t.d.s and temperature

[Colin O]

I've recently aquired a pretty basic tds meter and it got me wondering. The instuctions for many of these things say that they are for testing at a specific temperature, in my case 25ºc. Does this mean that if the water sample I want to test is not at this temperature the results are useless and I should heat /cool it until it is ? Is there a equation or graph that will covert the meter readings I get to the equivalent at 25ºc ? Sorry if this is a stupid question.

[chem geek]

First of all, why are you measuring Total Dissolved Solids (TDS) at all? It's pretty useless since no problems are directly associated with TDS. Problems with pool water chemistry that occur over time mostly have to do with increasing Cyanuric Acid (CYA) levels which make chlorine less effective often leading to algae growth unless a supplemental algaecide is used. TDS is mostly composed of calcium chloride and sodium chloride salt. Saltwater Chlorine Generator (SWG) pools have much higher TDS since they have around 3000 ppm salt (so their TDS is around 3200 ppm).

The TDS meter is measuring the conductivity of the water and then based on ASSUMPTIONS about what is in the water, the TDS is estimated. You cannot know what the TDS is unless you know roughly the components of the water. This is because TDS is a weight measurement while conductivity is more directly related to a molar (molecular) measurement and is also very affected by the charge of the ionic species. The conductivity is also affected by temperature. You can read more about this here where a rough rule of thumb for sodium chloride solutions is an increase in conductivity by 2% per ºC (so per 1.8ºF). Even more detailed information may be found in this PDF file.

However, as I said before, why are you concerned with TDS? The only real use would be to get a rough idea of your salt (chloride) level and there are other ways of measuring that as well.

Richard

[bobobi-wan]

I have 2 pentair heaters and both heat exchangers went down due to chemical erosion(pentair rep confirmed).both spas had consistently high fc.we have cat 4000 poolcomm monitors.high tds will cause false orp readings causing high chlorine.the spas are less than a year old and had a low bather load and so I didn't drain the spas completely altho they were partially drained every 6 weeks.this allowed the tds to accumulate thus the above result.has this happened to anyone else?

[Pooltech32]

not me, I have 22 buildings I look after and the only thing TDS has ever done for me is it makes my spas foamy, if you are going through exchangers it is usually due to made chemistry, low PH, high PH, high chlorine etc. I drain my spas completely every 2 weeks, I have several that I drain twice a week due to high bather load, for those I installed a pool to spa fill so they are getting balanced water and it also helps me get lots of fresh water in the pool.
I do not think that temp will affect your readings that much unless they are extreme.

[chem geek]

I suspect that since these sound like commercial/public spas, that they have no Cyanuric Acid (CYA) in them. That means that the active chlorine concentration will be very high, especially if the Free Chlorine (FC) level goes up due to an improperly functioning ORP control system. Now I know the "official" rule is not to use CYA in indoor pools nor in spas because there is no sunlight, but without a little CYA in the water, any chlorine is going to be MUCH stronger. CYA acts as a chlorine (hypochlorous acid) buffer moderating its amount.

If just 20 ppm CYA were used, then 4 ppm FC at spa temperatures would be technically equivalent to 0.9 ppm FC with no CYA (at lower temperatures, it would be equivalent to 0.2 ppm FC with no CYA). This is still high enough for fairly rapid oxidation of bather waste and rapid killing of Pseudomonas aeruginosa (hot tub itch bacteria) but not so high as to wear on equipment quite so rapidly.

If the TDS, and specifically the chloride level, got quite high, in the thousands of ppm, then this could corrode certain metals faster -- aluminum most especially, though stainless steel and copper to a lesser extent. So it is possible for the combination of high TDS along with high chlorine levels to cause faster corrosion if the heat exchangers were copper instead of cupro-nickel or titanium.

You can reasonably approximate the TDS buildup if you know how much chlorine you've added over time. For every 10 ppm FC added by sodium hypochlorite (chlorinating liquid or bleach), you end up with 16 ppm salt (sodium chloride). You get 8 ppm from the initial addition and then as the chlorine gets used up an additional 8 ppm.

Richard

[bobaz-union]

it's an outdoor facility.spa cya is usually 10 to 30.spa consistently read high chlorine at 6to8 range every morning unless I turned off the chlorine feeder the night before.I treated the high chlorine with thiosulfate which lowers ph and orp.to offset that I added a nonchlorine oxidizer which spikes orp to 700-800.I wonder if I somehow got a low ph (7.3)combined with low alakinity (60)and got aggressive heated water that caused the chemical corrision.I test the water 2x a day except on wknds, 1x.Could damage happen in less than 2 days?

[chem geek]

It's very unlikely for corrosion to occur in just a couple of days unless the pH was extremely low or the chlorine extremely high and not moderated by CYA. A pH of 7.3 with a TA of 60 would not cause rapid corrosion nor generally cause corrosion of metal at all, though it could dissolve plaster if the Calcium Hardness (CH) were not sufficiently high to have a near-zero saturation index. Also, 6-8 ppm FC with 10-30 ppm CYA at a pH of 7.3 and 104F temperature is technically equivalent to around 0.8 to 3 ppm FC with no CYA. Though higher than you'd want, it shouldn't cause rapid corrosion (certainly not in days).

So let me get this straight. You have a CAT 4000 controller. What is the source of your chlorine that is getting controlled? Is it chlorinating liquid fed by peristaltic pumps? You say that the CYA is usually 10 to 30 ppm, but how did you add that to the water? CYA only comes from explicit addition of Cyanuric Acid or use of stabilized chlorine (Trichlor or Dichlor). If Trichlor is being used, then this is very acidic and would result in lower pH and TA -- the pH can even "crash" (drop rapidly) if the TA gets too low. However, continued use of Trichlor would increase the CYA level over time. For every 10 ppm FC added by Trichlor, it also increases CYA by 6 ppm.

Richard

[bobobi-wan]

chlorine is fed by peristaltic pumps.cya is added thru the skimmers altho the 3000 gal. spa was hard to maintain cya at 25 to 35.I used to keep the chlorinators filled with tabs but since my chlorine readings were constantly high it was suggested I stop using the tabs.I dialed the pumps down to 3 to slow the chlorine feed which helped somewhat.

[chem geek]

So I can understand the corrosion when you were using tabs as they are very acidic, so if there wasn't a functioning one-way valve preventing water from flowing slowly from the tabs to the heater when the pump was off, then that would make sense. However, since you've removed the tabs, I don't know why the corrosion still rapidly occurred. Sorry I'm not of much help here.

[bobobi-wan]

I just posted a question about the ryznar stability index,I'm hoping this will shed some light on my aggresive water problem as mgmt is reluctant to replace the heaters until the problem is identified and resolved.

[Me...]

7.3 pH is not low and 60 Alk is not critical, well depending on other factors. You never mention Calcium readings. And a steady 6-7ppm chlorine reading would be brutal on a heater. When you use thiosulphate make sure to isolate your probes, hopefully you use a flowcell, or you will toast them.

it's an outdoor facility.spa cya is usually 10 to 30.spa consistently read high chlorine at 6to8 range every morning unless I turned off the chlorine feeder the night before.I treated the high chlorine with thiosulfate which lowers ph and orp.to offset that I added a nonchlorine oxidizer which spikes orp to 700-800.I wonder if I somehow got a low ph (7.3)combined with low alakinity (60)and got aggressive heated water that caused the chemical corrision.I test the water 2x a day except on wknds, 1x.Could damage happen in less than 2 days?