Salt water (ocean) immersion detection

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Dear all,

I'm toying with some ideas for a holiday project.

What I want to do is to reliably detect when a device is submerged in the ocean, or perhaps more importantly, when the device is *out* of salt water.

I was thinking either by pressure or two electrodes. Problem is, I have no idea how reliable either scheme would be, either as far as detection was concerned or long-term reliability (e.g. corrosion).

Does anyone has any suggestions as to what has worked for them, or anything I should look at in particular?

-- Damien

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What is the deepest point when the device is in the water?

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You can use capacitance between insulated electrodes.

You can measure resistance, but you need to keep the "average" voltage at zero; this can usually be accomplished with a moderately large cap in series with each electrode. Making electrodes out of stainless steel can help, when combined with the caps. But, reliability is not good.

You can use a pressure sensor, if you can tolerate at least a few inches of depth. You should be able to find semiconductor ones for less than $10US. You may need to isolate the pressure port of the sensor with a small diaphragm (think of something like a rubber balloon).

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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ka7ehk wrote:
You can use capacitance between insulated electrodes.

You can measure resistance, but you need to keep the "average" voltage at zero; this can usually be accomplished with a moderately large cap in series with each electrode. Making electrodes out of stainless steel can help, when combined with the caps. But, reliability is not good.

Looks like some experimentation then. $10 for a pressure sensor might be a bit much for what I want to do.

For the moment, I haven't yet thought enough about depth (my guess will be 50m max), but I probably should seeing I'll have to waterproof the thing anyway. A few inches of water isn't so good... I want to take a GPS measurement when I hit the surface, which is generally problematic under water :)

(The whole water detection thing is to prevent wasting power trying to acquire a signal when underwater)

-- Damien

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Is this a submersion/float detector in any type of water? Just how significant is the salt water reference. What happens to the sensor when you go up an estuary where the water becomes brackish & then fresh?

A floating piece of styro-foam when pushed down will provide a significant force on say a micro switch lever. This is what is often used for the "flood detector" in dish washing machines.

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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LDEVRIES wrote:
Is this a submersion/float detector in any type of water? Just how significant is the salt water reference. What happens to the sensor when you go up an estuary where the water becomes brackish & then fresh?

Actually, come to think of it, freshwater might be of use too..

LDEVRIES wrote:

A floating piece of styro-foam when pushed down will provide a significant force on say a micro switch lever. This is what is often used for the "flood detector" in dish washing machines.

How big are they generally? It might be useful - especially if I can get one COTS.

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In my Fisher Paykel dishwasher there is a small platform that is mounted on the base about 100mm x 100mm. A piece of polystyrene fits underneath it and is held in place by the mounting posts.
If water floods, polystyrene lifts and moves a micro-switch lever. Could use hall effect device or a optical sensor too.
The greater the surface area, the greater the force.

I though the saltwater might have been a distraction..hi hi!

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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It sounds like the object will never be totally out of the water. On the surface, it will be bobbing and rolling, which makes contact and capacitance sensors problematic.

Assuming that the GPS antenna is water proofed, or under some sort of water-proof cover, why not just turn it on at a few inches of depth. The time it should take to rise the last few cm should be pretty small compared to the time to rise from 50m.

The same sort of problem, though with greater depth, is encountered by the sat tracking tags we put on whales at Oregon State University. These were ARGOS tags that transmit from the ocean surface. Transmitting is far more power hungry than a GPS receiver. There, we just let it transmit on a a low duty cycle schedule, under water or not. A few C-sized batteries gave it a couple months life, if I recall correctly.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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My Citizen diving watch uses , what seems to be 2 rings of brass (embedded in the case) insulated by some material (looks like white plastic).

When moist hits the brass rings , the diving sensors are activated , and it says i'm diving when the pressure sensor is below 1m of water.

There is one serious flaw (room for improvement here), when it's summer and your wrist hits the watch the sweat activates the brass rings. And the watch activates the sensors .... Allthough i'm not diving , as i'm not on 1m water. The activated sensors uses a lot of battery , and at $100 to get the battery changed every 1..2 years. As it has to go the Citizen service centre to get new battery , O-Rings and a 200m pressure test. It becomes rather expensive.

The old model had a "mode" you had to activate diving mode via the watch settings , but ppl. complained that they couldn't operate the watch with 7mm neopren gloves on (i agree) , so they invented this new system.

I'd say that a combi of the above would be perfect.
1: Operate the watch via the buttons to activate the possibility to get into diving mode.
2: Use the brass & pressure sensors , to go into diving mode.

/Bingo

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One could use IR reflective optoelectronic coupler as in numerous industrial water/humidity sensors. An IR LED and sensor plus a Plexiglas prism will make nice full internal reflection (IIRC) sensor. For processing a single comparator will do.
Industrial sensors prices start at Eur, 25 per pc, indeed.

