02-05-2007 08:45 AM
I am attempting to monitor a hand full of sensors that
output resistance. I do not seem to be getting the expected results. I am sure
it is my lack of knowledge on the USB-6008 unit so I thought I would ask.
To make this as simple as possible
First I tried connecting the photocell the way the diagram in VI Logger Tasks
--> My Task1 --> Edit Task -->Connection Diagram shows
(PIN 2 --> Pin 3) This gave me readings all over the place! I then tried
connecting it the way I though it should connect (Pin 2 --> GND) again
nothing.
At this point I went to the test panel in the Ni-DAQmx software I changed the
setting for AI0 to single-ended (RSE) and tested it. To cut this short, I found
that in the Test Panel with AI0 set to RSE and the sensor hooked up between Pin
2 and Pin 32 (+5v) I get a perfect reading!
When I am in the software it does not work. Well it does work, if I put the
photocell between pins 2 and 3 then run a 5v line from pin 32 to the lead from
pin 2.
It would appear that I need to tell the unit that AI0 is single-ended; I just
can not find a place to do that. The Preferences selection in Devices and
Interfaces --> NI-DAQmx devices --> NI USB-6008 is grayed out.
02-06-2007 01:05 AM
02-06-2007 03:45 PM
02-06-2007 05:05 PM
02-06-2007 05:28 PM
02-06-2007 05:53 PM
02-06-2007 06:32 PM
02-07-2007 08:46 AM
Erik J,
What I need to do is measure resistance with an RSE setup.
The photocell is just the tip of the iceberg so to speak!
As I have stated, I can measure voltage and current in RSE
mode, and I can find no reason why measuring resistance in RSE mode should not available.
Another example of why I would need to measure resistance
would be in measuring the resistance when probes are placed in soil to determine
the moisture content of soil. (Ill need to monitor this)
02-07-2007 10:32 AM
CyberLink:
Can you post a sketch of how you have this wired? When you say:
When I am in the software it does not work. Well it does work, if I put the photocell between pins 2 and 3 then run a 5v line from pin 32 to the lead from pin 2.
I cannot make sense of it. Pin 32 is actually GND, not 5V. You cannot measure resistance directly across an analog input, just voltage. Thus you need a voltage divider to indirectly calculate resistance. And I doubt you can measure high resistances accurately such as that found in soil.
Also note that the input impedance of the 600x is just 144KOhm. This will drastly affect you measurement results.
02-07-2007 10:50 AM