Honestly, sometimes I think someone designed a pool 100 years ago and everyone has copied it. There are many things that affect swimming pool sanitation. Here we will look at how water circulation affects sanitation and what your chemical controller is telling you.
Sure, there are regulations recommending water turnover rates for the different types of pool. However, while the recommended requirement might be achieved, how this achieved leaves much to be desired in many of the older pools.
Well, while we cannot do much about that, we can and need to take that into account when operating a pool chemical controller.
Let’s just look at some of the differences between some 10m x 25m pools that exist.
1. The pool water is drawn by 5 skimmer boxes down one side of the pool with the returns on the other side of the pool.
2. The pool water is drawn by 5 skimmer boxes and suction points under the skimmer boxes down one side of the pool with the returns on the other side of the pool.
3. The water is drawn from 5 skimmer boxes down one side of the pool with the returns underneath the skimmer boxes.
4. The water is drawn from gutters on both sides of the pool with the water returned via several points up the center of the pool.
5. The water drawn from gutters on both sides of the pool with the returns along the walls of the pool and via a grid in the bottom of the pool.
Let’s assume that the water sample that the controllers see is taken in the same manner in all the above pools and that the turnover rate is the same which highly unlikely.
Obviously, there is a big difference between the pools in how long it takes between the time when the controller reads a sample, issues its instructions and it gets to see the result of those instructions.
Of course, we are only looking at a small volume of the water that has been turned over. Apparently for one turnover of the total pool volume only 63% of the water gets filtered as well as sampled for testing and it takes four turnovers of the total pool volume to filter 98% of the water.
No, I don’t know how these figures were arrived at, but common sense says it would have to be different for different pools, for example those above.
Needless to say, not a lot of the water that entered the pool 30 minutes ago would have exited the pool and found its way back to be sampled again.
If there isn’t much change in the pool's activity throughout the day then maintaining a balance without having your chemical levels swing all over the place is easy to do, as you will see them osculating within an acceptable range or even stable.
The rates of chemical supply will definitely affect the swings in chemical levels but that's another story and something else you need to know about a pool.
These relatively stable periods will occur when the pool is being continuously used or not at all, like in the middle of the night. The levels become a little less stable when there are sudden changes in pool activity.
Something as simple as the sun rising or the sun setting will have an effect on sanitation. Another example is if there is a sudden big jump in pool usage. What was a nicely controlled situation can suddenly become quite messy for a while.
The water exiting the pool will have a drop in ORP (or FAC) which will trigger chlorine addition. That chlorine addition will be accompanied by an increase in pH which the controller won't see for quite some time depending on the efficiency of circulation. When the change in pH starts to be registered by the controller, acid (or carbon dioxide) will be pumped into the pool and it will be playing catchup for a while, but because of the delay between delivery and detection it can overshoot the set point. Keep in mind the effect pH has on ORP as this will also affect the read. The opposite might happen at the end of the day when the sun is suddenly gone, and pool activity has ceased you will see you ORP level increase as the controller tests water containing chemicals added when chlorine demand was higher, that is, when the sun was out or the pool was in full use.
Think about the circulation difference between a 25m one lane lap pool and a 25m pool with 10 lanes, where both pools have the same number of skimmer boxes on one side of the pool and the same number of returns on the other side of the pool. Both pools have the same turnover rate. Which pool do you think will have the easier chemical management?
If you have an Aquarius Ultima Controller you can set a super-chlorinate for, say 1am for 1 hour on a suitable day. The next day you can download the data for ORP pump percentage, ORP value, pH pump percentage and pH value. This will give you a very good idea about what is happening when sudden events affecting pool chemistry occur. This is about understanding your pool and more importantly, how to interpretate what you see on your controller screen. With an Aquarius Ultima you also can download a chart for that day which will show the above information in graphic form together on the same sheet, but the real picture is in the data download.
If you can do the exercise, it would be well worth your while doing a dye test to see what is happening with the circulation, especially if you have just taken over the management of a pool. You need to be able to answer the questions. You can then use the super-chlorinate data in conjunction with your observations to give you a clearer knowledge of the pool's circulation.
Having this knowledge and knowing what is happening or about to happen to the pool will give you the ability to use your controller (especially if it is an Aquarius Ultima) to force an action such as adding a chemical for a short period of time to better manage the sanitation. Whenever I use an Aquarius Ultima to manage a pool, I use a timer to force a super-chlorinate for an hour once a week after the pool closes. On one particular pool I found that the acid addition wasn't being triggered until 30 minutes after the super-chlorinate had started, then it would overshoot, and the pH would drop too low. That was thanks to the circulation delay. To solve the problem, I set a second timer to pump acid for several minutes at the start of the super-chlorinate. That worked.
There is no formula, but understanding what is happening in the pool will allow you to use the tools that are available in an Ultima Controller to better assist in managing the health and wellbeing of your patrons.
In theory, a pool might be able to handle a certain number of people but, this may not be the case when you take all the factors involved in achieving the required sanitation level throughout the day. You may have to limit the number of patrons until your system is upgraded.
