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Old 13-07-2016, 23:43   #1
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Clamp ammeters: accurate or not?

And if so, what am I doing wrong?

I was curious about how much current my inverter used when it's switched on but not loaded. But the current going through the positive lead showed about 1/2 that going through the negative. (0.7 amps vs 1.7)

I checked a few other circuits with similar results. The positive showed about half the current the negative did.

Anyone else noticed anything like this?
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Old 13-07-2016, 23:53   #2
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Re: Clamp ammeters: accurate or not?

What brand/model? Did you zero it fully before taking the readings? They can be a bit touchy and require resetting a few times before they settle down.

Did you reverse the orientation of the clamp when switching between the +ve and -ve leads so that the electrickery was going the same way through the sensors?
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Old 14-07-2016, 05:23   #3
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Re: Clamp ammeters: accurate or not?

I've never noticed that with mine.

You may have a dud, or as mentioned above the direction may matter


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Old 14-07-2016, 05:31   #4
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Re: Clamp ammeters: accurate or not?

Not all clip on ammeters read DC. Some are AC only instruments.
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Old 14-07-2016, 05:43   #5
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Re: Clamp ammeters: accurate or not?

My fluke will do the same when the battery is low.
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Old 14-07-2016, 05:47   #6
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Re: Clamp ammeters: accurate or not?

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Originally Posted by 44'cruisingcat View Post
And if so, what am I doing wrong?

I was curious about how much current my inverter used when it's switched on but not loaded. But the current going through the positive lead showed about 1/2 that going through the negative. (0.7 amps vs 1.7)

I checked a few other circuits with similar results. The positive showed about half the current the negative did.

Anyone else noticed anything like this?
I assume you have a "cheap" DC clamp meter costing $100-$200. The first "calibration check" with these is to measure the current in a wire then reverse the clamp on the same wire and see if you read the same current except with a change of sign. If it passes this test then it's possible that you have a DC leakage path on the negative side so not all the DC current returns through the negative wire. Or it could be that some other DC load is using the inverter to return current on the negative side. Tell us which side reads higher current.

It sounds like you are measuring the inverter with no load. For low current you can use an ordinary volt/amp meter with 10A DC measuring leads and check that way. These meters are typically pretty accurate. Most DC clamp meters are not accurate at anything less than about 10 amperes. One way to increase the clamp meter accuracy is to wrap 5-10 turns of wire on the clamp. Then run the low current through this wire. Whatever current you measure then divide that by the number of times the wrapped wire goes through the inside of the clamp.

Note that almost all inverters draw a high amount of AC current plus DC when they are supplying power to an AC load. Thus the RMS current is higher than the DC current. Most "cheap" DC clamp meters don't measure this RMS current accurately. Neither can an AC clamp meter accurately measure the RMS current drawn by an inverter. The battery may not care much about the RMS current but the wires do.
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Old 14-07-2016, 15:15   #7
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Re: Clamp ammeters: accurate or not?

Thanks for the suggestions.


Yeah it is a relatively cheap one.


I'll try reversing the orientation on the same wire. If that produces different readings I might take it back to the shop.
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Old 14-07-2016, 19:13   #8
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Re: Clamp ammeters: accurate or not?

'accurate' cam be within 10% for a cheap meter or under 1% for a pro one but that is at full scale reading. Clamp meters typically measure high currants so a .3A error on a 100a meter wold be good.
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Old 15-07-2016, 00:26   #9
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Re: Clamp ammeters: accurate or not?

Yeah, but I'm seeing differences of over 50%.
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Old 15-07-2016, 06:00   #10
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Re: Clamp ammeters: accurate or not?

Yes, but you are probably measuring a current that is much less than 1% of the meter's full scale capacity. It is a bit too much to ask that such a low current be accurate for these cheap meters. You can use the loops of wire trick to improve the accuracy. But that defeats the easy clamp-on feature.
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Old 15-07-2016, 07:18   #11
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Re: Clamp ammeters: accurate or not?

Quote:
Originally Posted by 44'cruisingcat View Post
And if so, what am I doing wrong?

I was curious about how much current my inverter used when it's switched on but not loaded. But the current going through the positive lead showed about 1/2 that going through the negative. (0.7 amps vs 1.7)

I checked a few other circuits with similar results. The positive showed about half the current the negative did.

Anyone else noticed anything like this?
Most only read AC current and determine peak current through calculation. They tend to only be accurate in a narrow current range.

For DC current you need a shunt for accurate readings. I haven't tried clamp ammeters for nearly 20 years. Is your ammeter rated for DC?

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Old 15-07-2016, 07:54   #12
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Re: Clamp ammeters: accurate or not?

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Originally Posted by 44'cruisingcat View Post

Yeah it is a relatively cheap one.

Unfortunately with DC clamp meters you often tend to get what you pay for. I have to have both high amp capable DC clamp models, usually using my Fluke 376, and low amperage high accuracy models too Amprobe LH41A (Amprobe is a Fluke company) and an Extech 380942.

For low level DC current I have found my Extech to be the most accurate down to levels below 0.1A. It sure beats pulling out my Fluke 179 or Fluke 289 and inserting it into the circuit to measure current. The Extech is excellent for corrosion surveying and solar or wind spot checks but is not suitable for currents above 30A.

Some of the inexpensive DC clamp meters are pretty accurate for what you are doing and some are horrible. I even had one that started out okay, a Mastech MS2108, then rather quickly fell off the cliff and became one of the biggest pieces of junk I've ever purchased. It also physically melted in my car when in the same bag with four other meters that were perfectly fine. Heck even a red solo cup survived the heat better than the Mastech did.....
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Old 15-07-2016, 14:50   #13
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Re: Clamp ammeters: accurate or not?

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Originally Posted by leftbrainstuff View Post
Most only read AC current and determine peak current through calculation. They tend to only be accurate in a narrow current range.

For DC current you need a shunt for accurate readings. I haven't tried clamp ammeters for nearly 20 years. Is your ammeter rated for DC?

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Yes, it measures AC or DC, there are separate ranges for both.
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Old 15-07-2016, 14:57   #14
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Re: Clamp ammeters: accurate or not?

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Yes, but you are probably measuring a current that is much less than 1% of the meter's full scale capacity. It is a bit too much to ask that such a low current be accurate for these cheap meters. You can use the loops of wire trick to improve the accuracy. But that defeats the easy clamp-on feature.

I was using a 0-40 amp DC range, looking to measure around 1/2 - 1 amp.

There's no way to loop the wire through the clamp jaws, it's 25mm2 wire. (0 gauge)

I guess I could temporarily rewire with lighter stuff just to check the no-load current draw, but it was really just me trying to satisfy my curiosity.


Again, thanks for the advice!
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