Help with this new awesome automated acid feeder!

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flpoolgeek

In The Industry
Nov 2, 2017
15
Bradenton FL
Hi Guys,

I'm building an automated acid feeder and I would LOVE any Jedi wisdom you can contribute to this project.

The system will use:
  • a ph probe,
  • an arduino controller board,
  • and a tank of acid

I know programming so I would setup my Arduino controller to read the ph readings and then dose the pool based on the acid demand - keeping the ph of the water always where it needs to be.

I would 'tap' into the pvc pool plumbing to inject the acid. I've already done this part and it works well. No leaks. I already have the arduino controller and the ph probe. I've already built a sensor assembly for the probe where water flows into it (where the probe is submerged in the flowing water) and is constantly sending ph readings to the controller.

What I'm undecided about is how to move the acid from the acid tank into the pool plumbing. Can you give me your Jedi wisdom/experience? I don't doubt that I may be overlooking easier options due to inexperience.

Here are some options I have so far...

Option 1: Solenoid Valve. My initial idea was to 'tap' into the inlet pipe of the pump, connect plastic tubing from that to a solenoid which is connected via tubing the the acid tank. My controller would open the solenoid and the suction from the pump would pull in the acid. Yes, acid is corrosive to the pool equipment but I intend to use many tiny doses which should result in the acid being heavily diluted before it reaches the equipment. Also, I could always dilute the acid in the tank with water to reduce the acidic impact even further.

The upside is that these things would be easy to hookup with their quick-connect tube fittings, are inexpensive and will rarely fail (there's not many moving parts).

The downside is that I'm having trouble finding one that won't corrode with the muriatic acid (the plunger in the solenoid is made of stainless steel which apparently doesn't fare well with the acid). If I could find one that did, it would be perfect.

Option 2: Peristaltic Pump. Another option is the peristaltic pump. I do believe these are made of plastic so should hold up better to the acid.

One upside is that since it is a pump and can move the acid without needing to use the pool pump, I can probably tap into the return return side of the plumbing after the equipment instead of the inlet side (so the acid will not flow through the equipment).

A downside is that these have many more moving parts and I assume they will need to be repaired/replaced more often.

I would LOVE to hear what suggestions you may have.

Questions:


  1. Do you know where I can find a solenoid that isn't corroded by the muriatic acid?
  2. I'm think the stainless steel plunger solenoid valves would probably last several months. They've fairly inexpensive and extremely easy to replace so I wouldn't mind replacing them every 6 months or so. But my concern is that the corroded metal flakes off and ends up in the water causing staining on the pool surface. Is that a valid concern or am I overthinking this?
  3. Do you know of any OTHER solutions/options that I should consider?

Thanks! :D

John
 
  • Do you know where I can find a solenoid that isn't corroded by the muriatic acid?
There is a product on market using solenoids to feed acid and chlorine...Chemigem by Pool Controls. For a domestic situation this may be worth purchasing a solenoid for and solve the issue.

  • I'm think the stainless steel plunger solenoid valves would probably last several months. They've fairly inexpensive and extremely easy to replace so I wouldn't mind replacing them every 6 months or so. But my concern is that the corroded metal flakes off and ends up in the water causing staining on the pool surface. Is that a valid concern or am I overthinking this?
SS and HCL/Muriatic are about the worst mix possible. From experience will rust in under 24 hours and is likely to commence perforation in 48 hours. Article: Selection of stainless steels for handling hydrochloric acid (HCl)



  • Do you know of any OTHER solutions/options that I should consider?
I would be inclined to consider a true chemical dosing pump. It may be more money however it may be better control.

On the dosing logic I strongly suggest building a PID in the logic and making it fully exposed for adjustment. I think you will be able to get a better result this way. Most of the domestic controllers use a PI or On/Off logic and this is the main issue with them.
 
Thanks Spa Owner.

