pool water level sensor

[earckens]

Currently my pool is equipped with a probe (plastic 2cm diameter, 8cm long) with 3 stainless steel 3mm hex bolts. Due to -I think- electrolysis -the pool uses salt water for chlorination- the lower of the 3 bolts has started corroding.
The probe senses 2 levels: low and normal.
The sensor is used for automatic pool water addition.

I have been looking for replacements -excluding floater or mechanical sensors-, from Pepperl&Fuchs (too expensive) to chinese clones.

1. ultrasonic sensors: probably the most versatile; hard/imossible to find in IP66 min. version (immersible) for reasonable cost (less then USD50)
2. capacitive sensors: insufficient range
3. optical sensors: risk of clogging the optics
4. pressure sensors: insufficient precision
5. alternative sensors: https://www.adafruit.com/products/463 , looks promising, but anyone any experience?
6. self-made: piece of plastic tube, 3cm diameter 10cm long, mount 3 hex bolts, attach 3 wires, seal everything with glue/tec/silicone/resin/..

Any ideas, suggestions?
The best idea that gets implemented receives to spend 2 days poolside
(southern France)

Erik

 

[cowboybob]

All things considered, #6. Replacing the single bolt seems easiest and would not require whatever would be needed to interface #s 1-5 to existing system.

The new bolt will, of course, corrode as well. Buy several for future replacement.

That said, #5 certainly looks promising. As long as I have dealt with water sensors, both fresh and saline (levels and chemistry), it's the first of its kind that I've seen - Thanks for pointing it out (even if I don't recommend for your current system).

... The best idea that gets implemented receives to spend 2 days poolside
(southern France)...

If I win, can I give the award to a deserving ETO'er?

 

[tomizett]

I think it's certainly worth trying to replace the original bolt - it does sound by far the easiest.
I'm no expert, but I'm sure that there are grades of stainless that will survive swimming-pool water happily (after all, they must make the fittings out of something?) so if you can find a bolt of the correct material then corrosion should not be a problem.

 

[spec]

Currently my pool is equipped with a probe (plastic 2cm diameter, 8cm long) with 3 stainless steel 3mm hex bolts. Due to -I think- electrolysis -the pool uses salt water for chlorination- the lower of the 3 bolts has started corroding.
The probe senses 2 levels: low and normal.
The sensor is used for automatic pool water addition.

I have been looking for replacements -excluding floater or mechanical sensors-, from Pepperl&Fuchs (too expensive) to chinese clones.

1. ultrasonic sensors: probably the most versatile; hard/imossible to find in IP66 min. version (immersible) for reasonable cost (less then USD50)
2. capacitive sensors: insufficient range
3. optical sensors: risk of clogging the optics
4. pressure sensors: insufficient precision
5. alternative sensors: https://www.adafruit.com/products/463 , looks promising, but anyone any experience?
6. self-made: piece of plastic tube, 3cm diameter 10cm long, mount 3 hex bolts, attach 3 wires, seal everything with glue/tec/silicone/resin/..

Any ideas, suggestions?
The best idea that gets implemented receives to spend 2 days poolside
(southern France)

Erik

Hi Earckens,

How big is the bolt that corrodes?

What is the bolt's function

Can you post a picture?

spec

 

[cowboybob]

What is the bolt's function

spec, I'm assuming (you know the danger there... ) they're the probes. i.e., Gnd, Hi and Lo level (by length).

... so if you can find a bolt of the correct material then corrosion should not be a problem...

tom, I'm thinking they're probes for indicating water level. They have a slight potential on two of them (High and Low water) and one is ground. When conducting, they indicate water level(s). Pretty much anything will corrode under these conditions, especially in water with free ions.

 

[spec]

spec, I'm assuming (you know the danger there... ) they're the probes. i.e., Gnd, Hi and Lo level (by length).

I suppose by 'danger' your referring to corrosion due to potential difference.

But I would sill like to hear from Earchkins what the dimensions are and what the set-up is.

spec

 

[spec]

By the way, my understanding is that capacitive water sensing is the most reliable and the sensors do not require a conductor to be exposed to water so corrosion is not an issue.

spec

 

[cowboybob]

By the way, my understanding is that capacitive water sensing is the most reliable and the sensors do not require a conductor to be exposed to water so corrosion is not an issue.

spec

As you noted, we need more info - the circuit/device the SS probes are connected to (detection of the simple absence/presence of conductivity) may or may not (my suspicion is not) work with a different detection probe method.

... I suppose by 'danger' your referring to corrosion due to potential difference. ...

No. It's an old saying about the danger of using the word "assume" - that to do so can make an "ass" of "u" and "me".

 

[earckens]

Hi Earckens,

How big is the bolt that corrodes?

What is the bolt's function

Can you post a picture?

spec

Hi spec, the bolt is a 3mm hex, the three bolts measure conductivity

spec, I'm assuming (you know the danger there... ) they're the probes. i.e., Gnd, Hi and Lo level (by length).

tom, I'm thinking they're probes for indicating water level. They have a slight potential on two of them (High and Low water) and one is ground. When conducting, they indicate water level(s). Pretty much anything will corrode under these conditions, especially in water with free ions.

