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Old 03-24-2018, 06:45 PM
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Bill Bill is offline
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Default Choosing a digital clamp-on multimeter

Digital clamp-on multimeters are a very handy device to have in your RV toolbox. They have a set of jaws on the front, which form a more-or-less circular opening. To measure the current in a wire - normally a rather inconvenient thing to do - you simply pull a trigger to open the jaws, slip them around a wire, and close them again. The current flowing in the wire appears on the LCD display. But there are so many clamp-meters for sale, how do you choose which one to buy? I hope to help here.

I was reading one of Mike Sokol’s columns about RV Electricity the other day. Mike is a widely acknowledged expert in AC electricity (i.e., shore power) in all applications, and is well-regarded in RV DC power (battery) systems as well. I have corresponded with him a couple times on RV-related issues, and he has been friendly and helpful.

As part of the article I was reading, Mike recommended a particular clamp-on digital multi-meter. When I checked the specs on this meter, it turned out that it was great for AC power systems (Mike’s specialty), but not so hot for DC power systems. Mike readily agreed with this, and since RV owners deal with both AC and DC power systems, I went looking for something more RV-appropriate.

A quick search uncovered a lot of clamp-on digital multimeters - far more than I expected. This reflects their increasing popularity with anyone who deals with electricity. Most of them are intended for AC power systems only, but a few will handle both AC and DC systems. So if you want to buy one (and they are very handy), you need to know what’s important. Because there are a lot of candidates out there, I will talk in general about what is important in an RV-oriented clamp-on digital multimeter. In the end, I found one that looked good, bought it, and have been using it for several weeks now. And I’m loving it. There are undoubtedly others.

So here are some important tips on what to look for.

1. It needs to measure both AC and DC voltages. Every clamp-on meter I’ve seen does this, using a pair of test leads and probes. Nothing new about this.

2. It needs to measure both AC and DC current, using the clamp-on jaws. All clamp-on meters will measure AC current, but not many will also measure DC current. Again, this is important in an RV, where DC circuits outnumber AC circuits.

3. It needs to have a useful current range. It is common for these meters to have a single current scale of 0-to-400 amps. But for an RV, this is silly. In our TMs, we never encounter current greater than 30 amps because both the AC and DC circuits are fused at 30 amps. On the other hand, we often encounter much smaller currents, and you need to be able to measure them accurately. As an example, imagine that you are trying to read a current of 0.1 amp (about right for an interior LED light, for example). Or you are searching for battery-killing parasitic loads, which often run about 0.2 amps. On a 400-amp scale, these are not even a blip. And even if they show on the scale, the measurement accuracy will be terrible. Conclusion: a good RV meter should have one or more low-current scales.

4. Mike Sokol is a fanatic about hot-skin problems on an RV, and rightly so. Each year, a lot of people get a shock when they touch their trailer, and a few of them die. We’ve seen this hot-skin problem right here on the forum a number of times. Dealing with a hot skin is an issue beyond the scope of this article, but the first step, of course, is to find out if you have a hot skin. Mike has long recommended carrying a non-contact voltage (NCV) tester, a small plastic device about the size of a small cigar. I have a couple of them. You bring its plastic probe near the skin of your trailer, and if everything is OK, there is no response. But if your trailer has a hot skin, a light goes on and a tone sounds. This is your warning to fix the hot skin problem NOW, before somebody gets hurt. Carrying an NCV tester in your pocket is not a big deal. But if you also have a clamp-on multimeter handy, wouldn’t it be nice if the meter had a built-in NCV tester?

Prices of clamp-on digital multimeters vary from quite low ($20) to very high (several hundred dollars). Since I am retired on a fixed (and limited!) income, I went looking for one that is both good and inexpensive. And I found one. There are certainly others, but the one I bought is the Tacklife CM01A Digital Multimeter, $22 at Amazon, shown in the pictures. It has a number of things going for it.
1. It measures AC and DC volts, of course, and comes with the test leads and probes to do it.
2. It measures both AC and DC current with the clamp-on jaws.
3. It has the standard 400-amp range, and it also has a 40 amp range. This is much better for RV work, though I wish it also had a 4 amp range.
4. It has an NCV tester built right into the tip of the jaws. Just bring the tip of the jaws near the trailer, and it will sing if there is a hot skin.
5. It measures true-RMS AC current. This is kind of an exoteric function, but in some cases it matters.
6. It has an LED work light that shines on the area around the jaws. Handy if you’ve blown a fuse and have to work in the dark for a bit.
7. Finally, most clamp-on multi-testers, including this one, have a number of what I call "goody functions". These are functions that you won’t use very often, like frequency, pulse width, duty cycle, capacitance, diode good/bad, continuity, and so forth. Apparently they are easy to build into a meter, because they all have them. I don’t consider them a selling point, and would not disqualify a meter that doesn’t have them.

The attached pictures (copied from Amazon) show the meter I bought, the Tacklife CM01A. While I can recommend this particular unit based on my own use, there are many others out there. You just need to know how to choose one. With any luck, this article helps.

Bill

[EDIT: It has been pointed out to me that if you go to Amazon and read the large print for this meter, it says AC current, and implies AC current ONLY. But if you go down into the specs, it also shows specs for DC current. The markings on the function dial also say AC/DC current. And more to the point, the unit in my hand also says, and measures, DC current. My guess is that the top level description was pasted from the description of another meter by a lazy copy writer.]
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