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Using Arduino => Sensors => Topic started by: jpadie on Jun 11, 2017, 02:40 pm

Title: Deciphering ORP readings
Post by: jpadie on Jun 11, 2017, 02:40 pm
Hi

this is the first time I have played around with an ORP sensor, so I may have the wrong end of the stick. 

My understanding is essentially: the higher the free chlorine, the greater the oxidation reduction potential and the higher the +ve mV output from the probe.

The probe that I bought has specs that say the output is -2 ... +2v.

The results that I have been getting in various solutions are as follows:



These are not what I expected!  I would have expected that the higher the concentration of chlorine, the higher the +ve output in mV. 

In terms of measurement, I am not conditioning the signal at all: just attaching my multimeter leads directly across the output.  So far as I can tell there are no electronics within the probe.  There are no stray currents in the vicinity that could be affecting the readings. 

I understand that ORP is a relative measurement.  I am not looking to calibrate, just to understand what is going on and why the measurements I am getting are more or less the inverse of what I would expect! 

the Multimeter I am using has 10MOhm input impedance (Mastech MS8268).  Ultimately the idea is to automate the monitoring and dosing of a pool.  So far I am blindly dosing with 2L of chlorine per day and this seems to keep the pool around 0.5ppm.  I've been lucky and so far avoided algae blooms. 

Any help and advice gratefully received.

thanks
Justin


Title: Re: Deciphering ORP readings
Post by: wvmarle on Jun 11, 2017, 02:47 pm
Which ORP sensor and how do you connect it?

By the way, this just sounds wrong: "tap water with 50% chlorine (2.1% sodium Hypochlorite): 140mV" - that's a mere 1:5 dilution of pure bleach, and then your swimming pool water is even higher in chlorine concentration? That would mean anyone taking a dip in your pool comes out looking like an albino.
Title: Re: Deciphering ORP readings
Post by: jpadie on Jun 11, 2017, 03:27 pm
Hi

this is the orp sensor (http://www.ebay.co.uk/itm/121624290134?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT)

As for connection, literally just connected with crocodile clips to the sleeve and pin of the sensor.  

re your query: i meant 1:1 tap water:bleach and the relevant bleach is household bleach (2.1% Sodium Hypochlorite + surfactants and other stuff ).  So 1.05% NACIO in solution (assuming insignificant Cl- in the tap water), c.10,500ppm.  

But yes - I agree that the numbers/measurements don't make sense unless somehow I am misunderstanding the nature of ORP probes.  Using DRP tabs I estimate the free chlorine in the pool to be around 0.5ppm. 

Hence my post! 
Title: Re: Deciphering ORP readings
Post by: jpadie on Jun 11, 2017, 03:51 pm
just tried taking measurements again.  with a 5min soak interval.

results:

distilled water: 180-210mV
tap water: 185-195mV
50/50 household bleach/water: 40-80mV.

very odd - not consistent with previous readings and not consistent with expectations! 
(https://lh3.googleusercontent.com/oQkjDl9jenpzdOZw4joBnuaR5CgrRHojHRPgo_BnX-xcPXcjkHk8fbOc-E7wL84WqJJmjzsRHUf3MJ35SOdC6gCg8_RjVyNvNtXBD81SZXVxYY6-i2lyyZaAQkC2k1S_H2rt09U)


Title: Re: Deciphering ORP readings
Post by: jremington on Jun 11, 2017, 04:32 pm
Those sensors are reportedly quite tricky to use and subject to contamination, so read up carefully on how to use them.

I doubt if a multimeter has high enough impedance to make a sensible measurement. Get a standard calibration solution to check.

One place to start is here: https://www.ysi.com/ysi-blog/water-blogged-blog/2015/11/measuring-orp-top-tips-for-the-best-data (https://www.ysi.com/ysi-blog/water-blogged-blog/2015/11/measuring-orp-top-tips-for-the-best-data)
Title: Re: Deciphering ORP readings
Post by: jpadie on Jun 11, 2017, 04:51 pm
You may be thinking of pH meters.  ORP sensors (by my reading) are not tricky to use.  And you can't calibrate them (in the typical sense of the term): all you can do is "check" whether the change in reading is as anticipated between two solutions as different sensors will always read differently. 

A checking solution also presupposes that you have some kind of signal conditioning.  At the moment I am trying simply to understand the readings with no conditioning at all.

