Rising Salt Level

[bdavis466]

Correct. The pool would be around 1501ppm.

BUT... before adding the 10,000 gallons at 1ppm, the pool contained 10,000 gallons at 3,000ppm. Where did the other 1499ppm go?

You said you DRAINED it???? Only the refill would dilute it

Regardless. As you indicated in your original post, the only way to go from 3200ppm to 4300ppm is by adding 300lbs of salt which could not possibly have come from a couple of gallons of acid (or anything).

So, maybe your original reading of 3200 was before the salt had fully dissolved although salt will usually dissolve within a couple of hours (24 hours max).

My test results were consistent up to two weeks after adding the salt, then I began to see a gradual rise. I've added well over 100 gallons of acid in this time span.

I will admit that when I converted to SWG, I was using the salt test strips incorrectly resulting in a false low reading so I added more salt before realizing the error. I had to do a 40% drain/refill to fix it.

I've had the Taylor test from day one, knew the exact volume of water of my pool, tested prior to adding any salt (600 ppm), and added the correct amount of salt (per pool math) to get me to 3200 ppm (confirmed several days afterward).


Draining and evaporation are completely opposite. There was no mention of draining and refilling until you brought up your example which does not apply to this situation.

 

[chawkins99]

You said you DRAINED it???? Only the refill would dilute it



My test results were consistent up to two weeks after adding the salt, then I began to see a gradual rise. I've added well over 100 gallons of acid in this time span.



I've had the Taylor test from day one, knew the exact volume of water of my pool, tested prior to adding any salt (600 ppm), and added the correct amount of salt (per pool math) to get me to 3200 ppm (confirmed several days afterward).


Draining and evaporation are completely opposite. There was no mention of draining and refilling until you brought up your example which does not apply to this situation.

WOW!!! That's half a gallon of acid every day? I've used less than 2 gallons in 9 months. Although I have no experience of gunite pools (new or old) so not sure if that's normal but seems crazy high to me.

Maybe using draining as my example was confusing but the theory is sound.

Let's use an extreme example of evaporation...

If we start with 20,000 gallons at 3,000ppm salt and lose 50% (10,000 gallons) to evaporation, the salt concentration will be at 6,000ppm. Same amount of salt but half the water.

If we now add back the 10,000 gallons of fresh water at 0ppm salt, we get back to 20,000 gallons at 3,000ppm.

If, instead, we were to add 10,000 gallons at 6,000ppm (the same concentration as is already in the pool), the result would be 20,000 gallons at 6,000ppm. i.e. no change in concentration as we are adding water at the same concentration.

If, instead, we add 10,000 gallons at 3,000ppm, we are DILUTING the 6,000ppm water and would end up at around 4,500ppm. So, adding water at 3,000ppm to water at 6,000ppm results in a concentration somewhere mid-way.

I'm not sure I can explain this any clearer.

However, I'm still at a loss as to how you could go from 3200ppm to 4300ppm with anything other than raw salt.

 

[chawkins99]

Just had another thought.

Again, not my area, but could salt be leeching from the plaster?

I have no idea what the salt content of pool plaster is...

 

[bdavis466]

If you DRAIN half the volume and you refill with 0 ppm salt, the concentration will be cut in half (3,000 ppm in the scenario above) down to 1500ppm.

If half the volume EVAPORATES and you refill with 0 ppm salt, the concentration will stay the same at 3,000 ppm.

No one ever stated that the fill water contained 0 ppm salt (in this situation it contains 50 ppm) so any water lost due to evaporation is being replaced by water that contains salt, thus INCREASING the concentration (50 ppm per gallon DILUTED in 33,000 gallons of water at 4300 ppm) causing the salt level to increase. This was clearly stated in the original question, however, fill water alone does not account for the 1100 ppm rise in 7 months that was experienced.

Your explanation is perfectly clear but has bounced back and forth between draining and evaporation which have two very different effects on concentrations in the water, and neither accounted for the fill water and what it contains.

I appreciate your help nevertheless...

