Mind Swimming (pun intended)

[Doug Hensley]

I'm a new owner & 1st time poster to the Forum.
I have a 1750 gallon in-ground Fiberglass Spa. It has a Salt Chlorine generator.
I have a pH sensor and an ORP sensor. I have an automatic Acid Pump (diluted Muriatic acid 50:50)
I have a controller where I can Adjust pH up or down and I can "change the ORP" up or down.
Issues:
1) The alkalinity is ALWAYS low. It is usually 40-60. The pool store wants it 80-100. I can add sodium bicarbonate and get it up, but by the next day it is back down. If I add alkalinity, the "system delivers more acid to neutralize it. It seems to me that the only way for me to elevate the alkalinity is to Raise the pH on my controller. Right now, I have pH set at 7.6. I've been told that you don't want the pH too high because it lowers the efficiency of the Chlorine sensitizer. I'm under the impression that the main purpose of Alkalinity is to buffer the system to help stabilize the pH. I'm starting to think that with a pH sensor and automatic pump, the alkalinity just doesn't matter and I'm inclined to ignore it.

2) I have poor understanding of ORP. I've been told it "correlates with free Chlorine". My FC is ALWAYS low --- 0.7 to 1.2. I was under the impression that I needed to "increase" the ORP on my controller to raise the FC. I think the goal would be FC of 3-5. I do usually have a slight gap between FC and TC (1.2/2.4). I've been using Non-Chlorine Shock/Oxidizer.(Pool store suggestions). Is this faulty thinking? Currently, I have my ORP set to 790 (range is from 200-900) and today's FC was 1.2 (at the Pool store) (CYA - 5)

I saw some really nice posts from "chem geek" on this subject in 2011. I couldn't figure out how to Direct Message him.

Enlighten me.
Thanks in Advance

 

[Bperry]

I'm a new owner & 1st time poster to the Forum.
I have a 1750 gallon in-ground Fiberglass Spa. It has a Salt Chlorine generator.
I have a pH sensor and an ORP sensor. I have an automatic Acid Pump (diluted Muriatic acid 50:50)
I have a controller where I can Adjust pH up or down and I can "change the ORP" up or down.
Issues:
1) The alkalinity is ALWAYS low. It is usually 40-60. The pool store wants it 80-100. I can add sodium bicarbonate and get it up, but by the next day it is back down. If I add alkalinity, the "system delivers more acid to neutralize it. It seems to me that the only way for me to elevate the alkalinity is to Raise the pH on my controller. Right now, I have pH set at 7.6. I've been told that you don't want the pH too high because it lowers the efficiency of the Chlorine sensitizer. I'm under the impression that the main purpose of Alkalinity is to buffer the system to help stabilize the pH. I'm starting to think that with a pH sensor and automatic pump, the alkalinity just doesn't matter and I'm inclined to ignore it.

2) I have poor understanding of ORP. I've been told it "correlates with free Chlorine". My FC is ALWAYS low --- 0.7 to 1.2. I was under the impression that I needed to "increase" the ORP on my controller to raise the FC. I think the goal would be FC of 3-5. I do usually have a slight gap between FC and TC (1.2/2.4). I've been using Non-Chlorine Shock/Oxidizer.(Pool store suggestions). Is this faulty thinking? Currently, I have my ORP set to 790 (range is from 200-900) and today's FC was 1.2 (at the Pool store) (CYA - 5)

I saw some really nice posts from "chem geek" on this subject in 2011. I couldn't figure out how to Direct Message him.

Enlighten me.
Thanks in Advance

A TA of 50ppm is just fine. Increasing it will make the pH rise faster. Then you add bicarbonate, and the cycle continues. Any pH that starts with a 7 is equally fine. The pH doesn’t affect chlorine when there’s stabilizer in the water. And you definately want it in the water.

