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OWON XDM1041 the unknown multimeter...
Kleinstein:
The thermocouple probe gives 0 V if the TC tip is at the same temperature as the cold junction. So the cold junction temperature is added, not subtracted. The type K TC constant is relatively constant, so one can assume adding the temperatures. It actually is adding the calculated voltage to take into account a possible nonlinear TC response. Here the simple approximation of a constant thermopower should be OK.
So a copper short reads the cold junction temperature used by the meter.
The cold junction sensor away from the actual terminals is a weakness in the design, but with the normal banana plugs it is anyway difficult to get an accurate measurement. There is a good reason the for accurate TC readings they use special connectors.
Measuring the temperature at the volts input terminal would be very tricky with isolation.
The readings of 26.9 - 27.7 °C for the different probes and with 2 meters are not that bad. With the current warm weather and some self heating from the meter this sounds reasonable. Melting ice can be used as a reasonable reference temperature. The other simple test point is boiling water.
With the standard plugs one has to accept a few degree of uncertainty. TC probes are not made for high absolute accuracy and the 4 mm plugs and CAT rating of the meters add another complication. The initial reading can be off, as the meter may need to warm up before getting good readings. With the sensor at the main chip this would be normal. With turbulent air flow there can be some temperature variations at the chip. The heat up curve can be a bit more complicated than just a simple smooth exponential and the temperature reading my not be very frequent.
theHWcave:
The OWON XDM1041 temperature weirdness
I did a few measurements this morning and here are the results. Note that all this about firmware version 1.2. The graphs were taken by polling the measured value via SCPI every second into a spreadsheet.
From the cold-start and warmed-up graph you can see that the meter looks at the internal temperature sensor every 600 seconds (10 minutes). During that time (about 3 seconds) the meter does not respond to SCPI polls, i.e. 3 polls time out but at the 4th one you get 4 results (i.e. it buffered the responses internally). Note that this “pause” on SCPI every 600 seconds only happens if you are in temperature mode. If you are in millivolts for example there is no pause.
Interestingly, the very first temperature measurement after it updated the value it holds as cold-junction temperature is always a bit high. Not sure why. This you can clearly see in the warmed-up graph.
In temperature mode, the meter displays the “measured” value at the bottom of the display (what a brilliant idea! Seriously why don’t other meters copy that). This allows an easy check if it is “doing the right thing”.
Example: With just cables to my millivolt source plugged in, but no external voltage applied, it reads 28 deg. and -0.02 mV Using the NIST tables / polynomials for K-type, 28 deg. corresponds to 1.122 mV. This it stores as the cold junction offset (for the next 10 minutes).
If I now apply +1 mV to the input, the meter shows 52.7 deg.C and 0.98 mV. The meter adds the 0.98 to the stored 1.122mV and gets 2.102 mV. According to the NIST polynomial equation that would be 51.9 deg.C. Ok so its off by 0.8 deg. Maybe they use lookup tables instead of the reasonably complex maths.
If I apply -1 mV to the input, the meter shows 2.8 deg. C and -1.03mV. Doing the same calculation, the resulting voltage is 0.092mV and the NIST value 2.3 deg C. Again, 0.5 deg. off but I think in principle the meter is doing the right thing.
@davebb: If your meter shows the right value at startup but not some time afterwards, it seems it may not do its 600s refresh of the cold-junction temperature. The best way to find out is to repeat the same test I have done for the cold-start graph and see if you get that staircase (or the spikes in the warmed-up) graph. You could use the OWON software and its recording function but I am not sure how it responds to the “pause” after 10mins. Better to use a small script and simply send every second: “MEAS1?<LF> “ and store the response
theHWcave:
@davebb: coming to think of it. An easier way to check if the cold junction temperature is properly updated is to start the meter after it was off and cold with shortened probes and wait. Over 30-60 minutes, the temperature reading should increase (by several degrees) as it heats up internally.
Kleinstein:
The rather slow update of the cold junction temperature is a bit anoying. So one has to essentially wait at least some 30 minutes for warm up.
In addition there seem to be a few point's that are off just after a update.
I wonder if one could force an update by switching between a mV rading and a temperature reading. If the meter does not measure the temperature in mV range it may do an internal temperature reading just after changing the function. At least with program control one could this way force the internal reading more often if needed.
theHWcave:
A very good point and I tested that just now. Sadly the 10 minutes cycle to refresh the cold junction temperature reference seems to run independent of what mode you are in
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