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OWON XDM1041 the unknown multimeter...

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davebb:
Hi yes it reads fine until about 10-20 mins and it follows another meter until it has been on for this time,
Thanks Dave

theHWcave:
@Kleinstein:  I examined the board carefully and I don't think there is another sensor. I think they rely on the HY3131.

 I recorded the displayed temperature with shortened probes when the meter is turned on after being off overnight. Over 1 hour it climbed from 18.9 to 27 degC. But not smoothly. There were very distinct steps. This could be something else interfering as I was not paying attention, so I will repeat that again. Right now it shows 26.9 with probes shortened. With a K-type probe (borrowed from a BM235) it shows 27.7 The BM869S with a pretty good (=expensive) K-type probe shows 26.9 at the same location.

I will do some  more testing tomorrow



davebb:
Hi Thanks
if you could run some more tests that would be good,
Thanks Dave

wizard69:
Thermocouple could junction compensation needs to happen as close to the cold junctions as is possible.    Doing so on the PCB might result in a significant error.   

The first thing that should be done is to determine where the cold junction sensor is.   As has been explained that should be relatively close to the banana jacks, if not it is a design fail.


--- Quote from: theHWcave on June 05, 2021, 10:44:44 pm ---@Kleinstein:  I examined the board carefully and I don't think there is another sensor. I think they rely on the HY3131.

 I recorded the displayed temperature with shortened probes when the meter is turned on after being off overnight. Over 1 hour it climbed from 18.9 to 27 degC. But not smoothly. There were very distinct steps. This could be something else interfering as I was not paying attention, so I will repeat that again. Right now it shows 26.9 with probes shortened. With a K-type probe (borrowed from a BM235) it shows 27.7 The BM869S with a pretty good (=expensive) K-type probe shows 26.9 at the same location.

--- End quote ---
This shift is excessive and could indicate a part failure or simply the cold junction sensor being it the wrong place.   I'm thinking hard here but if the cold junction sensor was heating up then I'm thinking the displayed temperature should be going down.     Generally the offset associated with the cold junction is subtracted from the millivolt value at the measurement point.

A short, zero millivolts, represents 0ÂșC for a K-type thermcouple.   You would then subtract the reference junction voltage but here is the problem, you don't have another junction in an all copper connection scheme.   It would be interesting to see the behavior of the meter with a K type TC at the ice point.   This would create the second junction to be compensated at the meter terminals.

If you have a room temperature of 20 degrees a shorted input should result in an approximate negative 20 reading.    This if there is no second junction to offset the reading.  In any event my mind is a big foggy here and I worked extensively with type T TC's in the past where the copper conductor eliminated one joint requiring compensation.   My point is that reading room temperature might not make sense with a short.

--- Quote ---I will do some  more testing tomorrow

--- End quote ---

One thing to consider is air flow.   IF this is indeed a reference junction compensation problem cooling the unit down after it heats up should result in fairly fast response on screen.

In any event I may need to think more on this.   I'm not at work at the moment so I can't set up anything to test the idea expressed above.

wizard69:
As a follow up to the above post I remembered that my new EEVBlog meter does K type thermocouples and it indeed reads room temperature with the test lead shorted.    This is causing me a bit of confusion as it implies that they are adding the reference junction temperature to the reading.

It is getting late so I may end up sleeping on this.    As I've said it has been awhile since I was heavily involved in TC work and that was mostly with type T.  In this case though both TC are in very similar millivolt ranges at room temperatures.

in any event drift is the problem here.

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