 

[PoolStored]

I have pH set at 7.6. I've been told that you don't want the pH too high because it lowers the efficiency of the Chlorine sensitizer. I'm under the impression that the main purpose of Alkalinity is to buffer the system to help stabilize the pH. I'm starting to think that with a pH sensor and automatic pump, the alkalinity just doesn't matter and I'm inclined to ignore it.

You don't want TA to go lower than 50. Do not "ignore" TA, just stop adding bicarb. The risk of pH crash (not good), goes up with TA bellow 50. With a TA of 50-80, your pH bay be in balance with a pH of 7.8-8.0. I'd raise your pH to 7.8 to 8 and see if the swings go away. As to "efficacy of sanitizer," if you follow our recommendation to maintain your FC based on your CYA, you will have sufficient sanitizer. Link-->FC/CYA Levels

Managing pH and Total Alkalinity - Further Reading

www.troublefreepool.com

PH TA Relationship - Further Reading

www.troublefreepool.com

2) I have poor understanding of ORP. I've been told it "correlates with free Chlorine". My FC is ALWAYS low --- 0.7 to 1.2. I was under the impression that I needed to "increase" the ORP on my controller to raise the FC. I think the goal would be FC of 3-5. I do usually have a slight gap between FC and TC (1.2/2.4). I've been using Non-Chlorine Shock/Oxidizer.(Pool store suggestions). Is this faulty thinking? Currently, I have my ORP set to 790 (range is from 200-900) and today's FC was 1.2 (at the Pool store) (CYA - 5)

The issue with ORP is that is doesn't work with CYA above 20-30. Any ORP can be finicky. You really want CYA in your pool water. While it is true that cyanuric acid protects chlorine from excessive UV photolysis, it also has a much more important benefit - active chlorine buffering. CYA acts as a chlorine buffer (holds chlorine in reserve) and reduces the levels of active chlorine in water to ppb levels. This is VERY important for both bather comfort as well as the formation of disinfection by-products. 1ppm FC with zero CYA in the water has almost 5X the active chlorine levels (hypochlorous acid) as water with 3.75ppm FC and 50ppm CYA. The difference in bather comfort is very profound which is why people who use public pools a lot (public pools typically are barred from using CYA) find that their hair is dry, their eyes sting and their swim clothes don't last very long. I'd try to get 20ppm if you continue with ORP, or 50-60 if you can duty-cycle (see below).

I would not use non-chlorine shock. MPS will mess with your CC. MPS is an oxidizer and not a sanitizer. Chlorine is both an oxidizer AND a sanitizer. MPS breaks down and adds sulfates to the water, and sulfates, at high enough levels, cause metal corrosion. Best to use liquid chlorine, or get a salt water chlorine generator.

Which system do you have? Can you run it on a duty-cycle based method without any closed-loop feedback from a probe? Probes add a complexity to the management process of a pool that is really unwarranted in most residential situations. There is no need to control pH to any absolute value - a pool is perfectly swimmable with a pH anywhere between 7.2 and 8.0. Anyone that tells you it has to be an exact value, like 7.4 which everyone quotes as being suitable for the human body, simply does not understand the chemistry involved.

A simplified acid dosing system used in conjunction with reagent based testing will be more than sufficient to keep your pool clean and healthy to swim in.

How are you testing your water? Our recommendations...Link-->Test Kits Compared

Oh, and you really want CYA in your pool water. While it is true that cyanuric acid protects chlorine from excessive UV photolysis, it also has a much more important benefit - active chlorine buffering. CYA acts as a chlorine buffer (holds chlorine in reserve) and reduces the levels of active chlorine in water to ppb levels. This is VERY important for both bather comfort as well as the formation of disinfection by-products (DBPs). 1ppm FC with zero CYA in the water has almost 5X the active chlorine levels (hypochlorous acid) as water with 3.75ppm FC and 50ppm CYA. The difference in bather comfort is very profound which is why people who use public pools a lot (public pools typically are barred from using CYA) find that their hair is dry, their eyes sting and their swim clothes don't last very long.