Conductivity meters (TDS meters) are designed to measure the conductivity of the water, and they are therefore calibrated with a saline solution. However, silver has an electric potential two times lower than salt: to obtain an approximate estimate of the concentration of the silver solution, you must multiply the result provided by your TDS by two. 2 x 20 = 40 PPM. Furthermore, testers that measure up to 999 PPM have a margin of error of 2%, or 20 PPM. These devices are therefore somewhat unreliable. 

For qualitative results, you need to use a spectrophotometer instead of a basic meter for measuring dissolved solids, which was never intended to measure colloidal silver. 

You can find all of the analyses proving the concentration of KATHAROS 40 PPM solution here.