Fluke 79 179,... wrong readings - Page 1
Forum section: electronics / testgear

author: temperance
author_rank: Super Contributor
timestamp: October 31, 2023, 12:04:29 am
content: The setup:-A 1 Vrms 1 KHz sine wave, transformer balanced signal with 600 Ohm impedance, completely floating. (The source is powered from a 9V battery.)-ESD mat on the table and both the signal source and the Fluke are on the ESD mat.I was expecting to see 1 Vrms at the output but instead I got 0.56 Vrms. Expecting the Fluke to be broken I take the same measurement with an other Fluke. This one seems to show 0.8 Vrms. But after changing the polarity, this one also measures 0.56 Vrms instead of 1 Vrms.Confused I measure the signal with an oscilloscope and the signal is indeed 1 KHz, 1 Vrms as should be.I then put the Fluke which was still connected to the signal source aside, of the ESD mat. But to my surprise, the reading is now 1 Vrms.The Fluke manual states nothing about the source impedance for AC signals. What's wrong here?

author: bdunham7
author_rank: Super Contributor
timestamp: October 31, 2023, 12:11:20 am
content: Tell us about the test leads. Perhaps a picture? I don't think the source impedance is an issue here. Does the oscilloscope measure 1V when the polarity is reversed as well? How about if both are connected simultaneously?

author: temperance
author_rank: Super Contributor
timestamp: October 31, 2023, 01:47:33 am
content: Quote-The test leads are the regular Fluke supplied test leads.-The output is floating and the unit is battery powered. The oscilloscope measures correctly 1 V rms in both "directions".-I didn't try connecting them both simultaneously.The Fluke only measures 1 V rms as it should when not placed onto a conductive earthed surface.

author: bdunham7
author_rank: Super Contributor
timestamp: October 31, 2023, 02:26:30 am
content: Quote from: temperance on October 31, 2023, 01:47:33 amSo try connecting them simultaneously--that will answer a lot of questions. This is surprising behavior at 1kHz, so I'm curious as to the cause. I don't have either of the meters you mention, so I can't do any experiments. The most obvious idea is some sort of capacitive coupling, but 1nF would have an impedance of 159k at 1kHz. You'd need many times that to affect your 600R output impedance. Hmmmm.

author: temperance
author_rank: Super Contributor
timestamp: October 31, 2023, 02:42:40 am
content: This happened at a service company which I sometimes visit. In a few weeks I might visit them again. Pictures will follow.

author: temperance
author_rank: Super Contributor
timestamp: November 03, 2023, 11:19:15 pm
content: This mystery has been confirmed today by an other engineer (not a member of this board).The same Fluke. The setup: a diesel powered generator with, of course, a floating output. It should output +/- 230 V. It measures 150 V when the meter is placed onto a concrete floor. The measurement changes when picking up the meter from the concrete floor.He ended up measuring the current trough a heater to determine the output voltage. The current meter readings are correct.I wonder what's in those meters.Maybe Dave can setup an experiment to confirm this. Replicating the setup isn't difficult.

author: bdunham7
author_rank: Super Contributor
timestamp: November 04, 2023, 12:21:22 am
content: Quote from: temperance on November 03, 2023, 11:19:15 pmThe same model(s) or the actual same meter(s)?

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 04, 2023, 12:41:40 am
content: Quote from: temperance on November 03, 2023, 11:19:15 pmI think this meter does not like to be concrete. Correct readings are a fluke.

author: temperance
author_rank: Super Contributor
timestamp: November 04, 2023, 12:55:44 am
content: QuoteThis was also a Fluke 179.So there seems to be a problem with those meters when measuring floating AC voltages. Very strange.I tried finding schematics for those Flukes. But they seem hard to find.

author: chinoy
author_rank: Regular Contributor
timestamp: November 04, 2023, 05:29:20 am
content: Im having the exact same problem with a Fluke 223In fact I made a similar post about this just a few days back.I cant even get two Flukes to agree with each other. Facing this problem with both a/c and d/c.

author: tggzzz
author_rank: Super Contributor
timestamp: November 04, 2023, 01:56:35 pm
content: Quote from: chinoy on November 04, 2023, 05:29:20 amIf you mean this threadhttps://www.eevblog.com/forum/chat/how-do-you-calibrate-your-gear/msg5148885/#msg5148885thenmanypeople have asked you for important information, and you havenotgiven it.Without the information people have requested, we cannot determine whether the meters are faulty, your test techniques are faulty, your understanding is faulty - or whether there is no problem at all.Over to you...

author: temperance
author_rank: Super Contributor
timestamp: November 04, 2023, 02:37:00 pm
content: QuoteFor AC the Fluke might be wrong because the tape head is a floating 300...600 Ohm source.

author: bdunham7
author_rank: Super Contributor
timestamp: November 04, 2023, 03:46:43 pm
content: Quote from: temperance on November 04, 2023, 02:37:00 pmI can't find a Fluke 223 model anywhere, no idea what it is. Perhaps an Asian-only model that has managed to hide from the internet entirely?In any case, for a 179 the input is 10M/<100pF. That would present a load (Z) of at most 1.5M to a 1kHz signal, leading to an error of less than 0.05%. The specified accuracy of a 179 at 1kHz is 2%.

author: bdunham7
author_rank: Super Contributor
timestamp: November 04, 2023, 03:47:46 pm
content: Quote from: chinoy on November 04, 2023, 05:29:20 amCan you post a photo of this meter or a good link to a page for it?

author: BeBuLamar
author_rank: Super Contributor
timestamp: November 04, 2023, 04:15:33 pm
content: Quote from: bdunham7 on November 04, 2023, 03:46:43 pmI think he might mean the 233 with remote display

author: mendip_discovery
author_rank: Super Contributor
timestamp: November 05, 2023, 11:12:49 am
content: As a test could someone get this error to happen and then use your body to start with and approach the meter then use your hands to touch the test leads.I have observed issues with silicone leads being rather sensitive to humans, especially in MΩ ranges. PTFE is much less sensitive to this problem.