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.