What's interesting is where both meters make use of the same basic principal to measure water purity(between electrodes). The difference however is where the PWT meter presents its readings on a microsiemens(μS) scale, whereas the TDS meter converts this into PPM. - I'd also add that some meters will allow switching between measurements(EC, PPM etc), which can prove useful in certain cases.
That said, as to why you're not getting consistent readings between both units remains to be uncovered. Though I'd first question if one of your meters hasn't been calibrated for something other than drinking water?
- If so, then this could offset the base read value of the TDS meter beyond its minimum threshold. Which in turn, could lead the meter to display 0.0 rather than to coincide with your PWT(EC) readings. ie, < .5 PPM
Hope this helps.
PS. A well calibrated EC meter can prove helpful in obtaining relatively accurate PPM readings for a CS process. Whereas most basic TDS meters will not prove useful at all due to accuracy and conversion factors.