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Input drift in the DrDAQ?

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Input drift in the DrDAQ?

Postby Glovisol » Wed Apr 22, 2015 1:25 pm

I have been working on the DrDAQ barograph project (see post) for over 2 weeks, doing extensive tests to verify system accuracy and stability. From time to time I (seldom) noted unexplainable drifts in values of pressure shown, which apparently depended on shifts of pressure cell output voltage. Cell and ambient temperatures are accurately monitored, so the drifts could not be accounted for by temperature variation. Finally I resorted on installing a calibrated DVM at the DrDAQ EXT. 1 input and today proved that the variation is due to long term drift in the DrDAQ output. After several hours, while the DVM was constantly showing 1,060 mV from the pressure cell, the DrDAQ output shifted from the initial 1,060 mV down to 1,032 mV. Switching Picolog off and then turning it on again still leaves the reading at 1,032 mV with a stable error of 28 mV. I wished to upload the relevant PLW file and PC screens, but unfortunately the Forum software is temporarily out of order and no uploads can be effected.

I checked the input/output of the buffer Op Amp of the Universal Interface, but the buffer is O.K. and does not show any differential between input and output, so the only cause seems to be whithin the DrDAQ/Picolog combination. Will now check if this problem is due by an unexpected drift in the USB +5V of the PC and will report on the findings.
Last edited by Glovisol on Mon Apr 27, 2015 12:39 pm, edited 1 time in total.
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Re: Input drift in the DrDAQ?

Postby Glovisol » Thu Apr 23, 2015 8:35 am

Permission to upload attachements has now been restored and here is the PC screen of the long term drift in the DrDAQ. It can be seen that the cell voltage shown is constantly decreasing, with temperature nearly constant and with the external DVM constantly showing 1,060 mV. This drift does not appear at all times, but once in a while and on long runs, so it is very difficult to put the finger on the trouble....

This problem did not manifest itself in many previous runs (see posts on the DrDAQ Barograph) but only two or three times. I have now shifted the cell voltage from EXT. 1 input to EXT. 3 input and am doing another run, as it could just just be a mal-function of the EXT. 1 input alone and will report once this run is concluded.
Attachments
DrDAQ Drift.jpg
Apparent long term drift in the DrDAQ
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Re: Input drift in the DrDAQ?

Postby Glovisol » Mon Apr 27, 2015 6:42 am

TESTS DONE - THERE IS NO DRIFT ON THE EXT. INPUTS OF THE DrDAQ

In order to verify the stability of the DrDAQ, firstly I accurately checked the external control DVM and found a problem: on the 2 V range there is an intermittent drift to higher or lower readings, probably due to a worn contact on the rotary switch.

I then verified the connection between the pressure sensor and the DrDAQ and found a possibly faulty contact. In the test results shown in the previous post, most probably the DVM was drifting up, while the faulty contact was causing the reading of the DrDAQ go down.

Finally I then parallel connected the three EXT. inputs of the DrDAQ to a voltage standard of 1.9V based on the (Linear Technology) LT1021-10 IC, with a step down precision resistor divider. This IC provides a 10 V output with a long term stability of 15 PPM (0.15 mV@ 10 V and 0,030 mV @ 1.9V). I did a continous test for 96 hours (4 days) constantly reading 1,900.00 mV on all three inputs, with some fluctuation on the decimals. This test was done at a constant ambient temperature of 18 °C +/- 0.5 °C.

CONCLUSION: THERE IS NO NOTICEABLE MEDIUM TERM DRIFT IN THE DrDAQ. The DrDAQ can reliably measure voltage with excellent medium term stability.
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