SWG ORP and nothing makes sense.

Cookie Monster

New member
Jun 4, 2009

My wife and I follow this site regularly. We used BBB+SWG last year with our old plaster pool. This year we
refit the pool and all that is left of last years pool is the old gunnite shell.

Since she refused to post this from her account with a 10' pole, this may be the first post from my account...
I thought I would drop ot in the Deep end and see if anyone had suggestions for me. Especially after seeing.

learthur said:
don't take this the wrong way but I think you are pursuing a pathway that exceeds your technical knowledge.

For you I would recommend you not do ORP control, however pH control would serve you well.

Making ORP work has a lot of variables in it that the average pool owner shouldn't and most won't desire to deal with.

Yes ORP works for me , but there are lots of reasons why which I don't have the time to explain. There is a lot of research and undiscovered science related to ORP use in swimming pools. I enjoy studing it with my test pool, but this is not for most people.
please focus on pH control.

Good luck

I spent three years of High School, and a year of College studying Chemistry, and I currently work on
electronics and computers for a living, so I think I can wrap my head around this if I can just a bit of help
getting going.

The back story is:

We are often away from home, so to protect our substantial investment in pebble finish we chose to
up the stakes on automation system. We now have a Chemtrol 2000 with a full battery of sensors running
the show. This allows us to see and control ORP PH Temp and TDS from anywhere with internet access.

While most of the system is working very well, I have had a very hard time getting the ORP sensor to
be consistent with the FC from our Taylor set. We have been keeping the PH at 7.5 per the manufacturers
instructions on the chemtrol, but I keep seeing wacky curves that go the wrong direction, stay flat, or return
numbers that are completely anti-intuitive on the ORP readings.

We have reached our target chemistry, but still the ORP floats all over the place.

ORP (600-750)
FC 2.5
BDI 90 (Ideal) (Bartier Disinfection Index)
PH 7.5
TA 80
CH 400
CYA 30
BOR 50
NaCl 3200 ( reported by digi-200, backed by what we actually added )
TDS 7600 ( reported by chemtrol )
Temp 79 F
LSI 0.15
CSI -0.18

The CO2 system runs only about 10 min a day at 20 cfh, so I don't think
that I have a problem there. ( 50lb tank may last the summer at that rate )

I saw these two posts that got me thinking, but did not get me quite far enough.

JasonLion said:
The ORP effects you describe are most likely from dissolved hydrogen gas. Hydrogen gas is created by the cell when it is on, and if it dissolves it lowers the ORP reading. Then, when the cell is off, the hydrogen out gasses and the ORP reading goes up. Ideally the hydrogen gas bubbles out and very little of it dissolves. But in some pool, quite a bit of it dissolves and you get the behavior that you describe.
Is there anything that would be causing the gas to remain in suspension, or anything I can do that would help force it out?

JasonLion said:
You need to watch for failure to achieve ORP "lock". If CYA is too high, or you have one of the pools where things don't work, the cell can run constantly and FC levels will just go up and up and up during the day. Usually the cell will turn off for some while at night in this situation. Another, less likely possibility is that the cell may simply turn off and FC levels will fall rapidly. That can happen because of MPS, or a few other chemicals that are not recommended for use with ORP. Watch fairly closely for the first few days. If it starts out working, then it should keep working.
What is "ORP lock" exactly, and how do you best get there?

Am I missing something obvious. I know that in my research I came across an article mentioning Borates can change your
ORP ( I assume due to their buffering effect) We also have a trivial amount of "Jacks Purple" ( <2ppm ) floating about, and I
recall seeing that sequesterants can affect the ORP.

I honestly thought this would be as simple as dialing the pool in to the proper FC level at 7.5 PH and telling the system
to maintain the ORP that corresponded, but I am seeing that it is not that simple.

Any advice other that not using the ORP would be welcome. ( We are using the automation to supplement our Taylor testing
when we travel not as a replacement)

Thanks for your help.



TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
Silver Spring, MD
No, you aren't missing anything obvious, aside from the fact that you shouldn't be trying to get ORP to work. Getting ORP to work with a SWG can be extremely problematic sometimes, and there really isn't any point in an outdoor residential pool. Simple percentage based SWG automation works extremely well in outdoor residential pools.

"ORP lock" means a situation where there is a tight correlation between ORP and FC.

Automating PH control is very handy and works well. But the sensor technology for measuring FC levels really isn't suitable for use in a residential situation. There are simply too many things that change the ORP reading in ways that have nothing to do with FC levels, and the solutions to some of them are quite expensive/difficult to implement. There is a sensor that reads FC Levels directly, but it was around $2000 for the sensor alone the last time I looked.

Cookie Monster

New member
Jun 4, 2009
Journal Day one

Well, I am already knee deep in this project,
so I want to keep going rather than put the
controller in the garage to gather dust and
eat my investment.

Until I get this figured out I thought I would
use this thread to document my efforts for
anyone why may find them self in the boat
in the future.

Today is Day One.

As usual I face adversity with a spreadsheet.

The FC on the pool had fallen to 1.4 and I
documented the ORP and FC, then turned
on the SWG... The ORP promptly went the
wrong direction.

When the spreadsheet fails, fallback to
Google. Seems that I have a Platinum ORP
sensor, and for a salt pool you need Gold.

I have a call in to Chemtrol, I will let you
know if this helps.


TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
Silver Spring, MD
ORP falling when the SWG is on is caused by hydrogen gas produced by the SWG dissolving into the water instead of all bubbling up to the surface. Dissolved hydrogen gas lowers the ORP level. If enough hydrogen gas dissolves, it can drown out the increase from added chlorine. Sometimes you can counter this by pointing the first return up towards the surface, which can sometimes help the hydrogen outgas instead of dissolving.

Other Threads of Interest