08-05-2018, 07:59 AM
Join Date: Mar 2008
Location: Wake Forest NC
| | Re: TDS meter
The conversion factor from conductivity to TDS is about 0.50 for plain NaCl solution and about 0.67 for freshwater with a "typical" mix of other ions. So, a conductivity measurement of 100 uS corresponds to about 50 mg/L if NaCl is the majority of the dissolved material, or 67 mg/L in a freshwater sample containing an average mix of Ca, Mg, SO4, N, K, Cl, Fe, Mn, NO3, etc.
A chemically "accurate" measure of TDS (by evaporation and weight) is biologically meaningless, since it doesn't distinguish between the mineral ions essential for fish and plant growth and the dissolved organic compounds from wood and leaves. A sample could have high TDS (measured by weight) due to organics but have very few mineral ions. It's like measuring the nutritional content of your grocery shopping cart by weight.