I found a page that tells you how to make your own TDS calibration solution, and your own TDS meter if you don't want to buy one and know how to read a circuit schematic. It's cool that do it yourself instructions exist for a TDS meter, but I only care about the solution to calibrate the meter I bought. 250 ppm, 500ppm, 750ppm, 1000ppm, you name it you'll get it, just modify the recipe. Here's the link to the page. http://blea.ch/wiki/index.php/PPM_Meter And here's what the page says about making your own calibration solution. 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.