ACM04, clamp meter with True RMS and DC current measurement capability.

Today, a clamp meter landed on my workbench. When I was looking for one, I specifically wanted the ability to measure DC current with the clamp, and True RMS was a must.
I need both features for working on uninterruptible power supplies (and not only those).
As usual, all the details, teardown, and measurements are below the fold.

As I mentioned in the title, I sometimes repair uninterruptible power supplies and inverters. My colleague and I used to design and manufacture them; these days, I just repair them.
In some cases, it would be useful to measure current from the battery and accurately measure the output voltage. The advantage of a clamp meter here is that you don't have to break the power wire to connect it.
There are lots of spoilers in this review. If you can't find the information you're looking for, take another look—you may have missed one of them.

As always, I'll start with the packaging. I'll hide it under a spoiler; suffice it to say that everything arrived in good condition and very quickly. I placed the order on March 3, and it arrived in the mail on March 13.



Inside the package, the meter comes in its case. Again, everything is simple and neat.


And here's the entire kit.
Clamp meter
Case
Test leads
Manual.
Batteries aren't included, but perhaps that's for the best.


Next, I'll go through each item in the kit separately.
I'll start with the manual.
The manual is quite long and detailed, but unfortunately it's in English.
However, during testing I found that this device is a clone of the well-known Mastech MS2108, so the manual is identical to the original's manual, which is available in Russian.

The kit also includes test leads with probes. Let's just say the quality is pretty mediocre.
I wouldn't say they're completely awful, but it's hard to call them good, either.
I'll test them during the various measurements later; for now, here are some photos with brief descriptions.


They also included a pretty handy pouch, with a pocket inside for the test leads. But I think it could have been made a little bigger; otherwise, you'll have to pack the leads so they don't stick out as much. The pouch can be clipped to a belt, which is quite nice. We'll see how it holds up.


Now we've reached the star of this review.
I don't know if this is a brand name or what, but where the manufacturer's name would normally be, it just says B side. Whether that's the name or not is unclear. The model is ACM04.
The device looks very much like the MS2108; the only difference is the color of the clamps themselves.
It also has the True RMS marking.


Nothing particularly interesting on the back: the usual warnings about using only the correct batteries, the protection class 3 rating, and the maximum voltage of 600 volts. And a serial number sticker.


The sides of the case have inserts made of dense rubber, and it feels great in the hand. No complaints here.


Here’s a closer look at the meter.
The measurement mode selector has five operating positions and two end positions where the meter is off.
The meter shuts off automatically after 15 minutes (the manual says 30). It beeps a warning after 14 minutes, then shuts off one minute later.
There are also buttons for selecting different modes.
SEL — selects the operating mode. Some switch positions have multiple modes; this button also switches between auto-ranging and manual range selection. It also enables the zeroing function (useful when measuring DC current).
Min/Max — selects a mode in which the meter records the minimum or maximum measured value.
RAN — changes the measurement range in manual mode.
Hz% — switches the meter to frequency or period measurement; works in AC current or voltage measurement mode.
B.L / Hold — Press and hold to turn the backlight on or off; press briefly to hold the readings. The backlight also turns off automatically after 15 seconds.


The jaws are marked for polarity, since the meter can measure DC current.
When opening them, you first need to apply a little force (though not much) to release the jaws; after that, they move smoothly. It feels like there’s a small latch.


The markings are also present on the sides of the jaws.


Since batteries aren’t included, I stopped by the nearest store and bought a pack.
I didn’t like that there are three batteries, since they’re usually sold in even-numbered packs.
So you end up with one spare battery that’s no use for anything. They should have included either two or four.
But that’s more of a suggestion, since the original Mastech also uses three batteries.
It takes AAA batteries.


What I did like is the clear labeling and the fact that the mounting screw goes into a metal nut. That’s how it’s usually done, but I’ve come across versions with a self-tapping screw driven into a plastic post.


Since I’d already gotten as far as the batteries, I couldn’t resist taking the meter apart to see what was inside.
I figured I wouldn’t be the only one interested.
The meter is very easy to take apart. The battery compartment also comes off without any wires, and you don’t even need to remove the batteries from it.
By the way, the batteries aren’t simply connected in series; there’s also a tap from one of the cells.
So the meter runs not on 4.5 V, but on 3 + 1.5 V. I tried removing any one of the batteries in the circuit, and it wouldn’t work.



And here it is switched on for the first time.
The display is identical to the one in the original Mastech 2108, so I wouldn't be surprised if it's the very same one.


