How To Test Water Hardness With A TDS Meter

Trying to use your TDS meter to measure your water hardness? Well, you can't! Sorry if that's not the answer you were looking for. Here's why it won't work:

TDS stands for Total Dissolved Solids. It's the measure of all the stuff that is dissolved in your water. A TDS meter is actually measuring the electrical conductivity of the water, and then calculates the TDS value based on this.

The problem with trying to measure hardness by measuring TDS is that TDS is comprised of much more than just hardness. Yes - when the water is very hard, a good percentage of the TDS will be made up of hardness minerals, but a lot of it won't be. So your TDS value will always be significantly higher than the hardness value.

The other thing you can't do with a TDS meter is use it to figure out how well your softener is working. This doesn't work because of the nature of how water softening works. A water softener is an ion-exchange device - it grabs on to calcium and magnesium ions (aka hardness) and exchanges them for sodium. Well, the sodium carries the same electrical charge as the hardness. So the TDS reading of hard water and soft water is pretty much identical even if the water has been softened perfectly.

If you're looking for a water test kit that has a hardness test with it, here's a good one. If you want a TDS meter anyway, we've got those too!