Hey, maybe this might help you also.
http://www.tdsmeter.com/faqs/#best-readings
How can I get the best possible readings?
Shake
Always make sure to shake excess water off the meter before dipping it into a water sample, even if it's the same water.
Stir/tap
After dipping the meter in the water, always lightly tap it against the side and stir the meter to remove any lingering air bubbles or electrical charges.
Positioning
When taking the reading, always make sure to hold the meter straight up without it touching the sides or bottom of the glass/beaker/cup. The probes should be suspended as close to the center of the water sample as possible.
Time
The longer the meter is in the water, the more accurate the reading will be.
Temperature
25 degrees Celsius is the ideal temperature for conductivity readings, even if the meter has ATC.
Rinse
If switching between very low and very high ppm water, always rinse the probes with distilled water to avoid any build-up.
Are TDS meters really conductivity meters?
Yes. While EC and TDS are often used synonymously, there are some important differences to note. EC, when applied to water, refers to the electrical charge of a given water sample. TDS refers to the total amount of substances in the water other than the pure H2O. The only true way of measuring TDS is to evaporate the water and weigh what’s left. Since this is near impossible to do for the average person, is it possible to estimate the TDS level by measuring the EC of the water. Every digital TDS meter in the world is actually an EC meter.
All elements have some electrical charge. Therefore, it is possible to closely estimate the quantity of TDS by determining the EC of the water. However, since different elements have different charges, it is necessary to convert the EC to TDS using a scale that mimics the charge of that water type. The following are the most common water samples, and for the COM-100, each has its own conversion factor:
KCl
Potassium Chloride is the international standard to calibrate instruments that measure conductivity. The COM-100 is factory calibrated with a 1413 microsiemens solution is the default mode is EC-KCl.
442TM
Developed by the Myron L Company, 442TM simulates the properties of natural water (rivers, lakes, wells, drinking water, etc.) with a combination of 40% Sodium Bicarbonate, 40% Sodium Sulfate and 20% Chloride.
NaCl
Sodium Chloride is used in water where the predominate ions are NaCl, or whose properties are similar to NaCl, such as seawater and brackish water.
Measurements in EC (µS) do not have a conversion factor, but do require the correct setting for the proper temperature coefficient.
Other HM Digital products use the NaCl conversion factor.
