Author Topic: TDS and Water Hardness  (Read 852 times)

Offline quest4watneys

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TDS and Water Hardness
« on: November 22, 2013, 04:50:42 PM »
I bought a TDS meter so I could test the RO water I bought to make sure it was as pure (at least as far as TDS) as it should be. My tap water was 481. Is there any correlation between total dissolved solids and water hardness?
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Re: TDS and Water Hardness
« Reply #1 on: November 22, 2013, 05:00:01 PM »
I believe anything above 50 ppm total dissolved solids is considered hard water.

Offline hopfenundmalz

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Re: TDS and Water Hardness
« Reply #2 on: November 22, 2013, 05:48:00 PM »
TDS gives you an idea that there are minerals in the water. That is hardness and alkalinity. Hardness is not detrimental, high alkalinity is.

Read the Knowledge section in Brunwater for a good explanation.
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Offline mabrungard

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Re: TDS and Water Hardness
« Reply #3 on: November 22, 2013, 07:28:05 PM »
Is there any correlation between total dissolved solids and water hardness?

Sort of, but there is no direct correlation.  You can have high TDS and no hardness or you can have high TDS and high hardness. 
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Offline punatic

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Re: TDS and Water Hardness
« Reply #4 on: November 22, 2013, 07:44:21 PM »
Your TDS meter is actually measuring how well your water conducts electricity.  The more ions (dissolved solids) in the water, the better the conductivity.  Not all ions conduct electricity equally.  Thus, the conductivity to total dissolved solids (TDS) correlation is not an exact one.  It varies with the species of ions dissolved in the water.  The conversion factor can vary from TDS (ppm) = conductivity (µS/cm) x  0.54 to 0.96.  Generally speaking the conversion factor programmed into inexpensive TDS meters is TDS = conductivity x 0.67.  Higher end conductivity/TDS meters can be calibrated with calibration standards and to temperature.

While not good for exact determination of TDS, conductivity is good for tracking dissolved solids trends over time.   
« Last Edit: November 22, 2013, 07:49:58 PM by punatic »
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Offline quest4watneys

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Re: TDS and Water Hardness
« Reply #5 on: November 22, 2013, 07:57:07 PM »
Are the meters good for determining whether RO water I buy is what it's supposed to be? I didn't spend a whole lot of money on it because I read that ballpark is all you really need when using it for what I'm using it for.
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Offline punatic

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Re: TDS and Water Hardness
« Reply #6 on: November 22, 2013, 08:05:52 PM »
RO permeate (RO water) quality is dependent on the salt rejection rate of the RO membranes and the TDS of the feedwater.  Many RO membranes have a 98% salt rejection rate.  This means if the feedwater has 100ppm TDS, the permeate will have 2ppm.  However, if the feedwater has 10,000ppm TDS (brackish water), then the permeate will have 200ppm TDS.  Ocean water averages 35,000ppm TDS.

So the quick answer is, it depends.  Not all RO water is created equal.

I would say if your TDS meter is showing <50ppm TDS you should be fine.  Most likely it will show quite a bit less than 50ppm, probably less than 20ppm.

In water quality of the type used in brewing, ppm = mg/L.
« Last Edit: November 22, 2013, 08:12:07 PM by punatic »
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Offline quest4watneys

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Re: TDS and Water Hardness
« Reply #7 on: November 22, 2013, 08:14:14 PM »
I appreciate the info! I did use it to test a bottle of RO water and at room temp it said the TDS was 3ppm as opposed to tap water at 481  :)
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Re: TDS and Water Hardness
« Reply #8 on: November 22, 2013, 08:35:31 PM »
Great explanation, Carl. I have a (cheap?) TDS meter that came with my home RO filter. It shows ~350 in and ~15 out.

Offline punatic

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Re: TDS and Water Hardness
« Reply #9 on: November 23, 2013, 12:04:21 AM »
If I could only have two arrows in my water testing quiver, they would be a pH meter and a conductivity meter.

If I had to chose only one of those two, I don't know which I would pick.

My most valuable tool I have is a portable meter that measures pH, conductivity and dissolved oxygen.  It was an expensive purchase, but the payback period was about 60 days.

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Offline mabrungard

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Re: TDS and Water Hardness
« Reply #10 on: November 23, 2013, 08:00:13 AM »
For brewing, a pH meter is far more useful than a TDS meter. However for RO water users, the utility of a TDS meter is huge. There is nothing as useful as a TDS meter for ascertaining the 'relative' quality of that RO water for an exceptionally low price...and almost instantly too! 

I believe Carl already knows this. But the rejection rate of any RO or nanofiltration membrane is ion-specific.  Some ion rejection might be 98% while other the rejection rate for other ions may be higher or lower.
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Offline punatic

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Re: TDS and Water Hardness
« Reply #11 on: November 23, 2013, 09:37:07 AM »
Yes.  Product manufacturers being product manufacturers will choose the specification that shines the most positive light on their product.  The 98% salt rejection claim is pretty much industry wide for RO membrane manufacturers.  If they advertised on the radio you would hear that hyperfast lawyer disclamer mumble at the end of the commercial. However, in practice, it is not unreasonable to expect 96-98% overall.  Even at 500ppm TDS in your tap water (the recommended upper limit for TDS in drinking water) that gets your RO water down in the 10-20 ppm range.  Big Al's water is 350 in 15 out - that's around 96%.  Not bad for a point of use ("under the counter") RO system.

Nano filtration is good for softening and organics removal, but not for desalination.  Calcium, magnesium, and organic molecules are retained, but sodium and other monovalent ions pass through.

A TDS meter looks at all ions.  A pH meter only looks at hydrogen ions.  Most would agree that the hydrogen ion (the lonely proton) is the most important ion in water chemistry. 
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Offline punatic

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Re: TDS and Water Hardness
« Reply #12 on: November 25, 2013, 01:57:10 PM »
Two atoms were sitting in a bar, talking over a beer.  One atom says to the other, "How's it going?"
The other atom replies, "Not so good, I'm afraid."
"Oh yeah, what seems to be the problem?"
"I lost an electron the other day."
"Oh man, that's not good at all!  Are you sure?"
"Yep, I'm positive."
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Offline Three

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Re: TDS and Water Hardness
« Reply #13 on: November 25, 2013, 02:10:46 PM »
Two atoms were sitting in a bar, talking over a beer.  One atom says to the other, "How's it going?"
The other atom replies, "Not so good, I'm afraid."
"Oh yeah, what seems to be the problem?"
"I lost an electron the other day."
"Oh man, that's not good at all!  Are you sure?"
"Yep, I'm positive."

Wow....
Anyone who sings a tune so sweet is passin by........