Fully correct.

 

[earckens]

As you noted, we need more info - the circuit/device the SS probes are connected to (detection of the simple absence/presence of conductivity) may or may not (my suspicion is not) work with a different detection probe method.

No. It's an old saying about the danger of using the word "assume" - that to do so can make an "ass" of "u" and "me".

The circuit the probes are connected to is a commercial unit that measures conductivity between the three bolts and in result opens or closes a valve that injects tapwater in the pool(piping).

Besides the corroding probes there was an issue last year with an improperly working relay, the board had to be replaced under warranty.
This is French design and I suspect quality issues; I also suspect that the unit sends a continuous DC signal to the probes causing them -over time: 36 months by now- to corrode.
A solution may well be to replace the whole unit and fabricate a controller myself, using pulse signals (ie once every 2 minutes or so instead of continuous DC) to delay corrosion.

Or replace the probe with a ultrasonic waterproof unit. But I am reluctant to use Chinese technology; I don't want to image the pool flooding and setting the neighbourhood under water. And industrial sensors cost hundreds of euros. Something in between these two extremes would then probably be advisable?

 

[earckens]

If I win, can I give the award to a deserving ETO'er?

Sure. And the invitation stands.

 

[spec]

Hi Earckens,

Thanks for giving the size and function of the bolts.

There is certain to be a material that will not corrode so you could change to bolts of that material and/or eliminate any galvanic contact with other metals.

But the impression I get is that you are not generally happy with your pool water level controller.

So I would recommend that you use a dielectric approach, which can be either capacitive or inductive.

The idea, as you probably know, is to measure the capacitance or inductance of a probe, which will be different if the probe is in water and air.

Assuming a capacitive approach, each probe comprises two conductive plates, that are encased in a non corrosive, low dialectic, material, like plastic.

Then one plate of the probe is fed with an alternating voltage and you measure the signal on the other plate.

spec

 

[Pommie]

Why not make your own capacitive sensor. Two insulated metal strips 20cm long, 2cm wide and 5mm apart will have a capacitance of 570pF when under water and 7pF when in air (or with 200pF parallel cap 207 to 770pF) . Combined with something similar to Roman Black's capacitance meter and you've got all the basic parts.

Mike.

 

[earckens]

Hi Earckens,

Thanks for giving the size and function of the bolts.

There is certain to be a material that will not corrode so you could change to bolts of that material and/or eliminate any galvanic contact with other metals.

But the impression I get is that you are not generally happy with your pool water level controller.

So I would recommend that you use a dielectric approach, which can be either capacitive or inductive.

The idea, as you probably know, is to measure the capacitance or inductance of a probe, which will be different if the probe is in water and air.

Assuming a capacitive approach, each probe comprises two conductive plates, that are encased in a non corrosive, low dialectic, material, like plastic.

Then one plate of the probe is fed with an alternating voltage and you measure the signal on the other plate.

spec

Hi spec, thanks for your input. The issue with inductive and/or capacitive sensors is their lack of rang which is in the single centimeters while I need to measure levels between 2 and 20 cm distance.

 

[earckens]

Why not make your own capacitive sensor. Two insulated metal strips 20cm long, 2cm wide and 5mm apart will have a capacitance of 570pF when under water and 7pF when in air (or with 200pF parallel cap 207 to 770pF) . Combined with something similar to Roman Black's capacitance meter and you've got all the basic parts.

Mike.

Hi Mike, in theory that sounds great. But the actual fact of making an industrial quality version of this will take a bit of time, and I have no faith in my practical skills.

 

[Pommie]

Hi Mike, in theory that sounds great. But the actual fact of making an industrial quality version of this will take a bit of time, and I have no faith in my practical skills.

2 strips of PCB, solder on wires, glue 5mm spacer at each end, dip in paint, leave to dry, done. Or use heat shrink tube and hot melt glue to seal the strips.

Edit, you could even use a couple of 555s instead of a μC.

Mike.

 

[earckens]

2 strips of PCB, solder on wires, glue 5mm spacer at each end, dip in paint, leave to dry, done. Or use heat shrink tube and hot melt glue to seal the strips.

Edit, you could even use a couple of 555s instead of a μC.

Mike.

I appreciate your faith in my skills

 

[spec]

Hi spec, thanks for your input. The issue with inductive and/or capacitive sensors is their lack of rang which is in the single centimeters while I need to measure levels between 2 and 20 cm distance.

Hi Earckens,

What you say is quite true about limited capacitance sensor range; I did not explain fully.

I am suggesting that you have two capacitive sensors, one for the high level and one for the low level. You can then set the high and low levels to whatever you like. The sensors do not even need to be at the same end of the pool.

By the way, a single baby Arduino could do the control function well.

spec

 

[spec]

I appreciate your faith in my skills

I am with Pommie- it would be dead easy to make the sensors.

spec

 

[spec]

Update

With the capacitive sensors you would need to ensure that rain did not give false readings- it wouldn't be too good if every time it rains the pool overflows.

The safest approach would be to put a roof over the pool.

spec