Understanding ORP is the single biggest problem I run into when dealing with new staff. It's not a difficult topic but nobody really offers any training on automated controllers the way training is offered for manual chemistry and pool operation.

First ORP doesn't have anything to do with Chlorine. An ORP sensor has no idea what chlorine is and doesn't react to the presence of chlorine. ORP sensors are used in both Chlorine and Bromine pools. ORP sensors are essentially oxygen sensors - they sense how readily the pool water will let oxygen transfer from one molecule (reduction) to another (oxidation). Chlorine and Bromine do not kill pathogens in pool water, they just act like as a carrier for oxygen which is what does the killing.

Anywhere I mention Chlorine you can pretty much replace the name with Bromine and it's still mostly true.

When chlorine is added to pool water there are two compounds created (that we care about); one is Hypochlorite Ions (OCL-)and the other is Hypochlorous Acid (HOCL).

Hypochlorite ions are "free" chlorine but they are quite stable molecules and don't give up their oxygen very easily. OCL- contributes very little to the germ killing power of your water and is insignificant as part of an ORP value.

Hypochlorous Acid is "free" chlorine and very unstable. HOCL will give up its oxygen at the drop of a hat. It's the main sanitizing compound in the water and constitutes pretty much all of the water's Oxidation/Reduction Potential. Although technically incorrect you can pretty much consider an ORP sensor to be a Hypochlorous Acid sensor as that'll be the only significant oxidizer in your pool.

So your "Free" chlorine kit is showing you both HOCL and OCL- while an ORP controller is only showing you HOCL.

The difference between HOCL and OCL- is a single hydrogen atom. The more "free" hydrogen available in the water the higher the ratio of HOCL to OCL- will be. pH, or "Potential of Hydrogen" is a measure of how readily something will absorb hydrogen. The higher the pH value the more readily the substance will accept hydrogen.

Since Hypochlorous Acid requires OCL- to pick up an H+ the less basic your water is the more hydrogen will be able to bond with OCL- and the more HOCL vs OCL you'll have. The higher your HOCL the higher your ORP value and the more bug killing power you'll have. Free Chlorine, on the other hand, won't necessarily change.

Assuming a spa/hot tub:

- At a pH of 7.0 approximately 75% of the Free chlorine residual is in the form of Hypochlorous acid. So if your FAC is 4.0 then your HOCL is 3.0 and your OCL- is 1.0.

- At a pH of 7.5 approximately 48% of FAC is HOCL and 52% is OCL- so FAC 4.0 means HOCL 1.92 and OCL- 2.08

- At a pH of 8.0 approximately 22% of FAC is HOCL and 78% is OCL- so FAC 4.0 means HOCL 0.88 and OCL- 3.12

If your ORP control is set to 762 mV and your pH is 7.5 then you will have 4.0 PPM FAC and 1.92 PPM HOCL. If you have a PPM vs PH vs ORP chart around you'll see that's true (7.5 pH # 5 PPM FAC is 768 mV). You can work on the (technically incorrect but operationally OK) assumption that your ORP controller is set to 1.92 PPM HOCL.

If your pH drops to 7.0 then that 1.92 PPM HOCL will increase to 3.0 PPM which is too high and the controller will not feed chlorine again until the HOCL PPM has dropped to 1.92 PPM. For that to happen the FAC has to drop to 2.4 PPM. Why?

At 7.5pH HOCL is 48% of FAC or 1.92. At 7.0 pH there is more hydrogen available and the OCL- will pick it up and become HOCL. in order to drop to 1.92 HOCL you need 1.92 / 75 * 100 FAC = 2.56 PPM

If your pH rises to 8.0 then that 1.92 PPM HOCL will drop to 0.88 which is too low and the controller will feed chlorine until it sees 1.92 PPM of HOCL. This requires 8.73 PPM FAC: 1.92 / 22 * 100 = 8.73 PPM

In all cases above the controller has the FAC exactly where it needs to be in order to maintain the same pathogen destroying power. This is exactly what then ORP controller is supposed to do. The "problem" with the sanitizer levels above is the pH, not the FAC.

When doing manual sanitizer control you add the sanitizer then fix the pH after. When you have an automated controller it's kind of opposite. pH MUST be steady and correct before you can turn on the sanitizer system. I can't tell you how many times I've seen people trying to "fix" sanitizer levels that were too high or too low while completely ignoring pH. Heck, I did the same thing for the first couple of years I worked in a pool!

Here's a chart I stole from someplace online that shows the effect of pH on chlorine compounds. I wish I could remember where! It was a good site.

