Comparing current measurements of multimeters

Thread Starter

Leonardo Camargo

Joined Oct 30, 2017
9
Have you guys ever watched Youtube multimeter reviews, in which they connect several multimeters in parallel, and measure the same voltage with all of them at the same time? The goal there is, having one of the meters considered as the reference/standard, to compare their measurements and calibration.
I would like to do something similar, but in my case, I would like to connect several multimeters in series and proceed to measure the same current, specifically in the microamp range, with all of them. I'd like to be able to go from a couple microamps to hundreds of microamps.

I have two questions:
1)Is this technically acceptable?
2)Is there a cheap way to do it?

I've been looking into current sources and adjustable electronic loads and couldn't find something cheap (up to $200 US dollars) that would get the job done. A diy project rather than something off-the-shelf is fine too.
ps: the multimeters aren't included in the budget, I already have a bunch of them.
 

ebp

Joined Feb 8, 2018
2,332
You certainly can do that.

Ohm's law to the rescue. If you want a constant current, simply use a stable voltage source in series with a stable resistance. The internal resistances of the meters will be stable. If you use a good lab power supply, you can adjust the current simply by adjusting the voltage. If you don't have a good lab power supply with variable voltage, you can use a fixed voltage source and a good quality variable resistor, such as a wirewound potentiometer. [EDIT - you might find it helpful to use a fixed resistor in series with a potentiometer to get the adjustment range you want. Wirewound pots, while stable, aren't available at reasonable price in very high resistances.] Most bench power supplies have adjustable current limit, but only high-end supplies are likely to be able to produce accurately regulated currents in the micoramp range.

You don't need a source with super good long-term stability. It just needs to be sufficiently stable to allow you to read all of the meters for each setting. A power supply with a linear regulator will be better than a switch mode supply.
 

Thread Starter

Leonardo Camargo

Joined Oct 30, 2017
9
You certainly can do that.

Ohm's law to the rescue. If you want a constant current, simply use a stable voltage source in series with a stable resistance. The internal resistances of the meters will be stable. If you use a good lab power supply, you can adjust the current simply by adjusting the voltage. If you don't have a good lab power supply with variable voltage, you can use a fixed voltage source and a good quality variable resistor, such as a wirewound potentiometer. [EDIT - you might find it helpful to use a fixed resistor in series with a potentiometer to get the adjustment range you want. Wirewound pots, while stable, aren't available at reasonable price in very high resistances.] Most bench power supplies have adjustable current limit, but only high-end supplies are likely to be able to produce accurately regulated currents in the micoramp range.

You don't need a source with super good long-term stability. It just needs to be sufficiently stable to allow you to read all of the meters for each setting. A power supply with a linear regulator will be better than a switch mode supply.
I tried at first a resistor connected to a DC power supply (30v/5A, asymmetric), and a multimeter in series (with microamps range), but I couldn't get a reading even though the PSU was showing current consumption, and the resistor was getting pretty hot. I must not haven chosen a proper resistor for the job.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
4,055
I tried at first a resistor connected to a DC power supply (30v/5A, asymmetric), and a multimeter in series (with microamps range), but I couldn't get a reading even though the PSU was showing current consumption, and the resistor was getting pretty hot. I must not haven chosen a proper resistor for the job.
If you were unable to get a current reading but the other ranges of that multimeter were working, there is a problem within the meter. Sometimes there are two ranges, depending on which input connectors you use. In this case it seems like you may have been plugged into the 20 amps terminals instead of the correct terminals. that sometimes happens. So I suggest reading the manual or at least examining the circuit diagram.
 

Thread Starter

Leonardo Camargo

Joined Oct 30, 2017
9
If you were unable to get a current reading but the other ranges of that multimeter were working, there is a problem within the meter. Sometimes there are two ranges, depending on which input connectors you use. In this case it seems like you may have been plugged into the 20 amps terminals instead of the correct terminals. that sometimes happens. So I suggest reading the manual or at least examining the circuit diagram.
Only now that you mentioned the problem could be with the meter that I tried different meters. Now what happened was, I tried it with three chinese brands (Borbede BD-168B, PeakMeter PM8236, and Elecall MK72), the microamps measurements of all of them were flawless. The meter I was using originally is a Fluke 289. I also tried it with a Fluke 87V, and it also didn't work. I'm thinking this maybe has something to do with the Flukes' construction? I find it hard to believe both of them are defective.
 
