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Topic: Deciphering ORP readings (Read 2770 times) previous topic - next topic

jpadie

I haven't tried.

I shelved the project after getting limited help two years ago and picked it up again a month ago as I am in the process of rebuilding my pool

Is this a forum for advice and support or trolling?

wvmarle

Is this a forum for advice and support or trolling?
Can go both ways - depends mostly on the attitude of those asking questions, and whether those posters actually bother to do their homework.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

Delta_G

You may be right that ORP may be used for pools.  I never had a pool so I don't know what sorts of controllers you can get.  What I can be sure of is that some chemists were involved in that development.  It's not hobbyists building their own controllers. 

You're also right that you control the "general pH" whatever you mean by that.  But it doesn't take much movement in pH to change the relationship between your chlorine level and the ORP.  It is simply not possible to control the pH is a large outdoor body of water to that level.  A tenth of a point makes a difference and you've likely got that much difference between the top and bottom or from one end to the other.  Typically a controller would have a pH probe along with the ORP probe and would be adjusting the ORP reading in a calibrated way based on pH and temperature.  It's not a trivial matter. 

If you're missing anything it's a deep understanding of the chemistry involved.   This is something that sounds simple on the surface in theory, but in practice will prove to be quite complicated. 

Either way, you seem to doubt me.  I can't change that.  Just thought I'd offer advice from someone who knows how this stuff goes together.  If you don't want it that's fine.  I personally don't care if it works or if it burns you up the first time you jump in.  I just wanted to offer.  Take it or leave it. 
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Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

Delta_G

Quote
ORP sensors (by my reading) are not tricky to use.
You're right. ORP sensors are trivial to use.  It's using the ORP reading that they give you for anything productive that's complicated.  Getting the reading is trivial.  Interpreting it is another matter entirely.  Making decisions based on it is the tough part because it doesn't always mean what you think it means.  Sometimes the fall in ORP just means that the pool got shaded by clouds for a while.  Do you change the dose of chlorine based on that? 
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jremington

#34
Aug 28, 2019, 06:20 pm Last Edit: Aug 28, 2019, 06:27 pm by jremington
Let's first be clear: "free chlorine" and the active ingredient in bleach, hypochlorite (in either the neutral or anionic form) are completely different compounds. Hypochlorite spontaneously reacts or degrades into chloride ion and various products.

Delta_G

#35
Aug 28, 2019, 06:37 pm Last Edit: Aug 28, 2019, 06:47 pm by Delta_G
Close.  There's no neutral form of hypochlorite.  It's always an ion.  The only stable dry form I know of is calcium hypochlorite.   It also doesn't decompose into chloride.  It disproportionates into chlorine gas and hydroxide, a reaction accelerated by low pH taking the hydroxide to water.  This is why you never mix ammonia (usually sold as ammonium at an acidic pH) with bleach because it produces chlorine gas.  But even at basic pH the hypochlorite is at equilibrium with free chlorine gas.  As the chlorine is consumed by oxidizing the junk in the pool, the equilibrium shifts more hypochlorite to chlorine. 

Turning it to chloride involves a reduction, which means something else got oxidized and that's the reason you put the bleach in in the first place. 

But it can't spontaneously turn to chloride.  You gotta get the extra electrons from somewhere. 
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Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

jpadie

Quote
Do you change the dose of chlorine based on that? /quote]

I've not written the dosing code yet. However i would expect to make dosing decisions as the sun went down based on averaging over the day


jremington

#37
Aug 28, 2019, 06:51 pm Last Edit: Aug 28, 2019, 07:09 pm by jremington
Quote
There's no neutral form of hypochlorite.
Not true. The pKa is 7.53. But neither the acidic nor the basic form is stable in solution.

The chemistry is extremely complicated, depending on pH and other solution components, as outlined here: https://en.wikipedia.org/wiki/Sodium_hypochlorite

Delta_G

#38
Aug 28, 2019, 08:01 pm Last Edit: Aug 28, 2019, 08:01 pm by Delta_G
Not true. The pKa is 7.53. But neither the acidic nor the basic form is stable in solution.
If it is protonated then it is hypochlorous acid.   The hypochlorite ion in that compound is still ionic.  It's just balanced by a proton. 

Now we could argue over whether or not that O-H bond is truly ionic or covalent, and the true answer is that it is something in between.  But if you are going to argue that this makes it neutral then you have to stop calling it hypochlorite. 

Hypochlorite is an ion.  It has a single negative charge.  Taking another electron to make it neutral would give you chlorine dioxide. 
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Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

Delta_G

#39
Aug 28, 2019, 08:03 pm Last Edit: Aug 28, 2019, 08:08 pm by Delta_G
And yes the chemistry gets complicated.  Which is why I have my doubts about the feasibility of what the OP wants to do.   

