When diagnosing electronics, I often wished I could make current measurements without having to bridge out a component, or cut a wire. The inconvenience and potential safety issues are sometimes overwhelming and I just decide not to bother measuring the current at all. However, there is a way around that by measuring the magnetic field developed around a wire to determine the current flowing through it – namely, a clamp meter which works basically like a current transformer or hall effect sensor might.
While there are many models of cheap clamp meters available on the market, all of them only measure AC current using the clamp. AC current is relatively easy to measure, as the signal alternates back and forth and thus the necessary value can be determined from peak to peak response without regard for absolute measurements. DC current does not have such a property, and thus needs a unit that can measure the magnetism absolutely (across a range of field strengths) and is stable and sensitive enough to correct the background field by basically subtracting its value.
Of the units on the market, the most inexpensive seemed to be the Uni-Trend (or UNI-T for short) UT210E, for about AU$45 online.
The unit was shipped in a colour print cardboard box in their traditional red and grey colour scheme. Unfortunately, it wasn’t treated well by the postal service, and instead, became somewhat crumpled at the corner.
Luckily, the item was still safe, packed inside a zippered case, included in the box.
Inside the case, you are provided with a set of test leads which meet CAT IV safety ratings for 600V and CAT III for 1000V. Aside from that, you get the clamp meter itself. The meter range and mode is selected with a rotary thumb wheel and the clamp itself is a swing-open spring-loaded clamp marked with its geometric center and current flow direction convention. A warning LED is provided for high voltage, and an LCD provides the readings. The main unit only has a CAT III rating of 300V and CAT II rating of 600V, which may be a little close for comfort for certain mains voltage work.
The clamp itself is also marked with the safety ratings, and a reminder that it is suitable for 100A current.
The base of the unit has two banana jacks for connection of the probes. These are a little close together, as necessitated by the size of the unit. As a result, this unit is not environment-proof, so best not to use it in wet or damp environments. They are only used for voltage/resistance/diode/continuity checks, and no current measurements are preformed this way.
The LCD is read horizontally, with large four digit display and icons for indicating its other features, including non-contact voltage detector (NCV), variable frequency compensation (VFC), data hold (H), buzzer-continuity, true-RMS (Trms), low battery and auto-power off.
The rear has a battery compartment, secured by a screw. While it does keep the batteries from falling out, or from the compartment door being lost, it does make replacement somewhat inconvenient.
It requires two AAA cells, and two have already been installed at the time the unit was shipped. Unfortunately, the included cells are rather ordinary zinc-chloride type, which are prone to leaking once depleted – so best to replace with alkalines and remove them when not in use.
Also included is a warranty card, and a double-sided leaflet of instructions, although all in Chinese. Luckily, an English PDF manual is available here.
Operation of the meter was expected to be straightforward, however, there were some interesting differences from an ordinary meter.
The first had to do with DC current clamp measurements – becausae the unit is measuring absolute magnetic field, the meter has to be zeroed before any measurements can be made. Because it is so sensitive, it must be zeroed in the same direction as the measurement is to be made, with a minimal movement of the meter being recommended. As a result, you should hold it towards the wire with the wire outside the clamp in a position to read the screen, press the zero button, then clamp the meter over the wire carefully, making your best effort to align the wire with the geometric center of the clamp as marked by the arrows. Readings made quickly typically show good agreement with actual currents, although over time, the readings will drift (probably due to magnetization of the clamp head itself) and small readings are normally not reliable (as the claimed accuracy is 2% +/- 8 counts).
The backlight is also transient, to save on battery and reduce reading drift, but can be inconvenient where longer term lighting is desired.
The auto-power off feature is particularly annoying, as it will power off the meter in 15 minutes with no changes to settings. Disabling it entails holding the select button while powering on, but this is not persistent and has to be done every time you wish to disable the auto-power-off facility. Even when disabled, a few minutes prior to the 15 minute mark, the meter will beep its internal buzzer, and again at the 15 minute mark, and then every period after. It seems like the code to disable the feature was done in a sloppy and incomplete way, so while the meter doesn’t shut down, it can’t stop beeping. At least, you’re less likely to leave it on and with flat batteries that way.
The auto-power off feature comes in its own in ruining your day when combined with DC current clamp measurements, as it also resets the zero correction once the meter is woken up from sleep. As a result, if it sleeps, it must be unclipped, zeroed, and re-clipped to resume monitoring. A pretty nasty design issue, especially if you want to leave it clipped over a wire (e.g. from a solar panel), run out, do something, run back in and see what the reading is.
In no way is the buzzer disable-able short of removing the cover and cutting it out. As a result, when voltages exceed a certain threshold, the meter will beep as a reminder for safety.
Non-contact voltage detection worked, with beeping progressively more rapid depending on sensed field strength and an increasing number of dashes on the display. It’s important to remember that this does not necessarily correlate with the voltage carried in the wire though.
DC Clamp Accuracy
Seeing as this is the main claim to fame for this unit, I decided to try and give the accuracy a quick check. To do this, I used a Manson HCS 3102 power supply with my BK Precision 8600 electronic load. Wires connecting the two were passed through the clamp and the load left switched off for zeroing. The load was then set to a value, powered up and the (immediate, or second) reading displayed on the clamp was recorded. Additional time for the reading to stabilize was not provided, because it seemed to drift with hysteresis/magnetization of the clamp material.
The accuracy specification was 2% +/- 8 counts – and it’s fairly clear that it exceeded this accuracy. On an absolute reading, it only reached 8-counts of error by 0.35A, and at that time, that magnitude of error corresponds to about 2.3%. As a result, it seems that all my readings were within 14 counts of actual value, and maximum error was about 3.1% at low current values, falling as the current value increased. Not bad overall, and could possibly rival cheap meters, especially because adding burden resistance to a circuit can affect the operating point and produce strange results.
I thought to be a little devious and wrap the wire four times around the clamp, running in the 20A range passing 4.9992A through the wire. The expected reading was 19.9968A, the displayed reading was 20.06A, or about a 63.2mA error for a simulated 20A despite the less than ideal positioning of the wire. Its usefulness as an indicative tool is quite clear, but hey, it also proves the whole idea of induction, electromagnetism and coils/number of turns.
The Uni-Trend UT210E is a reasonably priced clamp meter. Complete with a protective case, test probes and batteries, it is ready-to-go and can measure voltages and resistances/diodes by direct probing, as well as current up to 100A by clamp. It also has a non-contact voltage detector. The display is large with blue-white backlight, and the warning LED quite apparent. It also has a firm clamp with adequate jaw separation.
While the specifications for accuracy are not that high, especially for DC current clamp and variable frequency drive AC measurements, that’s reasonable given the limitations of the technique. The unit met and exceeded its accuracy specifications under testing, and provides a very convenient feature at a reasonable price. Unfortunately, it doesn’t have a back-up shunt based measurement in the same unit, so if a more accurate reading is necessary, reliance on a traditional multimeter is necessary.
The CAT III 300V rating may also be a little on the low side for comfort with some mains work, and the auto-power off feature with buzzer is both quirky and at times, annoying. The zero nulling of the DC current clamp measurement mode does drift over time even maintaining the same orientation, and the response for current shows signs of not being instantaneous (likely due to the hysteresis in the material used to make the clamp).
Despite this, it is still a very versatile tool to have in your toolkit for quick magnitude-and-direction of current checks.