ORP vs. Temperature curve?

[stevet1]

Hello all,

My pool got built this past fall, and I had some initial problems with my Autopilot TC system. After finally getting the pH probe replaced, the pH control has been working flawlessly. However, I haven't been able to get the ORP control to work consistently. I have a feeling that the temperature of the water is playing havoc with my ORP readings, and turning on the SWG because the measured ORP seems to go down when the pool water temp goes down. Does anyone have any idea if there is a temperature correction curve for ORP vs. Temp that I can examine? I think it has something to do with the Nernst equation, but I'm too far removed from that kind of math to create a curve for ORP vs. temp, assuming all other variables remained constant.

 

[chem geek]

Unfortunately, ORP sensors don't exactly follow the Nernst equation for either hypochlorous acid concentration nor pH so I'm not sure they will follow it for temperature either. The Nernst equation is the following:

E = E_O - RT/nF * ln(K)

I can tell you that a 10F increase in temperature decreases ORP by 11 mV for Chemtrol sensors and 14 mV for Oakton sensors IF they follow the Nernst equation for temperature (that is, I've calculated the other parameters using real-world data and a formula that modifies the above Nernst equation for each sensor).

Richard

 

[JasonLion]

The sources I have read say that ORP varies with temperature in a more complex way than the Nernst equation implies, but that the variation is reasonably small within the relevant temperature range, say +-5% for 30 degree temperature swings.

There are a number of other factors that can affect ORP readings far more dramatically. Sunlight on the water tends to lower ORP readings. The ORP reading can be lowered by dissolved hydrogen gas in the water from the SWG cell. The hydrogen will normally bubble to the surface, but some conditions can cause it to become temporarily dissolved instead. As CYA levels rise ORP readings become less tightly coupled with FC levels and at high enough CYA levels the inherent noise in the ORP sensor overwhelms the signal. MPS can cause high ORP readings even in the absence of FC.

 

[stevet1]

Hmmm,

I'm sure it's all too technical for me. The water temperature is pretty frigid around 41-42F in the morning to 47-48F by the evening. Regardless, I'm not sure I can trust the ORP sensor to adequately regulate my SWG output. I oversized my SWG cell, so turning it on just a few hours creates a lot of chlorine for my pool. My CYA levels are not high, at 40. pH is constant at 7.7 (keeping pH a little on the high side in the colder weather to keep my saturation index from getting too negative). The weather is only a bit cooler than it was last week, the only difference really is that it's been a more sunny the last few days. Can sunlight really make that much of a difference and lower the ORP that significantly? Could it really be that as my SWG cell is in use, it's making chlorine and LOWERING the ORP via dissolved hydrogen gas? That would make the whole point of ORP completely useless with a SWG.

Last week: shady, windy, occ. rain: ORP readings around 680, FC levels around 6
Today, mostly sunny: ORP readings around 660, but FC levels around 13. pH the same 7.7. NaCl concentrations have been diluted 300ppm in between readings (rainwater). (yes, i know, waiting a whole week between testing is very bad of me, but I really thought my TC sytem had reached a steady state)

Any suggestions what to do? I'm looking for the least labor intensive way of maintaining my FC. I don't mind it fluctuating a few points in a safe range, but getting FC up into the teens is a little too high for comfort.

 

[JasonLion]

The simplest thing to do is to disable the ORP sensor, on the maintenance menu, and set the percentage to perhaps 1% and see how things go till spring. When the water warms up everything will probably work again and you can go back to ORP mode. Chlorine usage over the winter will be very low and fairly uniform so the ORP sensor isn't really doing anything for you right now.

The hydrogen gas effect is real, but it doesn't usually happen. In most cases the hydrogen gas will bubble out instead of dissolving. In my pool it dissolves and as a result the ORP sensor is useless. We don't really understand all the factors that lead to hydrogen dissolving instead of bubbling out. It is possible that temperature is one of the factors and that is why you are having a problem now, but it is impossible to say without more experiments.

Sunlight lowering the ORP is a much more common effect. Sunlight lowers ORP by anywhere from 5 to 30. This effect is normally factored into the calibration of the system and so only tends to cause problems when the CYA level is high. At high CYA levels it takes a great deal of chlorine to compensate for the sunlight reduction and FC can start swinging dramatically based on how sunny it is.

One final thing to keep in mind. As the water gets colder and the salt level gets lower you get closer to having the unit shut down due to low salt levels. The minimum salt level required to operate the cell goes up at cold temperatures. You clearly aren't having an issue with this yet, but the temperatures and salt levels you mentioned sound like they are approaching the limit. If the water temperature goes any lower, it would be a good idea to look at the display regularly to be sure the unit is still working.