Input impedance of my meter is 10Mohm.  I'd think that anything over 10k would be fine for ORP.  Ph is different.  If not what impedance do you suggest as a minimum and what is the rationale?

I had read the linked document earlier today.  Thank you but it was not helpful for this issue. 

My working assumption is now that the probe is badly constructed as I cannot see why else the result curve would be inverted. 

Title: Re: Deciphering ORP readings
Post by: MarkT on Jun 12, 2017, 01:27 pm
I doubt if a multimeter has high enough impedance to make a sensible measurement. Get a standard calibration solution to check.
Most DMM's are 10M input resistance, you probably want a lot higher than that, since you need to be
several orders of magnitude higher resistance than the water might have.  Also any current flowing could
potentially (no pun intended) lead to chemical change on the electrode surfaces.  A MOSFET input instrumentation amplifier might be more suitable, with input impedance more like 10^12 ohms.

Also you measure redox potential, water contains many species that affect redox potential, for instance
O2 gas might be an issue.  You also need to check for a particular electrode system if it has any incompatibilities with
dissolved species that could affect the reading.  Basically you need to be a chemist to understand all
the possible reactions at the electrodes!
Title: Re: Deciphering ORP readings
Post by: jpadie on Jun 12, 2017, 01:52 pm
Thanks

I get very similar results with a 100Mohm ADC. So at least one order of magnitude higher. 

The ORP probe has an internal resistance in the kOhm range. 

Whilst I accept that there are many factors that affect potential generation this in the end is just a battery creating a pd between a reference (probably KCL) and the solution it is submerged in.  Chemically this should be consistent and repeatable.  And for sure a higher Cl- concentration should result in a materially higher mV than distilled water!
Title: Re: Deciphering ORP readings
Post by: wvmarle on Jun 12, 2017, 03:02 pm
ORP = Oxidation Reduction Potential. Cl- ions are not oxidising or reducing, so shouldn't affect an ORP probe. ClO- (hypochlorite) is a very different ion, and indeed a rather strong oxidiser.

Lots of info also on the web, maybe it can help you find out where your measurements go wrong.
http://www.ozoneapplications.com/info/orp.htm
https://en.wikipedia.org/wiki/Reduction_potential
Title: Re: Deciphering ORP readings
Post by: jpadie on Jun 12, 2017, 03:17 pm
my mistake.  I should have type chlorine bleach rather than just refer to Cl-.  Hopefully all the previous posts mentioning Sodium Hypochlorite adequately contextualised. 
Title: Re: Deciphering ORP readings
Post by: STEFFI_G on Aug 23, 2019, 09:34 am
can you please suggest which MOSFET input instrumentation amplifier, with input impedance 10^12 ohms can be used in orp circuit?
Title: Re: Deciphering ORP readings
Post by: jpadie on Aug 23, 2019, 09:43 am
why do you need 10^12?  ORP sensors should be readable with a standard multimeter (despite the comments above). 

But if you really want a high impedance opamp then LMC6081 should work (10+TeraOhms).  That's what I am planning to use in the pH reader I'm building.

my gut feel is that also an LM358 would work.  Its input impedance is in the 4GOhm range. 
Title: Re: Deciphering ORP readings
Post by: Watcher on Aug 23, 2019, 09:50 am
Quote
Ultimately the idea is to automate the monitoring and dosing of a pool. 
I ve been trying to do the same for some time now both for ORP and Ph but I could never get consistent results from probes. I have tried 3-4 different ones as well and they also come with an instrumentation amplifier for signal conditioning .

Title: Re: Deciphering ORP readings
Post by: jpadie on Aug 23, 2019, 09:55 am
do they get consistent results when using buffer / calibration solutions?

do you have a schematic design to share?  I hope to finish the first cut of mine later next week.
Title: Re: Deciphering ORP readings
Post by: Watcher on Aug 23, 2019, 12:06 pm
do they get consistent results when using buffer / calibration solutions?

