AC/DC Clamp Meter Accuracy

[arcticfox2005]

This us a new Center brand 223 clamp meter ($88 Amazon). For DC amps in the 0-10 amp range it advertises a resolution of 1mA and Accuracy of 2% +10.

The Accuracy claim is what I don't understand what they are claiming.

In my class C the house batteries are new 6v. They are wired up using a knife switch on the negative post because in storage the parasitic draw could be a problem.

With everything turned off, the knife switch open, it says there is a current of 38mA to 45 mA, measured on the negative cable. I repeated the test about 5 times, turning the meter off each time and zeroing it before the next test. I can't get the meter on the positive cable because of tight quarters.

Are these readings possible? How could there be a drain with the isolation switch open? Or is this within the accuracy claim and the current is nonexistent?

 

[SeilerBird]

Switching the negative cable is wrong.

 

[Mark_K5LXP]

You can try duplicating the experiment with the positive cable. Any current through a battery must go through each terminal. You could also verify with a conventional meter in series with the terminals.

Mark B.
Albuquerque, NM

 

[DonTom]

Switching the negative cable is wrong.

Why? I don't see what difference it makes.

-Don- Reno, NV

 

[SeilerBird]

Why? I don't see what difference it makes.

-Don- Reno, NV

Because if the negative wire is switched off and someone touches the positive side and also contacts the metal frame of the vehicle he can get a hell of a shock.

 

[JayArr]

2% of 10 Amps is 200mA, 38-45mA is just an accuracy error.

 

[arcticfox2005]

The directions for the switch specifically say for the negative terminal only. The Accuracy Error (JayArr) is what I was thinking.
Thanks.

 

[Jim18655]

Because if the negative wire is switched off and someone touches the positive side and also contacts the metal frame of the vehicle he can get a hell of a shock.

12VDC not enough for a shock. If the neg terminal is open then there's no connection to the frame from the battery and it would be an open circuit.

 

[Lou Schneider]

Because if the negative wire is switched off and someone touches the positive side and also contacts the metal frame of the vehicle he can get a hell of a shock.

No, like Jim said there's no possibility of a shock because the metal frame is no longer connected to the battery after you open the negative side.

That's why mechanics always disconnect the negative terminal first when working around a battery. With it connected you can draw an arc if your wrench gets between the positive terminal and a metal ground. There's no hazard if your wrench gets between the negative post and the grounded frame, and once the negative is disconnected the positive terminal is safe.

 

[arcticfox2005]

Go back to my original post. I'm still a little fuzzy on the Accuracy Error of 38mA - 45mA. If the circuit is dead via the knife switch being open, why any reading at all? If the meter is turned on and zeroed and just left away from any wires, would it register current?
Why would clamping it on a dead battery cable make it register?

 

[DonTom]

Why would clamping it on a dead battery cable make it register?

Perhaps picking up interference from somewhere? Some cheap digital meters are way too sensitive to garbage signals. The wire you're clamping on could be an antenna for garbage signals getting into the meter.

-Don- Reno, NV

 

[Matt_C]

This us a new Center brand 223 clamp meter ($88 Amazon). For DC amps in the 0-10 amp range it advertises a resolution of 1mA and Accuracy of 2% +10.

The Accuracy claim is what I don't understand what they are claiming.

In my class C the house batteries are new 6v. They are wired up using a knife switch on the negative post because in storage the parasitic draw could be a problem.

With everything turned off, the knife switch open, it says there is a current of 38mA to 45 mA, measured on the negative cable. I repeated the test about 5 times, turning the meter off each time and zeroing it before the next test. I can't get the meter on the positive cable because of tight quarters.

Are these readings possible? How could there be a drain with the isolation switch open? Or is this within the accuracy claim and the current is nonexistent?

