TDS Meter

Does anyone have a recommended TDS meter? I'm interested in measuring the TDS of RO water from my grocery store. My city water is about 500 ppm TDS (Wards Lab test), and I've read that a well maintained RO unit should produce <20 ppm TDS water.

How often do you calibrate a TDS meter?

Don't know much about TDS meters, but I bought my pH tester from http://www.bghydro.com/. They have stuff for checking TDS too.

The HM brand seems to have 3 options:

http://www.tdsmeter.com/products

HM TDS-EZ, $17
HM TDS-3, $22
HM AP-1, $26

These are all calibrated at 342 ppm, where some other brands of handheld units are calibrated at 1382 ppm. It seems like the lower the calibration the better.

Anybody have experience with these?

TDS meters are fairly robust in my experience. Calibration is a good idea, but not a critical issue. If you know your local tap water's typical TDS level, it can be used as one check point. A jug of distilled water can be another check point. So, buying a bottle of TDS calibration solution may not be a requirement. The good thing is that as long as you keep the calibration solution tightly capped, it should have a long shelf life.

Definitely check the TDS level of RO water you purchase from a vending machine. Any breakdown of the membrane will mean that you will be left with water that is not as ion free as you expect. A TDS meter is an easy way to check the machine's output.

I ordered the HM TDS-3 based on the most online positive reviews for RO checking for reef tanks. I'll report back with some hands-on experience with this $22 tool.

The HM TDS-3 came and seems to be a nice enough product for home use. The readings take seconds to stabilize:

Distilled water = 1 ppm TDS reading
RO water from grocery store left over from last brew = 22 ppm TDS reading
Unsoftened tap water = 401 ppm TDS reading (478 ppm TDS on Ward report 6+ months ago).

It'll be interesting to see how the unit lasts and the calibration by checking with distilled water. There's a recessed trim pot on the back to calibrate the meter to any known concentration of NaCl TDS calibration solution.

I take it the kiosks at the grocery store are place to go for RO water? I was never sure what kind of quality of water those things kicked out. Well that sounds like the way to go instead of buying a bunch of gallons of distilled water. I'll have to find one of those TDS meters then.