do you have a schematic design to share?  I hope to finish the first cut of mine later next week.
This is one of the probes (https://www.ebay.co.uk/itm/PH-Electrode-Probe-BNC-for-Arduino-Liquid-PH0-14-Value-Detect-Sensor-Module/152988039335?hash=item239ecc38a7&var&_trkparms=ispr%3D1&enc=AQAEAAADIKvsXIZtBqdkfsZsMtzFbFsbX3WcW5fmB%2Fx7ZbaZTyexJbHD4RuITV3md3aohffe6asRZg4lX3XDnp0VH5T4DkbDmHWzqMuS9UY0TZSzvxO%2FzGEKG%2FTDySU%2Fvi29mNfwvNOYk9pQUQXKM1PNVrJTn2ytLw5%2Fm2YKKkZZPdHI9m7SbOnHz1XV7jH65xD13cNlKtMITDMxZNucHyHAt9IOAtF3a%2BmJyMKyP7aIYXGT2U1A6n%2FKSqEB5GeNVkWtYXHx03JIF2NZp%2BIycB9qrLhwjPIJLLsd%2FACxEMXZjkmhiRGpIBMM5okCT42yn1%2BZKlnfjuK7LUeT4WygQW9KYdu2MyfbfnEEn5VWy3lTkCeRVhlCeB6x5pQ8n%2F4lFRpR29VwiACKsclIhNsocdoOjPmiLhvSgRUvca9gjMEbACfFF0ZW89u%2BblGGwYzFgJG5XwyULMxXjbH%2FmBGzuGvgNBuHy5WjnBdLSsW7rkWz7p99flRLGRW9S8DJ%2BOjpaL8NJbVQ04xzie6xiFIlKn7Oz6h5Kxvk6tbfEo9av3ggsxzpIxNrwFXIpHpOBIMYoREhSDu2JEWpW6KMbfs%2F3gAhNT8v%2BhMyjHWPUG1OtWRzGnIzCjb7wUt%2FFJ36R6TKCtzRMrzy0mc3UqgGICWpt4uaCN0JgcasT4jSftFmSqzS%2Bp6TVFfqu67yWBgsmN213Zbf14T%2FYIU0GHs0wio66ZsuUIulXQRn29WcorXpGuIZGUhLzjtuDZtM%2FgDftfxUuY8NLSocMN4qhbmkwRysdtcYtMhMPPoaOy1EC1RK03mtEAtc9YE0gx2kf8y2D%2Fu8tdCWs2%2Fs1%2B2HFeLWswseKLL%2FK73I%2BGr%2B1EvXAS89XMEorlrWBlb%2BKcvxtubaCLhZGCtgkwx5QEGp6TaVDsy5wHMN4dOdW5ZgNG4LBx%2BSjBpj94EZHXnUaEgFKU%2FteDPYU8Mkzn09w9q9O%2Bhqq%2BI%2F6Lsi8SqONPnPAB0SsDKolJ7sAZVUrKfcne3ucBPmpLG5aZroksMOd%2Fh9jV3b2FyhoiXNE53b9mVwySExcbYDwr9Hi5YBDjr%2B&checksum=152988039335bb047165e6d442249b0a45ca82862ded) i tried.

It comes with its own buffer/ amplifier with calibration pots and instructions how to calibrate with it the provided solutions but I was unable to get consistent  readings.

No i don't have a schematic for it. I just connected it to a multimeter as well.

Another option which looks more on the industrial side is this (https://www.hackster.io/uFire/nano-33-iot-ec-ph-orp-webapk-82ab54).
This is much more expensive as well but I havent tried it.
If you do manage to get anything usable, do let me know as I am on the same track as well!



Title: Re: Deciphering ORP readings
Post by: jpadie on Aug 23, 2019, 12:23 pm
I'm trying to keep the design at a low price point, for probes too. 

I think the ph probes I have been using are £10 each.  or less.  the ORP probe I bought a few years ago and was much more expensive.  No conditioning op-amp provided.

Incidentally I am also designing an arduino control for a variable frequency device to power a standard three phase pool pump.  Much much cheaper than buying a variable speed pump
Title: Re: Deciphering ORP readings
Post by: Watcher on Aug 23, 2019, 05:19 pm
I'm trying to keep the design at a low price point, for probes too. 

I think the ph probes I have been using are £10 each.  or less.  the ORP probe I bought a few years ago and was much more expensive.  No conditioning op-amp provided.

Incidentally I am also designing an arduino control for a variable frequency device to power a standard three phase pool pump.  Much much cheaper than buying a variable speed pump

Thats interesting as well!

Never thought of doing that...
Title: Re: Deciphering ORP readings
Post by: jpadie on Aug 27, 2019, 03:23 pm
Quote
Another option which looks more on the industrial side is this. 
Just taken a proper look at these.  They appear to be an opamp connect to an tiny85. The ec probe is connected directly to the tiny85.   Then they all have some upc at the end that controls the i2c.  I'm not sure why that is necessary or desirable when the tiny85 can provide an i2c signal.

