When operating a pool where the sanitation is controlled by the ORP level you need to know what Free Chlorine (FC) level that the ORP level you see on your controller screen is telling you is in the pool.
Most of us are familiar with the DPD test which tells us what the Free Chlorine level is in a swimming pool which we keep within a required range. ORP is another way of measuring the sanitation in a body of water.
ORP stands for the Oxidation/Reduction Potential of a solution of water and is measured in millivolts (mV). It is measured using an electrical probe, the workings of which are of no interest here.
Simply, ORP measures the killing power of all oxidants in a body of water. Health Departments generally require a level of 720mV or 740mV. An ORP of 630mV is as low as you would ever want to see the ORP in a swimming pool.
When managing the sanitation of a pool using ORP you need to know what is creating that ORP read. What chemicals are creating it. According to the scientists, the ORP read that you see is that of the dominant oxidant in the water. Swimming pools need a strong oxidising agent to stay on top of the many nasties that find their way into a pool. For this we mostly rely on hypochlorous acid to do the job.
To ensure that this is the case we are usually required by health departments to do at least one daily DPD test to confirm the chlorine level in the pool. This is important. You need to know that the ORP read you see is caused by hypochlorous acid especially when using secondary sanitation systems. You also need to know what the Free Chlorine (FC) level is, that the ORP you see on the controller screen is telling you is in the pool. I call this the ORP-FC Relationship.
Before we go into that we need to state that the figure you see as the ORP is the strength of the ORP. You don't have to find out what the pH is to get a mental picture of what the strength really is.
Free Chlorine, on the other hand is affected by the pH. Your test result will tell you how much chlorine is in the pool but not how strong it is.
If you have an ORP probe sitting in a tub of water and you add acid to the water, you will see the ORP increase quite drastically. The reverse happens if you add sodium carbonate. If you do a DPD test before adding the acid and one after adding the acid, you will get the same result.
The following graph is taken from an Aquarius Ultima Controller. The pool is 1,300m3 old 25m pool with a club foot deep dive section. Circulation isn’t great and there is a duck problem. Turnover is not as high as we would like it to be due to the pipe size. The ORP level (instead of the FAC level) has been set to operate a liquid chlorine chemical pump. The levels are kept higher to compensate for the limitations just stated.
The ORP-FC Relationship changes everything about understanding ORP sufficiently for you to be able to use ORP in managing a swimming pool. You don’t have to wrap your head around trying to understand what is, quite complex science for most of us.
On a controller you need to be able to set the ORP set point at a level that will result in a pool having sufficient FC in it during the period of highest demand during the day. This is particularly important when using saltwater chlorination or when your chemical pump is not quite big enough.
Before you can start working this out you need to be certain that the pH is at you set point and that it remains there as you will need to take several tests throughout the day while you are establishing this important relationship.
You might find that you will have to keep the ORP much higher than expected in order to maintain an authorities' FC required level.
Another very important variable that you have to watch is when you change the cyanuric acid level in a pool, if you use cyanuric acid. When you add cyanuric acid, your FAC test result will be higher for the same ORP than it was before since the DPD test includes the chlorine locked up in the cyanuric acid. You need to be aware that there is a change that will affect the ORP-FC Relationship.
How simple is that? Ok, it will take a bit of fiddling around before you can work out the ORP-FC Relationship for your controller and your pool. Adjustments can be made as the circumstances change but once you have a start point which works, you are then up and running. Get lazy doing this and it will cost you or the owner money.
Stating that does not mean you can ignore the actual level you are operating at. You still want the ORP read that you see to have meaning, which means it needs to be as close to correct that it can be. You still need to clean, calibrate or be careful with the probe.
The following graph is an extraction over three days. It shows the relationship between FAC and ORP quite clearly which occurs when the pH is stable.
The following graph is an extraction during a super chlorination. The pH is difficult to keep stable when super chlorination thanks to circulation delay.
You can see in the centre of the graph how the FAC stays level while the ORP and pH are jumping all over the place, confirming that an FAC test result doesn’t tell you how strong it is without taking the pH level into account.
It’s amazing what you can get out of an Aquarius Ultima when you are trying to find out what the hell is happening. If you just don't have the time, then get us to do it for you.