It will definitely be way more complicated than
Code: [Select]
if(ORP < someThreshold) addMoreChlorine();
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Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

natew

#40
Sep 02, 2019, 11:04 am Last Edit: Sep 02, 2019, 11:06 am by natew
So I am a bit interested in doing the same as the OP.  I have not bought anything yet really other than the Arduino board I already have laying around.  You guys make this sound a bit impossible but, given that I can buy a hand held ORP/PH sensor for about $30 that measures chlorine and PH in my pool I am on the OP side thinking that this cannot be THAT complicated.

But, I am also not looking at really automating this fully.  I don't mind throwing a chlorine tablet in the pool every week... What I am more interested in is integrating this with my home assistant and throwing up a simple graph on the control panel in my house that will tell me the PH and chlorine level in the pool (and maybe sending me a message) so I have a chance of adding a tablet or two before the pool turns green..... 

Knowing that ORP and PH are related, I get that.  However, for non-pool owners maybe it is not well known that chlorine is not effective if your PH is anywhere outside the range of 7.2 and 7.4...   So.. if PH is off by even .1 then there is really no point in adding chlorine until the PH is properly adjusted.   

Also, my pool is 35,000 gallons.  Your statements may be correct when talking about a glass of water.. But to alter the PH of my pool by .1 I need to add more than a KG of either acid or a base.. So you cannot sit here and tell me that my PH is going to change from hour to hour, day to day. The same with Chlorine, I add about 1-2KG per week just to maintain a somewhat constant level.   Same with the temperature, it takes more than a week for my pool to move by 1 or 2 degrees in either direction....

The only thing that drastically skews these numbers is when it rains and dumps a few thousand gallons of new water in the pool.  And still, having some vague idea of how much chlorine to add or how much of which (acid or base) to rebalance the PH without having to go outside with these silly test tubes of yellow and red die and make a somewhat educated guess as to what the PH and chlorine level is (Is that a little yellow, or really yellow???)  (It looks kind of purplish to me, or is it really orangish.. not sure lets add a gallon of acid and see which direction it moves...)  Pool chemistry is already not exact science....

Delta_G

Quote
Also, my pool is 35,000 gallons.  Your statements may be correct when talking about a glass of water.. But to alter the PH of my pool by .1 I need to add more than a KG of either acid or a base..
It's funny sometimes to listen to people who don't know chemistry ty to make sense of pH. 

I'm not here to argue.  I'm just an analytical chemist who tried to help a little with a project involving chemical analysis.  No worries if y'all ain't interested. 

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Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

natew

Delta_G.. Indeed I am interested since I will admit I know little other than how to move around small numbers in a relatively large body of water.  But you, more or less make it sound impossible.

But then there is this: https://www.blueconnect.io/en/  .  Which I am sure is little more than an ORP sensor and a PH sensor in a plastic case.  And Im sure someone like you on the  figuring out how to interpret the data and write it into a program.  Im just not up for spending $300 on the thing when I see that the sensors total to about $50 at most and I have the knowledge and ability to do the electronics and programming myself.  Not to mention another thing floating around in the pool is not really exciting, was planning on mounting this somewhere off the piping for the pump or hiding the sensors in the strainer.

So.. any tips on how to interpret the data?  Or places to look?




jpadie

@natew

I have no idea whether the chemists on this board are being helpful or trolling.  I don't have the chemistry to tell the difference and I don't have the time to learn it.

I take the simple view that whatever we are being told here pH can be adequately balanced in a swimming pool to avoid killing or harming bathers.  And pools can be adequately sanitised to avoid infecting bathers and keeping the algae at bay.  Detailed analysis may be a precise science; dosing a pool is not and is well within the ambit of a consumer.

The liquid and tablet reagents that have been used for decades by pool owners are imprecise; the user interpretation against the colour chart is imprecise.  A calibrated pH sensor will be at worst as precise.  Likewise an ORP sensor should be at worst as useful.   There are published formulae for calculating pH and correcting for temperature.  Likewise there are published formulae for ORP correction for temperature.  And if you're not confident in programming there are pre-built sketches and libraries for pool management.

I am happy to share my board design once I've done it.  And could make one up for you if the prototype works.  My guess is that the cost of production and components is in the £10 range (excluding the sensors).   I don't see that anyone has much to lose in trying such a solution.  Compare against manual tests until you're confident.


Delta_G

Oh I'm definitely not trolling you.  I just have a bit more experience working with these types of measurements.  I've been down the road you're starting down and was hoping to let you know what I found ahead of you.  But you can't hurt anyone.  Worst that happens is you waste some money.  So carry on.  Hope you make it work.  Please let us know. 
|| | ||| | || | ||  ~Woodstock

Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

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