CG K/ J/ S/ B to 4-20mA 0-1800 Celsius Thermocouple Converter TC Input 4-20mA Output Head-mounted Temperature Transducer – first impressions

CG K/ J/ S/ B to 4-20mA 0-1800 Celsius Thermocouple Converter TC Input 4-20mA Output Head-mounted Temperature Transducer.

So the concept is that one can program the transducer module for a range of input sensors, and lower and upper limits to the conversion range. Another significant benefit is that it does cold junction compensation (and it does not seem possible to disable it if you wanted)..

Yep, the software (which you can only get after making a purchase… gotta be a bad indicator), has an English language option but reverts to Chinese every time it starts. The software is glitchy, loocking up sometimes, so that exacerbates the problem.

The tranducer received was correctly configured as selected, and labelled.

I also purchased the relatively expensive programming cable as I could re-program the thing to my requirement.

So, here are the specs from the Aliexpress listing.

Yes, there are errors in there.

The programming cable does appear to have electronics in it, it instantiates a CH340 USB-serial driver on the port. The transducer does have a mini-USB jack on it, but it is NOT USB, the Chinese often appropriate standard USB connectors for non-USB use.

The programming software was a bit tricky

I interpreted the graphic above statement of response time of 1s as meaning that it provided valid output within 1s of power up (which is relatively slow compared to ‘fixed’ transducers… but this no doubt has a microcomputer doing smart things.

Above is a scope capture of applied 12V (blue trace) and the voltage across a 100Ω load resistor (red trace).

In fact, the 4-20mA output is updated about every second or so, but the response time for the cold junction sensor is much longer than 1s.

The red trace is quite noisy, and the noise seems to depend on the start up process inside the transducer… demonstrating that the noise comes from inside the thing. It may be possible to filter this noise externally.

The other thing that the capture shows is that is takes almost 2.5s before the output stabilises at the correct value. It is very slow (compared to ‘fixed’ transducers).

Many of my applications are battery powered, and they wake occasionally, power the 4-20mA loop, make a measurement, shut the loop down, report the measurement and go to sleep. For the other projects with ‘fixed’ transducers, I allow 400ms for the loop current to stabilise, this transducer needs 3s or more, 8 times the power consumption.

So, it is possible to program the sensor type and the upper and lower limits for the 4-20mA output, and these can be read back from the transducer at a later time. It is also possible to program a correction to be added to the measurement. It seems most of these settings can be read back from a programmed tranducer, but the English language manual does not explain most of them.

A test with a Pt100 RTD

Above, a PID temperature controlled stirred beaker of water to provide a stable measurement target to explore jitter on a Pt100 probe.

Above is a plot over an hour of the temperature controlled water bath. Note that the jitter is due to both the temperature regulation system and the measurement system. For this dataset, mean is 40.43° and standard deviation is 0.10°, that sort of total jitter would be quite acceptable for many purposes.

Trial run measuring soil temperature with a Pt100 RTD

Above is a capture with the probe pushed 100mm into a garden bed. This may form the solution for longer term monitoring of soil temperature to inform planting.


The following conclusions are made:

  • the configuration / diagnostic software client has an English option but reverts to Chinese language on each start, with more emphasis on visual effects than function and reliability;
  • the programming cable is proprietary, and inexpensive for what it is;
  • the cold junction compensation in a basic test of a type K thermocouple seemed to work, it is slow, but it is more important that it tracks the cold junction accurately than that the system responds to ambient changes quickly;
  • in concept, a transducer that is configurable for a wide range of sensors / inputs, and programmable scaling to 4-20mA which seems to work for the type K thermocouple case tested;
  • the transducer is relatively inexpensive, and the flexibility is a plus;
  • the accuracy of conversion from any of the input sensors to 4-20mA was not tested.

I purchased two pairs of cable and transducer, one pair described at EARU Programmable Thermocouple K J PT100 to 4-20mA Converter TC RTD Input 4-20mA Output Head-mounted Temperature Transducer – first impressions and this CG branded pair.

I would not buy EARU product again, but I have purchased some more of this CG branded product.