Hi all,
how do they compare when using both meters to read ro?
They should read the same value for conductivity (how low it is will depend upon a lot of factors like the RO membrane, pump pressure, how hard the tap water was etc.), but they are likely to read differently for pH. As an example of this is that in 100% RO one meter could read pH4, and the other pH8, whilst both being reasonably accurate.
I've put the reasons (below) why conductivity is simple to measure in nearly all water, and pH is easy to measure relatively accurately in hard, salty, buffered water (Marine, Rift Valley Lakes etc), and very difficult to measure accurately in soft water.
cheers Darrel
Conductivity
The TDS meter would read 0ppm in pure H2O (It is more likely to read 10 - 20ppm), even cheap TDS meters are fairly accurate, because they actually measure electrical conductivity. If you know the area of the electrodes (S), and the distance (L) between them, you can use Ohm's law to work out the theoretical conductivity, and then use a calibration solution to get an accurate cell constant. This cell constant shouldn't then change during the meters working life.
Method
When the probe is placed in the tank water, the meter applies voltage between two electrodes inside the probe. Electrical resistance from the solution causes a drop in voltage, the voltage arriving is read by the meter and converted to micro-Siemens.
H2O
Pure H2O is an electrical insulator and the voltage drop is dependent upon the ions in solution. This is a linear relationship, the more ions you have the more current flows, and the higher the reading. Pure H2O = no ions = 100% voltage drop.
Temperature
Because conductivity is temperature dependent, the meter should show the conductivity of the water at 25oC, not at the actual temperature. This results in a standardized reading.
Calibration
You can calibrate the media using a bought calibration solution (or KCl), but they should be pretty close even without calibration. The typical conversion of conductivity to TDS ppm is that 100 μS/cm is equivalent to 64 ppm TDS.
pH
This is much, much more complicated to measure, pH is really just a proxy ratio we use to estimate acidity and alkalinity, rather than a measurement itself. From Wikipedia:
In a solution pH approximates but is not equal to p[H], the negative logarithm (base 10) of the molar concentration of dissolved hydronium ions (H3O+); a low pH indicates a high concentration of hydronium ions, while a high pH indicates a low concentration. This negative of the logarithm matches the number of places behind the decimal point, so, for example, 0.1 molar hydrochloric acid should be near pH 1 and 0.0001 molar HCl should be near pH 4 (the base 10 logarithms of 0.1 and 0.0001 being −1, and −4, respectively). Pure (de-ionized) water is neutral, and can be considered either a very weak acid or a very weak base, giving it a pH of 7 (at 25 °C (77 °F)), or 0.0000001 M H+.[
H2O
The meter probably won't read pH7 even if it is correctly buffered.
In RO pH is a totally meaningless measurement. Water is such a efficient solvent that water that has been exposed to air is mildly acidic. The water absorbs carbon dioxide from the air, which is then slowly converted into carbonic acid, which dissociates to liberate hydrogen ions, more hydrogen ions = acidic:
CO2 + H2O <-> H2CO3 <-> HCO3− + H+
If you keep marines, or Rift Lake Cichlids, your pH will be relatively stable and useful, in soft water it needs some interpretation and an understanding of what that pH actually means. Additionally at around pH7, accurate measurement of pH is problematic.
This is an acid/base titration curve for neutralising hydrochloric acid (HCl) with sodium hydroxide (NaOH) HCl. Because pH measures the ratio of acid and alkali ("bases"), it doesn't change the graph whether we start with a strong solution (molar) of each, or a weak (0.001 molar).
As long as the concentrations of H+ and OH- are the same the pH curve will look the same.
The small amount of base neutralizes an equal amount of acid with an excess amount of acid remaining. The hydrogen ions supplied by the excess acid exist in increasing amounts of water produced from the neutralization. As a result, the pH changes slowly.
The midpoint of the vertical section of the curve is the equivalence point indicating when equal amounts of acid and base are present. At the equivalence point, the H+ has been neutralized by the OH - leaving a neutral (pH7) solution:
H+(aq) + OH -(aq) —> H2O(l)
As you add more NaOH, the ratio changes so there are more OH- ions and the pH rises, quickly at first and then apparently slowing, due to the log10 nature of the pH scale.
You can think of it like putting coffee and sugar into water. You can neutralise the bitterness of the coffee with the sugar, so that a cup with 4 spoonfuls each of coffee and sugar might taste similar to one with 1 spoonful of each, this would be the "pH7" scenario. The colour of the coffee, and how "thick" it was, would differ between the 2 cups and this is the TDS measurement.
Whatever you do it is difficult to get back to "water" by adding anything else, or by trying to remove the coffee and sugar.
Any-one who still has the will to live can have a look at this thread:
"
My water conditions, help, do I need RO or HMA or neither" for some details and a practical example: <
http://www.plecoplanet.com/forum/showthread.php?t=8904>
cheers Darrel
. He was going to use it for checking his pond but has let me try it out first. 
