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Fluke 87V & 28II problem at the ohms range .

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Kiriakos-GR:
Well I just discover an true problem with the Fluke 28II  by comparing it with the Agilent U1272A and the Fluke 8050A (bench top) .

And the problem is that the Fluke 28II adds  100mOhm on the 600 Ohms range , because it just does so .
The same thing was happened also at my Fluke 87V and now I know why … because it just does so too.

And why it did bother me ?  simply because all those tests that I did on test leads and their tiny resistance,
I was getting an a resistance  of  00.15  when I was crossing the leads and I was believing that the meter was saying the truth .
But no it was not !!
The true resistance  was 100mOhm less.
And in this example would be  00.050  that is just 50mOhm

I did a simple test by measuring the inner resistance of my own Resistors decade box .
With a fixed set of cables with bananas at the ends.
And the numbers that I got from them : ( with No REL) (cables & switches & bridges)
Fluke 28 II  ……………. 0.15  ( High res 20.000 counts )
Agilent U1272A …..….00.049  mOhms   (30.000 counts)
And the Fluke 8050A    00.06  (07 fluctuates)    (20.000 counts)
And the point is that all of them was measuring about 50 mOhm !!

And I got smokes from my ears , because I had take about a hundred  of pictures with measurements so far,
and I had posted them in the EEV for over one year now, with both the 87V or the 28II , and all my measurements today looks wrong.

Are those Fluke meters damaged? , hell no but there is a bug in them ..
Do they measure accurately?  Yes if you use the relative first so to manually zero out the resistance of the leads .

And then where is the problem ? 
The problem is when you need to measure the resistance of the leads it self.
This additional 100mOhm makes the measurement to look fake.

Is there a cure ?
No there is not,  the only thing that I can think of , are that in order to measure the resistance of the test leads,
I will had to use a dual banana bridge on the DMM, so to zero out  ( RELative)  first , and then to add the leads so to measure their own resistance.
But this sounds crazy too .. 
Because even if I need to get close to the correct measurement, this bridge banana it should be made of gold !!   

Before you call me as nerd because I care that much about micro ohms, just think what you are when you care that much for the micro amperes.

And now that we got even, I am ready for your comments if you have any. 

Joshua:
Wow, that's kind of dissapointing. You'd figure that if it's consistent like that, they could almost fix it in software.

Richard W.:
I don't see any malfunction. Lets do some math:

We want to measure a resistance 50m?. The Fluke 87V uses a measuring-current of about 1mA.
According to ohms law this means a voltage of about 50µV at the resistors terminals.

Then consider things like:
~ thermoelectric voltage (because not every contact has the same temperature and doesnt consist of the same material, there is copper, brass, solder, ect)
~ electrochemical voltage (humidity, residue of fingerprints,... ect..)

Your Agilent U1272A has a function called Smart ? which seems to compensate some of this effects.

I could not find any data to the measuring current of the 8050A, but i guess its higher than 1mA.
This also lowers the influence of thermoelectrical and chemical voltages in the contacts.

Kiriakos-GR:
Well for the moment, what matters is to make a list of the models with this awkward behavior .

The Old Fluke 8050A looks healthy .

The 87V and the 28II is not.
I would expect from the members in our forum,
to do similar tests at the older versions of the 87,  like  like 87 & 87III.
Or at any Fluke meter that has 20.000 resolution at list.
And report your findings here. 

Just cross the leads, and tell us the reading. 


 

Kiriakos-GR:

--- Quote from: Richard W. on June 23, 2011, 11:24:47 am ---Your Agilent U1272A has a function called Smart ? which seems to compensate some of this effects.

--- End quote ---

Hi Richard, about the Agilent and Smart Ohm, this is another function by it self on the ohms range and needs to be manually activated.

All that I do here is to compare the ancient classic Ohms range, and nothing more. 

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