As a Controller Manufacturer, I always get this question from CPO's asking for my opinion in the matter. PPM or ORP Control? - the question really should be... Control Water QUALITY, or Control my RESIDUAL.
Interestingly enough, both options offer COMPLETELY different Pro's and Cons...Lets take a closer look:
With that said, you can see that the answer really depends on what's more important to you - do you want consistently Clear and pristine water, regardless of the Chlorine level, or do you want a consistent Chlorine Residual, regardless of overall Water Quality?
We use ORP control for our chloinator, but I find that I am having to constantly adjust the ORP setpoint in order to get the correct amount of sanitizer in the water.
ORP will not give you a steady Residual. Its best to do the following:
that's the best solution for you. again, you have to resolve yourself to understanding that ORP will NOT give you a steady PPM.
ORP control can be very confusing and frustrating if you don't really understand what ORP is and what the sensors detect.
PPM Sensors actually detect chlorine while ORP sensors have nothing to do with chlorine. ORP Sensors measure the oxidizing power of the water. The oxidizing power of the water is a result of oxidizers in the water (chlorine, bromine, hydrogen peroxide, or various other possibilities) AND the pH of the water.
The ORP reading is dependent not only on the sanitizer level but also the pH. If the pH rises the effectiveness of the sanitizer is reduced, the ORP value drops and the controller will add sanitizer to keep the ORP value at setpoint. If the pH drops the sanitizer becomes more effective, the ORP value rises, and the controller will not feed sanitizer until the ORP drops back below setpoint.
So as you can imagine, good pH control is vital if you want to keep a steady sanitizer residual (regardless of which sanitizer is used). Also, the ORP setpoint is not a residual setpoint - it is an oxidizing power setpoint and the controller will raise or lower the residual to meet that requirement.
Another issue with ORP sensors is they get dirty which slows down the sensors response speed. I've seen fouled sensors take upwards of 30 minutes to respond to changes in the pool water. In this case a sanitizer feed wouldn't start until 30 minutes AFTER the actual ORP value dropped below setpoint (thats how long it takes for the water to penetrate the fouling) and wouldn't stop feeding until 30 minutes AFTER the actual ORP value passed setpoint.
Cleaning the sensor is easy - use some dish soap to clean any oily deposits, rinse well, then soak the probe in the manufacturers cleans/storage solution overnight. I actually use pH 4.01 buffer solution which is almost the same thing (a local salt water aquarium store has lots on stock).
Check the response time of the probe by switching it between a container of tap water and a container of sanitizer solution (or just pool water). It should be able to swing from one extreme to the other within a couple of minutes. If longer than 4-5 minutes that probe would not be good for a small pool but would do fine for a large one.
I wrote this article earlier this year, can give people insight as to how ORP works...
Another part of the ORP/Residual problem has nothing to do with the ORP controller but rather the people performing chemistry tests and the testing reagents themselves.
In almost every facility I've been to in the last 25 years people performing chemical tests did not follow anything resembling best practices. Equipment is invariably too dirty, reagents are often spoiled for various reasons, and the gathering of samples was done very poorly. Not that I'm throwing stones - I was guilty of all of the above my first couple of years in pools (its what everyone did so it must be right).
To get consistant test results:
- Keep reagents at a constant cool (not cold) temperature - NOT in a fridge. Reagents are harmed as much by temperature changes as they are by overheating or freezing.
- Keep caps on reagents. Exposure to oxygen will degrade the reagents
- Don't mix up caps or cross contamination can degrade the reagents
- Hold reagent bottles straight up and down, not at an angle. Angled bottles deliver smaller drops of reagents.
- Keep test equipment clean
- Rinse containers at least twice in the water to be sampled (I use three times)
- Hold container upside down and push it into the water until it reaches your elbow. Organics tend to float so water near the top has a higher organic content.
- Flip the container right-side up and withdraw to keep the deeper water in the container.
- Perform the test immediately as organics in the water will continue to be oxidized which reduces the sanitizer result and increases the oxidation by product level. Taking the water from the first couple of inches in a busy hot tub and waiting just 5 minutes will likely result in zero free sanitizer and high by product results (i.e. zero free chlorine and high chloramines).
For more detail visit your test kit manufacturer's website and peruse the vast trove of invaluable information they have. You'd be amazed at what you can find.
P.S. Did you know that, depending on the condition of a pH reagent, you can start getting false high pH results after a free chlorine residual of 5 PPM? Anything over 15 PPM free will always give a false high pH.
That's why I use Palintest for my testing needs: