Yes thanks need to think sometimesAnd the answer is...
Which of the following is a TDS meter measuring to determine the purity of RO/DI water?
1. Changes in the voltage drop between two electrodes in the sample
2. Changes in the magnetic field induced by an electric current between two electrodes in the sample
3. Changes in the electrical capacitance between two electrodes in the sample
4. Changes in the electrical current passing through the sample in an applied electrical field
The way commercial conductivity meters (TDS meters) work is to have two electrodes and the sample to be tested in between them. An electric field is applied across the electrodes, and the charged ions in the water "feel" the electric field. They then move in the direction toward the electrode they are attracted to, and away from the electrode they are repelled from.
For a bunch of reasons, the electric field is typically a AC field reversing polarity several thousand times per second. One reason is that we do not want to drive a lot of electrochemistry at each electrode (say, chloride (Cl-) ions being attracted to a positively charged electrode , dumping their electrons into it, and becoming chlorine (Cl2). Another reason is that we simply do not want ions piling up at the electrode, setting up a static field of positively charged ions around a negatively charged electrode (polarization of the electrodes), etc. We want them constantly moving.
Of course, moving charged ions form an electric current, and the more ions present, the higher is that effective current. Consequently, the extent of the flowing current is an indication of the number of ions in the water. That current can be quantified with a unit of measure called a Siemen per cm, or S/cm. For seawater, conductivity is about 53 mS/cm or 0.053 S/cm. For saturated kalkwasser, it is about 10.3 mS/cm. For RO/DI water we are usually aiming for 0-1 uS/cm, or 0-0.000001 S/cm. Totally pure fresh water does have some conductivity (~0.055 uS/cm) from the inherent concentration of H+ and OH- in pure water.
TDS (total dissolved solids, ppm TDS) is a poor unit of measure, since it relates to the amount of some solid (usually NaCl, but not always) that gives the same conductivity, when that solid is measured in ppm. For example 702 ppm TDS (NaCl) equates to about 1413 uS/cm and has the same conductivity as a sodium chloride solution with a concentration of 702 ppm. Likewise, 10.7 ppm TDS (NaCl) equates to about 23 uS/cm. Totally pure fresh water is by definition 0.000 ppm TDS, but does have conductivity. The conversion of TDS to uS/cm is not even completely linear, just to complicate things further.
So, back to the answers. I do not want folks to think I understand how the electronics of these devices work to know or evaluate the current flowing in this AC field. I expect some of you do, but I do not, and cannot expound on that. My understanding is the chemistry, not electrical engineering, but I will endeavor to discuss the other possible answers.
1. Changes in the voltage drop between two electrodes in the sample
If one thought of the sample as a resistor, and the more ions present the lower the resistance, then this could work out to measure ion concentrations. Aside from the issues relating to a DC field vs an AC field needed to prevent polarization of the electrodes and extensive electrochemistry, this should work, and it is even possible that answer 1 is not actually different than answer 4 because perhaps a voltage drop is how the devices know the current. Sorry, no wisdom here, but this is not the way conductivity meters are normally described.
2. Changes in the magnetic field induced by an electric current between two electrodes in the sample
Definitely there is no measuring of magnetic fields between the electrodes of a conductivity meter. A flowing current does create a magnetic field, and thus one could create such a device if one were able to detect the small magnetic fields involved, but that's not what is done.
3. Changes in the electrical capacitance between two electrodes in the sample
This one is even trickier than #1 for a chemist to understand, but I do not think the capacitance between two electrodes is determined by the ability of the charges in the medium between them to move about. The ions will short circuit any capacitance given enough time to move. This answer seems like it couldn't work, but perhaps some sort of AC capacitance or the decay of the capacitance might be able to give ion concentration.
Hope that was at least somewhat enlightening!
Happy Reefing!
