Bad readings in static calibration

[kenkar]

I want to sample four grounded and one floating analog signals together. Two channels are grounded signals from hot wire anemometry electronics. The third grounded signal is an amplified thermocouple signal. A pressure transducer gives the last grounded signal. The floating signal measures the voltage drop of a heating element which is powered by a transformer, i.e. at 50 Hz. I am using BNC-2090 rack-mount BNC accessory with the MIO board, PCI-MIO-16E-1. I have configured the BNC-2090 and MIO board to DIFF mode.

All channels were defined in MAX Explorer measuring voltages with no scaling, except the one thermocouple channel whose output was conditioned to 10 mV/�C, so I applied a linear scaling to it.

I used the intermediate VI to acquire the multiple waveforms because it allowed me to set the interchannel delay. My VI was based on the simple-buffered analog input with a write to spreadsheet file example in figure 7-8 in LabVIEW Data Acquisition Basics Manual.

Before I collected any data from LabVIEW, I ran the E-Series Calibr VI to self-calibrate the MIO board. When I did a static calibration comparing the acquired data to known references, I found that the values collected from the pressure and thermocouple channels were significantly different to the references (e.g. 0.54V vs. 0.50V). To troubleshoot the problem, I had switched to use the easy VI- AI Acquire Waveforms instead. I did have the correct values from the pressure channel but not the thermocouple one. Because the thermocouple output was conditioned to 10 mV/�C, I had a BNC splitter connected at the output of the thermocouple amplifier. One end connected to the BNC-2090 and the other end connected to a digital voltmeter. During the tests the temperature was kept constant, the digital voltmeter always read 185 mV (correspond to 18.5�C), whereas the average value (to remove the effect of noise) from LabVIEW varies from 18.5�C to 21�C. LabVIEW only gave false values at high flow speed, a condition which made the voltage outputs in all other channels increases significantly. How do I solve the problem? Any suggestions are welcomed.

[Filipe A.]

Hello;

As you are dealing with temperature systems, that are slow by nature, you can increase the interchannel delay to the maximum values that the sample rate you chose allows.
Furthermore, you can even decrease the sample rate, and interchannel delay for test purposes.
Hope this helps.
Filipe

[kenkar]

The system I am measuring will be fast-changing, a step change only takes about 0.4 s to settle. I will try to increase the interchannel delay for test purposes, though I don't know by doing so how much error in the measurements will be introduced.

Do you think it was a crosstalk problem?