Fluke 5100B for 6-7 digit DMM - Page 1
Forum section: electronics / general

author: Badwolf
author_rank: Contributor
timestamp: August 29, 2021, 04:11:00 pm
content: Hi allI have a quick question about the FLUKE 5100B and Fluke 5101B. I know that these devices are originally designed to calibrate 4- or 5-digit DMMs.However, have you made any measurements on the stability and noise of these devices?My idea would be to use one as a standard transfer to calibrate 6- or 7-digit DMM.To do this, I would check the value with a recently calibrated 34470A before using it (without changing any parameters) on the DMM to be calibrated.To do this, the device must be stable (without drift) for only a few minutes and not too much noise.What do you think of my idea?

author: bdunham7
author_rank: Super Contributor
timestamp: August 29, 2021, 04:55:51 pm
content: Were you not the 'victim' of exactly such a scheme with your 8.5 digit DMM from eBay?I have some experience with this line of calibrators as in I have repaired and refurbished a half dozen of them. I currently have two complete working units plus a complete set of modules that I'm working on. They're very useful units to have around if you have the room, but there's no way that they are good enough for what you propose. I use mine, in conjunction with a good 6.5 digit DMM, to calibrate any 4.5 digit DMM and most 5.5 digit models--and even that can be a bit tricky.I could go on and on all day long about the various issues that get in the way of extracting better performance from these beasts, but the first and main showstopper is the way that voltages from 20 to 1100V (DC and AC) are generated. Essentially there is a DAC, a control board, an oscillator module, a power amplifier, a transformer/bridge rectifier/stabilizer (the HV module) and a ranging module. So to generate 20V or more, the control board and oscillator are input to the power amplifier (with the specified AC frequency or a 2kHz flattish-wave for DC) which drives the transformer. After rectification and stabilization (that stabilization circuit is another story...) the result goes to the ranging module where it is both output and divided down and fed back to the control board. So you can see there is a very long feedback loop. The result is, that even in at unit in tip-top condition, the overall stability isn't that good and even worse, the short-term response to small load changes is slow and often oscillatory. Even the small load and current changes from the autozero function of many DMMs will cause significant destabilization of the outputs. Put another way, the output impedance is low at DC, but not low at even a few Hz.Here are the DC specs. The noise and ripple is 10X the rated accuracy. Pay attention to the 'transient recovery' section because the meaning there is not entirely clear to me. And note that the stability spec alone would give you a potential 19 count error on a 6.5-digit DMM at 10VDC or a 13-count error at 100VDC. This appears to be a 'typical' spec, not k=2 or anything like that and in my experience, in real life you will do better than that at 10VDC but would be lucky to even meet that spec at 100VDC.Then there's the issue of resistance, current and relays in the ranging module, the AC control system, INL in the DAC (not and issue if you are using a reference meter) and so on. So no, not sufficient to do a good job for any decent 6.5+ digit device. I do use mine to calibrate the AC boards on Fluke 8840A/8842A 5.5-digit DMMs and that is with a great deal of care and cross-checking--and the knowledge that I don't have to certify anything.