 

[bdavis466]

Just had another thought.

Again, not my area, but could salt be leeching from the plaster?

I have no idea what the salt content of pool plaster is...

Possible but I have no way know for sure...good thought though. I did a bicarbonate start up, I don't know if it is possible for some of the sodium in the sodium bicarbonate to have made its way into the plaster during the formation of calcium carbonate.

I'd always assumed that my initial 600 ppm of salt after the fill was from the chlorine, bicarb, fill water and a small amount of calcium chloride.

 

[JoyfulNoise]

There is no salt (chloride) in plaster. Some cement formulations use calcium chloride as an accelerant for curing BUT that is typically NOT done for pools.

Were you adding any baking soda to control the TA or just neutralizing the TA build up from your tap water supply?

Remember that muriatic acid contains some residual salt (minor amount) from the manufacturing process but forms chloride (Cl-) as it eventually reacts with the alkalinity in your water -

20 Baume MA = 370.88 kg/m^3 HCl in water.

So,

370,880ppm * (100/33000) = 1124ppm Cl-

That assumes all of the muriatic acid reacts to eventually become chloride and assumes full strength MA. Backwashes and water exchanges would lower the chloride level slightly over the swim season.

Your salt is coming from the acid use. Remember MA is "hydrogen chloride" and your "salt" test measures chloride, all chlorides not just that from salt. Since the chloride is not volatile and stays in the water, it will register on your test.


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[JamesW]

How did your calcium level get so high?

If possible, try to get a salt test with a conductivity meter.

 

[bdavis466]

Matt, thank you so much. I've never seen a formula for calculating acid additions, and that definitely solves the mystery. I knew acid added chloride but everything I've read indicated it was an insignificant amount....1124 ppm is far from insignificant!

The baking soda was for the bicarbonate start-up and ended up being around 350 pounds. I haven't added any additional since I brought the TA down to normal levels. I struggle enough to keep up with the rate that the TA rises. I don't backwash and have yet to change out any water, but my days are numbered.

Thanks again...where were you when I needed you most????

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How did your calcium level get so high?

If possible, try to get a salt test with a conductivity meter.

Fill water is 125 ppm, 1" of evaporation is 843 gallons and pan evaporation charts indicate 90-100" per year of evaporation. I have a lot of surface area and get tons of evaporation (in addition to an average depth of 3.25 feet).

Doesn't the salt cell determine salinity through conductivity?

 

[JamesW]

Note that the chloride in muriatic acid is chloride in the bottle and in the pool without needing to react with anything.

Every gallon of acid contains about 2.92 lbs of chloride, which equates to about 4.81 lbs of salt per gallon. Every gallon of acid would add about 17.4 ppm salt. 100 gallons of acid would add about 1740 ppm salt.

Some salt systems use a conductivity meter and some use the performance of the cell. Some might use both, but I don't know if any do. I'm pretty sure that the IntelliChlor does use a conductivity meter.

If the system and the K-1766 agree, the level is probably accurate. However, when there's doubt, I like to triple check.

 

[JoyfulNoise]

Thanks again...where were you when I needed you most????

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Doesn't the salt cell determine salinity through conductivity?

I actually remember seeing this post when you first put it up and I was about to answer it but then had to run out to school to get my kids. When I got back, it slipped my mind and got buried under newer posts. It wasn't until it got bumped again that I saw it and remembered your question.

The IntelliChlor line has a built in conductivity probe which is actually nothing more than two small Cu electrodes to measure conductivity. The one thing I don't like about the IC's is that they are programmed to "measure" the "salinity" once at startup (after a 2 min warmup period) and then once again every 12 hours. There's no way to make the unit do an "instant read" without cycling the power on the cell. Kind of an annoying feature but I guess it keeps the Cu probes from wearing out from excessive conductivity measurements.


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[JamesW]

If you're using that much acid, then you're probably trying to keep the TA too high and the pH too low.

