Fnirsi DMC-100 Clamp Meter

During a Fnirsi promotional campaign, I was recently offered the chance to review one of two of their instruments: a measuring tweezer or a clamp meter. As it happened, the clamp meter interested me both in its own right and because I had a very practical use for it. So today, I’m reviewing the DMC-100 clamp meter.

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First, why did I choose the clamp meter? Sure, the tweezers would have been interesting too, but they’re more useful for repair work, which I do much less often, so they took a back seat.
The clamp meter, on the other hand, has a very practical use for me. When I started building electronic loads for currents above 100 A, I wondered how I could calibrate them—or, more precisely, check them—since I already have an Owon clamp meter for currents up to 100 A.


I also have a clamp meter rated for up to 600 A, so you might think I wouldn’t need anything else. But it turned out to be noticeably inaccurate, reading much higher than the actual value. For example, it shows 124 A instead of 120 A. Of course, the obvious question is whether it might be the other meter that’s wrong. Unfortunately, no—but more on that later.


So, I ordered it and it arrived. No complaints about the packaging—it was packed very securely.
The box lists the clamp meter’s main features.


There’s also a brief specification list, but most of it concerns the multimeter functions, since current measurement almost gets lost among everything else.


Yes, I know everyone and their dog has reviewed this clamp meter. One well-known site alone has five reviews, and of course I’ve read them all. The problem is that products like this often end up with reviewers who don’t really understand what they’re reviewing. One says it measures DC current only up to 100 A, another tries to post as many photos of the box as possible, and someone else says the continuity buzzer is really fast... Only one of the five actually made a proper attempt to check the current measurement accuracy—at least as well as his available instruments allowed.

Anyway, let me have a go at writing my own review of this instrument.
I’d call the package average. Besides the clamp meter itself, it includes:
1. Test leads
2. Thermocouple
3. USB-C cable
4. Manual
5. A handy carrying case—which, unfortunately, isn’t included.



The thermocouple and cable are about as generic as they come.
The test leads are pretty standard too. The cable is flexible and marked CAT III, but I have my doubts that they actually meet that rating. The thing is, the probes don’t have safety caps to make the exposed tips really short, and the contacts that plug into the meter aren’t insulated at the ends—not even with a bit of colored insulation.



Here’s what a safety cap for a probe tip and insulated terminal ends look like. As you can see, the included test leads have neither.


The clamp meter looks a little angular, but overall it’s quite nice. I like it.


Here it is next to my main Owon CM2100 and a certain Bside clone of a Mastech.



There’s a fairly large display and five control buttons on the front, a QR code sticker on the back, the clamp jaws and a flashlight at the top, and the test lead sockets at the bottom. It’s a classic layout, except for the lack of a rotary switch, which takes some getting used to. By the way, a quick note before I forget: the meter remembers its last mode when switched off. Handy.
The display is slightly recessed below the surface of the case. At least, you can safely set the clamp meter down face-first, as long as the surface is reasonably flat.



The display is great—I really like it. I’ve liked this type of display ever since I bought my UT181A. Unfortunately, displays like this use quite a lot of power, so battery operation is out of the question; you need rechargeable batteries. That’s why both Uni-T and Fnirsi use rechargeable batteries.

For some reason, it powered up in Chinese, but not for long: a low-battery warning appeared almost immediately. Well, that’s a good excuse to charge it and check its capacity at the same time.
On the right are the flashlight button, a USB-C port, and a reset button hole. The port is covered by a rubber flap and is also set deep into the case. This feature is necessary for safety, since measuring instruments usually have a circuit that is directly connected to the probes. That’s why Uni-T went a step further: the battery is charged through a special adapter connected to the input jacks.


While charging, an animation is shown on the screen, and the charging current is about 0.72 A. The meter charges slowly; a full charge took 3 hours.
When charging is complete, the battery icon appears full. But while it’s charging, there’s no way to tell how much charge the battery has gained. Not great...
About 1800 mAh went in, but the current draw didn’t drop to zero—it fell to 100 mA, which is the current consumed by the meter itself. It all adds up: 1800 mAh went into charging, of which 3 × 100 = 300 mAh was used by the meter itself, leaving 1800 − 300 = 1500 mAh, which is what’s claimed.


You can change the language in the menu. It’s easy enough to figure out, even by trial and error.


I forgot to mention the side button. A long press turns on the flashlight, which is basically just there to illuminate the clamp itself. A short press switches to the light theme, but since working with it is almost impossible, I think it’s just an alternative kind of flashlight :)


The main operating modes are selected with the left/right buttons; additional modes are activated with the center button.
1, 2. DC/AC current
3, 4. DC/AC voltage
5–7. Resistance, continuity, diode test.
8. Capacitance
9, 10. Frequency and temperature
11, 12. Phase detection and non-contact voltage detection.


