Multimeter

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Multimeter

August 14, 2010, 10:55:26 am

I need a lesson on the use of the multimeter to measure amperage. We've been discussing the amp draw of the coach batteries in another topic "Heart Interface Inverter/Charger". Where do I place the multimeter leads to measure the amperage being taken by the batteries. I would like to measure the amperage values for the coach & engine batteries. I generally know that the amperage can be measured by placing the red lead at the entry point of the current into the resistive load and the black lead at the current exit point of the load. But where do I find these points for both battery banks?

Re: Multimeter

Reply #1 – August 14, 2010, 11:32:43 am

Peter,

Most multimeters can only read up to a few hundred milliamps, and you run the risk of frying the device if you try to read out of their capable range.

You're MUCH better off getting a clamp-on style non-contact device, especially when we're talking amps, not just milliamps.

Michelle

Re: Multimeter

Reply #2 – August 14, 2010, 12:18:02 pm

CANNOT use mutilmeter to measure the high amperage flows that our alternator and battery chargers put out. Must use permanent add-on shunts.

To measure volts, one just touches probes to circuit to 'pressure'. To measure amps all of the 'flow' must go THROUGH the meter, which can easily overheat a multimeter.

Shunts can be designed for very high amp flows without being damaged, although they may heat up. We use 500-amp shunts.

Mutlimeters are useful for measuring lower amps, some as high as 20, most as high as 10, some only less than 2 amps. Often multimeters will use a different meter probe plug that is fused to not blow meter if too many amps flow through meter.

There are also meters that have a split to go around and close over a single wire to sense amount of amp-flow in that wire. They are called clamp-meters and generally work better on AC than DC electricity. Also they are easily remotely readable as they have to go to the cable, but are a handy diagnostic tool.

Re: Multimeter

Reply #3 – August 14, 2010, 12:23:17 pm

What about this device?

http://www.homedepot.com/h_d1/N-5yc1vZ1xkc/R-100341467/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053

Will this measure amperes for DC & AC? Does it make any difference?

Re: Multimeter

Reply #4 – August 14, 2010, 12:54:17 pm

The specifications don't specify that it will read DC amps with the clamp.

I bought a meter in that price range and found that it only measures AC amps with the clamp. It was more bucks for the meter that will read DC amps. Unfortunately, by the time I realized my mistake, it was too late to trade the meter for the better one. In the 12V system on the coach, we need to measure DC amps.

Re: Multimeter

Reply #5 – August 14, 2010, 02:03:59 pm

I don't see any indication that it reads DC _current_. Am I overlooking it?

The Model 61-702 will indicate AC or DC voltage. It will indicate AC current. I don't see anything about DC current (amps to/from the battery/charger/alternator) on the list.

I wouldn't want Peter to spend buck for a tool that doesn't do what he wants. (I've been there, done that!)

Re: Multimeter

Reply #6 – August 14, 2010, 02:10:05 pm

I see that the DC amperage is not included. I found this one and seems to do it all for a reasonable price:

http://www.amazon.com/Sheffield-CM100-Clamp-Digital-Research/dp/B0015PI7EA

Not sure if the 1/2 inch clamp is big enough to wrap around battery cables. Any thoughts?

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Re: Multimeter

Reply #7 – August 14, 2010, 02:51:25 pm

Quote

I don't see any indication that it reads DC _current_. Am I overlooking it?

My eyes are seeing funny things again

I hate when it does that

Re: Multimeter

Reply #8 – August 14, 2010, 05:50:11 pm

The 100 amp meter with 1/2 clamp would not have the capacity I would want for checking a current in a coach. I would expect that a 200 amp dc clamp meter would be minimal. The dc sources and drains might easily exceed 100 amps.

If you want to check current to the starter, you will want to borrow a meter. Better yet, have an expert "demonstrate" the procedure. I put two new chassis batteries in my coach today. Each claims to be able to deliver about 800 amps and they should be able to deliver 1,600 amps from the parallel configuration. I might check charging current to the starting batteries, but I won't be checking power to the starter!

Re: Multimeter

Reply #9 – August 14, 2010, 06:28:34 pm

JD,

I agree. I found the likely candidate in the below link. It's priced about the same as the others and has all the features necessary.

http://www.professionalequipment.com/extech-true-rms-ac-dc-clamp-meter-non-contact-voltage-detector-ma640/clamp-meters/

I guess another way to do the same thing is to test the batteries with my regular multimeter and when the voltage reaches about 12.8 I'd be ready to go so I don't tax the alternator too much.

