# Thread: ORP probe has some issues, it reports 770mV regardless of FC

1. ## ORP probe has some issues, it reports 770mV regardless of FC

Guys,

I have a 25K inground fiberglass-concrete pool (painted with expoxy paint).
I bought an aquarite-pro with 40K cell a few months ago. It works perfectly (purchased from inyopool they gave me a good deal beating the competition..). Two weeks ago I bought a sense-dispense kit (from ezpoolz) and hooked it up.
Warning: the plastic of the plastic case is quite stiff so the normal teflon tape tends to tear with associate leaks. I am currently using the thicker yellow teflon (the one for gas lines). No leaks.
Anyway, the PH probe is perfect. I calibrated with a buffer solution that we use for reference in my place and it is straight to the digit. The PH probe controls a stenner acid pump. I will switch to CO2. I`m aware that it will cost more (0.5-1lb of CO2 per day) but safety is important (especially because I`ve already got burned in a fire and I do not want to experience endless pain anymore).
Well, the ORP probe has some issues. It reports 770mV regardless of FC. With FC 0-1-2-3 it always says 770mV.
My CYA is 80 (suggested by the aquaritepro), so I`m wondering if the CYA can fool the ORP so much. Hayward says that I have a defective ORP and they are sending me a new one. Let`s see.

Anyway, the numbers are CYA=80 TA=90 CH=250 ph=7.4 T=67F.

BTW, if you want the saturation index, I rearranged the formula to have the ph which gives zero saturation.
If T is in F, TA,CH are in ppm (and log is natural log), then the correct ph is
ph_optimal=12.1-(1.5461*log(T+50)-6.87)-(0.4308*log(CH)-0.372)-(0.4308*log(TA)+0.280)..
For instance with the above numbers I get
ph_optimal=7.38~7.4
Hence ph=7.2 will be the min (corrosion under) and ph=7.6 will be the max (scale above).
Since I do not want to go beyond 7.8 (copper comes off solution) and under 7.1 (epoxy paint chalks too fast), then I`ve a functional which connects CA,TA, and T: ph(CA,TA,T)=optimun=7.4
As CA is fixed (calcium goes away very slowly with leaks or backwash), and T changes during the year (slowly, remember that the specific heat of water if big and near freezing is infinite), then I have a rule connecting T with TA (with the +-0.2 tolerance given by the saturation index range). Finally I plan to make some simple diagrams TA versus T at fix ph and CA so that I see how I need to tune TA to keep the saturation index in the perfect positon.

Well, to make things simple, the water of the pool has never been so clear (borax helps too) and transparent.
Now I need to:
1) wait to fix ORP so I have automatic Cl generation (otherwise I switch to % and that`s it).
2) observe for the next several weeks how TA drifts so I understand how to keep SI in the right range. SI and PH are linearly related so if I do functional derivative of ph versus TA I also get the range of TA to keep the SI in line.. The power of math.
3) find out if TA drift with acid or CO2 are in the same direction. I would prefer if TA goes naturally down as adding baking soda is cheaper than areating... and waiting.

BTW, I plumb, glue, install, program, things by myself. It must be age: I developed a strong allergy to contractors. I am cheaper (think about how many hours you have to work in your job to make 100\$/your after taxes to pay 1 hour of some unliterate contractor who has no clue of what he/she is doing) and I am more precise.. and things work ! The last pool contractor I had is still waiting for 1/2 of the bill. I did put a few internet cameras and I proved that he was NOT working the hours that he was charging. I gave him the option: take 1/2 or meet in court. After he threaten me and wish me bad diseases he chose the 1st option... Security cameras.. a good investment against thieves.

Stefano

2. ## Re: ORP probe has some issues, it reports 770mV regardless o

I suggest you just give up on the ORP controller right now. They are not even close to worth the trouble in an outdoor pool.