1. A long press of the right button switches to wattmeter mode. It basically multiplies current by voltage. But while voltage and current are displayed in large characters, the power reading is tiny. Why didn’t they make the font bigger?
2–4. A long press of the center button switches to graph mode. I don’t find it particularly useful, since the readings update relatively slowly. And while it makes sense to have graphs for current, voltage, and resistance, I don’t know why there’s one for capacitance.


Menu.
Everything here is fairly straightforward:
1. Stores measurement results; press Save to record. It can’t work as a logger—the readings can only be saved manually.
2–4. Language, display brightness, and auto-off time.
5. Reset settings
6. Firmware version.

There’s also a calibration menu, but I don’t know the algorithm, so I don’t want to mess with it. It’s easy to open: hold down the left and right buttons and turn on the meter.



The non-contact wire finder works pretty well; it can detect a wire in the wall.
The screen has three indicators: blue means nothing detected, yellow means something is somewhere nearby, and red means a live wire has been detected with confidence. It triggers at a distance of about 2 cm.


The power button has another function that isn’t labeled: zeroing the reading. This only works in DC current measurement mode.
The thing is, DC current measurements are affected by everything, even the Earth’s magnetic field. Before measuring, you need to zero the clamp and then take measurements in exactly the same position. I usually do it differently, when possible: turn off the current in the circuit, clamp around the wire, zero the meter, then turn the current back on.
Below are a couple of photos. The clamp hasn’t been zeroed; it’s just lying on the table in two different positions.


A few tests
First, some measurements that aren’t related to metrology, but may still be useful.
1, 4. Voltage and current during resistance measurement—the lower, the better
2, 5. Voltage and current during diode testing—the higher the voltage, the better
3, 6. Voltage and current during continuity testing.

Basically, it’s like many modern meters—nothing unusual. Though I did like that the probe polarity is the same in all three modes. For example, on my Uni-T, the polarity is different for diode testing and the other modes, which is annoying.

The continuity buzzer is a little sluggish. I’ve more often come across one that either turns on and off quickly, or turns on a little more slowly and turns off more slowly. This one feels more like it turns on slowly and turns off quickly.
No, it works, and it works reasonably well, but I wouldn’t call it fast. It doesn’t respond at all to brief touches.



Checking voltage measurement accuracy. I checked it across the 1 mV–50 V range using the 1-2-5 sequence, with voltage supplied by the RD6012P.
No complaints. There is a small error, noticeable above 10 V, but I wouldn't call it critical.


I'm much more interested in checking current measurement accuracy.
Here, too, I applied fixed current values using the 1-2-5 sequence, starting at 10 mA. The meter can detect both 10 mA and 20 mA, but the readings are unstable: it might show 0.01, then jump to 0.03 A. It displays readings fairly reliably starting at around 50 mA, which is pretty good.
Accuracy is also fine here, though the reading starts to drift noticeably as it approaches 5 A.



But these currents are easy to measure with practically any multimeter. What matters more to me is how it performs at higher currents.
1. At 10 A, the actual current was 9.97 A, while the clamp meter showed 9.71 A. That's quite a bit off, in my opinion.
2–6. For these measurements, I wound the wire into 10 turns to get a current multiplier of ×10 relative to the current flowing through it.
And yes, the clamp meter noticeably underreads: at a current of 10 A (100 A), I got just 96.5 A instead of the expected 99.7 A...



My first thought was, no, Fnirsi can't be wrong; maybe my Uni-T or Riden is at fault. I got out the Owon clamp meter and ran the same test. Nope, three meters can't all be wrong—the Owon readings were almost perfect.


I got out the Bside and saw the same thing I'd seen when testing the electronic load: the readings were too high.


I got a more powerful 30 A power supply. With the ×10 multiplier, that gives an equivalent current of 300 A—half the scale, so it should be accurate.

I compared two clamp meters at once. Both are rated for 600 A, so it was a fair comparison. It turned out one reads high and the other low. That's how it goes, folks. Looks like I'm out of luck when it comes to properly testing my loads for now, and I've already got 200 A models planned.




I'd also like to point out a small oddity in its behavior. Right at the beginning, after unboxing it, I just wanted to take a few measurements for myself before taking photos for the review and so on. That's when I noticed something strange.
First, on most meters the display changes like this: 11.1, 11.0, 10.9, 10.8. Here it's a little different: 11.1, 11.0, 10.0, 10.9, 10.8.
Yes, there's a moment when the display shows 10.0 instead of 10.9, and only then changes to 10.9.

I also happened to notice that the clamp meter can keep displaying 10 A for quite a while when 11 A is applied—definitely up to ten seconds. I noticed it by chance, so the photo isn't very good. I tried to recreate the situation later with everything set up more neatly, but couldn't.
In any case, I recommend keeping an eye on how it behaves.



