Trying to measure voltage through SCXI-1127/1331 1-wire connection

[ERT_LSU]

We have NI hardwares like: 1) Chasis SCXI-1000, 2) DMM PCI- 4060, 3) MUX SCXI-1127 and 4) Terminal Block SCXI-1331

We have a system with two large glass plates standing vertically and placed parallel to each other each having 24 electrodes on them. Distance between them is 4-5 inches. The plates are places in such a way that the electrodes are on the outer side and accessible to the user. We are trying to flow different liquids of different density through the channel in between. Please find attached a picture of it.

So a total of 48 electrodes and hence, wires are connected, 24 on both sides. On each side there are 3 rows with 8 electrodes/row, making a total of 24 wires. Wire#1-24 are on side A (say) and wire# 25-48 are on side B (the other side). The medium between the two sides will be salted water (say for now).

We want to measure potential/voltage in such a manner so that we can calculate the voltage difference between all possible electrode combinations between side A and side B. Like V1-25, V1-26, V1-27,...,V1-48,V2-25,V2-26,........,V24-48, all 24 X 24 possible combinations. We are connecting two end of a power source to side A and side B so that there is some current flowing in the water.

I have connected wire# 1-48 to 1-48 in SCXI 1331 (left out the zero to avoid confusion). Now that I'm measuring values through the program attached ("Scanning_....vi"), I can see that the first 16 values are showing some values of standards E-2. But from 17-48 are showing values of standard 5E-3 and I guess they are not taking any values at all. Can you tell me why this might be happenning?

Can 1-wire measurement be applied for this scenario. If not then let me know how to achieve our goal. I can manually take a Multimeter and measure all the combinations  one by one, but measuring all the values at the same instance would be better and that only can be done by automation and I hope that the NI hardwares I have, should be sufficient to do that. Please let me know if it is otherwise.

Another thing, in one of my previous contact with NI someone mentioned connecting 1_WIRE_LO_REF. Is it needed in this case? If yes, then if I connect one end to 1_WIRE_LO_REF, where should I put the other end?

In case the program is wrong or more than 16 values can not be taken please let me know alternate path or show me how to make changes in the program. I would appreciate your help in this issue. Thank you.

Message Edited by ERT_LSU on 09-17-2007 12:16 PM

[chericks1]

Hi ERT_LSU,

I believe the person you previously had contact with at NI is correct.  You need a voltage reference.  Without it, the DMM + signal is connected to your electrode, but the - signal is floating.  Tie 1_WIRE_LO_REF to a system GND (or common reference potential that does not change) and see what happens.

Hope this helps!

Chad Erickson
Switch Product Support Engineer
NI - USA

[ERT_LSU]

Thanks for helping.

Can you just explain what is a system GND?

Should it work if I connect the 1_WIRE_LO_REF to a battery (say 5 V) and keep my power source voltage at 12 V?

I actually want to use A side as + and B side as -. But I think in that way too we can't have 24X24 values for one instance. The only way to measure all the values in one instance is possible by takig potentials for all the elctrodes and subtracting them according to the need.

As I said, voltages should be like V1-25 etc. So, the way you are suggesting is actually getting the potentials for each electrodes and getting V1-25 as V25 - V1. Please correct me if I'm wrong and you meant something else.

[chericks1]

Hi ERT_LSU,

I think I understand what you are attempting to accomplish with your application.  You continue to describe it as a 24x24 matrix.  However, you do not need the functionality of a full matrix (which is to connect x rows to y columns simultaneously).  Rather, you simply need two 24x1 multiplexers.  One to select an A electrode, and another to select a B electrode.  A is always routed to DMM+, and B to DMM-.  As you've correctly determined, the SCXI-1127 does not have a "dual" topology like some other NI switches do, so you're using an alternative method of measuring one potential and subtracting it from another.

To answer your questions:
A system GND is a ground reference common to the entire system.
The 5 V battery may not work as a common reference potential.  What's keeping the battery reference fixed relative to the power source's reference?

Can you use the power source's GND as the 1_WIRE_LO_REF?

Chad Erickson
Switch Product Support Engineer
NI - USA

[ERT_LSU]

Ok. GND is ground, I initially thought it as an abbreviation of something else. Yes, I'll use the system GND as 1_WIRE_LO_REF. Things are much clear now. Let me try it and then I'll get back to you. Thank you for your patient answers.

[ERT_LSU]

I connected the COM of my power source to the 1_WIRE_LO_REF. There are 4-5 points to connect. I choose the middle slot. Is there any significance of assigning so many slots to 1_WIRE_LO_REF?

No change in result. Can you please test run the program I attached and see whether it's correct or not. You'll see 48 segments getting the output. First 16 of them are showing some seemingly proper values. rset are either too small or as big as 5 V.

I'm providing 5Vs from the power source. You might have seen the old analog ones. I'm using one of them. there are different Voltgae option. I'm using the 5V and so I've connected a wire from the 5V connection to one of the electrodes on B side (28th electrode) and the COM ports goes to the A side (4th electrode) and also to 1_WIRE_LO_REF.

Now I'm thinking if the program if not working. Because it seems that we are getting (seemingly) proper values for the first sixteen and something's messing up the values for the the rest of the electrodes.

Do I have to use two 24X1 MUXs i.e, use physically different boards and also two different programs to measure them? Or I can just use 1-24 as 24X1 and 25-48 as another 24X1 in the same board and measure them through the same program? Thanks in advance.

Message Edited by ERT_LSU on 09-17-2007 04:00 PM

[Ima U]

Hi ERT_LSU,

As Chad mentioned you would be able to do this measurement on the same card with some of our other switch card options.  Cards such as the SCXI 1130 and the PXI 2530 offer Dual Multiplexer modes that would allow you to connect both to one card. Otherwise you will need two SCXI 1127 modules.