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'[EE]: Help water level'
2000\10\16@065222 by Andy Shaw

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Hi Folks,
I'm trying to put together some sort of sensor to measure the amount of
water in a model sub ballast tank. The requirements are:
1. Must be small (not a lot of space).
2. Easy to seal against water.
3. Continuous range from say 1 cm to say 30 cm deep, with 1mm accuracy.
4. Possibly best not to use anything that depends upon pressure (since this
may vary depending upon sub depth).
5. Easy to hook up to a PIC (either digital or analogue OK  - I have both).
6. Cheap!
Not asking much!

I seem to remember seeing a rain gauge project that used two (insulated)
wires separated by a fixed gap as part pf some sort of oscillator. But guess
what I've lost the mag. Anyone got the details of know if this would work
for me?

Thanks

Andy

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2000\10\16@070300 by Alan B. Pearce

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>3. Continuous range from say 1 cm to say 30 cm deep, with 1mm accuracy.

Does it really need to be perfectly linear instead of stepped?

One way to do this is have a long piece of Perspex which thins down in steps. shine an opto reflector down it, and then measure the light coming back. As the steps in the Perspex go under water they stop reflecting the light. For your purpose the rod could be at an angle in the ballast tank.

It may be possible to make a truly linear one using a tapered rod, especially if it is grooved in minute steps all the way down, a bit like a fresnel lens.


at the top I would be tempted to have a tiny groove between the led and the sensor to try and remove crosstalk between the two caused by light going straight across instead of down the rod.

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2000\10\16@070306 by John Perkinton

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I saw an ultrasonic range finder project somewhere, I don't know how
accurate it is though.

-----Original Message-----
From: Andy Shaw [spam_OUTandysTakeThisOuTspamSCO.COM]
Sent: 16 October 2000 11:45
To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
Subject: [EE]: Help water level


Hi Folks,
I'm trying to put together some sort of sensor to measure the amount of
water in a model sub ballast tank. The requirements are:
1. Must be small (not a lot of space).
2. Easy to seal against water.
3. Continuous range from say 1 cm to say 30 cm deep, with 1mm accuracy.
4. Possibly best not to use anything that depends upon pressure (since this
may vary depending upon sub depth).
5. Easy to hook up to a PIC (either digital or analogue OK  - I have both).
6. Cheap!
Not asking much!

I seem to remember seeing a rain gauge project that used two (insulated)
wires separated by a fixed gap as part pf some sort of oscillator. But guess
what I've lost the mag. Anyone got the details of know if this would work
for me?

Thanks

Andy

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2000\10\16@074242 by Andy Shaw

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Thanks Alan see comments...

----- Original Message -----
From: "Alan B. Pearce" <A.B.PearcespamKILLspamrl.ac.uk>
To: <.....PICLISTKILLspamspam.....mitvma.mit.edu>
Sent: Monday, October 16, 2000 12:01 PM
Subject: Re: [EE]: Help water level


>>3. Continuous range from say 1 cm to say 30 cm deep, with 1mm accuracy.

>Does it really need to be perfectly linear instead of stepped?

No steps would work so long as the transistion from one step to another is
good and repeatable. Seems to me like your solution would work fine in this
case. Any info on the mechanical construction of this (like width of rod,
how deep to make each step ec.)

Andy

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2000\10\16@085637 by Alan B. Pearce

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>No steps would work so long as the transistion from one step to another is
>good and repeatable. Seems to me like your solution would work fine in this
>case. Any info on the mechanical construction of this (like width of rod,
>how deep to make each step ec.)

I do not have any direct experience with the method, but heard of it being used for fluid level, I think it was in petrol tanks, where it was important to have no electric's inside the tank.

Time for some ASCII art

------\_________
               \---------\_________
                                   \
                           ________/
      _________/----------/
------/

If you get the idea. The LED and sensor (e.g. a reflective sensor unit) would be looking down the rod from the left hand end. each step causes some light to be reflected back, and the liquid covering each step fouls up the reflection coefficient of that step, meaning light does not get reflected back.

If you have access to a suitable lathe you may wish to try turning a piece of Perspex rod to try out the idea. The other trick which may work better for you, as I envisage the rod being on a slope inside the tank, is to use a rectangular rod, and steadily mill away steps on one side only. It may require some experimentation to determine if you get better results with the steps on the top or bottom side in this case.

happy experimenting

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2000\10\16@101257 by John Walshe

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I've looked at this method before for a single level reading, but a problem
arose in that the fluid(water) was dirty and contained some oil which coated
the reflective face and therefore can a false reading. My conclusion was
that this method would only work with a clean fluid.
John
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2000\10\16@104300 by M. Adam Davis

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Given that it is for a sub, the water shouldn't slosh around all that much.  Why
not use a float?  It is probably a little larger than you want, and perhaps
you'd rather have a non-mechanical sensor, but for ease of assembly and reading
and simple float setup with a pot or rotary encoder can't be beat.

When you do go to another expiremental method, it would be benificial to have
the float working as well so you can compare the readings and improve your
design, before getting rid of the float.