Arcticfox,
There is a very simple answer. I knew it in the first line. It is a two part answer.
What you may not know is that to sense DC current, you have to sense a steady magnetic field that is created. If you put the probe near or around the cable and zero it then close the switch, you have a chance. Measuring DC current this way has never been easy. I have a DC probe that I have owned for 25 years. It has gotten very good care and I would still expect some offset in a case like yours. A big problem here is that sensing the magnetic field is just not easy. I love showing people that want to know more that my probe is also a pretty good compass.
Another truth, though I hate to say it like it will sound, is that the replacement cost for my probe alone would be over 200$us today. That is the probe only - no meter.
What you have is probably a very usable field diagnostic tool. You can use it to check starter current or alternator output, but to measure the current at the levels you are looking, put the meter set for DC current in series with the negative cable.
If you really have to measure the current in that situation with the circuit complete, wind ten turns of lighter gauge wire and use the clap-on that and divide by 10.
Matt

 

[JayArr]

It may work off the eddy current produced in the iron ring that gets clamped around the wire. It's robust enough to measure 10 Amps and placing anything inside the ring may change or produce the eddy current enough to cause the error.

 

[John From Detroit]

Ok I've seen this "Switch in the positive or negative lead" argument many times.

IT DOES NOT MATTER in fact there may well be an argument for the negative lead. here it is

You turn the switch OFF (Disconnect the batteries) And grab your wrench to remove the battery cables prior to replacing the batteries.. and like an idiot you start with the POSITIVE lead. the wrench (metal) shorts the positive lead to the chassis.. Negative lead is "Open" no problem.

Try that with the switch in the positive lead Sparks and possibly a serious burn on your hand. Not to mention a melted wrench.

NOTE 2: This assumes only ONE wire is hooked to the battery negative. if more than one.... Well the 2nd wire needs to be connected to the non-battery side of the switch.

 

[JayArr]

You turn the switch OFF (Disconnect the batteries) And grab your wrench to remove the battery cables prior to replacing the batteries.. and like an idiot you start with the POSITIVE lead. the wrench (metal) shorts the positive lead to the chassis.. Negative lead is "Open" no problem.

This is how it was explained to me in automotive manuals when I was younger, I've been doing it this way ever since.

 

[TonyL]

Why not fit a double pole switch, problem solved

 

[Lou Schneider]

Go back to my original post. I'm still a little fuzzy on the Accuracy Error of 38mA - 45mA. If the circuit is dead via the knife switch being open, why any reading at all? If the meter is turned on and zeroed and just left away from any wires, would it register current?
Why would clamping it on a dead battery cable make it register?

Like JayArr said, the probe is measuring the steady state (DC) or fluctuating (AC) magnetic field that's formed when current flows through the wire. This magnetic field is weak enough that the meter also picks up the Earth's magnetic field so you have to compensate for it by zeroing the meter first.

It's likely the disconnected wire is acting like an antenna and is amplifying the Earth's magnetic field when the probe is clamped around it. It's not unusual to read what looks like a little induced current, even if no actual current is flowing through the wire.

 

[John From Detroit]

Why not fit a double pole switch, problem solved

a doupole switch is two single pole switches with a single operating device. 2x the things to go wrong, additional cost no advantages.

 

[TonyL]

a doupole switch is two single pole switches with a single operating device. 2x the things to go wrong, additional cost no advantages.

What's to go wrong? It's a simple switch much like a circuit breaker. I've fitted hundreds in control panels and can't think of any I've replaced.

 

[Kirk]

Because if the negative wire is switched off and someone touches the positive side and also contacts the metal frame of the vehicle he can get a hell of a shock.

That concern is exactly the reason you should always remover the negative cable first. And much more probably that a shock when working the other way around is the possibility of shorting from positive to ground(also negative post if still connected) with a tool. I have actually seen that happen and if the battery is fully charged the spark made is memorable as is the damage to the tool.

No, like Jim said there's no possibility of a shock because the metal frame is no longer connected to the battery after you open the negative side.

Like myself, Lou has made his living working on equipment that often has a battery backup supply and has probably seen the sparks.

It's likely the disconnected wire is acting like an antenna and is amplifying the Earth's magnetic field when the probe is clamped around it. It's not unusual to read what looks like a little induced current, even if no actual current is flowing through the wire.

Remember that 45 milliamperes is only equal to .045A. In all probability the meter accuracy is best in midscale like most meters so the error could be part of your readings. And as Lou said, it can be an antenna effect and even your meter leads could be the antenna.