use of multimeter to calibrate refractometer?

[spamreefnew]

is there a way one could calibrate/mod a multimeter to measure salinity? i was thinking you could mount the probes a certain distance and measure the "resistance" of seawater and or r/o water ,,,do some conversions and whalloha,,you have a salinity meter,,,no?

[Randy Holmes-Farley]

No.

I discuss that here:

Using Conductivity to Measure Salinity
http://web.archive.org/web/20040604034508/http://www.aquariumfish.com/aquariumfish/detail.aspx?aid=1804


from it:

Can I just drop some electrodes into the water and measure the resistance with a meter? No. Several factors make that impossible. The size and shape of the electrodes are significant, but more important is what happens at those electrodes. If you apply a DC current to seawater, numerous reactions take place when the ions hit the electrodes. Some ions will plate out on the electrodes, some may bubble off as gasses, and the electrodes themselves may dissolve. These and other effects all serve to change the nature of the solution at the electrode, impacting the measured conductivity.

So how do conductivity probes get around this problem? They, in fact, use an AC current rather than DC. Using fields that oscillate, there is no overall movement of ions toward one electrode or the other. The ions move one way for a tiny fraction of a second, and then back the other direction for the second half of the cycle. Overall, the solution and electrodes stay unchanged and the conductivity is accurately measured. Modern conductivity meters use complex AC waveforms to minimize additional complications such as capacitance, which can interfere with simple conductivity measurements.

In practice, commercial conductivity probes have either two or four electrodes, with the four-electrode version being more resistant to fouling and other effects that can cause degradation of the measurement. The electrodes are made of nonreactive materials such as epoxy/graphite, glass/platinum or stainless steel. The choice depends primarily on the nature of the solution to be tested. For occasional use in seawater, all of these are acceptable.

One final complication is that the conductivity of ions in water depends upon temperature. There are a number of factors that cause this in seawater, but one big one is simply that the ions are naturally moving around faster as they get warmer. When the same number of ions are moving faster, the apparent conductivity is increased. The conductivity of seawater at 41 degrees Fahrenheit, for example, is a little over half of that at 56 degrees. For this reason, all conductivity meters simultaneously measure the conductivity and the temperature. The internal electronics then take the temperature into account, and normally provide a value that is “corrected” to what the conductivity would be at a standard temperature (47 degrees). Consequently, you can measure the salinity of water regardless of the temperature of the sample.