-Adam

Andy Shaw wrote:
{Quote hidden}

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2000\10\16@104649 by Arthur Brown

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Hi Andy,

Subject: [EE]: Help water level

How about a gas tank level Indicator.
you could use a magnet and a hall efect amplifier into analogue port.

We used 32 hall effect switches to indicate the position of a piece of test
kit while under development.
The final kit was a bit steal from a Mouse one of the wheels was used as a
up/down pulse counter, so position of tool was known.

Sorry if not too much of details are here as I don't know if I could release
details as I do contract work in the Electronic Industry, and don't know who
listens to the list.
I met a list listener in York last month he worked for a competing company I
was working for. He knew all the names of Contributors like Tony Nixon and
Olin.

Regards Art

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2000\10\16@111110 by Andrew Kunz

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Andy,

What's the purpose of measuring the water in the ballast tank?

Given the initial volume of air (which I hope is constant in your situation),
the volume of water will change as the air pressure changes (or vice versa,
depending on your field of view).  So a pressure-based system should be
adequate.  And the pressure will exactly correspond to sub depth.

Why not use one of Skip's ADCs?

Andy

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2000\10\16@132231 by Andy Shaw

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Thanks Adam,
any advice on which float system to use and where I can get hold of one in
the UK?

Andy

{Original Message removed}

2000\10\16@132238 by Andy Shaw

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Hi Andy,
I thought you might reply! What I'm trying to do here is to go closed loop
on my ballast tank control (I use a pumped air system). One of the things I
would like to be able to do is to take on a fixed amount of water (during a
dive for instance), and also to be able to perform small trim operations
during the dive. The problem as I see it with a pressure system is that
a) I already have one of those (used for depth control) but the response
time is connected to how long it takes for the sub to settle at a depth (the
sensor is not inside of the ballast tank).
b) I'm not sure that it is accurate enough to repeatedly take on a fixed
amount of water in the tank (I seem to be able to measure down to about
5/10mm of water with it).
c) For closed loop I would really like something that gave a direct measure
of the actual amount of water in the tank. I'm worried that the additional
pressure from the depth the sub is at etc. is going to complicate things
rather a lot.

I had thought about using the depth sensor for this (and maybe moving the
sensor into the tank to do it). But I'm concerned with the effects on the
closed loop of the lag that you get between taking on water and the sub
actually moving to a depth. Maybe if I move the sensor I won't have this
problem (since it would be inside of the tank?). But I guess I would still
have the problem of sensitivity/accuracy.

Just a quick note here. The way I like to run my subs is not to spend ages
ballasting them so that taking on a full tank of water takes them to a
neutral buoyancy (which is what most people do). Instead what I would like
to do is to automate this part so that I can just do a simple pond side
set-up of taking on water until the sub is neutral then store the value
associated with this state in eeprom. Then I just command the sub to take on
the same amount of water when I dive. I currently do this open loop (timing
based) but this tends to be a bit hit and miss as the batteries go down...


Andy

{Original Message removed}

2000\10\16@134118 by Andrew Kunz

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>a) I already have one of those (used for depth control) but the response
>time is connected to how long it takes for the sub to settle at a depth (the
>sensor is not inside of the ballast tank).

If you are using pumped air, the air pressure in the tank will accurately relate
to final depth.  Assuming your tank floods rather freely.  If slow flooding (due
to constricted porting) it will not, as the air will be at a lower pressure as
the tank floods.

>b) I'm not sure that it is accurate enough to repeatedly take on a fixed
>amount of water in the tank (I seem to be able to measure down to about
>5/10mm of water with it).

A colored floating ball in a vertical tube parallel to the main tank will give
you depth indication.  Getting that visual indicator into the micro will be a
challenge...

>I had thought about using the depth sensor for this (and maybe moving the
>sensor into the tank to do it). But I'm concerned with the effects on the
>closed loop of the lag that you get between taking on water and the sub
>actually moving to a depth. Maybe if I move the sensor I won't have this
>problem (since it would be inside of the tank?). But I guess I would still
>have the problem of sensitivity/accuracy.

On Skip's control, I had better than 1/4" resolution of depth.  It was "dumbed
down" for the mass market.

>Just a quick note here. The way I like to run my subs is not to spend ages
>ballasting them so that taking on a full tank of water takes them to a
>neutral buoyancy (which is what most people do). Instead what I would like
>to do is to automate this part so that I can just do a simple pond side
>set-up of taking on water until the sub is neutral then store the value
>associated with this state in eeprom. Then I just command the sub to take on
>the same amount of water when I dive. I currently do this open loop (timing
>based) but this tends to be a bit hit and miss as the batteries go down...

I've only been out with Skip once, but there was no neutral setting.  He set the
boat up so that with a dry tank it had a scale waterline (lead ballast) then we
just drove it.  The APC and ADC worked to get the depth and pitch right, and it
just held itself there.  I think on this boat, a flooded tank would have allowed
it to continue diving a long time, and it was flying itself up instead.  We
stuck it to the bottom a few times (in weeds) and blew the tank to recover it.

Pretty cool, but not enough fun to get me to forsake race boats and airplanes!