 

[JoyfulNoise]

I think the primary issue here is the OP had a new pool (plaster) and used the bicarbonate startup approach. The bicarbonate startup used 350lbs of baking soda. Given that amount of bicarbonate and the fact that the plaster is fresh, I am not at all surprised that he used 100 gallons of acid.

Going forward, the prescription would be to lower the TA (and possibly add borates) to find a stable point for the pH so that acid additions can be minimized. Strategic draining and the use of rainwater collection could help in lowering the salinity.

And, like me, the OP should get a pool cover deployed to reduce evaporation.


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[JamesW]

350 lbs of baking soda would take about 50 gallons of acid to neutralize and would add about [edit]870 [end edit] ppm of salt. That should have mostly happened in the first 30 days. The salt level was measured before adding salt. If we assume that about 63 gallons of acid were added after the salt was added, then that could explain the salt rise.

 

[bdavis466]

I actually remember seeing this post when you first put it up and I was about to answer it but then had to run out to school to get my kids. When I got back, it slipped my mind and got buried under newer posts. It wasn't until it got bumped again that I saw it and remembered your question.

The IntelliChlor line has a built in conductivity probe which is actually nothing more than two small Cu electrodes to measure conductivity. The one thing I don't like about the IC's is that they are programmed to "measure" the "salinity" once at startup (after a 2 min warmup period) and then once again every 12 hours. There's no way to make the unit do an "instant read" without cycling the power on the cell. Kind of an annoying feature but I guess it keeps the Cu probes from wearing out from excessive conductivity measurements.

You put your kids ahead of the salt level in my pool???? You must have forgotten that I've been keeping tabs on your calcium level after all!

If you're using that much acid, then you're probably trying to keep the TA too high and the pH too low.

I am far from a chemist, but I do understand the basic pool chemistry (hopefully chloride from acid additions isn't basic ). I really only hang out in the deep end and 201 anyway....

I maintain a pH of 7.6-7.8, a TA of 60 and a CSI of -0.1 to -0.3. I've tried getting the TA down to 50 but it does not stay. I run the spillway 30 mins a day (for spa chlorination) and have the air vent closed for the jets. I have minimal aeration, in fact, it seems my TA rises faster than the pH does. I haven't noticed any difference in acid demand with the TA anywhere between 60-90.

I think the primary issue here is the OP had a new pool (plaster) and used the bicarbonate startup approach. The bicarbonate startup used 350lbs of baking soda. Given that amount of bicarbonate and the fact that the plaster is fresh, I am not at all surprised that he used 100 gallons of acid.

Going forward, the prescription would be to lower the TA (and possibly add borates) to find a stable point for the pH so that acid additions can be minimized. Strategic draining and the use of rainwater collection could help in lowering the salinity.

And, like me, the OP should get a pool cover deployed to reduce evaporation.


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My TA being at 60 and 50 ppm of borates (in the water from 30 days after the initial fill) keep the CSI very negative. Taking the TA lower would prove to be very difficult and likely would be of little benefit. I can deal with the salt level, that's easy enough to lower, its the calcium that I know will be the constant battle.

I will be trying to get everything El Nino has to offer in my pool this winter.

350 lbs of baking soda would take about 50 gallons of acid to neutralize and would add about 840 ppm of salt. That should have mostly happened in the first 30 days. The salt level was measured before adding salt. If we assume that about 63 gallons of acid were added after the salt was added, then that could explain the salt rise.

You both definitely nailed it now that I've seen the math behind the acid additions. I am certain that this is where my salt rise has come from. Thanks to you both.

 

[JamesW]

Note that the Intellichlor does compensate for temperature in the salt reading. If the temperature sensor fails, then the reading will be off at any water temperature other than 77 F. At 55 F, the salt reading will be low by about 750 ppm, and at 90 F, the salt reading will be high by about 600 ppm.

 

[bdavis466]

Thanks, my water temp is currently 68 and the cell and the Taylor test are both indicating 4200.

Its unfortunate that I will have continued troubles with salt and calcium even though my fill water is nowhere near as bad as some of the others I've seen.