Making your own standard will yield the most accurate results. This is done by making a mixture of salts in relative proportions that simulate the solution to be tested, then dissolving the mixture into distilled water. This should be done according to the following formula:
1 mg salt mixture/litre of distilled water = 1 ppm TDS, or in other words, X ppm TDS = X mg of salts + one litre of distilled water.
Remember that “X mg of salts” is the number of milligrams of a mixture of salt whose proportions simulate your test solution. An appropriate value for “X” is determined by the following rule:
Choose a ppm value for a calibrated solution as close as possible to the expected ppm values of the test solutions. If the ppm content of the test solution is expected to vary a great deal, it is best to choose a ppm value for the calibrated solution in the upper 1/3 of the TDS conductivity range.