It's Easy,
Now for the WHY?
Controlling pH is probably the most important thing you can do in managing a swimming pool. So many things depend on it. The FC (Free Chlorine) level must be at the minimum required but there is a far bit of tolerance (up to 10ppm in Queensland) if it is higher. That’s not the case with pH. You want that as close to the set point as possible, all the time.
Controlling pH in a swimming pool economically can be a tricky process. If this isn’t managed well, you can waste a lot of money.
One of the first things I remember learning in science was,
ACID + BASE → SALT+ WATER
Remember that one?
Well, hydrochloric acid, obviously an ACID, is added to reduce pH.
Sodium bicarbonate is a BASE (within the desired pool water pH range), which is added to increase alkalinity.
Given the above, care is needed when adding these chemicals to a pool as they can knock each other out before they achieve their purpose.
The pH will increase when,
The pH will decrease,
What we are looking for is a pH in the range of 7.2 to 7.8 as recommended by most regulations. We usually choose 7.4, 7.5 or 7.6 when using a chemical controller.
Even with 24-hour dosing, when using a chemical controller, chlorine addition does not happen continuously (and shouldn't). There will be long periods of time when there is no chlorination happening.
During those long periods when there is no chlorine addition, there is also no mechanism for increasing the pH. That upward pressure just isn’t there. This is where the alkalinity level comes in.
To have a pH above 7 (pure water) the water needs to be alkaline to some degree. There needs to be more OH-than H+ ions in the pool water. The more alkaline the water is then the higher the pH. The required pH range is barely above neutral.
To achieve this
This is where it gets difficult. With either method if we add or create too much sodium bicarbonate ($) the pH will get and be held too high which will require another chemical, usually hydrochloric acid) ($) to bring the pH down to where we want it.
When the pH is controlled by acid, a controller will keep adding the acid until it belts the pH down to a stable level and subsequently lowering the alkalinity to a useful level, at which point the acid addition will slow down. When pH is controlled by carbon dioxide injection, the only way to bring the alkalinity down is to turn carbon dioxide injection off and add acid until you achieve the desired alkalinity level. This can be managed quite easily by the right type of chemical controller.
In a large pool it certainly can be a big expensive ($) deal if pH isn’t managed diligently. With a backyard pool you may prefer to buy a carton of beer rather than waste money on chemicals. Diligently means it needs to be supervised like a 2-year-old.
This is not too onerous when using a quality controller with remote monitoring which will alert you to a problem. It puts an alarm on your 2-year-old. Otherwise, you can sit on the side of a pool with a test kit, a stack of chemicals and no beer.
Even with a chemical controller the pool will need a physical attendance at least once a week by a service technician to test and add the correct amount of sodium bicarbonate and top up the acid drum, plus everything else that needs doing. Duties otherwise completed by full time attendants.
The more the chlorine is added to your pool, the more your pH increases, the more acid is added and subsequently, the quicker the drop in alkalinity. The risk here is that you can be tempted to add heaps of sodium bicarbonate in one go. If you do that you had better increase your acid order. It is far better to test and add smaller amounts of sodium bicarbonate more frequently.
Remotely monitoring your pool will reveal cause and affect with chemical additions. Remote monitoring will teach you what is happening in your pool.
The chart is from a remotely monitored liquid chlorine pool. You can see the acid additions needed and the chlorine addition to the pool.
pH control clearly demonstrates the power of remote pool monitoring which can be greatly enhanced with site service reports. It helps when you know what has happened on site when you look at a chart like the above. Site service reporting is a very easy and a quite inexpensive system to set up. Site reporting can also be setup to be accessed remotely. It's as simple as placing the reports on One Drive.
Alkalinity Level
As you are adding chemicals to a pool your bank account is going down. Understanding the relationship between alkalinity level and pH is important when it comes to saving costs.
We can set the alkalinity to the top level in the recommended range and fill a big acid drum to the brim and let it rip or, we can manage the alkalinity level that will give you the pH result you want most of the time. The first method will more than double your acid and sodium bicarbonate costs. This isn't a guess. I achieved this cost saving by better handling pH management on a pool.
Before we can choose a correct alkalinity level, we need to look at what causes persistent drops in pH. Increases in pH can be responded to quickly by pumping in chemicals. It is not so easy when the pH drops below your setpoint.
The two things that I have identified as causing the drop are,
When the alkalinity drops below 60 ppm the resultant pH, when it drops, will remain stubbornly too low.
The second cause is more difficult. Pool water circulation has a big effect on how quickly the pH will settle down throughout the pool after suddenly adding chemicals at a high rate.
Keeping the alkalinity between 60ppm and 80ppm is the correct range. To achieve this you need to take into account,
Pool Usage - The higher the pool usage, the higher the chlorine demand and addition, the greater the upward pressure on the pH and, the greater the demand for pH reduction which also will lower the alkalinity. This will result in the need to respond quickly with testing and adjusting. Just a note here, the chemical pumps and/or saltwater chlorinators should be well able to handle the demand for chemicals such that they do not run continuously for extended periods of time.
Testing Frequency – Testing alkalinity frequency should be a response to the rate of chlorine addition. For a heavily used pool testing and adjusting should be done daily. It is far easier to control the alkalinity level and manage chemical additions if you are adding small quantities at a time. You can keep your range to say 60 to 80ppm instead of too high an upper limit.
A high upper limit will increase your acid usage and make managing both pH and alkalinity levels more difficult if you are using carbon dioxide injection.
When using a chemical controller with remote monitoring you can see when the alkalinity is out by the way the pH is behaving. If you are using both carbon dioxide and acid to control your pH you will be able to see when you should swap your pH control from carbon dioxide injection to acid addition and visa, versa. You can switch them over remotely with the right controller. How good is that?
The following is the pH graph for a week from a pool I manage using the above method. The sharp spike on the 16th occurred during a backwash. As you can see there is very little acid consumption, and the pH mostly stayed very close to the set point and even at its worst it didn't drop below 7.4. The acid pump operates at 5.6 litres per hour on a 1,300,000 litre liquid chlorine pool. Sodium bicarbonate weekly consumption sits between 10 and 15kg.
A different pool would show different rates of consumption depending on the rate of pool usage. You should still be able to maintain pH stability.
The following graphs show the difference between a pool that has good alkalinity management and not so good alkalinity management.
You can see that the controllers have maintained pretty good pH stability for both pools. The acid consumption in the Fig. 4 pool is way higher than for the Fig 3. pool. This tells me that the alkalinity level in the Fig. 3 pool is better managed than in the Fig.4 pool. Acid and sodium bicarbonate both cost money.
This also demonstrates the power of an Aquarius Ultima Chemical controller and the very real benefits of remote monitoring of a pool by someone who knows what he/she is looking at.
Several years ago, I attended an aquatic conference when I was approached by a swim school operator whose complaint was that he was chewing through the sodium bicarbonate. At that point I hadn't questioned the commonly recommended alkalinity range. If I was asked that question again, I would want to know more about the
Gee, that's a bit isn't it. There would have been further questions as the interrogation went on. Still, it comes down to "Manage your alkalinity to control your pH to contain running costs".
Don’t forget that once you decide to operate on a lower alkalinity level you will need to increase (usually) your calcium hardness level in order to maintain water balance.
