Ideal ORP level?

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JasonLion

JasonLion

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May 7, 2007
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I have the AutoPilot Total Control system which lets you set an ORP level which the system will then maintain.

I am trying to figure out what the ideal ORP level to shoot for should be. Various sources suggest an ORP level of 650-750. The Total Control system defaults to a level of 650. Other sources suggest a chlorine level of 3-5, which on my pool/sensor seems to correspond to an ORP level of about 590. Now I know that ORP readings can be affected by the CYA level and most of the people who suggest an ORP of 650 seem to ignore CYA or assume that it is zero.

The other thing I have noticed, which seems to complicate things greatly, is that any external addition of chemicals can send the ORP reading off wildly in one direction or the other for hours, or in one case days, at a time. Since I just opened the pool and was adjusting things for most of a week the ORP readings were varying wildly. It seemed like the ORP sensor was broken for a while. Plus, I have no idea if my sensor is properly calibrated, or what standard I would use to calibrate it against.

Sadly the Total Control manual says nothing on this subject.

Poolsean? Anyone else?

Thanks
Jason
 
650 mv was determined by WHO (World Health Organization) to be the ORP level at which the sanitizer will instantaneously destroy e-coli.
650 mv is the factory setting for the Total Control, and you'll find most all other ORP/pH control systems too.

In real world applications, there are many influences to the ORP measurement.

In my opinion, you're best to adjust your chlorine level to where you want to maintain it, then set your ORP setpoint to match the sensors' measurement.

It usually takes a few hours for the sensor to get "acclaimated" to the water conditions the first time. This should not happen on subsequent operation though.

Any chemicals that are added to the pool, particularly if added directly into the skimmer, will affect the ORP readings. I'll have to check the manual, but I believe it recommends closing the water sample valves when chemicals are added...especially cyanuric acid.
 
650 mv was determined by WHO (World Health Organization) to be the ORP level at which the sanitizer will instantaneously destroy e-coli.
650 mv is the factory setting for the Total Control, and you'll find most all other ORP/pH control systems too.

In real world applications, there are many influences to the ORP measurement.

In my opinion, you're best to adjust your chlorine level to where you want to maintain it, then set your ORP setpoint to match the sensors' measurement.

It usually takes a few hours for the sensor to get "acclaimated" to the water conditions the first time. This should not happen on subsequent operation though.

Any chemicals that are added to the pool, particularly if added directly into the skimmer, will affect the ORP readings. I'll have to check the manual, but I believe it recommends closing the water sample valves when chemicals are added...especially cyanuric acid.
 
It could have been a PH change.

I understand ORP to be influenced primarily by the combination of chlorine and PH, and that you can't simply interpret the ORP levels to BE chlorine. I do not recall all of what I studied, but I think that a system can be tricked into believing that the chlorine level is high, low, OK, or non-existant just by manipulating the PH.

If you alter ONLY the chlorine levels, and the PH stays consistent, then you can use ORP as a relative chlorine indicator. But ORP without a corresponding PH reading is useless.
 
Ohmboy, Exactly!
That's why you will usually find ORP controllers with pH controllers.
The Total Control system also has both so that we have some degree of "accuracy".

There is no direct correlation between ORP and pH unless all parameters are held constant. Cya, water temperature, TDS, grounding issues, UV exposure can all affect ORP measurements.
In laborator environments, you can plot the correlations but in real world pool applications, you can only hope to be close... therefore my suggestion of disregarding the ORP of 650 and just set it to whatever the ORP is measuring when the FC is 3 - 5 ppm (or whatever desired ppm level you wish to maintain).

If you use an ozone system, you can maintain 0.5 ppm and have an extremely high ORP reading, because the ORP measurement will be high. ORP, Oxidation Reduction Potential, is a measurement of the amount of oxidizing strength is in the water.
 
Actually, I pondered grabbing one of these - http://www.vernier.com/probes/orp-bta.html at one time... I could be taking ORP measurements for really cheap. Turns out that vernier.com has a fair selection of educational sensors and collectors which bear looking at. Unfortunately, I never actually justified it as a toy at the top of the priority list before my oldest daughter decided to get married, rendering the whole idea moot.

Maybe it would be fun for someone to play with...
 
I use a handheld combo pH/ORP pocket tested from Hanna instruments with reasonable success.
It uses a stainless steel probe whereas the good ones use a platinum tip but mine is OK if you are willing to let the ORP scale indicate a realative condition.

There are both ORP calibration solutions... But they won't verify the linearity of your probe.
There are also low and high ORP "conditioning" solutions which are probably only rarely helpful (Milwaukee instruments).

What I have found most helpful is to always rinse the probe with reverse osmosis water after use and AlWAYS stores the probe tip wet by keeping it in a good quality potassium based pH probe storage solution. Never let the probe dry out.
 
learthur said:
Some of you may have trouble using automation ORP to regulate your FC levels.
This article provides some interesting insights on the effects of CYA on ORP levels and bacterial kill rates: http://www.texaswaterworks.com/overstabilization.pdf
Click to expand...

Indeed, CYA levels above about 50 render ORP automation essentially useless. Of course anyone who is using ORP automation should already be aware of that as that information is going to be in the ORP automation systems user manual.

The bacterial kill times listed in that article, while correct, are misleading. They assume that you hold the FC level constant while the CYA level varies. We recommend raising the FC level as the CYA level varies, which restores the bacterial kill rates to appropriate levels.
 
The use of ozone in combination with chlorine in my pool has added tremendous stability and correlation of my FC levels vs ORP.

