No. As explained above I'm not familiar with this kind of devices. But we have to start somewhere...
I assume I cannot determine whether it is a good one or not...
You mean that those devices need to be stored somewhere in a special liquid, and you are only allowed to submerge those into water when you want to do a single measurement?
this is true for some probes, yes. Therefore, I mentioned it. I think (please note I have not used those personally) that those from atlas scientific can be constantly in water.
I would suggest starting with monitoring first and doing this with NR.
Have you already thought about the case where that stuff sits in the the power supply? Water proof etc.
Interesting concept. 
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Ok, very good tip. Thanks a lot!!
Yes think you are right.
This swimming pool is some kind of test. I would like to build on my own a real swimming pool, but my wife is still hesitating. So we will try it for a couple of years, and then we will see whether it was nice or a failure. But would indeed be useful if I could monitor the temperature, pH and ORP values in Node-RED. Then I get a feeling about all those values, and monitor them remotely (e.g. to monitor how the values react when I'm adding chemical stuff manually).
Yes, that is no problem in my garden...
On the subject of automatic ph control:
As I eluded in my reply: Run away scenarios can happen.
The system starts adding substance x to set the PH level right and the actuator gets stuck and it dumps a lot in: messing the PH level the opposite way.
opposite being a bit academic.
You adjusting the PH manually is it a lot harder to do this, unless you fall in the pool with a bucket of substance x while adjusting the PH level.
That is possibly why the auto adjustment devices are so expensive. I can't really be sure.
But can you see the problem with trusting something so cheap to do something which may have catastrophic health problems to anyone who uses the pool if the levels get accidentaly messed up.
That is absolutely true, so this has to be taken into account. But keep in mind that by measuring it continuously, Node-RED offers enough possibilities to warn you in many kind of ways. Moreover I assume it might be possible to do the 'controlling' only during the night hours, to further avoid risks...
Yes, that too is true.
But measuring is (to me) problematic how it is being done here.
You have a pool. Say.... 2000 litres. (Arbitrary number to keep things easy)
Your sensor is measuring the PH of..... 250ml/sec (say) water.
I'm guessing the PH sensor will have a fan of some sort to move the water past it and it isn't just sitting there.
To get the real PH level of the pool it would take...... 4 x 2000 seconds. ( / 60 = minutes)
133 Minutes. 2+ hours.
So:
It detects wrong PH that requires solution x to be added. It knows how much to add because of maths, etc...
BUT
That is only true for that 250ml of water.
Water will have pockets (pools if you prefer - and the pun) of different PH levels through out the mass. Much the same way air can have different temperatures at different points in the room.
Can you see my concern? Not that it is absolutely correct. (Remember who I am) But it is a problem I can see and I have explained it as best I can.
Just throw a handful of chlorine in each night and your done! 
Also, just use test strips for a week and find out which way your water ph drifts and then just throw a handful of + or - in each night as well
#JobDone
No need for fancy Node-RED!
Simon - just call me Pool Boy - Walters
Proud owner of two pools AND a jacuzzi
#Expert
I have been drinking........
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Going just by the image, those look to be the Atlas Scientific carrier boards with circuits, specifically the non-isolated carrier board with BNC. The EZO circuits are going for 40 euros each, the non isolated carrier boards are at ~22 euros each. The sensors depicted don't look to be Atlas Scientific probes, so I checked the link and found their Bill of Materials page: https://github.com/segalion/raspipool/wiki/BUM-(Build-of-Materials), which links to cheap probes on Aliexpress.
The way they are connected in the diagram is one probe connected over UART, the other over I2C. Carrier boards without isolation means they're not protected from interference, ground loops and high voltage. Especially the interference part matters here. For most Atlas Scientific connecting schemes I've seen so far isolated carrier boards are used when running several probes from the same controller, while the non-isolated carrier works fine for connecting a temperature probe.
I'm not sure how I feel about the mixture of high end Atlas Scientific boards/circuits with cheap probes that might be hard to calibrate (or can arrive already dried out), or with the interference you can run into with non-isolated carrier boards. On the other hand, the price increase otherwise is cheap. For another project of mine I'm looking at the T3 board (which mounts on top of a raspi and extends the gpio headers upwards) which comes with 2 isolated carriers and 1 non-isolated carrier, and you can quickly see the difference in costs there.
I'd say go with pH strips for the first weeks and see if you think it's worth the investment first. The linked BOM has an expected cost of 350 to 500 USD for the entire project. And keep in mind that the lower part goes with a pH probe fo 4 dollars, where even consumer grade probes for Atlas Scientific start around 50 dollars.
Also when you have a chance - watch Bad Boys 2 from the early 2000's with the pool scene and the dog - when i saw your picture of the pool it reminded me of that !
Craig
I was going to say:
Add a couple of fizzy tablets and you can turn your pool into a jacuzzi.
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Deal!
Well that is a good question.
But there is very nice documentation on the RaspiPool project that explains it:
And that bypass looks like this in detail:
So the water is pumped along the sensors.
But I need to have a look at the code, to see how they calculate the global measurement of the entire water volume ...
Ok, thanks for the tip!! So I need to stick with the real Atlas probes...
You cannot do that Bart, you can only measure the pH at the probe!
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Already answered but I want to add that if there will be a pump circulating the water, it will be mixed enough to measure from a single spot.
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I shall agree - as I previously mentioned.
The only trick you could do is (say):
Say the pump measures 250ml a second.
The pool is....
(Deja vous kicking in)
If you take a measurement, you will need to calculate a time period that will be one cycle of the entire volume of water and measure it again.
My example was 2+ hours.
So you could take a measurement and then wait 2+ hours and take another one.
Then act on what you read.
But that is a very slow method and I don't think it is worth the effort.
But, I have been wrong before and I am sure I will be wrong again.
Andrew, a quote from the great scientist Niels Bohr:
" We are all agreed that your theory is crazy. The question that divides us is whether it is crazy enough to have a chance of being correct."
Said lightheartedly my friend
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Most pools (that are not DIY) have the water return and the water intake for the pumping system create a whirlpool type effect to try and get all the water to mix.
In the documented scenario above by Bart you would only sample the water when the pump was running - and would sample over a period of time (say 1 hour), (most people run pumps at night to save electricity here in Australia) so you would after sampling for an hour then make a decision to start microdosing of the appopriate additives (presumably either liquid chlorine or liquid acid). As they are only using a small pump to dose the water it would be difficult to get a runaway condition
Craig
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Sounds logical. And that is not in a DIY pool? I assume because the intake and teturn tubes are too close together?
But what is the difference with non-DIY pool, because their pumps also run during the night?
Thanks a lot for this very useful feedback!!!
Probably not a good idea.
You want them as far apart as possible.
Otherwise you may get local flow and not be seeing the bigger PH level for the rest of the pool.
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