I discuss what the different units of measure (442, NaCl, etc. ) mean here:
What is TDS? by Randy Holmes-Farley - Reefkeeping.com
http://www.reefkeeping.com/issues/2004-04/rhf/feature/index.htm
from it:
True TDS meters (that only give output in TDS units, such as the
Oakton TDS Tester shown in Figure 4) typically convert the conductivity reading into the ppm concentration of some salt that would give the same measured conductivity. For example, it might be set to give as its output the concentration of sodium chloride (NaCl) that would give that same conductivity. So if the device detected 447 mS/cm, it might display that as 215.5 ppm, as that is the concentration of sodium chloride that gives that same conductivity.
Unfortunately, there is no single exact conversion between conductivity and ppm NaCl as the conductivity of a sodium chloride solution is not linear with concentration (that is, 20 ppm NaCl is slightly less conductive than twice that of 10 ppm NaCl, the reasons for which are beyond this article, but in a sense, the more ions there are in solution, the more they interfere with each other in terms of sensing the voltage, and in terms of moving in response to it). Nevertheless, for values in the range sensed by most TDS meters, a rough conversion is that 1 ppm NaCl = 2.1 mS/cm.
Now the fun really begins. The problem with such devices, at least when not carefully calibrated, is that it is not clear whether it is referring to ppm of sodium chloride equivalents, or to something else.
Potassium chloride (KCl) is actually used as the standard more often than sodium chloride. Additionally, researchers often use something called
442, which is a mixture of sodium sulfate (40%), sodium bicarbonate (40%) and sodium chloride (20%). The 442 mixture is designed to mimic the ions often present in natural fresh water systems.
These systems all have slightly different relationships between concentration (in ppm) and conductivity (in mS/cm). Table 2 shows this relationship for some commercial conductivity standards made by
Oakton. For many aquarium purposes, it may make little difference whether the true value corresponds to 10 ppm NaCl or 10 ppm 442, but there is a 30-70% difference in the measured conductivity and hence in the total ions present.
Consequently, two aquarists using different meters (which may use different standards) may get substantially different results on the same water. Of course, if you calibrate the meter yourself (if that is possible, not all meters can be calibrated), instead of accepting a factory calibration, then you eliminate this concern since you know exactly what you calibrated with.