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An acoustic transducer (ceramic, piezo) could also work. They change imedance significantly when "loaded" with water instead of air. Ie the resonance frequency change.

An acoustic transducer can be simple like a piezo disk with some tape or glue on it to prevent it from corrosion. The difficul part is to build an oscillator or the like that can sense the impedance.

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I've built an overflow alarm for bathtub that is very simple.
Two wires slightly separated from each other.
One connected to GND, the other to an I/O pin setup as input with pull-up.
When both wire ends are submerged in water Input goes low and alarm goes off.
Don't know if this will work in your scenario, but it's as simple and cheap as it can get.

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Heihopp wrote:
An acoustic transducer (ceramic, piezo) could also work. They change impedance significantly when "loaded" with water instead of air. Ie the resonance frequency change.

An acoustic transducer can be simple like a piezo disk with some tape or glue on it to prevent it from corrosion. The difficult part is to build an oscillator or the like that can sense the impedance.


That's brilliant idea, indeed. Due to viscosity of water the transducer losses will increase significantly when immersed; thus monitoring amplitude, current draw or, alternatively, resonance frequency will do the trick.
The generator itself could be built with one BJT or mosfet, but this will require transducer with three terminals (two usual plus an extra tap from the center). There are commercial beepers, too, that have built in generator - cheap and fast solution. Yacht or epoxy coating seems to be enough for corrosion protection.

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Kas wrote:
Heihopp wrote:
An acoustic transducer (ceramic, piezo) could also work...

That's brilliant idea, indeed. Due to viscosity of water the transducer losses will increase significantly when immersed; thus monitoring amplitude, current draw or, alternatively, resonance frequency will do the trick.
The generator itself could be built with one BJT or mosfet, but this will require transducer with three terminals (two usual plus an extra tap from the center). There are commercial beepers, too, that have built in generator - cheap and fast solution. Yacht or epoxy coating seems to be enough for corrosion protection.

If using a generator with fixed frequency, the transducer can be connected in series with a resistor to ground where the AC current can be measured (resistor voltage is proportional to current).

Or what would happen to the frequency of an simple RC oscillator were the transducer itself is the capacitor? If that works, only an amplifier stage and a frequency counter is required to detect the change.

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Mount the transducer to a small circular closed end housing containing the electronics and sealed at atmospheric pressure. With the other side subjected to changing water pressure, you have crude depth indicator.

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Quote:
An acoustic transducer can be simple like a piezo disk with some tape or glue on it to prevent it from corrosion.

Quote:
Yacht or epoxy coating seems to be enough for corrosion protection.

You mean, let the transducer inside of you enclose? How you will protect from water?
I think that I'm missing something... :oops:

Regards,

Bruno Muswieck

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@Brunomusw:
business side of the transducer must have close contact with media - air or water. Through a water tight orifice, hole or window in an enclosure. And it do needs to be protected against corrosive salts and water. Well, a membrane close to the transducer shall work, too - the media impact will be weaker, nevertheless.
Personally me prefer IR optical solution.

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Oh dear, the weather is not "good one day, perfect the next" in QLD then... :?

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Steady there John,

Its the weather from Sydney thats come to spoil our fun.

For those not in the know Sydney is allegedly the wettest capital in Australia.

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I recently acquired a digital tire pressure gauge for hacking purposes. The pressure transducer in such a gauge may be useful for what you want. Some of my initial tests show it has a sensitivity of about 20uV for one inch of water. 50 meters works out to about 71psi well within the range of the transducers sensing capabilities. A diaphragm as mentioned earlier would be needed to protect it from the seawater. In my particular gauge the sensor is glued to the circuit board, but that portion could easily be cut off and used as is. My intent is use the sensor for measuring water depth in a bubbler. My gauge I picked up for about $6 you may be able to find them for less over the internet.

Roger

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Just a thought here: how about a loosely coupled concentric transformer, such as one might find powering a rechargeable toothbrush? I have a feeling that a conductive dielectric between the two parts would have an unpleasant effect on its efficiency, and this would have the advantage of both no moving parts and nothing electrical exposed to salt water.

Experimentation would not be too tricky; stick an ac voltage on the primary, measure the secondary, add water|salt water and see what happens.

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js wrote:
Oh dear, the weather is not "good one day, perfect the next" in QLD then... :?

We tried sending it to Melbourne for the Test, but it didn't arrive on time :(

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I still like the idea of electrodes as they're passive and therefore (at a guess) going to consume less power than a transducer, optical beam or concentric coil.

@Bingo,

Any idea if those citizen watch pressure sensors are available as a commercial-off-the-shelf component?

-- Damien

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I like the idea of two exposed disks... a cap with air or water dielectric... you measure the rise time and calc the C from the rise time. Isnt this how the qtouch algorithm works?

