 4 of these meters were left running overnight reading an EDC VS-330 voltage reference (which has been "massaged" with some better components here and there to improve stability) set to 10V.
Over the 8h period, the four meters and the VS-330 tracked each other as follows:
DMM1 +/- 1.2ppm
DMM2 +/- 0.5ppm
DMM3 +/- 0.6ppm
DMM4 +/- 0.85ppm
This was at night, during the summer, where the temperature is very stable overnight in this location.
I just used the Peak function on the meters for this, so I don't have any logs.
Bottom line, this old school stuff is all "good enough for Australia" when in good shape and given a quiet place to work! :D
bdunham7:
--- Quote from: SilverSolder on January 26, 2021, 11:21:02 pm ---I get the full compliment of numbers in averaging mode.
It sounds like the A/D converter may need trimming, if it is missing codes (all the steps are adjustable...)
--- End quote ---
I actually did the A/D calibration procedure, although I went through it pretty quickly, not aiming for perfection at the time. I don't have references that good anyway. It took about an hour and I recall doing the first loop thrice, then on the rest of the procedure I just rechecked to make sure I was pretty close. It seems pretty good throughout the ranges, but I'll keep an eye on it and see if those 'missing codes' really are missing.
--- Quote ---That is serious project, though. The manual explains it, but you need a ton of precise equipment and it is still an iterative process... (I have never attempted it.)
--- End quote ---
It's just fiddly and takes time. And if you do have accurate references, you just have to keep doing it until you just aren't improving anything anymore. I've done similar adjustments on the 5100-series calibrators and they never seem quite happy. This one seemed pretty amenable to adjustment.
--- Quote ---Over the 8h period, the four meters and the VS-330 tracked each other as follows:
DMM1 +/- 1.2ppm
DMM2 +/- 0.5ppm
DMM3 +/- 0.6ppm
DMM4 +/- 0.85ppm
This was at night, during the summer, where the temperature is very stable overnight in this location.
--- End quote ---
In my case, unfortunately, the room it is in is not stable because the furnace cycles and I'm getting variations of 5K on a daily basis. The 731B and the 8505A both seem to have a negative tempco, and between them they move about 5ppm total over 24 hours. That's still about as much bang-for-fifty-bucks-plus-some-work as one could ask for.
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