I'm kinda leaning towards the peristaltic pumps now after not having success finding a solenoid that won't corrode with muriatic acid. I think I will try it and see how long it holds up.

I would be inclined to consider a true chemical dosing pump. It may be more money however it may be better control.
Click to expand...


Since I will be getting constant feedback from the ph probe, I can program some logic into the controller so that it will start to learn what dosing rate it needs to maintain in order to maintain the desired ph. If I want it to stay at 7.5, it won't dose until it gets to 7.6. And if the dose doesn't succeed in getting it to 7.5, it will double the next dose. And it will keep doing that until it gets it down to 7.5. At which point it will stop dosing until it rises back to 7.6.
On the dosing logic I strongly suggest building a PID in the logic and making it fully exposed for adjustment. I think you will be able to get a better result this way. Most of the domestic controllers use a PI or On/Off logic and this is the main issue with them
Click to expand...


I'm not familiar with PID. Can you explain that for me?

Thanks!
 
flpoolgeek said:
I'm kinda leaning towards the peristaltic pumps now
Click to expand...


I think the peristaltic pump is your best bet.

The great thing about these is that the fluid you pump never ever makes contact with the pump mechanism - only the supply tube itself, so you don't have to worry about acid damaging the pump.

You still need to replace the feed tube every so often as the tube will deteriorate over time (tends to harden).


flpoolgeek said:
I'm not familiar with PID. Can you explain that for me?
Click to expand...

PID controllers are common in many control loops - Google 'PID Controller' and you will find plenty of hits...!
 
flpoolgeek said:
Since I will be getting constant feedback from the ph probe, I can program some logic into the controller so that it will start to learn what dosing rate it needs to maintain in order to maintain the desired ph. If I want it to stay at 7.5, it won't dose until it gets to 7.6. And if the dose doesn't succeed in getting it to 7.5, it will double the next dose. And it will keep doing that until it gets it down to 7.5. At which point it will stop dosing until it rises back to 7.6.[/QUOTE]

I don't like this. Before setting the logic this tight do a flavour please.

With the equipment running, using a manual test kit take maybe 6 or 8 samples from the water body at different points and different depths and test for pH. I think you will find differences (normally we do but the bodies are bigger). These differences are due to the circulation delay in the water body and the efficiency/not of the circulation design.

In practical terms this usually requires a bit of a relaxing on the logic. Allow the pH to drift at the probe and do some averaging in the background before responding. As it is your own pool and there is not a rush you can take as long as needed to get the settings right. As long as the control logic is fully exposed and able to be adjusted.
Click to expand...
 
flpoolgeek said:


Since I will be getting constant feedback from the ph probe, I can program some logic into the controller so that it will start to learn what dosing rate it needs to maintain in order to maintain the desired ph. If I want it to stay at 7.5, it won't dose until it gets to 7.6. And if the dose doesn't succeed in getting it to 7.5, it will double the next dose. And it will keep doing that until it gets it down to 7.5. At which point it will stop dosing until it rises back to 7.6.
Click to expand...

Its fair to say i have a lot of experience with chemical controllers. Increasing the dose rates each time its somewhat 'overthinking' things in my opinion. If you let me know what kind of dosage rate your pump/solenoid valve can deliver along with your pool volume i am happy to give you a recommendation for dosage times and wait times.
The other problem with increasing the dose amount each time is that if you have some kind of failure in the probe you will end up with a lot of acid in the pool before you know it.

As for the hardware to actually put the chemical in the pool, solenoid valves are the least best answer. It seems like they cant work but if you buy the correct type you should have no issues. A good quality media separated valve wont have any issues with armature corrosion because the diaphragm will keep the chemical away from it. I will give you the best tip i can. Burket valves are the way to go. Solenoid Valves / Products / Products Applications - Bürkert Fluid Control Systems
They may be a little more expensive but they are well designed and they work. The Burkert website has a whole bunch of handy tools for selecting the right valve.
I offer this advice with over 10 years specialising in chemical controllers.
 
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