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Q & A: measuring salt concentration

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Most recent answer: 10/22/2007
Q:
I am trying to measure how well various soutions ( salt and distilled water,salt and mineral water,salt and tap water)conduct electricity using a digital multimeter. My results do not seem to make sense. I would imagine that the higher the concentration of salt in the solution, the lower the resistance reading on the meter. Is this correct? Can a digital multimeter be used to measure salt concentration?
- Steve Colomby (age 41)
Toronto, Canada
A:
Your general idea is ok but there can be technical problems. The ohm-meter uses small dc probe currents, then measures the voltage to determine resistance. The problem is that the dc current causes different ions to build up around each electrode. Even if you switch off the current (by switching fom ohm mode to volt mode) youíll still see some voltage. So the results donít really reflect the resistance accurately.
You may be able to do better by rapidly switching the leads back and forth between the two connections to the meter, then averaging the two types of reading. Or if you can find a low-voltage ac source (say from a step-down transformer, but using precautions) you can use your meter in ac current mode to measure conductance. Use of big electrodes helps reduce problems at the electrodes.

Commercial conductivity meters combine the use of ac currents with some good electrodes, typical made of platinum to avoid problems with surface chemistry.


p.s. I just tried this with some saltwater and my home DMM. The results were nonsense, because even on the volts setting, there was about an 0.4 volt difference between the two nominally similar probes. That happened before I tried the ohms mode, so it was caused by some difference in the surfaces of the probes, not by ions built up by the current.  The voltage was too big to allow the meter to register any resistance, so I couldn't average resistance values with the leads switched. That indicates the importance of using clean, stable electrodes, preferably with big areas, not just any old wires.

Mike W

Lee H


(published on 10/22/2007)

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