Resistance measurements with 75 Ω and 510 Ω resistors, and 1.6, 2, 47 and 180 kΩ resistors—no complaints.


Capacitance measurements with 1000, 1800 and 7850 pF capacitors, and 0.1, 0.39025 and 100 µF capacitors. The readings are a little high, but that may be because this meter has no relative measurement function, so you can't zero out the capacitance of the test leads.


Frequency: it's rated for up to 10 MHz. In reality, I tested it from 1 Hz to 20 MHz, and it gave up at frequencies above 19 MHz.

A small aside. Dear Fnirsi developers, just so you know: megahertz is written MHz, while mHz means millihertz.



An assessment of AC voltage measurement, using a square wave. From 10 Hz to 1 kHz, there are no particular complaints; at 10 kHz, it doesn't show anything.


A little about what's inside.
It's very easy to take apart, but make sure to remove the top half, since there's a spring fitted to the bottom half. It may be awkward to put back, and it could accidentally short something.

You can almost completely separate the top and bottom halves by disconnecting the measurement section's ribbon cable and the battery connector. The only connection that can't be unplugged is the small green wire, which is soldered in place.
I'd call the assembly neat, but I'll take off at least half a point for the single wire from the measurement section to the main board being pinched—the very wire I mentioned above.



The battery has a capacity of 1500 mAh, as advertised. I agree with other users: they could have fitted a larger battery, since there's plenty of room.


The measurement section, unfortunately, has the markings on the main chips rubbed off. The EEPROM chip is visible, as is the charger built around a 4056 chip.


A trimmer potentiometer turned up near the measurement section connector. I haven't tried adjusting it yet; I suspect it has something to do with calibration, but I'm not sure.
There's also a relay near the probe terminals. It switches on when measuring resistance, capacitance, and frequency.

The protection circuitry looks very basic, but without a schematic it's hard to say anything specific.



Of course, all this is fine, but I wanted to see how it would perform in a real test—that is, in a typical use case for me. A high-power load will help with that; I've just finished assembling the fourth one. This unit differs from the previous versions in a few ways: circuit changes aimed at improving performance, as well as some mechanical changes. I was asked to fit an XT-60 connector on the front panel, so it's now included as standard.


The rest is simple: a large 230 Ah battery, the load, cables, a clamp meter, a thermal camera to quickly check for any unwanted hot spots, and a stool :)

This load is calibrated at 90 A, as agreed with the controller developer to improve accuracy. I'm checking it at 100 A, the maximum my clamp meter can handle.


So, what are the results? As expected, the Owon reads 100 A, since that's what I used for calibration. The Fnirsi reads about 98 A. I increased the current to 120 A; the Owon went into overload, while the Fnirsi displayed 116.8 A.



The obvious question: is 98 A when the actual current is 100 A a large or small measurement error?
Even if we calculate it relative to 100 A rather than full scale, as is usually done, that's 2% + 10 counts. According to the table, the allowed range is 96.5–103.5 A, so the clamp meter is technically within its specified error. If we calculate relative to full scale, at 600 A the deviation is ±16 A.

When building loads, I try to keep the error within 0.5%, which at 100 A means a deviation of no more than 0.5 A. Of course, I'd like even better accuracy, but that gets noticeably harder. Even so, the Owon clamp meter can provide calibration accuracy at that level.
Yes, in practice, a 1% error would be enough at these currents, especially given how I use this series of loads. But as my grandfather used to teach me, it's better to do things as well as you can; doing them badly will happen on its own. So I try to get the best possible result.

So, unfortunately, I won't be giving up the Owon for now, especially since my repeat tests showed that it delivers more than respectable measurement accuracy. Maybe I'll try recalibrating the Fnirsi later, but I'm not happy with its initial accuracy.



Conclusions.
Subjectively, this is a decent clamp meter. Like many products from this company, it has a few flaws that I mentioned in the review, as well as some “teething problems”—for example, odd behavior when switching readings, a sluggish beeper, displaying mHz instead of MHz, and so on.
On the other hand, just because it's comfortable to use and even looks good, I can give it a pretty high score. The main thing holding it back is its relatively low current measurement accuracy.
On the plus side, accuracy is quite good in the other modes, it measures frequency up to 19 MHz in practice rather than the advertised 9.999, and it does a good job measuring low currents.

It's a shame there's no description of the calibration procedure. When you enter the calibration menu, it starts with some resistance values that automatically step upward. I don't know what comes after that, and I didn't want to experiment.

That's all for now. The store sent me a coupon for a small discount: FNSCD87. It's valid until 7.04.2025, and the price with the coupon will be $34.99