Author Topic: Fluke 79 179,... wrong readings  (Read 2045 times)

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Online temperanceTopic starter

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Fluke 79 179,... wrong readings
« on: October 31, 2023, 12:04:29 am »
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?
 

Offline bdunham7

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Re: Fluke 79 179,... wrong readings
« Reply #1 on: October 31, 2023, 12:11:20 am »
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?
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Online temperanceTopic starter

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Re: Fluke 79 179,... wrong readings
« Reply #2 on: October 31, 2023, 01:47:33 am »
Quote
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?

-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.
 

Offline bdunham7

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Re: Fluke 79 179,... wrong readings
« Reply #3 on: October 31, 2023, 02:26:30 am »
-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.

So 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.
« Last Edit: October 31, 2023, 02:33:13 am by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Online temperanceTopic starter

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Re: Fluke 79 179,... wrong readings
« Reply #4 on: October 31, 2023, 02:42:40 am »
This happened at a service company which I sometimes visit. In a few weeks I might visit them again. Pictures will follow.
 

Online temperanceTopic starter

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Re: Fluke 79 179,... wrong readings
« Reply #5 on: November 03, 2023, 11:19:15 pm »
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.
 

Offline bdunham7

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Re: Fluke 79 179,... wrong readings
« Reply #6 on: November 04, 2023, 12:21:22 am »
The same Fluke.

The same model(s) or the actual same meter(s)?
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline SiliconWizard

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Re: Fluke 79 179,... wrong readings
« Reply #7 on: November 04, 2023, 12:41:40 am »
It measures 150  V when the meter is placed onto a concrete floor.

I think this meter does not like to be concrete. Correct readings are a fluke.
 
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Online temperanceTopic starter

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Re: Fluke 79 179,... wrong readings
« Reply #8 on: November 04, 2023, 12:55:44 am »
Quote
The same model(s) or the actual same meter(s)?

This 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.
« Last Edit: November 04, 2023, 01:01:23 am by temperance »
 

Offline chinoy

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Re: Fluke 79 179,... wrong readings
« Reply #9 on: November 04, 2023, 05:29:20 am »
Im having the exact same problem with a Fluke 223
In 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.
« Last Edit: November 04, 2023, 06:35:56 am by chinoy »
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Offline tggzzz

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Re: Fluke 79 179,... wrong readings
« Reply #10 on: November 04, 2023, 01:56:35 pm »
Im having the exact same problem with a Fluke 223
In 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.

If you mean this thread
https://www.eevblog.com/forum/chat/how-do-you-calibrate-your-gear/msg5148885/#msg5148885
then many people have asked you for important information, and you have not given 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...
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
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Online temperanceTopic starter

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Re: Fluke 79 179,... wrong readings
« Reply #11 on: November 04, 2023, 02:37:00 pm »
Quote
Im having the exact same problem with a Fluke 223
In 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.

For AC the Fluke might be wrong because the tape head is a floating 300...600 Ohm source.
« Last Edit: November 04, 2023, 02:46:45 pm by temperance »
 

Offline bdunham7

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Re: Fluke 79 179,... wrong readings
« Reply #12 on: November 04, 2023, 03:46:43 pm »
For AC the Fluke might be wrong because the tape head is a floating 300...600 Ohm source.

I 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%. 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline bdunham7

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Re: Fluke 79 179,... wrong readings
« Reply #13 on: November 04, 2023, 03:47:46 pm »
Im having the exact same problem with a Fluke 223

Can you post a photo of this meter or a good link to a page for it?
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline BeBuLamar

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Re: Fluke 79 179,... wrong readings
« Reply #14 on: November 04, 2023, 04:15:33 pm »
For AC the Fluke might be wrong because the tape head is a floating 300...600 Ohm source.

I 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%.

I think he might mean the 233 with remote display
 

Offline mendip_discovery

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Re: Fluke 79 179,... wrong readings
« Reply #15 on: November 05, 2023, 11:12:49 am »
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.
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So everyone is clear, Calibration = Taking Measurement against a known source, Verification = Checking Calibration against Specification, Adjustment = Adjusting the unit to be within specifications.
 


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