Outrageously priced at 60$.  Component cost looks like a couple of dollars disregarding the connectors.   Not entirely clear where the water temperature is being measured either. 

The other issue is that the pH probe doesn't look to have any negative voltage on the opamp.  So difficult to see how that's being conditioned to handle the range of values seen from commons probes. Less of an issue for orp since negative readings (technically possible) would be very unusual and in any event you're really looking for above/below a threshold (taking hysteresis into account)

I'd have thought the need for electrical isolation could be circumvented by dropping a ground probe into the water being measured.  That's my plan.   
Title: Re: Deciphering ORP readings
Post by: wvmarle on Aug 27, 2019, 04:10 pm
I'd have thought the need for electrical isolation could be circumvented by dropping a ground probe into the water being measured.  That's my plan.   
May work - if your probe is not being biased, as is often done to make pH values > 7 easier to read with a single sided power supply.
Title: Re: Deciphering ORP readings
Post by: jpadie on Aug 27, 2019, 04:16 pm
I'm biasing with a negative rail.

Title: Re: Deciphering ORP readings
Post by: wvmarle on Aug 27, 2019, 04:28 pm
Outrageously priced at 60$.  Component cost looks like a couple of dollars disregarding the connectors.
You forget the cost of development. You may be able to reproduce that - and make it cheaper, if your time is free.
Title: Re: Deciphering ORP readings
Post by: jpadie on Aug 27, 2019, 04:49 pm
true.  but once you've got the product in the market, the development value is lost.  it's reverse engineerable after the first one hits the ground.

so these days you can't have an expectation of making a big mark up over the cost of production and distributions themselves (unless you can control access to the design).
Title: Re: Deciphering ORP readings
Post by: Delta_G on Aug 27, 2019, 06:50 pm
Are you correcting for pH?  Or anything else?  ORP is strongly affected by pH.  As the pH changes the ORP changes.  ORP isn't really a good indicator of free chlorine unless everything else is tightly controlled, which won't be the case in your pool. 

The effect you're seeing with bleach in water is probably more an effect of the strong base in the bleach rather than the oxidizing power of the chlorine. 
Title: Re: Deciphering ORP readings
Post by: jpadie on Aug 27, 2019, 06:53 pm
thanks Delta_G

the pH is corrected for so we are testing for ORP when the pH is balanced. 

the initial work I did a couple of years ago was very inconclusive.  the readings for pool water and for tap water changed wildly over moments so i think there must have been stray capacitance from somewhere.

Title: Re: Deciphering ORP readings
Post by: Delta_G on Aug 27, 2019, 07:20 pm
Quote
the pH is corrected for so we are testing for ORP when the pH is balanced.
So there's a pH probe also involved in this measurement?

So what was the pH in the bleach solution compared to the tap water?
Title: Re: Deciphering ORP readings
Post by: jpadie on Aug 28, 2019, 02:00 pm
hi Delta_G

this thread spans two years.  yes - originally when I did the measurements I took the water from my pool which was pH controlled.  I did not check the pH of tap water, but it would have be within normal ranges.  Evidently adding NACIO will change the pH and that may have skewed those results, I agree.  But they should still have been repeatable.  I did not check it however.

thanks
justin




Title: Re: Deciphering ORP readings
Post by: jpadie on Aug 28, 2019, 02:11 pm
i should add that my comment above, that you captioned, referred to the general use case for this design which is to check for pool water quality.  there is a pH probe, temperature probe and ORP probe all within the design.  In my case, the attached uProcessor logs the data and decides whether to add acid, base or, if the pH is in range, chlorine (or to turn on the electrolyser).  the liquid dosing is done by cheap peristaltic pumps although I'm having trouble sourcing liquid ph+ so I may also develop a dosing auger for the powder.  the electrolyser is turned on and off by a mains relay. 

currently the design is to have the probes in line with the chlorine cells (before) as the system has 30mm threads ready to take the ground probe and the sensor probes.   but i have wondered whether there would be significant stray current from the electrolyser, so was thinking that instead it might be an idea to t-off and run some aquarium tube up into a separate analysis area physically distant from the higher voltage electronics.  it's all a job for October.
Title: Re: Deciphering ORP readings
Post by: Delta_G on Aug 28, 2019, 02:56 pm
1.  The pH control on your pool isn't tight enough to make for stable ORP readings.  It isn't.  I don't care how tight you think your control is.  It's a big open body of water and the slightest change in pH can affect that ORP reading.

