Total Control - ORP vs. Chlorine PPM - Confused

[PoolESQ]

Hi all - hoping JasonLion can chime in here:

I have an AutoPilot Total Control System. Right now, it's in total control mode, which presumably means the system automatically generates chlorine and adds acid as necessary. What I cannot figure out is how to calculate the relationship between ORP and Chlorine PPM readings. I've added enough stabilizer to get my CYA to 40, which I know is on the low side for a SWG, but I'd like to make sure my SWG is keeping the FC level at 5.0, the recommended target level. Right now my ORP level is set at 650. What I cannot figure out is how I determine what that equates to in terms of Chlorine PPM. For example, if I want the system to maintain an FC level of 5.0, what do I set the ORP level at to achieve that FC level? I cannot find a menu/submenu that permits me to set a "goal" FC level in terms of PPM. Any help or explanation would be greatly appreciated!

 

[jblizzle]

Until Jason maybe chimes in, I will just say that we generally do not recommend controlling the SWG using an ORP sensor ... they can end up fluctuating wildly and it is just simpler to make the SWG out set on a % output for some amount of time.

 

[tim5055]

I might recommend a search of the forum for "oxidation reduction potential" (ORP is too short a search string) and there are several threads where Jason, Richard (Chem Geek) and others talk well above my head about the subject.

 

[PoolESQ]

Thanks Jason & Tim! I will look up those threads. I know the ORP isn't particularly reliable, esp at higher CYA levels. This system recommends a CYA in the 30-40 range, unlike other SWGs that recommend 60. I'm hoping with the lower CYA, the ORP may actually do its job...haha.

If it does not work (and I suspect it will not), I will set it at a consistent %age. I suppose that will require some give and take until I get it to the sweet spot. Thanks for the comments, guys.

 

[chem geek]

They require a lower CYA because low active chlorine levels are noisier at the ORP and higher CYA levels may also foul the sensor membrane more quickly. You will end up using more chlorine so requiring a higher SWG on-time and have a shorter SWG cell life at a lower CYA level (even with a proportionately lower FC level).

At any rate, don't try and mathematically correlate the ORP to FC since it varies by manufacturer and on the state of the probe. Instead, use a manual test kit (preferably FAS-DPD) to measure the chlorine level and then read what that ORP is and just set the ORP setpoint to whatever ORP gives you the desired FC level. I could point you to various formulas, but I think that's pretty pointless. Just treat the ORP number is a "dumb" reading that you just read off when at the FC level you desire. Chemtrol sensors roughly increase by 22 mV for every doubling in active chlorine level (so at a constant CYA that would be for every doubling of FC level). It's a logarithmic relationship so ORP is proportional to the logarithm of FC (at constant CYA level) so for the example I gave this is ORP = 31.7 * LN(FC) + constant.

You may find your FC goes up and down as the ORP sensor isn't properly reading the active chlorine level since instead it's reading an ORP proxy that is affected by other oxidizers in the water such as dissolved oxygen and by the hydrogen gas from the SWG. It is also affected by sunlight in ways that don't correlate properly with what is going on in the pool. The UV in sunlight breaks down chlorine into very powerful but short-lived oxidizers called hydroxyl radicals, but the ORP system is affected by sunlight in ways that imply the chlorine level has dropped when in fact it has not. It takes 35 minutes for half the FC to get broken down near the surface of the water yet the ORP responds almost immediately as if something has changed. If you are able to get your ORP system to reliably maintain an FC level, then you are fortunate.

 

[PoolESQ]

chem geek, thank you for the thoughtful (and understandable) explanation. You'd think it wouldn't be that difficult for manufacturers to print such a straightforward message in the manual..haha.

I am not at all latched onto the idea of having the ORP regulate chlorine, but it would be nice. We use our pool irregularly during the week - somedays not at all, others all day long but mostly a few hours here and there. I'm afraid if I set it on a given %age then I will end up with high chlorine when the pool goes unused several days in a row or end up with low/no chlorine on busy pool days. I notice the system has a "boost" mode. Perhaps I could just use that mode on days with high bather loads and keep the percentage set at an otherwise reasonable rate?

Thanks again to you three for jumping in and helping me figure this out. My understanding is much more clear now.

 

[chem geek]

Unless your pool use involves a dozen people or some urinating kids, then I wouldn't worry about the varying bather load at all in an 18,000 gallon pool. Every person-hour in an 18,000 gallon pool consumes around 0.06 ppm FC. It would take 17 person-hours to use up 1 ppm FC of additional chlorine.

What is much more likely to vary in terms of chlorine demand is the amount of sunlight on the pool if you are in an area that gets some sunny days and some cloudy days. However, in Albuquerque it looks like the weather is sunny and partly cloudy, not very cloudy.

 

[PoolESQ]

It's mostly just 2-3 of us and a couple dogs that get in the first step to cool off for a moment. Your statement re: the consumption of FC by person/hour was very informative. It does give me a level of comfort knowing that I don't need to worry about the bather loads very much. I think I will just play with it a bit until I get it set to a percentage that will keep the FC at 5. Thank you again for the helpful information!

 

[chem geek]

Well dogs use a lot more chlorine than people unless you regularly give them shampoo baths. Normally you figure a dog is like having 5 or so people in the pool. You can just see the difference in chlorine demand with and without the usage of the pool and adjust accordingly.