Imagecraft compiler user

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@damien_d:
shall the capacitive sensor be an option, please see:
http://elm-chan.org/works/capsens/report_e.html
Two notes - enclosure is supposed to be dielectric (plastic or composite), and some minimal sensor plate dimensions shall be taken into account.
Virtually no hardware costs at some AVR resources expense.

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damien_d wrote:
I still like the idea of electrodes as they're passive and therefore (at a guess) going to consume less power than a transducer, optical beam or concentric coil.

@Bingo,

Any idea if those citizen watch pressure sensors are available as a commercial-off-the-shelf component?

-- Damien

@damien

Nope .. I have no idea at all.

They might be expensive though , as the watch is a fully functional diving pressure sensor (and acts as backup for my diving computer).

/Bingo

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I think more info is needed...

How big is the device, how long is it submerged, how often does it surface, tethered, free floating, autonomous, or RC controlled, power supple?

Does it ever hover just below the surface? Is it sufficient to know it is near the surface vs at 50 m?

Is it just a "surface" detector, or is it desirable to know its depth below the surface?

And the ever present question: Building one or many?

In addition to many great ideas above one could also point an ultrasonic transducer upwards, facing the surface, and measure one's depth. Depth < 10 cm = Surface, etc.

JC

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The electrodes seens to be cheaper and simplest one, simmilar like lennard said or feed the electrodes to the AD... If you have problem with power consuption you can even use a transistor to cut the electrodes power, and only use it when you will sample it...
But I don't know if you need to have some concern about salt water..

Regards,

Bruno Muswieck

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Some experiments & projects have inherently more risks than others...

So who is up for some prototype testing? :)

JC

Attachment(s): 

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JC is your little toy? :shock:

Regards,

Bruno Muswieck

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I wish!

I saw that while on vacation in New Hampshire, USA, near a large lake.
I stopped to take a photo, and would have talked to the owner, but had the family with me. ("Dad, what are you doing?! Why are we stopping?!, etc., etc.").

I figured Damien needs a good platform to test all the suggestions given above!

JC

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They always don't get why we stoped on this kind of stuff... I know how it is...
My girlfriend, she already knows... :)

Regards,

Bruno Muswieck

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The OP's requirements are not clearly stated and any suggested solution is a mere guess so here is mine. Put a magnet in a float. Put said float in a tube that allows the entrance and exit of water. Place magnetic reed switch on tube in such a position so it is closed in either the above or below water position. Wire to circuit. Mount on vehicle.

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Someguy22 wrote:
The OP's requirements are not clearly stated and any suggested solution is a mere guess so here is mine. Put a magnet in a float. Put said float in a tube that allows the entrance and exit of water. Place magnetic reed switch on tube in such a position so it is closed in either the above or below water position. Wire to circuit. Mount on vehicle.

True.... I haven't been very clear because I haven't yet entirely sorted out the concept of operations.

However, as some have guessed, the idea is whether to tell if I'm in water or not. Once I hit the surface, I'll take some GPS measurements and do "stuff" with it :)

Thank you all for the above discussion. I like the idea of the electrodes and I might do something about it next time I'm on holidays :) .... they went so quickly!

-- Damien

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Quote:
the idea is whether to tell if I'm in water or not.
In QLD you are IN WATER ... in case you haven't looked out the window.. :wink: or are you holydaying in Sydney? :lol:

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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js wrote:
Quote:
the idea is whether to tell if I'm in water or not.
In QLD you are IN WATER ... in case you haven't looked out the window.. :wink: or are you holydaying in Sydney? :lol:

Fortunately Brisbane learned after the '74 floods and built Wivenhoe Dam, otherwise we would have copped another one.

Unfortunately, Bundaberg, Rockhampton, and and an area the size of France have not been so lucky. Some rivers were at 15m or more.

-- Damien

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damien_d wrote:

Unfortunately, Bundaberg, Rockhampton, and and an area the size of France have not been so lucky. Some rivers were at 15m or more.

-- Damien

I blame those fools who prayed for rain :lol:

Ross McKenzie ValuSoft Melbourne Australia

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I have to confess it's my fault. I got a water tank and it hasn't stopped raining ever since. :?

It seems there is not much one can do to prevent floods in the affected areas: The land is soooo flat, great for sugar cane.

Back on topic: if your device is fully immersed in water the float and magnet seems the best idea. Any other device like probes will need to contend with residual water/humidity. If you go for probes you may want to consider AC for the sensing voltage to prevent oxidation. In the good old days when I used to make water sensing modules for industrial coffee machines we used 6V ac from a transformer. There was also a NSD chip that was designed for the purpose (LM1830) and would generate the AC signal as well as detecting the water, I think it has been discontinued for a while but I may have a few around somewhere.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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js wrote:
I used to make water sensing modules for industrial coffee machines ...
I guess those were the megalitres per hour versions :lol:

Ross McKenzie ValuSoft Melbourne Australia

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Lygon Street caffes.

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Pretty much. :-) you can make up to 8 cups of expresso (or other lower grade coffee :lol: ) simultaneously, then load and repeat.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Gentlemen,

"espresso" for the beverage, please. Regarding mega-liters an hour: each espresso shall, sorry, must be brewed within ca. 25 sec. and result in ca. 30ml of drink. Once js points "other lower grade coffee", it stands probably for varieties. These, nevertheless, are espresso based, too. Extra water, milk, milk foam or whatever makes these.
BTW You do have a very good local coffee supplies down under, that's for sure - mostly arabica.
Coming back to serious water sensing thread - one of main EU espresso machines producer uses probes for water level control in a boiler. Second biggest - float with small magnet inside and reed switch outside the vessel.

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I'm thinking a float is going to be a hollow tube with a ball. Any problems with it caking with mud and other stuff (especially inside where the water has to get in, which won't be all that accessible for cleaning)?

Will a tube like that (say, 20mm in outside diameter) be able to withstand 50m of pressure? Well, I suppose they'll be equalised inside and out, but what about the ball near the top of the float.

The other reason I'm not thinking the ball/magent is because I don't have a set "up" direction, which makes it challenging in other ways (especially for GPS)?

What sort of current is the ball/magnet likely to draw vs the probe arrangement?

The LM1830 looks cool... I think a couple of hours of googling are in order...

-- Damien

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Quote:
The LM1830 looks cool...
...but discontinued from memory. I haven't worked with them for many years but still have couple on hand. So if it is for a one off then I can send you one otherwise look at other alternatives.

There is no current to speak of with the ball/magnet arrangement. The sensor could be a hall effect sensor so the whole thing can be protected from water unlike probes. Even if you use stainless steel (which you should) the salt water can still attack them.

If the tube cannot be guaranteed to emerge the right way up then you have a problem.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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A half a dozen would piss it in to put it colloquially

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damien_d wrote:
I'm thinking a float is going to be a hollow tube with a ball. Any problems with it caking with mud and other stuff (especially inside where the water has to get in, which won't be all that accessible for cleaning)?
Damien, my image of what was suggested was a tube open at both ends except for a single plastic "nail" or grid that prevented the pingpong ball with magnet attached from floating out the top of the tube. At the surface, the ball would be at the bottom of the tube and activate your reed switch. The at depth crushing forces would only act on the pingpong ball; the tube is unaffected.

damien_d wrote:
The other reason I'm not thinking the ball/magent is because I don't have a set "up" direction, which makes it challenging in other ways (especially for GPS)?
True "up" is simply a matter of managing your ballast and your flotation elements correctly.

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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Ross noted:

Quote:
True "up" is simply a matter of managing your ballast and your flotation elements correctly.

In my little buoy using a thermos as the case the batteries sat in the bottom of the case and provided the needed ballast. Easy in this case.

JC

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Quote:

Thank you all for the above discussion. I like the idea of the electrodes and I might do something about it next time I'm on holidays Smile .... they went so quickly!

IME electrodes is indeed >>the<< way to do it. Assuming when surfaced, you can get them mostly out of the water.

Now, you may want stainless steel as mentioned.

Using DC as was suggested above for a simple switching will work--but you'll probably find your electrodes get plated and/or corroded. So you excite with an alternating signal. Square waves work fine. Usually for "real" conductance work a few hundred Hz is used.

Then you filter the return signal to get a DC level for the ADC. These circuits would be quite similar to a current probe. (Not ocean current, mind you.)

For simple go/no-go the circuit does not need to be elaborate.

Check www.discovercircuits.com , looking for moisture testers and current probes for ideas.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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millwood wrote:
Quote:
IME electrodes is indeed >>the<< way to do it.

try to work with salt water for a while.

or simpler yet, google water sensors and see how others do it.

Agreed... interestingly, having a look around for the LM1830 led to all sorts of replacement circuits and ideas, most based around a square wave as suggested by theusch.

-- Damien

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Quote:
any solution that relies on the measurement of "resistance" via the fluid is a terrible design, especially those that use DC - you only need to have done it once to know why.

Tell us your bad experiences?

Regards,

Bruno Muswieck

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millwood wrote:
any solution that relies on the measurement of "resistance" via the fluid is a terrible design, especially those that use DC - you only need to have done it once to know why.

the lm1830 belongs to that categories as well. or a 555-based solution for that matter.

any one proposing a hall-effect sensor based solution has never done it before.

I'm asking the question precisely because I haven't done it before :)

Which method would you recommend and why?

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