I don't know the answer here, but I do have some knowledge of how TDS meters work, and this might be useful.
To measure TDS, the meter actually measures the conductivity of the water, as this is directly proportional to the TDS. Alas, it is not possible to simply apply a DC voltage to a couple of probes because of polarisation effects, so the measurement is made using an AC voltage and some kind of synchronous rectifier.
Unsurprisingly, this measurement is easily upset by stray voltages and currents in the water. Drive harmonics seem a likely villain here.
Now, as to why our poster is having problems, it seems to me that we need to identify just what it is that has changed since the TDS readings started to play up. And this is not clear from the original post.
I hope the above is of some help.
Mark Monson
