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Conductivity & Total Dissolved Solids: What Do They Tell Us?

Conductivity is defined as the amount of electrical current that can be passed across a 1 cm gap between two electrodes at a potential difference of one volt. One microsiemen/cm means that you can pass one microamp across this gap (Sorry, every now and then, the physical chemist in me comes out). So what does this mean?

The greater the conductivity, the greater the amount of current that passes across the gap. But what causes the conductivity to increase? Contaminants in the water, especially those that are able to be ionized (dissolve into positive and negative ions). Pure water has a conductivity of about 0.055 microsiemens/cm. Critical water as defined in AAMI TIR34:2014 has a conductivity of <10 microsiemens/cm. Tap (or utility water) has a conductivity of <500 microsiemens/cm. The less pure the water is, the higher its conductivity.

And what of total dissolved solids (TDS)? This is usually derived from conductivity, but that’s not the whole story. TDS can also include dissolved impurities that do not ionize, and are therefore not detected by a conductivity measurement. However, these materials are often inert, so conductivity provides a good measure of the impurities in water that can cause trouble for an SPD. It doesn’t tell you what those impurities are. But if you are able to decrease the conductivity, you know that your water is purer than before you started.

As we saw last month, water that is too pure can also cause trouble if the plumbing isn’t designed for it. So, don’t get carried away without a full system examination by an expert! See you next month!


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