TDS vs conductivity

ratio

I'm looking at trying to measure TDS on two boilers, one water, one steam. It looks like what's commonly sold as a TDS meter is really just measuring conductivity and calibrating that somehow to show TDS. Would one of those meters give me a reasonable number out of the box, would I need to calibrate the adjustment factor myself, or ???

Any suggestions for a good TDS meter? www.tequipment.net has a selection, starting at about $45. There's a cheap(?) combo unit that measures TDS, salinity, & pH at the same time. Good idea?

Jamie Hall

TDS is Total Dissolved Solids -- and is a measure of the concentration of ions in the water. The ions are what give the water conductivity. Hence conductivity is a very good measure of the TDS (it's not exact; it depends a little on exactly what ions are present. Since most of the ions -- and most of the conductivity -- are the commonest ones (Sodium, Calcium, Chloride, Carbonate), the difference can be ignored.

ratio

Thanks, @Jamie Hall. I was reading a little about TDS, it said the only real method was to boil a sample down and weigh it; that conductivity was only an approximation. I wasn't sure how close it'd get me, or even if it would make a big difference.

Jamie Hall

ratio said:

Thanks, @Jamie Hall. I was reading a little about TDS, it said the only real method was to boil a sample down and weigh it; that conductivity was only an approximation. I wasn't sure how close it'd get me, or even if it would make a big difference.

That's correct. The evaporate to dryness approach is exact -- and a colossal bore (and requires special filters, very good temperature control, a lot of patience, and very very accurate balances). Except in rare instances, conductivity is a dandy surrogate.