Quote Originally Posted by herbie the love bud
Go to your hydro store and get one of the premixed TDS calibration solutions from GH. Comes in a small foil packet that looks like a big ketchup packet. Should cost about $1.50.

Dip your pen in that and see if your pen is not accurate first of all. TDS is SO simple its very hard to go wrong. Then if its off and cannot be calibrated or won't hold calibration then get yourself a HM Digital meter. About $40 and use EC from now on. TDS is not accurate to begin with.
TDS is only the type of measurement and measures the same thing as does EC. TDS is indeed as accurate as EC, if not more so (considering it typically has more significant figures per readout). As long as you set the correct conversion ratio, TDS is fine.