PatM, all essentially true if you make a couple of assumptions, which are unfortunately not always true. First, everything you said assumes that there isn't any CYA in the water. CYA dramatically changes several of the relationships you mentioned. For example the HOCL vs OCL ratio will be dramatically different with CYA in the water, and far less sensitive to PH than what you indicate.

Second, All of the specific ORP numbers you mention are arbitrary and may, or more often will not, correspond with reality. Different ORP sensors will read out different numbers and there is no standard for calibration. Readings vary from brand to brand noticeably and even from sensor to sensor of a single brand. This is not simple lack of precision, the numbers are just different and there is no way of saying this one is "correct" and the others "wrong", since all of them are by the definition of ORP correct even though they are all different.

Third, there can be various things that could be in the water, dissolved hydrogen gas or metal ions for example, that can dramatically change the ORP reading without having any significant effect on HOCl levels or sanitation. Your charts and numbers all assume that the water is free of these "contaminates", but in practice it is impossible to make such a guarantee.

Some of these issues are explored in this topic in far more detail.

I posted elsewhere about what ORP is and how pH affects it. Now we'll talk about the actual numbers.
Topics merged. JasonLion

There is actually an absolute scale for ORP. What there isn't is an absolute way for you to get your sensor/controller to give you that exact scale.

ORP and pH Sensors are batteries and all a chemical controller does is read the voltage that battery is supplying to it. I've found that new sensors, regardless of manufacturer/price are all very close to an ideal sensor - at least when we're talking about how close we need for pool operations.

There are two seperate types of error a probe can have and usually a new probe will both to some degree.

Scale error is like when a thermomether reads 32F at 32F but only 93F at 100F. The amount of change is not correct even though it's correct at 32F

Offset error is like when a thermometer reads 35F at 32F and 103F at 100F. The change in reading is at the proper scale but the readings are always offset by 3F.

I've yet to come accoss a new sensor that had enough scale error to make a difference. However, just like any battery, as it ages and gets used the voltage will drop and your readings will change. A sensor's offset error doesn't change much but it's scale error continually increases until the sensor is dead.

I've found I can use most sensors in a public pool for two years, after which they can suddenly die at any time. I did manage to keep one going for three years once but it was very difficult to work with.

On top of the sensor error is your controller's error. It may not have exactly the right scale set up and it's ground (0V) might not be the same as the sensor's zero output (e.g. the sensor's idea of 7.0 pH is 0V but the controller's ground is +0.021V compared to the sensor). Controllers also have issues with electronic noise and biasing by their own electronics.

So, with all the potential for error you can probably see why having the "exact" readings is virtually impossible. So what to do?

What I do is manually get the pool to the pH and FAC I want then adjust the controller to make it's readings match what I have in the pool so that it will do it's best to keep the chemicals there. I set a hot tub to 7.5 pH and 762 mV ORP when I have 7.5 pH and 4 PPM FAC. I don't really care if my ORP sensor is sending out 800mV or 400 mV, I adjust the reading to say 762.

As the sensors age the ORP reading will drop (FAC rises) and the pH reading will get closer to 7.0 (water pH rises). Once per week I check the pool logs and seeif the AVERAGE pH and FAC PPM levels are good. If not, I'll adjust the offending sensor reading to where it should be. So if the average pH for the week was 74.5 I wouldn't touch it. If it was 7.40 I'd lower the controller reading a bit (0.05) so the water pH will run a little higher. Same goes for FAC PPM. If a hot tub averages less than 3.5 PPM or more than 4.5 PPM then I'll adjust the ORP reading a bit.

How old is too old for sensors? Well, the thing to remember is that once set, the controller will feed or not feed based on being above or below a setpoint. How far above or below that setpoint is irrelevent.

If I have a very old pH sensor and I zero/standardize it to read 7.5 pH when the water is at 7.5 pH do I really care if I get a reading of 8.0 at 8.0 pH? Perhaps it's so old that immersing it in a 8.0 solution only results in a reading of 7.7. Does that mean the sensor needs replacement? NO! If the pH rises above 7.5, where I've zeroed the pH reading to, then the controller will feed soda ash or stop feeding acid. If it goes below 7.5 then the opposite will happen. it doesn't matter how big the scale error is as long as the reading is zeroed at the right spot.

Just don't zero a sensor reading when you are NOT at the optimal chemical residual/level as this will cause all sorts of headaches.

For example, you set a pH reading to say 7.8 when the pool is at 7.8 but it's an old sensor that's lost 50% of it's scale. Sure, the pHs match at 7.8 but when the water drops to 7.5 (-0.3) the controller will only drop half that distance to 7.65. An acid demand pool would still be feeding acid until pH dropped by 0.6 or 7.2 pH in the pool while the controller showed 7.5.