I have data points every day for the last several months. I dose 12 g of ozone per day in a 24000 gal pool which is the minimum ozone dose I would recommend. The ozone functions as a primary oxidizer of organic contaminates which has reduced my chlorine consumption by at least half, let's me run 1.0 -2.0 ppm FC with an ORP over 600.

It did take 6 weeks of ozone to get the pool chemistry to this point. Ozone is expensive and installation can be more complex so it is not for everyone.

I predict that in 5 years most manufacturers of devices such as the Autopilot Total Control will be offering integrated ozone to make the automation via ORP measurement more reliable.
 

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I very much doubt that it was specifically the ozone that helped your system become stable. More than 2/3rds of ORP systems work just fine right off. It is most likely that you simply hit upon a combination that worked at some point after getting the ozone system.

In general ozone is unsuitable for indoor pools because of air quality issues, and a waste of money for outdoor pools, where chlorine alone can perform the same function more reliably and less expensively.
 
JasonLion said:
I very much doubt that it was specifically the ozone that helped your system become stable. More than 2/3rds of ORP systems work just fine right off. It is most likely that you simply hit upon a combination that worked at some point after getting the ozone system.

In general ozone is unsuitable for indoor pools because of air quality issues, and a waste of money for outdoor pools, where chlorine alone can perform the same function more reliably and less expensively.
Click to expand...


Agreed 100%. I'd also note that an ORP of "over 600" may not mean much. You need to compare the FC/CYA chart and then evaluate your ORP. I would normally expect to see an ORP of around 720 minimum in a properly sanitized pool with a clean and functioning ORP electrode.
 
I should clarify my "over 600mV comment". ORP in the 600 mV range is not necessarily a magic number for several reasons but ther is one important reason that isn't common knowledge for some reason. Many companies have latched onto the 650 mV standard from WHO and use that as a default. Unfortunately that standard is 30+ years old and no longer relevant.

That standard was arrived at in the late 60's or early 70's when calomel reference electrodes were the norm. Today we use silver chloride reference electrodes almost exclusively; certainly in pools and water treatment. Calomel reference electrodes tend to read 30-50 mV lower than silver chloride reference electrodes. That means the minimum standard guideline should be 700 mV at least.

In fact, WHO has updated their recommendation to reflect this.

Today they recommend 650 mV for a calomel reference electrode and 720 mV for silver chloride.

It's here on page 17 (5.10.4):

http://www.who.int/water_sanitation_hea ... 2chap5.pdf
 
The situation is even worse than that. As described in this post different sensors from different manufacturers will read differently, not only in absolute terms, but in the mV rise per doubling of FC relative change (slope). At 3.5 ppm FC with 30 ppm CYA, pH of 7.5, Temp of 80F, a Chemtrol unit will read around 695 mV while an Oakton will read around 654 mV and a Sensorex (if one believes their chart ) will read about 600 mV. Even if one adjusts a "setpoint", the differing slopes will result in different readings when one moves away from that setpoint. That makes these sensors fine for control of chlorine at a specific level, but not for comparing against an "absolute" standard.

Richard
 
learthur said:
The ozone functions as a primary oxidizer of organic contaminates which has reduced my chlorine consumption by at least half, let's me run 1.0 -2.0 ppm FC with an ORP over 600.
Click to expand...
First ,realize that ORP means oxidation-reduction potential and that ozone is a potent oxidizer so it will have the effect of raising ORP! (In fact the use of ozone to raise ORP is it's main use in aquaria!) The correlation between ORP values over 650 mv and sanitized water are arbitrary at best, IMHO. In many cases the use of ozone actually increases chlorine demand because of ozone's ability to destroy chlorine.
 
If you really want to trick up your ORP number then use some of the non-chlorine shock (MPS). It boosts the ORP level drastically while obviously not raising the CL level any.

It could actually cause problems with the probes if used enough.
 
Re: AutoPilot's Total Control System Review

UPdate: We now have our One Touch Autopilot syetm installed and running for 10 days now. It seems to be running very well. The oppressive heat here in the midwest is really making it work though. Sidebar: I have since discovered that our pool is actually 54k and not 36k as was previously told. :eek:)

My question now is does anyone have an "ideal" ORP setting. The manual seems to indicate 650 but only through sample display pics. It never directly addresses what a standard start up setting should be or suggested mean and my internet queries turn up nothing. Anyone have any ideas/recommendations/sources?

Jon
 
Please only ask questions in one place. I moved the other one here, as this seems like a better place for this. JasonLion

Just posted this same question in another forum and then found this discussion and after reading, I still am unsure of a suggested ORP setting other than 650. The detailed discussion is great but at times, over my head. Still, just looking for others who have the Autopilot Total Contorl system up and running and what their ORP settings is at.

Our sytem is 10 days old and I just changed from 650 to 700.

Jon
 
By far the best thing to do is to test the FC level, and then adjust the ORP level until the FC level comes out where you want it to be. When doing this, you should always test the FC level at about the same time of day, as the relationship between the FC level and the ORP level will vary somewhat at different times of day. Even after you have it set somewhere appropriate, continue to test the FC level every week for several weeks to make sure the FC <--> ORP relationship is remaining stable. If that goes well, you can switch to testing less often, perhaps once a month, just to be sure than nothing is going wrong.
 
I can hardly wait for the people using a salt cell with it's appreciably higher CYA recommended level to discover Pentair's Intellichem and wonder why things aren't working quite as they expected. I, for one, won't be recommending Intellichem and SWCGs.

I expect the best situation will be when a cell dies and the home owner has had enough of it. I can see that for Hayward and Jandy systems. Then installing a liquid chlorine/MA feed system goes in. If the CYA is high, say 70-80 ppm, dilution is in order and then things should work well.

Scott
 

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