Also, balancing SI to zero is not ideal. You want SI a little negative. It will be higher inside the SWG and heater. Also SI tends to go up, and not go down, so you want to start a little negative. Beyond that, focusing on SI generally leads to to much micro managing. Just keep your levels generally in the recommended ranges and you never need to worry about SI. SI is most useful when you have no choice but to be outside the recommended ranges for some reason.

3. ## Re: ORP probe has some issues, it reports 770mV regardless o

Uhmm interesting. If SI goes up with ph fixed and T fixed then TA must go up. Anyway let me check for 2-3 weeks what happens so I understand what to expect. It is true, SI brings micromanaging, but having a pool give fun also on the technical aspects.
I`ll make graphs with countours of SI=-0.2 -0.1 and 0.0 so the users will see it right away.S

4. ## Re: ORP probe has some issues, it reports 770mV regardless o

I`ve some updates. After doing experimenting and plenty or reading (look at this nice review, on poaa.org Cyanurics ~ Benefactor or bomb? by Kent Williams) I realized that my CYA makes my water uncontrollable. The recommended CYA for SWG is quite high and the aquarite-pro suggests cya=80... If you look at picture 1 you see that with CYA=70 the curves of ORP vs FC collapse and there is no connection between ORP and FC anymore. Since I guess my water is around 80, the ORP vs FC must even have the wrong sign and the more FC the less ORP I would get... Amazing.
Well, now I`m stuck with a worthless ORP system because of high CYA. Since I do not want to change water, I`ll let the water age and by cleaning/splashes/backwash the CYA will slowly go down. North Carolina gets 40-55 inches of water per year, and I know the surface of the pool , I`ll get around 45m^3 per year which is ~half of the pool (25000gals=98m^3). In an year my CYA will be ~40.. In the mean time I`ll remove the probe, cap only the ORP hole with a 1/2" cap, short the ORP inputs (you do not want electric discharge to smoke it) and use chlorine %..

They should have told me before. I`m disappointed of the Hayward manual with a warning: "if you plan to use sense&dispense do not put too much cya as if you do not plan to use the control..."
Now I`m stuck waiting for rain.

S

5. ## Re: ORP probe has some issues, it reports 770mV regardless o

You should be careful about what lessons you take from that article. While there is a lot of good information in there, it is also full of errors and misleading statements. ORP can work at CYA levels up to around 50. Without ORP, CYA levels up to around 80 are not at all a problem. In both cases you simply adjust the FC level to compensate for the CYA, as described in our Chlorine/CYA chart in Pool School.

ORP never goes down when the only change is to raise the FC level, it always goes up as FC increases (again assuming nothing else changes). What is probably happening in your case is that you are using a SWG, which adds dissolved hydrogen gas to the pool. In some situations the hydrogen gas can cause the ORP level to go down by more than the increased FC level causes it to go up, causing a net decrease. In that situation, ORP goes down while sanitizing ability is actually going up!

This is one of the main reasons I said you should simply give up on ORP. Getting ORP to work in an outdoor pool with a SWG is very problematic. Hayward does say to keep CYA lower when using the sense and dispense system, but it is easy to over look. However, even if you do lower your CYA level, your ORP level will still go down when the SWG runs, just not by as much as it does now.

Simple percentage based automation is just fine for an outdoor residential pool. There really isn't any point in trying to get the ORP control to work.

6. ## Re: ORP probe has some issues, it reports 770mV regardless o

Yes, there are a lot of strange conclusions because even if the chemistry is well understood the rate at which things happen are not that studied. The many body effects are just too entangled to make simple generalizations.
Anyway, I`ll save the orp probe and plug its home until normal rain+refill will lower my CYA to 40. This should take 1 year. Next fall I`ll try again. In the mean time I stick with % which works quite well.

My PH is automated with the stenner tank and HCL in 3-4 dilution rate. It works fine. I would like to put CO2 but there is a thread in this website which describes the many TA issues that it raises.

S

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