Measuring PPM/EC/TDS
TDS is the measuring of the amount of salts in a solution. For alot of applications the amount of salt is indicitive of the levels of other stuff in a solution. TDS/PPM meters sold for gardening and aquariums figure the amount of salt in Parts Per Million by measuring the Electrical Conductivity of the solution under test. So a PPM/TDS meter is a EC meter that converts the EC value into PPM values. There are alot of descriptions online about why and how to measure PPM/TDS or EC so google for them if you want alot more details. EC is a measure of Electrical Conductivity from two probes 1cm apart. 1 EC is = 1 microsiemens, to convert from EC to siemens multiply by 1E-6. EC can be converted to PPM by multiplying by 500. PPM can be converted to EC by dividing by 500. To convert from siemens to Ohms is s=1/ohms, you can also go the other way and do ohm=1/s for siemens to ohms. Siemens is also known as Mhos, which comes from ohm written backward.
The number 500 used to convert between PPM and EC is called the Conversion Factor. Different salts will have different conversion factors because some conduct better or worse than others. NaCl's is 500, this seems to be the most common standard used, and is what was used for the calibration solutions.
If EC/PPM is just measuring the Conductance (or resistance) then why not use a volt/ohm meter directly?? Because they pass DC current thru the probes and you cannot measure conductivity of salts with DC current because it will rip the molecules apart, and since the molecules are what conducts the electricity you get a constantly changing reading that is useless. Overcome this by using an AC signal. If the frequency is high enough (>1khz it seems) the molecules dont have time to move apart before they are pulled in the opposite direction. Sort of like how high DC current will throw a person off of it, where as AC forces them to hold on and get shocked to death or something.
AC Molecular Workbench model
DC Molecular Workbench modelThe screenshots above are from Molecular Workbench (its free!!) and show what happens to NaCl and H2O molecules when exposed to AC and DC fields. If you download the models you can press play and watch the atoms move around. Normally the molecules look about the same as in the AC simulation staying in about the same spot. When AC is applied they just shake back and forth. With the DC simulation you can see the sodium and chloride atoms are attracted to the simulated charged plates at top + and bottom - of the model container.
Something very important, temperature will effect the conductivity! The chart on the right shows the same solution measured at 3 different temperatures. If your connecting this to a microprocessor you can use a thermistor to compensate. If not then make sure the calibration solution is about the same temperature as the solution to be tested. One way this can be done taking a sample from the fish tank or whatever and letting it set until its room temperature. If your temperature will vary by only a few degrees then it may not even be a problem.
Measured Conductivity
at different Temperatures
EC Temp C 3.43 13.72 4.16 22 4.73 30
How to Make Calibration Solutions
Needing something to calibrate the meter, a friend suggested i measure out salt by weight, and put that into a measured volume of water. Most calibration solutions have a mixuture of 2 or more types of salts such as sodium chloride (NaCl) and potassium chloride (KCl). However all i have around the house is NaCl table salt so...
Looking around the net, i found that 1mg of solute per 1L of water would equal 1ppm. So to get a 2Kppm solution would need to put 2g of salt into 1L of purified water.. or just put 1g of salt into .5L of purified water. The water MUST be purified, pure water has allmost no conductivity, compared to tap water which has all sorts of contamination (tapwater here reads about 80ppm). So get some distilled water for this.
Once you have the 2K solution from 1g into .5L, you can put some of that in a jar, the pour out the rest till you have only 250ml, pour in purified water till you get 500ml again, then you have 1000ppm! Keep doing this if you want to create a range of solutions, just be carefull to measure perfect and STIR IT UP WELL.
