Question about TDS

[Bwdonohues58]

I just purchased a TDS meter from Hanna Inst. Pretty easy to use and cost $52.00. I checked my fill water and it's about 86. My pool does not use CYA or SWG. The only chemicals I add on a regular basis are Chlorine and Muriatic acid. The TDS is consistently about 1050. The CH is 200, ALK around 60-70 and I keep the temperature at 95 degrees F for Physical Therapy. I try to keep the PH at 7.5-7.6. Just wondering if a TDS of 1050 is acceptable, high, or low?

 

[mgtfp]

That TDS is basically salt. The chlorine you are adding will be turned into salt after it's done its job. I don't think there is anything to be worried about. SWG pools have usually about 3000-4000 ppm of salt. Your level of about 1000ppm is pretty normal for liquid chlorine pool after some usage.

 

[Bwdonohues58]

Thanks mgtfp. I'm thinking about dumping the water after 1 year, around July 1, to start over, clean the filters, etc. The tap water is pretty cheap compared to everything else here in Alaska.

 

[ajw22]

Measuring TDS does not tell you anything good or bad about your water chemistry. You have to measure the components of TDS we care about- CH, CYA, Salt - and determine if their levels are good or not.

TDS - Further Reading

www.troublefreepool.com

 

[JamesW]

If the meter has a salinity reading, you should set it to read salinity (NaCl).

If the reading is calibrated to a 442 “Natural Water” solution, you should convert the reading to salinity.

1,050 ppm 442 solution is about 1,500 Microsiemens Per Centimeter (µS/cm) (aka micromhos/cm), which is about 750 ppm in units of NaCl as seen on the below graph.

Standard Solutions & Buffers | Myron L® Company

Ensure specified water testing instrument accuracy. See a list of our Standards Solutions and Buffers (custom solutions available).

www.myronl.com

 

[Bwdonohues58]

The meter comes with a 1382 ppm buffer solution for calibration. It does not measure NaCl Separately, My CH is 200, ALK is steady at 65, zero CYA. Tap water measures 86 ppm TDS. I have not added any salt. I assume the 1050 reading is mostly salt resulting from adding liquid chlorine on a daily basis with a Stenner unit plus PH correction weekly with around 8 ounces of muriatic acid. I assume TDS will continue to rise. So far, since June 15, I have used about 35 gallons of 12.5% liquid chlorine and about 3/4 gallon of MA. I used dry acid for the first 2 months and put in about 10 pounds of that. I do not add other chemicals. I did dump about ⅓ of the pool volume a few months ago to reduce the CH from 350 to 200. I did not like the white buildup around the water level. I vacuum the pool almost every day and get mostly a little hair and white stuff that might be skin cream residue. ORP stays at 745 plus or minus. The water is very clean and clear. People like it. I am just wondering at what point the TDS would become an issue?

DiST 1 Waterproof TDS Tester (0-2000 ppm) - HI98301

The Hanna Instruments HI98301 DiST®1 TDS tester is a rugged and reliable pocket-sized tester that offers quick and accurate low-range readings of TDS.

www.hannainst.com

 

[JJ_Tex]

I am just wondering at what point the TDS would become an issue?

That is the problem with using TDS. There is no magic number on TDS as not all solids are created equal in pool maintenance.

The same would be true if you had a sensor measuring the air quality inside of your house. If it told you there was too much "stuff" in the air, you would want to know what exactly is that stuff. The "stuff" could be intentionally added like an air freshener or your son going crazy on the Axe body spray, it could be seasonal/environmental like pollen, or it could be something really concerning like smoke or carbon monoxide. That is why houses have smoke detectors and carbon monoxide detectors vs just "stuff in the air" detectors.

 

[JamesW]

I am just wondering at what point the TDS would become an issue?

I would suggest that you get a K-1766 salt test kit and check the result against the meter.

 

[JamesW]

Salt pools run between about 3,000 and 4,500 ppm salt. Anywhere below that should not be a problem. The salinity is a component of CSI, so it's helpful to track. Maybe even consider switching to a salt system when the salinity gets in range.

 

[Bwdonohues58]

Thanks to all for the good, sound advice. I opened this pool in mid-June and had no experience managing a pool before that. I did take the 2-day CPO class here in Alaska last February and got a CPO certificate. Other than that, I have obtained almost all my good info from Trouble free pool people and I do appreciate it very much.

 

[mgtfp]

So far, since June 15, I have used about 35 gallons of 12.5% liquid chlorine and about 3/4 gallon of MA. I used dry acid for the first 2 months and put in about 10 pounds of that. I do not add other chemicals. I did dump about ⅓ of the pool volume a few months ago to reduce the CH from 350 to 200.

Thanks for sharing those numbers. I just put these into chem geek's PoolEquations spreadsheet to calculate the effect on TDS. The effects are:
35 gal of 12.5% LQ: 800ppm TDS (PoolMath says 700ppm)
3/4 gal of MA: 30ppm TDS
10 pounds of dry acid: 100ppm TDS

So, with the TDS of 86ppm of your fill water that adds up to pretty much 1000ppm. You dumped 1/3 of water after a couple of months and replaced that with 86ppm TDS fill water. But I assume you will also have considerable evaporation losses (heated water in an A/C environment, I assume), here you replace basically 0ppm TDS evaporated water with 86ppm TDS fill water, so that will account to a steady TDS increase over time. On the other hand, you will probably also have some splashing losses (your's is a therapy pool, right? - so probably not as much splashing as in a backyard pool), that lead to a TDS reduction.

So, exact TDS increase is hard to quantify, but 1000ppm doesn't sound unreasonable.

As others have mentioned, SWG pools run above 3000ppm, my one runs between 4000 and 5000ppm (Aussie SWGs are designed for a higher salt range). I still hardly notice the slightly salty taste.

I don't see a concern with a bit of salt in the water that would require dumping the water, unless there are legal TDS requirements for a therapy pool (does that make it a commercial pool?).

As James suggested, getting a SWG could be an option once you get above 3000ppm. Since you operate your pool without CYA, you could consider an ORP based automated system (no idea how reliable those are...). Bit of an investment, but it adds a lot of convenience, and you don't have to add LC anymore (apart from the occasional manual adjustment dosage) which will stop a lot of the salt increase. And most people here with a SWG find that the salt makes the water feel more enjoyable - might actually be just the thing for a therapy pool...

 

[JoyfulNoise]

If your therapy pool is being used for a physical therapy business that services customers, then you need to follow the advice of your local health department and in-pool equipment manufacturers when it comes to water chemistry. TFP teaches RESIDENTIAL pool owners how to care for their privately owned pools; TFP does not teach commercial pool operators how to care for their public pools. A commercial pool used in a business must follow local regulatory requirements, whatever those may be. If your equipment manufacturers have specs for TDS or other water parameters, then you need to follow those guidelines.

 

[Bwdonohues58]

Thanks Joyful. I called the Alaska DEC office lastApril and they described pool in a town north of here (Wasilla) that had installed a Hydro Works therapy pool two years ago. It was completely out of line with their regs so they decided that Physical Therapy pools were private and not regulated by the State DEC. So I am sort of on my own up here in the last frontier. I do follow all State regs and stay in touch with the ARS lab that tests my water every month. I appreciate very much that TFP does not give advice to commercial pools such as mine and do realize that your advice is just that, no guarantees whatsoever. That said, I think you guys are really smart and experienced and I do appreciate the input. Please realize that all my decisions are mine and mine alone. Nuff said about that.
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