ORP settings for algae prevention

[miltos]

Hi.

I haven't found an answer searching the forum (instead i've been confused a bit)

Quoting from this post:

Note that the red in the linked chart above represents a cutoff of 0.011 ppm HOCl which roughly corresponds to the 650 mV ORP level that the U.S. and WHO set as the minimum required for disinfection. The green color is a guess at 0.05 ppm HOCl of the minimum level of chlorine needed to prevent algae.

What is the ORP level correspoding to 0.05ppm HOCL? Is there an ORP chart?

 

[Bama Rambler]

There is no chart because things beside HOCL affects orp. You need to adjust the FC and then note the orp and use that as the set point. Be aware that CYA can make the orp unreliable. Keep a close eye on it for a while.

 

[KenGood]

What I would suggest is that you get everything set manually or with everything except the ORP and note what OP reading is. If you want a minimum then again set everything manually to that point and note the ORP reading again. See if that will get you started. But keep in mind that, as stated by someone else, other things attribute or maybe make contributions that affect the reading. So if they change, then this would effect the set points. I am betting that over time you will have several sets of set points: for normal summer, fall, heavy use, etc.

Good luck!

 

[chem geek]

KenGood summed it up well. You want to get your FC and CYA levels based on the Chlorine / CYA Chart and also have your pH set to where you want and then take your ORP reading and make that your setpoint.

The minimum FC at each CYA level for non-SWG pools in the chart is an FC that is around 7.5% of the CYA level and at pH 7.5 that very roughly corresponds to around 680 mV on a Chemtrol and 630 mV on an Oakton and ridiculous values on Aquarius and Sensorex (according to their charts). There is no single absolute number consistent from all manufacturers of ORP devices even measuring the same water. Variations of 100 mV are not uncommon -- see this post where 23% of the pools with two ORP measurements from different controllers had 100 mV or greater differences.

 

[miltos]

Thank you all.

But keep in mind that, as stated by someone else, other things attribute or maybe make contributions that affect the reading.

Don't these things have the same influence at lower FC levels? Is that the reason why 0.011 ppm HOCl roughly corresponds to the 650 mV ORP, but on the other hand there is no (at least approximate) correspondence for higher FC levels?

There is no single absolute number consistent from all manufacturers of ORP devices even measuring the same water.

This is just a measurement error. Does that error have the same % magnitude for the hole ORP range?

 

[chem geek]

Hydrogen gas bubbles from saltwater chlorine generators interfere with ORP readings regardless of FC level. Cyanuric Acid (CYA) levels and other organics can "foul" the sensors depending on the membrane that they use.

The variation in ORP with different sensors is NOT just measurement error. The multiple sensors measuring the same water had large errors at higher FC levels as well as lower ones though it is true that the lower levels had more error and that is seen in the graph in the link I gave -- but the variation even at higher FC is quite high (that graph is just from one sensor, but it is relative to chlorine/CYA levels). It's ORP sensor design and variability and it makes using ORP as an absolute standard for disinfection in swimming pools practically useless if you try and use an absolute ORP mV level as a standard. ORP is OK for process control where you use a setpoint based on other more reliably consistent methods such as FAS-DPD testing.

The main reason people like ORP for swimming pools is that it roughly measures something proportional to the active chlorine level, but you can readily calculate that knowing the FC and CYA levels instead -- it's just that hardly anyone understands the chemistry of chlorine/CYA in spite of it being definitively determined in 1974.

What exactly are you trying to do? Do you want to set an active chlorine level sufficient to prevent green and black algae growth? If so, then follow the chlorine / CYA chart and then once those levels are set, see the ORP reading and make a setpoint for that reading to have your chlorine controller feed chlorine to keep that level. Ignore the actual ORP mV reading since it isn't relevant unless it's way out-of-whack in which case you should consider servicing/replacing the sensor.