Conductivity/Resistivity Measurement with an Ohm Meter?

I'm sure wikipedia has a good section on conductivity measurement, or google will find you some info.
The size and shape of electrodes makes a huge difference. In theory the conductivity is measured between two parallel plates say 1unit square 1unit apart, the size of unit doesnt matter as the bigger the plates the further appart they are. In practice conductivity probes come in many configuration depending on what the liquid is etc.
You don't want to use an ohm meter as a permanent measuring device as it can polarize the liquid. AC is usually used. In my conductivity measuring circuits I use a short and infrequent 5v pulse which is AC coupled.

Beware that ph and other factors also effect the conductivity in a very nonlinear manner.
Del

The resistance reading on an ohm meter also depends upon the wetted area of the electrodes and whether AC or DC is being used.

Commercial conductivity meters run on very low voltage AC so as not to electrolyse the water while carrying out the measurement.

Verification of the calibration of a commercial conductivity instrument is usually carried out by using a resistance box.

The ohm meter would have to be very sensitive in the range of meg ohms/volt plus the test leads would have to be a low loss type. A Wheatstone bridge may work if you can find one.

I'd suggest you get one of these

http://www.fann.com/public1/pubsdata/Brochures/Resistivity_Meter_88C.pdf

Useed to have one many moons ago and did a damn fine job. Your local Baroid mud distributor will likely be able to help and may even be prepared to let you take a look at one. If they say they don't have one, they bloody well should!

Using an Ohm Meter will give you some idea of the resistance if you know how to convert the measurement to Ohm /unit section and unit length. BUT this needs a very sensitive metre. In any case, because of electrolysis and shapes of the electrodes, there will be errors that will have more importance as the value is smaller... In your trials, you will notice that the results vary a lot in precision.

The reason to use specialized instruments is to standardise the way the measurement is made so as to have a meaning when compared with other people measurement results! Also, to have a consistent repetitive result, without altering the media/electrodes during the measurements, ... reliability..!

Measuring the conductivity or resistivity of water is based on the same instrument design. only readouts are varied. Conductivity = 1/resistivity or resistivity = 1/conductivity. The reason we don't use ohm meters is that the length of the probes and separation of the electrodes are standardized in the manufacturing. It is important to be accurate and that the standards of measuring be accurate. The idea of measuring was really meant to give the user an approximation of the total dissolved solids. Corrections are made for temperature (usually 2%/degree C but will vary in a nonlinear mode). It is common practice in many labs to use temperature baths in the testing to bring the sample up to 25 degrees C and help to eliminate temperature errors. Some solutions will also not impart an electrical charge such a silicon with a neutral charge or not exhibit a proportional charge such as sulfate. Check out this site for a better understanding. I think in the long run you will also see that the measuring of ohm or mho is the same and we don't need to invent the wheel again.