2.  Don't think your tap water has "normal pH ranges".  The "normal" range for tap water is about two and a half pH units wide.  City water goes from high 6's all the way up to the low 9's.  So there's a pretty big range there.  If you have a well then it's wider than that. Well water could be anywhere from upper 5's to low 9's.

3.  Having used ORP for years to try to dose ozone into an aquarium I can tell you it's a fickle beast.  Make sure you got plenty of backup and make sure that your system can't take any drastic actions on its own.  You'll get spurious readings all over the place.  Especially if this is an outdoor pool. 

As an analytical chemist this sounds a bit crazy.  Sounds like someone who knows pools but hasn't sat through many chemistry courses.  But I hope it works for you. 
Title: Re: Deciphering ORP readings
Post by: jpadie on Aug 28, 2019, 03:20 pm
Thanks.

I think the point is that ORP  _is_ used for pool measurement all over the world so there must be a fashion to make it adequately accurate for those needs.

pH may well not be controlled perfectly but again, it seems that in general pH can be controlled adequately for the needs of pool users. 

Or am i.missing something?

Title: Re: Deciphering ORP readings
Post by: jremington on Aug 28, 2019, 05:04 pm
Quote
Or am i.missing something?
You are evidently missing whatever it is that people "all over the world" know about using ORP sensors for "pool measurement".

Quote
ORP sensors (by my reading) are not tricky to use.
But after two years, you still have no idea how to use one.
Title: Re: Deciphering ORP readings
Post by: jpadie on Aug 28, 2019, 05:27 pm
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?
Title: Re: Deciphering ORP readings
Post by: wvmarle on Aug 28, 2019, 05:32 pm
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.
Title: Re: Deciphering ORP readings
Post by: Delta_G on Aug 28, 2019, 06:12 pm
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. 
Title: Re: Deciphering ORP readings
Post by: Delta_G on Aug 28, 2019, 06:16 pm
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? 
Title: Re: Deciphering ORP readings
Post by: jremington on Aug 28, 2019, 06:20 pm
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.
Title: Re: Deciphering ORP readings
Post by: Delta_G on Aug 28, 2019, 06:37 pm
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. 
Title: Re: Deciphering ORP readings
Post by: jpadie on Aug 28, 2019, 06:48 pm
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

Title: Re: Deciphering ORP readings
Post by: jremington on Aug 28, 2019, 06:51 pm
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
Title: Re: Deciphering ORP readings
Post by: Delta_G on Aug 28, 2019, 08:01 pm
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. 
Title: Re: Deciphering ORP readings
Post by: Delta_G on Aug 28, 2019, 08:03 pm
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();
Title: Re: Deciphering ORP readings
Post by: natew on Sep 02, 2019, 11:04 am
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....
Title: Re: Deciphering ORP readings
Post by: Delta_G on Sep 02, 2019, 07:37 pm
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. 

Title: Re: Deciphering ORP readings
Post by: natew on Sep 03, 2019, 09:29 am
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?



Title: Re: Deciphering ORP readings
Post by: jpadie on Sep 03, 2019, 12:04 pm
@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.

Title: Re: Deciphering ORP readings
Post by: Delta_G on Sep 03, 2019, 03:46 pm
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. 
Title: Re: Deciphering ORP readings
Post by: tf68 on Sep 04, 2019, 08:38 am
@jpadie

This paper addresses the relationship between pH, chlorine and ORP.

http://anrcatalog.ucdavis.edu/pdf/8149.pdf
Title: Re: Deciphering ORP readings
Post by: natew on Sep 04, 2019, 11:05 am
@jpadie

Indeed sounds great, maybe we can collaborate a little as I may start down the path on my own.

I agree that with a few known constants and formulas this should be straight forward.  I don't see how this would be any less accurate that approximating the quantity of water in two tiny vials, putting 4 approximate drops into those vials and then, more or less, guessing as to which color the vials have turned...  Especially since said color varies depending on how much ambient light you have outside.  I would think that even if the readings are off from reality by 10-15% they are as good if not better than what Im doing today.

Not looking for a precise quantity of chlorine, not trying to put a rocket on the moon here by mixing hydrogen and oxygen in precise quantities of atoms....  Just trying to keep the pool from turning green too often and keeping it more or less comfortable for swimmers.