How accurate is a multimeter when measuring current

[kma]

Hi. I want to ask a quick question, how accurate are multimeters? Like is every current detected on a multimeter a legitimate current or can it sometimes detect false currents, and can it miss any currents that are there? And how accurate are the values on there when it comes to what the current is?

I hope this question makes sense

 

[hutchphd]

Ther answer is that it depends upon the meter. And it depends upon whether it has been calibrated (as is required in a host of circumstances where certification is required).
Absent other indicators, I would treat the least reported digit with great skepticism. If you need precision then worry about it, quantify what you need, and obtain necessary calibration. Much of this is spelled out in, for instance, ISO9001 certification requirements

 

[Tom.G]

If the meter came with a manual, the specifications may be listed therein. Of course if the meter is well-used it could have been overloaded sometime in its life, making some readings well out of tolerance; and you would not necessarily know without calibrating it.

A very rough quick test is to take the same measurement on two different ranges and see if the radings are "reasonably close." Minor differences are to be expected because the meter impedance is often different on different ranges.

Cheers,
Tom

 

[Rive]

I hope this question makes sense

It does.

Usually, even cheap multimeters are fairly accurate for current measurements, as long as it's within range and is DC or some 'clean' 50Hz sinusoid AC current.
Overloading the multimeter even once can make things messy, though. Higher frequencies may be tricky, and measuring non-sinusoid or noisy signals is matter of luck on a cheap one.

Could you please tell us more about the circumstances?

 

[berkeman]

And how accurate are the values on there when it comes to what the current is?

In addition to the other good replies, keep in mind that standard multimeter current measurements are invasive. That is, they alter the circuit where you are trying to measure the current (sometimes significantly).

The reason for this has to do with the fact that most multimeters insert a test resistance in series with the current and measure the voltage across that test resistance to calculate the current that is flowing. But if the test resistance is a moderate (or significant) fraction of the circuit resistance, that can decrease the current flowing in the circuit.

Usually, a different value of test resistance is switched in for each different range of current to be measured on the multimeter. So when switched to the "1A" current range, perhaps a 0.1 Ohm test resistance is used. But when switched to the "1mA" range, the test resistance may be 100 Ohms for example. You can see that if you are trying to measure small currents in a circuit that has fairly low values of resistance in it, the test resistance becomes a significant source of error.

You should be able to find the values of the test resistances in the Owner's Manual for your multimeter. If not, you can just use a 2nd DVM on its "resistance" measurement setting to measure the first multimeter's test resistors as you switch between its different current measurement range settings.

 

[Vanadium 50]

Some multimeters come with a clamp which allows you to measure current, if not exactly noninvasively, at least less invasively.

 

[berkeman]

Some multimeters come with a clamp which allows you to measure current, if not exactly noninvasively, at least less invasively.

Yes, AC current. And for oscilloscopes you can use clamp-on Hall effect current probes that work for both AC (up to moderate frequencies like 10MHz) and DC. Very handy to have for some measurements.

 

[DaveE]

Hi. I want to ask a quick question, how accurate are multimeters? Like is every current detected on a multimeter a legitimate current or can it sometimes detect false currents, and can it miss any currents that are there? And how accurate are the values on there when it comes to what the current is?

I hope this question makes sense

Yes, it's a great question. Previous answers are great too. Like most technical subjects it can be rather complicated.

One of the things EEs have to get good at is understanding their test equipment. This is often more complicated than it appears. This web site lists lots of DMMs for sale. They neither cost the same or work the same, in features, accuracy, reliability, useability, etc.

If you haven't yet, read a bit about the AC measurements; True RMS (TRMS), Average AC, measurement bandwidth and peak to average specifications. Read the whole data sheet* for an expensive meter and see if you understand each entry. You don't need to make a career out of this, but you should know that those specs exist and sometimes matter.

