Products > Test Equipment
OWON XDM1041 the unknown multimeter...
Kleinstein:
The type K sensor is a thermocouple and it thus makes no realy sense to look at the reading, when open circuit. This would be effekted by the input bias and the relatively high input impedance (likely 10 M or even higher).
It make sense to look at the reading with shorted inputs. This should read the internal temperature used for the cold junction. Dave has a relatively recent video on this.
davebb:
Hi Thanks for the info, is the internal temperature used for the cold junction in the HY3131 chip or is it external to the chip,
Thanks i am trying to find out what is going on as i do not rely want to send it back to china as the meter is working well apart from this problem ,
Thanks
Dave
Kleinstein:
Connecting a type K thermocouple just to the banana plugs is always tricky, as the banana plugs are usually copper / brass or similar. So the cold junction would be somewhere out at the connector. So it really depends on how the connections are made and it could be just a poor probe quality. The test with just a copper short would be more reliable than with a real probe. Some banana cable are also not that great and could cause a few µV of offset if one connector gets warm.
Chances are there is some sensor near the COM terminal (due to isolation requirements it would be hard to measure at the other connector).
One could test this by using a short with a copper cable and than warm up the COM connector (e.g. touch / warm with the hand). The closer the sensor to the terminal, the faster it can react.
For an accurate reading a PT1000 or NTC temperature probe would be better. Thermocouple with just 4 mm connectors is more like a crude relative estimate with some extra uncertainty.
P.s. : The linked teardown pictures don't show an extra sensor near the connector. So likely the HY3131 internal temperature only - ideally with some offset in software.
davebb:
Hi Thanks for all that info, i have shorted out the input and yes temp reading is good but after about the same amount of time the temp goes up high about the same amount, just like it does with a probe, There is a relay in line in the circuit, so the next thing to try is to link it out to see if it is that,
Thanks Dave
Kleinstein:
I don't expect the relay to be the problem. Chances are the relay is a latching type and not getting really hot.
The more likely problem is that the whole meter gets warmer with time. With a copper short the meter should show the temperature of the connector(s). So some rise above the room temperature is correct. There is still a chance that the correction for the meter getting warmer is not correct (e.g. to much, as the chip likely heats up more than the connector).
With no sensor at the connector the meter can not reliably get that temperature - the cables can have quite some effect on the temperature at the connectors. So one can not expect a really good conpensation of the cold junction.
Besides the problem inside the meter, it is also the question where the actual cold junction is. Ideally it would be on the inside of the terminals, like the TC wire going all the way to the tip if the plugs or plugs made from thermocouple equivalent material. The more likely construction is having normal cheap plugs made from brass and thus a cold junction a bit outside. This could add some error, though likely less than 50% of the cold junction variations. So I don't hink the probe could explain a similator effect as with the short.
Is the thermocoule at least reading external temperature changes about right (e.g. some hot/cold water) ?
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