Fluke 27 mA/milliamp reading not accurate?[Solved: 'Burden Voltage' differences]

[Charles7]

Hi, first post so hopefully I am in the right spot here. I did some searching on this but found no answers, solutions or even anyone else talking about the same thing.

I have two different Fluke 27's (older all yellow one, and newer yellow with grey face), also a Fluke 23 series II around the same vintage, that are doing this same thing.

When I use the DC mA current setting all meters are close or right on accuracy at the lower mA, one test 5.01mA on all meters and verified with newer Fluke 87V and an Agilent meter.
I test a 121mA current on the newer (presuming accurate meters) but I get 200-220mA reading on the 3 older flukes...   

 The Amps and the microamps both seem good, just the mA/milliamp is not accurate at the higher mA currents, while being accurate at the lower currents (at least at the current I mentioned).

Is there an easy fix/adjustment?  I am guessing this may be a somewhat common issue with these older meters?  I mean, I am unlucky sometimes but hopefully not THAT unlucky!? for all 3 of mine to have the same issue, LOL! 

Thanks for any help.   

[barry14]

Did you perform the test by putting all of the meters in series to be sure they were all measuring exactly the same current? Different meters have different voltage drops on their current ranges which could account for the differences.

[Charles7]

I did check the same current with the meters in the Amp setting. I.e. All three of them measure the 121mA current at 0.12 Amps

Just did one test with my newer Fluke 27 in series with my Fluke 87V:

Both in mA range I get ~168mA on both meters...  (incorrect - should be 121mA, still seeing ~220mA with the 27 when measuring it just with that)

With the 27 in Amp range and 87V in mA range I get 0.12A on the 27 and 121mA on the 87V..  (which is correct)

They both read the same (or really close) in series, with both in the mA range but it is not a correct reading, whatever is messed up in the 27's mA range circuit just skews the results for the 87 also.

So, I'm still not sure what is going on...

[retiredcaps]

What does your test setup look like?  How are you sourcing the DC current?  DC variable power supply?  What voltage are you using?

This sounds like it could be related to "burden voltage"?

When you put a multimeter in series, you are adding a resistor to your circuit.  The resistor varies depending on the uA/mA/A setting and every meter, even made by the same manufacturer will be different.

The Fluke 27 burden voltage for the 320mA is 5.6 mV/mA and for the 10A is 50mV/A.
The Fluke 87 burden voltage for the 400mA is 1.8 mV/mA and for the 10A is 0.03V/A.

In the 10A range, both resistors values are fairly close.  In the mA range though, it is quite a difference and might make a difference depending on your circuit.  See

[retiredcaps]

The Fluke 23 II burden voltage for the 320mA is 6 mV/mA and for the 10A is 50mV/A.

Also Dave's explanation of burden voltage leading to ucurrent.

https://eevblog.com/files/uCurrentArticle.pdf

[Fungus]

What are you measuring? If it's something low voltage then the burden of the multimeter may be altering the circuit.

If you've got all those meters then measure the voltage across the multimeter that's measuring the current. That might give you a clue.

[Charles7]

Thanks for the info and ideas, I'll be trying to spend some time on this later today... I'm a bit of a newbie with much of this so I'm learning as I go here, thank you for your patience.  

[Charles7]

Ok, so after watching the video and doing more tests/measurements this is all making a lot more sense to me now.. I should have included what I was measuring in my first post. Thank you guys for not laughing at me too hard trying to figure this out, haha.

I was measuring the current of a flashlight at a set mode/level, 18650 cell around 4.15V. The main thing I was trying to figure out was why I was getting such different readings with the new meters vs the older ones. The answer is exactly what you guys said - the 'Burden Voltage'. After learning this is a thing, now I know that the reason my vintage flukes all read close to the same (but different than the newer ones) is that they all have a burden voltage very similar to each other. The same for the newer meters, they are pretty close to the same burden voltage - quite a bit lower than the older ones.

Apparently, what I was measuring was pretty sensitive to the burden voltage load in the circuit which translated to voltage drop hence the mA increase etc.. (if I am thinking about all that correctly). 

The suggestion to measure it with the meters in series was a good idea, when I did that they measured very close to the same. Just a different reading than I was expecting (newbie me)... 

I presume that if I was using a power source that maintained a set voltage under load I would get a pretty similar reading with the new and the old meters, or at least a lot closer than the readings I am getting now?

Thank you all for your help! I've learned a lot, hopefully someone else can learn from this also. That is part of the fun right?