Hi all,
But I believe that's done automatically by the meter so when you read off the TDS in ppm the conversion factor has already been applied.
I expect so, analytically you don't use a meter to measure TDS, you use evaporation and a very accurate balance, so the lab meters only measure in microS or salinity etc.
The calibration solution can use either potassium chloride (KCl) or salt, (NaCl) you actually get pretty close with water softener salt or cooking salt if you can't get either chloride as a pure salt.
The easiest to buy or make is probably the 342ppm NaCl solution, this is 700 µ(micro)S and 448ppm TDS. This is the conversion factor for KCl.
342ppm NaCl = 362ppm as KCl and 700 µS at 25oC
This solution is 342 micrograms (0.000342g) of NaCl in 1 litre of RO water, but because working with very small weights is difficult, I'd go for 34.2g of NaCl in 1 litre of RO as a stock solution= stock solution 1.,
Take 10cm3 of stock solution 1. (you can do this as 10g), and make up to 1000cm3 with RO, this is now 0.342g of NaCl in 1000 cm3 of water = stock solution 2.
Take 10g of stock solution 2 and dilute to 1litre with RO, this is now 0.00342g NaCl in 1 litre = stock solution 3.
Take 10cm3 of stock solution 3, and make this up to 100cm3 with RO, this solution is now the 342ppm NaCl and should read 700 microS or 448 ppm TDS.
cheers Darrel