WHY WOULD YOU WANT BOTH AN ORP PROBE & A FREE CHLORINE PROBE IN A SWIMMING POOL CHEMICAL CONTROLLER?

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Having two different methods of testing the sanitation levels in a swimming pool has to improve the management of your pool out of sight. This is especially beneficial when monitoring a pool remotely.

Let’s look at the ORP Probe and the Free Chlorine (FAC) Probe and the DPD test.

ORP Probe

• The ORP probe gives you a measure of the oxidising power of the of the water in the pool expressed in millivolts (mV). The ORP read that you see is the sum of all positive voltages less any negative voltages. The ORP read that you see is mostly determined by the dominant redox couple in the water e.g. chlorine/chloride.
• pH changes will result in changes in ORP. An increase in pH will result in a drop in ORP, while a decrease in pH will result in an increase in ORP.
• There is no formula that will convert an oxidant addition into the increase in millivolts. When chlorine is added to the pool the resultant hypochlorous acid will increase the ORP level, but there is no direct relationship. The relationship is non-linear.
• You will need to establish the ‘ORP-FC Relationship’ for the pool. All pools will have a different relationship as there are many other factors which will affect this.
• Maintenance and handling care is important when using these probes.
• Depending on the authority that the pool operates under you will need to do at least one DPD test a day to confirm that the pool has a chlorine level that lies within the authorities required range. This is to confirm that it is chlorine that is generating your ORP millivolt read.

Free Chlorine Membrane Probe

• The FAC probes gives you a measure of the hypochlorous acid level in the pool water expressed in parts per million (ppm). 1 ppm = 1 milligram per litre.
• pH does not significantly affect your FAC read but is does affect the strength of the sanitation.
• FAC probes need to be calibrated using your DPD test result.

DPD Test

“DPD No 1 tests for Free Chlorine or hypochlorous acid (HClO) to be more precise, a compound not an ion.  The important thing about DPD No1 powder, tablet, liquid or whatever is its buffering capacity (the ability of the DPD test chemical to lower the sample pH to the required level).  The pH of the sample must be between pH 6.2 and 6.5 otherwise you get interference from chlorinated compounds that may be present in the sample.  At pH 6.35 all the free chlorine is present as HClO, none of it is dissociated into its constituent ions, H⁺ and ClO^-.

DPD No 3 is potassium iodide which when added to a sample containing DPD No 1 will react with chlorinated compounds such as chloramines.  DPD No1 plus No 3 provide an indication of all the chlorine present, free and combined.”

(As advised by Noel Sampson CEO of Select Chemicals NZ.)

Given that both the probes require a DPD test confirmation that they are performing as they should you need to be sure that your testing equipment and methodology are good.

Importance of pH

Time and time again we keep coming back to the importance of keeping the pH accurate. Given that the probes behave differently you can expect to see different graph patterns for each probe. We don’t want these very important patterns being thrown out of wack thanks to tardy attendance to accurate pH. We want to know that the ORP we see is telling that the hypochlorous acid level is at the correct level in the pool. We also want to know that the FAC figure we see does tell us that there is enough hypochlorous acid in a molecular state.

What are the probes telling us?

All pools will present their own unique graph patterns which you will become familiar with once you get used to checking your pool chemistry remotely using an Aquarius Ultima Controller.

We do want the graphs to tell us when onsite conditions have changed or when there is a problem developing. Of course, on-site reporting plays a very important in this communication process

Having both an FAC and ORP probe gives you the ability to keep an eye on

• the performance of the probes.
• the degree to which onsite procedures are being followed.

The big plus is having two different tests giving you information that tells you if the pool is sanitised when you are looking at it from miles away. I was quite happy working with ORP probes and really didn’t see the need for an FAC probe. Then it hit me. For years I had been looking for a way of putting a secondary test system on a commercial pool that I was responsible for. In my thinking it was to reduce human error. Then the light-bulb moment hit me. There it was, right in front of me. Two different tests in the same unit.

It is generally accepted that FAC is easier to work with than ORP. You can look at an FAC figure (and the pH figure) for any pool and know the state of the sanitation level in the pool, while looking at an ORP read we really need to know that the ORP-FAC relationship has been set correctly for that pool. That is. Is the set point correct?

For further information

ionode manufacture probes for different chemical controller manufacturers. This is the link for ORP Theory

The video linked here shows how a membrane probe works.