Hi everyone,
I'm sure this is like the easiest question in the world, but I'm new at this so please bear with me 🙂
All I'm trying to do (at this point) is measure the resistance of 0.1 M salt water. I'm basically sticking some wires into a beaker, applying a voltage, and measuring the current using LabVIEW.
The attached circuit diagram is how things are setup right now, using a PCI-6229. First, analog output: the AO3 and AO GND terminals used to apply a 5 V potential into the salt water. Second, analog input: a set of wires correspond to AI24 and AI GND, and there's a 100 ohm shunt resistor setup in parallel with the salt water.
The parameters being used in LabVIEW: RSE terminal configuration, N sampling (2.5K @ 100 kHz), the external shunt resistor specified as 100, and a max/min range of 4mA/-4mA.
The readings I'm getting now are 0.000001 A when the AO3 wire is out of the salt water (i.e., no potential applied), and 0.06 A when it is in the salt water (i.e., potential applied). Using that 0.06 A reading and plugging it into the basic parallel resistor equation of I_total = V_applied ( 1/R_shunt + 1/R_saltwater), I get something like 500 ohms for the resistance of my salt water.
Some questions...
1. Do things look correct the way they're setup?
2. Should I be using a smaller resistance for the external shunt? Is there a rule of thumb for ratio of orders of magnitude of shunt vs. load resistances?
3. Anyway to eliminate noise from the LabVIEW / DAQ end of things? Right now my 1 microamps of noise (i.e., the 0.000001 A reading when no potential is applied) is much smaller than the 60 milliamps of signal, but eventually I'll be moving to much higher solution resistances, and the current will drop down to nanoamp or even picoamp levels, a signal which will be drowned out by the microamp noise.
Thanks!
Message Edited by clas004 on 08-26-2009 10:08 PM