Step 1 is the specifications for the meter. Manufacturers like Fluke, Keysight, Keithley... have pretty good specs available, but they are expensive. These are meters for people that need good tools. The cheap DMMs you can buy on eBay and Amazon for $20 or less, don't (or I wouldn't trust them if they did). You get what you pay for. The cheap ones are good too, I have both types in my lab, I use them for different things.

Step 2, no matter how much you paid for your instrument, is to work with it in your experiments to somehow verify that it's working OK. Small errors are nearly impossible to detect, but sometimes the errors aren't small because the instrument isn't appropriate for the task, has EMC issues, maybe broken, or set up incorrectly.

This sort of lab work is sometimes just as important as choosing IC's or transistors and follows a similar, often iterative, process: define what you need; read documentation; design, setup and VERIFY your test; collect data, including calibrations or sanity checks; think about if it makes sense (i.e. matches models and such). Other times you just want to know if the power is on and ±20% with minimal effort is good enough.

However, the truth is that practicing engineers simply don't have the time or money to always do all this stuff. We are usually operating on experience and faith, hoping we get good data but also keeping in mind the instruments might be wrong enough that we need to care. Knowing the circumstances where you need to be careful is a bit of an art.

*Tip: if you are reading a whole data sheet, don't skip the footnotes in tiny fonts. They are often the most important part.

 

[DaveE]

Some multimeters come with a clamp which allows you to measure current, if not exactly noninvasively, at least less invasively.

In my experience they are insanely convenient and not very accurate (or repeatable). I'll spare you my diatribe, but just say that geometry and environment both in and next to the clamp often matter. My standard measurement with these when I'm lazy is to do a measurement and then repeat it after I've moved the clamp a bit and/or unclamped it and rotated it 180^o as a test of the data quality.

 

[Averagesupernova]

My Klein multimeter measures AC as well as DC current with the clamp. I have not tried it on DC. The clamps I've used on AC are fine for what they're used for. Why is it important to have .1% or better accuracy when measuring mains current? Afraid your clothes dryer draws a fraction of an amp more than spec?

 

[berkeman]

My Klein multimeter measures AC as well as DC current with the clamp.

Interesting! Which model is it?

 

[Averagesupernova]

Interesting! Which model is it?

CL700

https://www.google.com/search?q=kle...s-wiz-serp#scso=_aBUlY4C2MOi-0PEPn4eH2AM_30:0

 

[berkeman]

CL700

https://www.google.com/search?q=kle...s-wiz-serp#scso=_aBUlY4C2MOi-0PEPn4eH2AM_30:0

 

[Averagesupernova]

View attachment 314245

Hmm. Might be the 800. I'll be able to actually pick it up and look in a couple hours.

 

[DaveE]

Fluke has some that look good for high currents. I've only ever used the really old ones from them which were sort of OK, but not great. The magic words are hall-effect. They usually have a manual DC offset adjustment, since it's hard to avoid some magnetization of the core.

 

[Averagesupernova]

@berkeman

 

[The Electrician]

Interesting! Which model is it?

$50 gets you this: https://www.amazon.com/dp/B0188WD1NE/?tag=pfamazon01-20

The build quality is excellent and I can measure DC currents down to the 10 mA range. I used mine to track down a vampire load in my old Town & Country van. You don't need .1% accuracy for something like this.

When measuring DC currents that low you have to watch out for the effect of the Earth's magnetic field.

 

[Averagesupernova]

The reason I posted the pic showing the jacks the probes plug into with text showing their function was to show that current is NOT measured on that meter except in the clamp. Not a practical unit for lower currents.

 

[Rive]

Guys, we just scared away the OP

 

[berkeman]

Guys, we just scared away the OP

Well, you do know what his username stands for, right?

Spoiler

Use acronymfinder if you don't know. Anyway, the OP hasn't been back since posting, so we probably didn't scare them off (yet).

 

[hutchphd]

Kevin Mitchell Anderson?

 

[Averagesupernova]

Kevin Mitchell Anderson?

Instead of: kiss my...
Kisses many ...