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Voltcraft VC940 (alias Unitrend UT71E) problem - false readings

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Ricardo:

--- Quote from: csadam on March 20, 2012, 10:14:51 pm ---New problem appeared, or old problem but I just noticed it now :)

I measure 1.000 volt. It displays 1.000 volt. OK.
But when I activate the PEAK mode the displayed value changes to 1.400 Volt. Then I deactivate the PEAK and it shows 1 volt again.


--- End quote ---

I own a UT71E Multimeter and it has the same problem as you describe with the peak mode. This mode is useless, I think it is a software bug. It also has other strange problems, perhaps you might check yours:
1: When doing AC-Current measurement, if a small value is measured in a fixed measurement range,  it is not displayed if value is < 200 digits, displayed value is 0. E.g. if 10A AC range is used, the Multimeter can not display values <200mA in this range, it jumps to 0, while >200mA is displayed correctly.
2. In tempereature measurement range, there is a discontinuity fom 0°C to -2°C, no changing values are displayed between, it jumps from 0.0 to -2.0 or vice versa. The problem is also shown on fahrenheit-dispay, jump points are 32.0 to 28.4.

I must say as other users do, the UT71 series seems to be very unreliable, I would not buy it again.

Bored@Work:

--- Quote from: Ricardo on March 23, 2012, 01:57:46 pm ---1: When doing AC-Current measurement, if a small value is measured in a fixed measurement range,  it is not displayed if value is < 200 digits, displayed value is 0. E.g. if 10A AC range is used, the Multimeter can not display values <200mA in this range, it jumps to 0, while >200mA is displayed correctly.
2. In tempereature measurement range, there is a discontinuity fom 0°C to -2°C, no changing values are displayed between, it jumps from 0.0 to -2.0 or vice versa. The problem is also shown on fahrenheit-dispay, jump points are 32.0 to 28.4.

--- End quote ---

Sounds like they try to fake a stable reading of Zero. I think the UT71E uses an off-the-shelf multimeter IC. You could try to find the datasheet and check if this is a known feature of the chip, and if it is maybe a configurable option.

saturation:
Sorry to jump the gun, it represents Fluke's attention to detail, at least with DMM. 

Keeping an in house safety testing lab is expensive unless they use such labs frequently, which implies they do more frequent quality checks on their production, which suggests high quality control.  Most folks oursource such testing to dedicated labs.

http://www.dtbtest.com/

If Fluke can spend this much on safety testing, its very likely less costly quality testing of boards, components, and cases, are done with much attention too. 




--- Quote from: csadam on March 22, 2012, 07:51:21 pm ---Interesting video.
But I'm not sure how this Fluke promotion relates to my problem  ???

--- End quote ---

csadam:

--- Quote from: Ricardo on March 23, 2012, 01:57:46 pm ---
--- Quote from: csadam on March 20, 2012, 10:14:51 pm ---New problem appeared, or old problem but I just noticed it now :)

I measure 1.000 volt. It displays 1.000 volt. OK.
But when I activate the PEAK mode the displayed value changes to 1.400 Volt. Then I deactivate the PEAK and it shows 1 volt again.


--- End quote ---

2. In tempereature measurement range, there is a discontinuity fom 0°C to -2°C, no changing values are displayed between, it jumps from 0.0 to -2.0 or vice versa. The problem is also shown on fahrenheit-dispay, jump points are 32.0 to 28.4.

--- End quote ---


Hi Ricardo

I tested your 2. problem and it is the same at me. So it is very likely a software issue in the controller.

BoredatWork,
I've done that. The measuring IC takes a command to switch to a mode, then it starts to send the measured counts. In temperature measurement mode it is switched to 400mV DC range. There is no compensation at measuring level. Every correction is done by the controller.
Seems you are right with the fake zero, but there is also a resolution issue.

I've simulated -2°C to 0°C and sniffed the communication. I've found that the the display switches to 0 when the measured counts are around 0 and 8. But there is many missing values there, the measured counts vary between -4, 0 and 6 when the display switches to 0°C. Then the measurements linearly increase until 16  and just then switches the display to 0.2°C. So it shows constant 0°C. from -4 to 16 measured values.


Based on the above I suspect buggy firmware. So my next step is to get an MSP JTAG and read the software. I hope it is not protected :D

Thanks for the replies

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