Last edited:

Reloadron

Joined Jan 15, 2015
4,591
I would like to do something similar, but in my case, I would like to connect several multimeters in series and proceed to measure the same current, specifically in the microamp range, with all of them. I'd like to be able to go from a couple microamps to hundreds of microamps.
I guess if we look at Kirchhoff's circuit laws and those pertaining to current that should work fine. It's all in the numbers. However, if I did this and for example read let's say 5 meters how would I know which one was telling me the truth? This is like a man with two watches never knowing the correct time.

Let's say I place 5 meters in series to measure current. Each meter will have a burden voltage per unit of current. Just for example a Fluke 87 DMM on the 400.0 mA range has a burden voltage drop of 1.8 mV/mA. If I place 5 of them in series and apply 200 mA of known current each meter will have a burden voltage drop of 200 * 0.0018 = 0.360 Volt so that becomes 0.360 * 5 = 1.8 volts so my compliance voltage at my current source must exceed the total burden voltage. Not always a concern but worth knowing.

The best way to calibrate a current meter is with a known precision current source which has a test uncertainty ratio of at least four times greater than your unit under test. So if your unit under test has an uncertainty of 1.0% on a current range, your current source should have an uncertainty of 0.250%. That gets you a 4:1 test uncertainty ratio.

The meter I was using originally is a Fluke 289. I also tried it with a Fluke 87V, and it also didn't work. I'm thinking this maybe has something to do with the Flukes' construction? I find it hard to believe both of them are defective.
How were things setup?

Ron
 

MisterBill2

Joined Jan 23, 2018
4,055
My Fluke 77 has a fuse inside and I have never had to replace it so I don't know what it protects. I got the meter used for $1 so if it does not measure amps that is not a big deal to me. If you connected to the 10 amps jack you will not see much current displayed if it is not fairly large. AND, if the meter has been modified by somebody it may not be right at all.
 

Thread Starter

Leonardo Camargo

Joined Oct 30, 2017
9
How were things setup?

Ron
The simplest way possible. I connected a 1 MOhm resistor in series with only my Fluke 289, and then connected both to my power supply. I stepped up the voltage from zero to 20V and got no reading on the Fluke. Same issue with the Fluke 87V. Now with the chinese brand meters, right out of the gate I got readings, they increased as I increased the voltage, as one would expect.
 

Thread Starter

Leonardo Camargo

Joined Oct 30, 2017
9
I did a different setup this time. I connected my fluke in series with one of these electronic loads, and then connected both to 12v/10A power supply. Current goes through just fine, but no readings of either amps, miliamps or microamps from the Flukes (289 and 87V), but normal readings from the chinese brand meters.

I think indeed both of my Fluke meters are defective.
 

Reloadron

Joined Jan 15, 2015
4,591
The lowest current range on a Fluke 87 DMM for DC current is 60 mA with 0.01 mA resolution and an uncertainty of ± (0.4 % + 4). With 20 Volts applied and a 1 Meg Ohm load your current will be about 20 uA and you have 10 uA resolution so I would expect to see something in that last decimal place on the fluke. However, there is still the allowable four counts on the last digit. You are sure the Fluke is set for DC current? When I turn my 87V on it defaults to AC current and I need to hit the yellow button to switch to DC current. My older 87 (not a series V) defaults to DC current.

OK, just read your subsequent post but I think it would be strange you have two failed Fluke meters as that is a strange coincidence. No way the current fuses are blown is there? Not unusual to find blown fuses for the current inputs.

Measuring the input resistance on the 400 mA fused input it should be about 1 K Ohm and the 10 Amp input about 0.1 Ohm if even that high.

Ron
 

MisterBill2

Joined Jan 23, 2018
4,055
That was the problem, both Flukes were set to AC current. Damn I made a fool of myself now. Sorry everyone and much thanks for the inputs. Especially you Ron.

Thanks everyone.
Actually, we did get a worthwhile exercise in considering what the problem could be, so it was not all a waste. AND, besides that you did accept the advice and checked the setup and the settings. That is better than a whole lot of people who ask for hrlp and then ignore the advice. given.
 
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