What does it do?
When measuring continuity, modern multimeters have the annoying habit of using their ADC to check if there's continuity or not. Depending on how fast that ADC is, how long it needs for sampling and how good/bad the software inside the multimeter is, modern multimeters paradoxically often have a much longer delay before they actually detect continuity than old meters (which did that in hardware, no ADCs involved). I bought a UNI-T (Unitrend) UT71E, a very fine quality meter with great specs for a great price -- however I was extremely disappointed over the most crappy continuity measurement I have ever experienced. So I decided, not knowing if that was my prejeduce or not, that I would measure the continuty. In other words, I built a continuity tester tester.
How does it work?
It's extremely simple: A generic microcontroller circuit (I used a Atmel AVR Tiny2313) controls a relay which shorts out the leads of a multimeter. The multimeter is in continuity mode. As soon as it starts to beep, a microphone registers that. If the noise is above a certain threshold (adjustable by pot), a comparator triggers and signals the MCU that audio was detected. The MCU then deenergizes the relay and starts from the beginning. To cancel out the delay caused by the relay itself, a second switch inside the relay is fed back into the microcontroller. Measurement only starts when the relay has really switched.
The results were quite interesting. I compared a really really cheap meter (10 EUR or something from a local grocery store - seriously) to a Mastech PM334 to the Unitrend UT71E. The cheap meter actually is a "King Craft MD10759", however since I found that name out after I made all the graphs (it says only on a tiny sticker in the back of the meter), I will from hereon call it "El Cheapo". It is the only meter with a real continuity tester and therefore the absolute clear winner in this test. The Unitrend has a "fast" 4000 count mode and a "regular" 40000 count mode. I tested both, the effect on the continuity seems to be non-existent, however. Also, all meters with ADC show weird artefacts in the measurement graph (sawtooth pattern). I double and triple checked this, this is actually correct. There seems to be a beat inbetween the testing frequency and the sample frequency of the meters causing that pattern. If the delays are non-constant, but a random pattern is used, the graph also looks noisy. Look for yourself:
|Meter||Number of Measurements||Avg. Delay||Min. Delay||Max. Delay|
|El Cheapo||46||1.2 ms||1.2 ms||1.2 ms|
|Mastech PM334||200||65.9 ms||26.0 ms||103.9 ms|
|Unitrend UT71E 40000 Counts Mode||100||314.1 ms||224.4 ms||378.6 ms|
|Unitrend UT71E 4000 Counts Mode||50||329.8 ms||200.2 ms||393.3 ms|
When trying to switch to a new working horse, the UT71E was my choice. However, after the first day I returned it immediately, since one of the functions which I need often and which need to work well is the continuity measurement. 300ms may not seem like much, but - trust me - it is absolutely horrible and useless for doing real work. Actually the Mastech also seems quite bad to me, but the Unitrend performs 4 times worse. Oh, and see where the red line in the first graph is? No? That's because it is at the very bottom with its 1ms response time.
I am absolutely baffled on how this crappy performance can be achieved. The Unitrend UT-61 seems to perform much better (from what YouTube videos show), has a much lesser accuracy however. Why can't they build a UT71E and include a simple stupid El Cheapo continuity tester? The whole El Cheapo meter cost less than 10 EUR, so it really cannot be that hard to do. Very disappointing since the UT71 does make a otherwise very good impression on me (feels very robust, has a nice grip, switching feels nice, display is pretty). I hope Unitrend do that better with their next meter, if it has the UT71 specs, I'll surely buy it.
- Schematic of the testers as PDF
- Schematic of the testers as Gschem Schematic
- PDF of the "all results" graph
- PDF of the "El Cheapo" meter
- PDF of the "Mastech PM-334" meter
- PDF of the "Unitrend UT-71E" meter in "regular" mode
- PDF of the "Unitrend UT-71E" meter in "fast" mode
- All raw data and scripts I used to create the graphs