Next, a rundown of the meter's operating modes.
From left to right, top to bottom.
When you switch the meter to AC current measurement, it simply enters auto-measurement mode; I didn't include a photo of that.
1. Inrush current measurement mode. As I understand it, the meter detects a sudden rise in current followed by a drop, then displays the inrush current value. It's suitable for checking motor inrush current and possibly that of high-power power supplies, but the minimum current is fairly high: the measurement range is 30–600 A.
2. Frequency measurement while measuring current.
3. Duty cycle measurement mode, 10–95%. Not particularly useful for me personally.
4. The main DC current measurement mode.
Warning! Before measuring in this mode, be sure to press the SEL button to zero the meter. When measuring DC current, the meter responds to everything—even the Earth's magnetic field (which is hardly surprising)—so you must zero it before taking a measurement.
Ideally, first clamp the wire, then zero the meter, and then apply current. But you can also zero it first and then clamp around a wire that already has current flowing through it.
In DC and AC current measurement modes, you can select a 60 A or 600 A range. Auto-ranging is enabled by default, and you can also enable maximum and minimum value hold.


1. In the previous photo, you could see a random value on the display when DC current measurement mode was selected. After pressing the zero button, the display showed 0000-0001.
2. AC/DC voltage measurement mode. You can also measure frequency and duty cycle with the probes in this mode. AC voltage measurement is selected by default.
3. Resistance measurement mode. Automatic range selection is enabled by default. Pressing the RAN button switches the meter to manual range selection; press the same button again to cycle through the ranges. Press and hold to return to automatic mode.
4. Semiconductor test mode.


1. Continuity test mode. It works reliably, but I have a strong feeling the sound is smoothed out by a capacitor: if you keep the probes shorted for a while, the beep doesn't stop immediately, but after a fraction of a second. The delay is noticeable. Overall, it's fine.
2. Capacitor capacitance measurement mode. The manual claimed protection up to 250 V in this mode, but I would still recommend discharging the capacitor before testing it.


I mentioned above that the meter has a backlight. It is turned on by pressing and holding the B.L/Hold button. It turns off automatically after 15 seconds, or you can turn it off manually by pressing and holding the button.
The meter's backlight has two operating modes, selected automatically depending on the measurement mode.
When voltage, resistance, or capacitance measurement mode is selected, only the display is illuminated.


When the meter is set to current measurement mode, the clamp's working area is illuminated as well.


To test the meter further, I got out my old, trusted multimeter. I'll use a few other things too, but this is my main meter and I trust it completely.


But here I'd like to take a brief detour and focus on the probes.
You can see in the previous photo which probes I use. I want to compare those with the ones supplied with the meter.

To check the meter's accuracy, I once again used my homemade assortment of components: precision resistors and capacitors, regular and Schottky diodes, and electrolytic capacitors.


There's no point showing all the test photos, so I'll put everything in a table and include comparisons with meters I've tested before.


Here are a few photos of the testing process, with comments to go with the table.
1. The meter behaved as expected when measuring resistance. The photo shows a 1600 Ω resistor with a tolerance of 0.25%.
2. The meter can measure capacitance up to 66000 µF. The photo shows a 4700 µF capacitor, which it checked without any problems. The measurement took about 5–7 seconds. I actually tested it with capacitors up to 30000 µF; I didn't try any larger ones.
3. But there's a downside: the meter noticeably overreads capacitance. That was disappointing—not a huge deal, but I expected better. I may have to calibrate it. Though I rarely measure capacitors with a multimeter in practice.
The photo shows a 0.39025 µF capacitor.
Yes, the meter can't zero out the capacitance measurement. This is especially critical when measuring small capacitances, roughly up to 0.01–0.047 µF.
4. When measuring resistance, the voltage across the meter's probes is 0.5 V.


I've put the rest of the tests under a spoiler so the main review doesn't get too cluttered.

And finally, a couple of photos of the meter in my hand.




Summary.
Pros
DC current measurement
True RMS measurement for AC current and voltage.
A fairly wide range of functions
Good accuracy when measuring DC current and voltage.
Sturdy, convenient design
Protective case included

Cons
Capacitance readings are too high (for capacitance below 1000 µF).
I didn't quite understand the error when measuring current on the primary side of a switching power supply, so I'm not sure whether this is actually a drawback.

My take: Overall, it's a pretty good meter, but I was disappointed by its low accuracy (or rather, the high readings) when measuring capacitance, as well as the puzzling AC current measurement. Otherwise, I liked it. A decent all-in-one option for field work.

The clamp meter was provided for review by the eachbuyer store.

This item is currently unavailable from the store and most likely won't be back. As an alternative, the store offers a clamp meter with similar (and even slightly more extensive) functionality for $52.80; with the NEWORDER8 coupon, the price comes to about $48.5.
One thing to note: it runs on 9 V.

I hope this information was useful. If you have any ideas for tests or suggestions, I'd be glad to hear them and, if possible, try them out.