? Relationship Between TDS and Hardness.

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sidmoore

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Well, I understand that a TDS reading lumps all dissolved solids together and that these, mainly, consist of calcium and magnesium. It is also my understanding that "hardness" is a measure of the amount of calcium (mainly) in the water. What I don't understand is: How can one possibly derive a reasonably close hardness estimate from a TDS reading? Witness my latest conundrum.

I came to realize that our water softener was no longer softening and decided that I had been pushing its capacity by running scheduled 1100 gallon cycles (this is a Culligan Twin 9100 with a Fleck controller), so, I figured a good first step was to do a few "on-demand" regens and did two for each tank. The practical result so far is the fact that the water now has that "slippery" feeling of softened water when showering. However, my pocket HM Digital TDS meter now reads about a hundred points higher (before: 640).

So, my main question is: How useful is a TDS meter for measuring water softener performance?'

Second, if not a TDS meter, what can I, as a homeowner, use to measure hardness?

Thanx for listening!

Sid
 

Tom Sawyer

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A TDS meter is pretty much usless for calculating hardness. You need a Hatch 5B kit, about 30 bucks on amazon.
 

David A.

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TDS meters read electrical conductivity (EC), which is related to charged particles in the water (ions). TDS stands for total dissolved solids, but that's not completely accurate, since non-charged particles don't increase the conductivity of water (such as oxygen, nitrogen, and any nonpolar molecules that don't dissociate in a polar solvent).

There is a relationship between hardness and TDS (1 grain = 17.1 PPM), so the harder the water, the higher the TDS reading. But since the TDS meter can't distinguish between calcium and sodium, it's not a direct correlation, so you can't say 34 PPM = 2 grains of hardness because some of that will be calcium but some of it will be sodium, potassium, etc. And since 2 sodium ions replace each calcium ion, your TDS will increase after softening.
 

Bannerman

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Not sure why an almost 4 year old thread is now being replied to. Sidmoore has not visited this site since March 2015.
 

ditttohead

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Very odd. Oh well, I will add another thought. In general, TDS is not an accurate measurement as it is more accurately just an algorithmic estimate. conductivity and resistivity are actual measurements. TDS is estimated from these actual numbers but since TDS is more easily understood it has become the standard.
 
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