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Old 09-18-2012, 04:34 AM   #1
LargePiece
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Default is a tds meter much use?

I've just got a ph meter which sounds like it will be helpful for checking my mash ph. I'm considering a tds meter for testing our water hardness but if it doesn't give me any information on the makeup of the dissolved solids is it going to be of much use? We're on tank water so I haven't got access to a water report.

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Old 09-18-2012, 12:22 PM   #2
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A TDS (total dissolved solids) meter can be useful in a small number of cases. It can give the user an idea of the gross mineralization of the water supply, but with no indication of what ions it contains. If the TDS varies, the cause of the variation in the shift in TDS would have to be understood before any conclusion on water profile could be made. If the water company draws water from two or more starkly differing sources, the meter might help the user discern when this has occurred. If the water profiles from those sources were known, then the user might be able to guess what the current profile is. That's pretty iffy!

The other use that a TDS meter is well suited for is when using RO treatment. RO water from a properly operating unit should have a TDS of less than 10 ppm. In many cases, it can be less than 5 ppm. If the water presents a reading higher than this, it is an indication that the unit is failing and the assumption of nearly ion-free water cannot be used. Its then time to fix the unit or find another source (assuming the RO is purchased from a vending machine).

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Old 09-18-2012, 12:53 PM   #3
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A handheld TDS meter has been handy to know how good my grocery store RO unit is working when buying brewing water. My city water is about 400ppm TDS and the grocery store water is consistently <25 ppm, usually about 15 ppm. So far I would have been fine without the meter, but it's a $20 tool to eliminate a variable.

http://www.tdsmeter.com/products/tds3.html

I lent it to a co-worker too who was curious about his city water TDS and ended up finding a plumbing issue with his refrigerator water dispenser that showed TDS the same as his softened water.

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Old 09-19-2012, 04:03 AM   #4
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Quote:
Originally Posted by mabrungard View Post
RO water from a properly operating unit should have a TDS of less than 10 ppm.
I agree in part, but that's not completely true. We would all like to have our RO put out lower than 10ppmTDS, but most don't. I try to explain to customer's why I can get their RO to produce twice as good as they can all the time. Some listen, but sadly most don't.

The rule we generally go by is percentage. We want the RO rejection to be 85% or better(90% for rental customers.) Usually, you can get a quality RO to run at 90+% for 10+years. A lot of times the lower end "box store" units will give you 90+% for only a couple of years.

In short, if your supply water to the RO is 400ppmTDS we strive for a maximum of 40-50ppmTDS from the RO. A little higher isn't a big deal, but still the lower the better.

In closing; my supply to my RO is 360-500ppmTDS. My RO has ran at 10ppmTDS or lower for over 13 years. (Just showing that you CAN get a quality RO to produce great RO water for much longer than most people think or say.)
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Old 09-19-2012, 12:33 PM   #5
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I have to agree that TDS < 10 from feed with a TDS of 500 would be impressive indeed - average rejection of 98%. Keep in mind that some ions are rejected more than others so what you get depends not only on the TDS but on the composition of it. The real value of TDS with respect to RO systems is in looking for and noting change. The meter in my unit has two probes - one in the feed stream and one in the permeate stream. If I see permeate TDS increase I can push the button and check feed TDS to see if that's the cause. If it isn't then I'd better have a look. I installed a Hobbs hour meter in the system and so am able to record TDS against hours in the log and would be able to detect a trend. This system has been in service about 2 yrs now and output TDS has remained less than 5 (touch wood). I used a couple of the cheapie GE units from Home Depot for 5 or more years without problems. I try to take care of my membranes which doesn't require that I do much except feed them with softened water. My silica runs about 28 mg/L and that concerns me but it hasn't gummed up (or glassed over) a membrane yet (again touch wood).

Actually, maybe 98% isn't all that impressive after all. My system seems to produce 97% average rejection. I really need to get into it and calibrate those conductivity probes.

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