Re: Multimeter

Reply #10 – August 14, 2010, 06:40:29 pm

That one looks good. I thought I was buying that one and unfortunately bought the next one down in the series. I think it was $20 cheaper, but doesn't do DC current. Doh!!

Re: Multimeter

Reply #11 – August 14, 2010, 07:08:53 pm

You found a winner with
http://www.professionalequipment.com/extech-true-rms-ac-dc-clamp-meter-non-contact-voltage-detector-ma640/clamp-meters/ Extech makes good stuff and reasonable prices. I have some of their meters. True RMS AC voltage measurement is a very good thing also and 600 amp limit is a good rating that will also measure starter current.

http://www.amazon.com/Sheffield-CM100-Clamp-Digital-Research/dp/B0015PI7EA 100 amps may be too low of a limit for alternator, starter, but probably ok for battery charging.

http://www.homedepot.com/h_d1/N-5yc1vZ1xkc/R-100341467/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053 Had to research it at manufacturers site to learn it does not measure DC amps.

http://www.idealindustries.com/products/test_measurement/clamp_meters/700_series_clamp_meter.jsp
The market seems to have lots of choices for AC amp clamp meters. It seems that many clamp meters do not measure DC amps. If you want to measure DC amps with a clamp meter, the specifications must state that it does DC and what the amp limit is.

As an Amazon Associate Foretravel Owners' Forum earns from qualifying purchases.

Re: Multimeter

Reply #12 – August 14, 2010, 08:45:25 pm

Well I just placed an order for the Extech MA640 at the below web site:

http://www.tequipment.net/ExtechMA640.asp

No shipping charges or tax for a total of $149.99! I thought that was the best deal.

Thanks to all for the great input...as usual.

Re: Multimeter

Reply #13 – August 15, 2010, 12:22:23 am

Looks like a winner. It does everything, so you can toss the old multimeter.

Re: Multimeter

Reply #14 – August 15, 2010, 04:59:48 pm

Tom,
I wouldn't toss it. Peter is going to get real adept with T/S'ing and want two meters on different circuits. (like playing with relays) And, everyone knows a "real" mechanic has a back-up. Ask Tim the toolman. ug! ug!

Re: Multimeter

Reply #15 – August 15, 2010, 06:13:54 pm

Small note, if you wrap the wire you need to measure 2 times, the reading will double.
For example, run wire thru the jaws and you get for example 4.5A, wrap wire thru again, reading will be 9A.
This becomes more important when trying to read very light loads.
Dave

Re: Multimeter

Reply #16 – August 15, 2010, 06:18:25 pm

Dave,

I truly have a lot to learn about using the multimeter with the jaw that takes the amperage readings. So what would the result be if the jaw is placed to read an appliance plug-in wire? Anything?

er

Reply #17 – August 15, 2010, 06:47:54 pm

It can only read 1 wire at a time. To read amps on a toaster for example you need to separate out the wires and read 1 only.
Putting the clamp over the cord, you get nuttin, they cancel each other,
Dave

Re: Multimeter

Reply #18 – August 15, 2010, 06:55:13 pm

Dave,

Thanks fo the explanation. I thought as much. That old 'rithmetic...+ one way and - the other way = 0, right?

Re: Multimeter

Reply #19 – August 15, 2010, 07:07:13 pm

You can put both wires inside the jaws of the clamp meter. But it will read amps flowing in one direction and amps flowing in the other direction at the same time, which cancel each out so the reading will always be zero. This is true for AC or DC amp flow measurements.

One way to measure AC amps in a lamp wire for example, is to have an extension cord where you carefully separate the two wires from each other and clamp just one of the wires.

But a better & safer way is to buy an AC line splitter like
http://www.extech.com/instruments/product.asp?catid=30&prodid=114 This AC unit also optionally 'amplifies' very low current so it will register on the meter.

This from Extech:
"The Line Splitter provides a means to cleanly "open" a standard 120V AC line cord in order to make clamp type current measurements. When connected between the 120V AC wall outlet and the device under test, a clamp meter can then be clamped around one of the two test openings in the Splitter. One opening provides a one-to-one current reading and the other provides a times-ten (X10) reading so that small current will display with better resolution on a clamp meter."

Re: Multimeter

Reply #20 – August 15, 2010, 07:54:11 pm

For the toaster and any plug-in AC appliances, I'd use a Kill-A-Watt. Plug it into the wall to measure voltage and frequency, and plug an appliance into it to measure Amps and Watts.

Meritline puts it on sale about once a month.

http://www.meritline.com/p3-international-p4400-kill-a-watt-the-electricity-detector---p-22980.aspx

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