Hmm, I'm getting to the babbling stage I think. Hopefully this is clear enough and, if not, I'll try and respond more lucidly another day.

Last thing I wanted to say is that a controller can show you any value you want it to regardless of the sensor output. It's the operators job to make it say the numbers they want and it's the controller's job to keep the pool at those numbers, regardless of what those are.

Topics merged. JasonLion
I elaborated a bit on that in a different thread. This was meant to show the relationship between pH and ORP and PPM FAC. The exact numbers are NEVER the real numbers but sufficient when talking about these relationships. Specifically, if the pH changes then the ORP sensor reading will change even if the FAC hasn't.

HOCl does not work by giving up oxygen. It works by taking electrons from other atoms. Specifically, the chlorine goes from a +1 state to a -1 state, which means that it takes two electrons.

pH is the negative logarithm of the molar concentration of hydrogen ions. Or, the negative log of the hydrogen ion activity.

Unfortunately, this is not true. If you looked at this post you would find HUGE discrepancies in the mV ORP per doubling of HOCl (or in FC at constant pH). The differences between Chemtrol, Oakton, Aquarius, Sensorex, etc. are HUGE even for brand new sensors using the manufacturer's own calibrated tables! As described in this post, 23% of pools that had built-in ORP controllers were off by more than 100 mV from the Oakton portable ORP sensor measuring THE SAME WATER (and yes, both sensors were properly equilibrated).

So while you might be able to get some consistency with a particular manufacturer's sensor, they do not work reliably enough to report the same values over a wide range, even when one fixes offset error at one point. Also, they have absolutely no direct correlation to the thermodynamics (like they are supposed to) since theoretically a 2-electron transfer should have 9 mV per doubling while 1-electron transfer should have 18 mV per doubling. Yet sensors vary 22 mV for Chemtrol to 28 mV for Oakton to 46 mV for Aquarius to the absolutely ridiculous 84 mV for Sensorex. These higher mV per doubling imply thermodynamics involving less than 1 electron transfer per molecule. No one has explained such discrepancies and since ORP measurements vary so much there is probably chemistry related to the specific composition of the electrode material itself and to contamination of that electrode.

As James noted, ORP is NOT measuring anything to do with oxygen, but rather is a thermodynamic measurement of the oxidation-reduction (redox) potential difference between the water being measured relative to a silver/silver chloride reference electrode. If you don't have any strong oxidizers such as chlorine in the water, then ORP will roughly measure the dissolved oxygen since that is the next strongest oxidizer in most water.

As for pH, ORP varies with pH not only because of the hypochlorous acid vs. hypochlorite ion, but also DIRECTLY due to the pH itself. Though this would be explained if the standard reference electrode were hydrogen, it's not explained with the silver/silver chloride standard electrode so is yet another mystery and the "best fit" formulas for different manufacturers vary in their pH dependence.

For your 762 mV example, assuming 80ºF (since there is also temperature dependence), at a pH of 7.5 this is 0.8 ppm FC and 0.41 ppm HOCl with Chemtrol, 1.5 ppm FC and 0.73 ppm HOCl with Oakton, about the same with Aquarius as with Oakton, and 1.6 ppm FC and 0.78 ppm HOCl with Sensorex. What manufacturer of sensor are you using for your analysis? Whose "PPM vs PH vs ORP chart" are you using? The links I gave above show completely different charts for different manufacturers -- wildly different, not just a little different.


This is also not true. ORP is affected by things that are not related to killing power. For example, the use of non-chlorine shock (MPS) will register as high ORP yet MPS does not kill pathogens nearly as quickly as hypochlorous acid. Similarly, hydrogen gas from saltwater chlorine generators can affect ORP readings usually lowering them yet has nothing to do with the killing of pathogens. ORP is a thermodynamic measurement, not one of reaction rates for specific compounds which would be much more relevant to determining kill times. Our bodies are thermodynamically favored to oxidize with the oxygen in the air, yet this process is fortunately very slow otherwise we would burn up almost immediately. Also, some compounds can kill pathogens slowly, such as metal ions, linear and Polyquat algaecides, and boric acid yet these do not show up in ORP because their mechanisms do not involve oxidation (electron transfer).

The net of all this is that ORP can be useful for process control with a setpoint for separately measured FC levels as well as knowing CYA, pH and temperature. It is only a very rough indicator for absolute water quality at the extremes and even then only when one understands the specific characteristics of their particular manufacturer's sensor.