Why differnent channel can affect each other in voltage measurement using 2532

Question

I use PXI 2535 to measure lots of channels (connected with electrodes) voltages using the sample vi: niswitch DMM swtich synchronous scanning.vi. Among these channels, one channel was  -0.6V and other channels were about -1.0V. But after running the vi, the -0.6V turned into about -0.9~-1.0V. It is very strange. I connected another electrode (-0.6V) to a channel (all other electrodes still connected), whether I used the soft front panel or the sample vi its voltage all turned into about -1.0 V. I didn't find this phenomenon when I used 2532 finished the above measurement. I suspect there was current flew between channels with high and low voltages.    
The red line (HI) was connected to r0, and the black line (LO) was connected to a reference electrode. The V diffence between different  channel vs the reference V were recorded.
When I measure voltages with low difference (lower than 100-200mV), each voltage can be measured correctly (using 2535).
thanks advance
小木虫
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Answer 1

 I used a setup shown in Fig. 1 to measure potentials of 121 electrodes (120 zinc wire and 1 iron wire, 1mm in diameter, embedded in epoxy resin, each electrode allowed one surface exposed) in NaCl solution against a reference electrode (with a constant potential). The HI line of DMM was connected to r0 and the LO line was connected to the reference electrode.  
Fig.1
These 121 electrodes were connected to the terminal of 2535 with 5 pin Ribbon cables, each of which has 25 lines.Generally, the potential of Zn is about -1.1V and -0.5-0.6V of Fe in NaCl solution. I used the niswitch DMM switch synchronous scanning.vi, sample of LV8.5 to measure these potentials. But I found that the potential of Fe was about -1V, which was similar to that of Zn. This did not occur when PXI 2532 was used. I used a portable DMM to measure the potential of Fe in the 121 electrodes and the results was -0.56V whether I run the sample vi or not. Then I suspected that the switch bring some effects on the measurement.To clarify this problem, I connected a Fe wire to c135 channel (as shown in Fig.1) and measure the influence of other 121 electrodes on the potential measurement of the c135. 1. I did not connect pin Ribbon cables to the switch and used the DMM waveform demo to express the potential of the c135 channel evolution. I opened the soft front panel of the switch and made the c135 connect to r0, which was connected to the HI line of the 4071. When the demo was run, the potential was constant and about -0.56V, as shown in Fig. 2.  
Fig.2
2. I connected one pin Ribbon cable (25 zinc wire connected to switch terminal). When the demo was run, I found that the potential was constant and about -0.56V, as shown in Fig.3.  
Fig.3
3. When two pin Ribbon cables (50 zinc wire connected to switch terminal) were connected to the switch, similar result was obtained, which was shown in Fig 4.  
Fig.4
4. When three pin Ribbon cables were connected to the switch, the potential of c135 fluctuated, which were shown in Fig 5 a-c.
Fig.5 (a)
Fig.5 (b)
Fig.5 (c)
We conclude that the number of the Zn wires have an effect on the Fe potential measurement using PX I 2535. But why not using PXI 2532 ?We also found other interesting phenomenon: when one or two pin Ribbon cables were connected to the switch (25 or 50 wire electrodes connected to the 2535), the waveform of the potential of c135 (Fe wire) can be affected by the nearby body action, e.g. stand up and sit down, go by the test table. The waveform changes were similar to the Fig.5.  I’d like to know the reason for this phenomenon and how to prevent it? Thanks advance.
Attachments:
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