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OWON XDM1041 the unknown multimeter...

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davebb:
To test the internal chip sensor i used 2 good 4MM plugs and the copper core from some rg213 coax cable between the to plugs so is very short,
 The meter has been running a long time now and is reading 35.2 with the k type , and 33.4 c with the short,
the room temp is 25.5c
Thanks Dave

Kleinstein:
The socket inside the meter should get warm. So the 33.4 C for the short are plausible. The strange thing is the low voltage measured for the thermocouple. So it the the thermocouple working, e.g. showing about the right difference for something like boiling hot water or ice water.
The numbers shown would point to a failure with the probe or maybe the 50 mV voltage range. Chances are the temperature measuremetn would use the 50 mV voltage range.  So one could do a test with a more or less stable voltage in the 20-40 mV range and measure this in both the 50 mV and 500 mV range.

davebb:
Hi thanks for the info yes the k type is working, it does not jump temp on any other meter  i have tested with other k types and get the same problem,
i have now tested with 30.842mv from a psu and when i ended the test it was 30.927mv showing at the end of the log file When i switch over to voltage,
and that was correct with another meter,
Thanks all for your Help
Thanks Dave Browne 2E0DMB

theHWcave:
I am using a K-type that came with the BM235. It has 2x 4mm banana plugs on one side and the K-wire pair comes directly out of the other end. For my BM869S I use a specially made adapter that has 4mm banana plugs on one side and a small 2-pin socket especially for thermocouples on the other side. My RS-component K-type  thermocouples I bought for this meter end in those plugs that fit the sockets in the adaptor.

In both cases the cold-junction (i.e. the non-sensor end of the K-type wires) actually happens outside the meter.  Depending how “hot” the XDM1041 terminals are and how well the banana plugs conduct that heat, the cold-junction may be quite a bit lower in temperature than what the XDM1041 thinks it is. In handheld meters this seems to be less of a problem because they are seldom on for long periods and generally more power optimised. This means the inside temperature of a handheld is closer to ambient.

This could well be the problem of your meter which seems to run hotter than mine. I have never seen more than 28 deg after hours of operation but you seem to run at 31 or so.  It shows correct for the first couple of 10-minute cycles until it adjusts the cold-junction to 31 deg which is way higher than the real ambient temp.

The first question is why does it get hot inside? The power supply is always on, regardless of the power switch so that is not it. The HY3131 with the actual temp. sensor consumes max 0.5W and the processor even less. In the end I had to open the meter and examine it with a crude self-made thermal imaging camera (MLX90640 sensor & Raspberry Pi). The only thing that gets quite hot is a 1 ohm SMD resistor next to the LCD connector. When I turned the backlight to the lowest setting, the heat disappeared.

So here is a test you can do. Run the meter at the lowest backlight intensity and check if you getting better accuracy. I have done this now for 30 mins and the temperature has dropped by 2 deg.

Although I think mine is generally ok as long as I use the BM235 probe which probably gets warmed quickly through the sockets, I am seriously thinking of opening some of the “vents” at the back with a drill. They seem to be more style than function.

davebb:
Hi
ok i will try lowering the back light and see if that helps
but i did run the meter open and it did not make much difference,
Thanks for your help
Dave

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