pH/ORP water chemistry controllers?

[stevet1]

Hello,

I have been considering the AutoPilot Total control (with built in ph/ORP controller). However, i have been able to find a couple of posts alluding to some special concerns related the using a ph/ORP controller, but nothing specific.

What are the pros / cons of using a chemistry controller?
Any specific concerns related to CYA? or to salt?
Any recommendations as to which controllers and which sensors are the most reliable?
If you were to use a chemistry controller, would you use it to control your SWG, or would you just hook it up to a bleach pump/ resevoir and forgo the extra cost of a SWG?

Here are some links to some controller manufacturers that I have been considering:
http://www.chemauto.com/
http://www.sbcontrol.com/
http://www.rola-chem.com/

 

[JasonLion]

The Total Control system is the only one I know of that is designed to be sold for home pools. There are any number of systems out there for use with large commerical pools, but they tend to be much more expensive. There are all kinds of things a DIY type could do, bleach pumps for example, but not very many people go that route.

When using an ORP controller you need to keep your CYA at 50 or below and you need to have something regulating the PH. ORP varies dramatically based on PH, so if you don't hold the PH fairly constant you will get large chlorine level swings.

ORP sensors can sometimes be a pain. ORP readings vary based on many factors, not just chlorine levels. If you don't control the other factors you can get inconsistant chlorine levels. In the commercial pool world users of ORP sensors often "recalibrate" based on manual water testing very frequently (sometimes as often as every four hours). In a home pool you can usually keep all of those other factors more or less constant and only re-evaluate your target ORP level occasionally.

 

[stevet1]

Thanks for the info Jason, I really appreciate it.

I guess I'm considering buying a commercial controller and putting into my future residential pool.

Now, in regards to CYA, do you know why levels above 50ppm make it so difficult? From my reading of the materials, ORP should more accurately measure sanitizing ability compared to FC ppm (provided all other factors remain controlled, including pH), and it's supposed to take into consideration CYA and it's effect on sanitizing chlorine.

So is it the CYA itself causing direct interference with the electrode measurement at the higher levels, or is it the high CYA levels causing lower levels of sanitizing chlorine, thus causing lower ORP readings?

In any case, isn't it strange that Autopilot calls for the higher CYA levels , when those levels may make it hard for it's the ORP sensor/SWG to keep up or accurately control the system?

In regards to the cost of these systems, I found prices for the Total control system around $2000-$2400 online. I've found ph/ORP control systems from the $1300-$2000 range, but these normally don't include peristaltic pumps. Adding on the cost of 2 pumps and tanks, flow cell assemblies, flow sensors, etc., I usually get a range of about $2600-$3000. So yes, these commerical controllers are definitely more expensive than the Autopilot TC, but are still in the same ballpark. The neat thing about most of these commercial controllers is that you can have it control a pump, or even a SWG, so future proofness is definitely there.

 

[JasonLion]

Now, in regards to CYA, do you know why levels above 50ppm make it so difficult?

The higher the CYA level the less the ORP changes when the chlorine level changes. Eventually the ORP change caused by changes in the chlorine level becomes smaller than the ORP changes from background noise and the sensitivity of the sensor. 50 isn't an absolute limit, you could go a bit higher, but the signal to noise ratio gets to be more and more of a problem.

AutoPilot recommends CYA at 30 to 50 with the Total Control system, and at 60 to 80 without.

 

[Guest]

If you want to spend the money there are directy reading chlorine electrodes that are coming out on the market that do not have interference from CYA and actually measure chlorine levels. However, they are EXTREMELY expensive at the present time so they are being aimed at commercial installations.

 

[stevet1]

Not interested in the ppm electrodes, as ORP seems to more accurately measure disinfecting ability compared to actual FC PPM measurements, (within a reasonable CYA range).

However, PoolSean PM'd me and said that using gold ORP electrodes for the noble metal (rather than the standard platinum) is preferred for swimming pool applications. Can anyone shed light as to why gold ORP electrodes are better for swimming pools? The only info I could dig up is that gold is preferred when measuring ORP in cyanide applications. I don't believe we use any cyanide (NOT related to CYA) containing chemicals in pools, do we?

 

[JasonLion]

I was told that gold electrodes were better in salt water (ie 2500-3500 ppm) and platinum in non-salt, but I have no idea what the chemisty behind that might be. The commercial systems generally have platinum and are primarily designed for non-salt water.

 

[JasonLion]

Not interested in the ppm electrodes, as ORP seems to more accurately measure disinfecting ability compared to actual FC PPM measurements, (within a reasonable CYA range).

There has been some debate about this. Ben used to say that all the studies that said ORP was the number to go by were funded by ORP controller companies and were misleading at best. Richard looked into this at one point and appears to agree that ORP isn't the best indicator. Regulations for public pools in the US have been moving away from ORP and requiring ppm measurement.

As a practical mater, ORP readings vary based on a number of factors, some of which clearly have nothing to do with disinfecting ability (like disolved hydrogen gas or metals in the water). Because of that it is frequently is not good enough to simply aim for an ORP of 650 and assume that the pool is fine. You need to do other testing of the water, get it where you want it, and then aim for the ORP reading that your particular sensor gives in that water, and then hope the other factors stay reasonably constant.

Direct ppm reading sensors eliminate that "calibration" step and allow you to narrow things down to worrying only about CYA levels when picking the target for the controller. Since CYA tends to stay reasonably stable over a season (assuming nothing like the recent rain in Texas) that is much simpler to manage.

 

[chem geek]

I talk a little about ORP controllers in this thread. You will note the HUGE variation in absolute and relative ORP values by controller.

 

[stevet1]

OKay,

So, if I use an ORP controller, just make sure to calibrate it against actual FC levels. Once properly calibrated, and pH is controlled, then the ORP controller should roughly be able to maintain a constant level of FC? Take home message is don't trust the actual ORP reading?

Thanks for the links. Very informative. But any ideas about the gold vs. platinum electrodes? Thanks again.

 

[JasonLion]

You are getting the idea. Basic setup sequence is to get the system installed keeping ORP automation off, verify that PH regulation is working, which will give the ORP sensor time to acclimatize to your water. Balance the water using external testing and calibrate the PH sensor. With the water balanced the way you want it, take the current ORP reading and use that as the target for automation. Follow the system closely for a couple of days to verify correct operation. Once everything looks good you can let it run and only recheck the chemistry weekly (or less as you get more experience with how things go in your pool).