Andy

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2000\10\16@141027 by Dale Botkin

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On Mon, 16 Oct 2000, Andy Shaw wrote:

> Just a quick note here. The way I like to run my subs is not to spend ages
> ballasting them so that taking on a full tank of water takes them to a
> neutral buoyancy (which is what most people do). Instead what I would like
> to do is to automate this part so that I can just do a simple pond side
> set-up of taking on water until the sub is neutral then store the value
> associated with this state in eeprom. Then I just command the sub to take on
> the same amount of water when I dive. I currently do this open loop (timing
> based) but this tends to be a bit hit and miss as the batteries go down...

Just a thought:  There are available pretty accurate flow meters, as used
in automotive applications to sense fuel flow.  Couldn't you use one of
these (or even a home-brewed one using a paddle wheel and slotted disk or
whatever) to measure how much water you pump into and out of the ballast
tank?

Dale
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2000\10\16@145120 by M. Adam Davis

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I've found that the 'pretty accurate flow rate meters' require large flows and
are expensive.  But it was some time ago when I looked for them, and I only
looked at those suited for high flow rates.  Perhaps a bellows type flow meter
would work here...

If you don't mind going slowly you could always use a parastaltic (sp?) pump to
pump a known quantity in and out of the ballast tank.  Put a pressure sensor on
the outside and inside, and use the pressurized air to make the job easy for the
pump.

-Adam

Dale Botkin wrote:
{Quote hidden}

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2000\10\16@145332 by Bob Ammerman

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> Just a thought:  There are available pretty accurate flow meters, as used
> in automotive applications to sense fuel flow.  Couldn't you use one of
> these (or even a home-brewed one using a paddle wheel and slotted disk or
> whatever) to measure how much water you pump into and out of the ballast
> tank?

Or just count revolutions of a positive-displacement pump?

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2000\10\16@151002 by M. Adam Davis

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I was considering a home-built unit.  It shouldn't be too difficult to build
such a unit youself, although I've seen industrial type units (but are very
expensive!).

You could take a straight bar of stainless steel, at the required length, which
is 1mm thick, and 7.5mm wide.  Twist it, say one full twist every 72mm.  Put a
float on it, with a slit in the center of the float that accepts the twisted
bar, and put guides on the float that keep it from twisting.  As the float
travels up and down the bar, the bar is twisted 1 degree for every .2 mm the
float moves.  Put a 72 position rotary encoder at the top and you get 1 pulse
per mm.  You can work it from the other end by finding the encoder you want to
use, and then twisting the bar to give you your 1mm resolution, but the steaper
the twist, the more likely the float will bind, and your resolution will go out
the porthole.  Use a 360 degree pot instead, and you can get as good a
resolution as your A/D can handle (excepting mechanical limits), and you won't
have to constantly track a rotary encoder, either.

If you want to trade resolution for ease of assembly, just mount a float on an
arm connected to a pot.  It is a non-linear relationship, As the arm nears
vertical you'll see the pot change little for a large change in water level.  If
you think it through, though, you can probably find a way to mount it such that
it measures full scale from, say 45 degrees below horizontal to 45 degrees above
horizontal.

-Adam

Andy Shaw wrote:
>
> Thanks Adam,
> any advice on which float system to use and where I can get hold of one in
> the UK?
>
> Andy
>
> {Original Message removed}

2000\10\16@155704 by reroy

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My vote is to use differential pressure sensors, one internal
one external, process the difference. Hey you might even
find out what depth your sub is listing!

{Original Message removed}

2000\10\16@172521 by Tony Nixon

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Andy Shaw wrote:
>
> Hi Folks,
> I'm trying to put together some sort of sensor to measure the amount of
> water in a model sub ballast tank. The requirements are:

Any chance you could measure the weight of the tank instead? More water
- more weight etc.


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2000\10\16@220544 by Gennette, Bruce

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Maybe there are solutions to this other than measuring the liquid *AFTER* it
enters.

Could you use a free wheeling positive displacement pump connected to a
rotary encoder on the (lower) input as a measuring tool - it should tell you
*EXACTLY* how much liquid passes *IN* or *OUT*.

Just a *NEW* thought.

Bye.


> {Original Message removed}

2000\10\16@223709 by staff
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Alan B. Pearce wrote:
{Quote hidden}

I think that is a really clever idea!

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2000\10\17@045550 by Simon Nield

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>A colored floating ball in a vertical tube parallel to the main tank will give
>you depth indication.  Getting that visual indicator into the micro will be a
>challenge...

if the little coloured ball is opaque then it would be pretty easy to put an IR led and a photo
transistor either side of the tube. if daylight is a problem you could put the tube inside the sub,
or use daylight filtered opto parts, or pulse the IR led and do a comparison with the ambient level.
or whatever.

Regards,
Simon

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2000\10\17@053345 by staff

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Hi, if you are going to use a float I would use the sharp opto
distance sensor. They are only a few bucks, have a serial interface,
and give a pretty fine resolution up to about a meter max. You may need
to put a white flat surface disk on top of your float ball or choose
a float with a flat top.
There is some data on this Sharp sensor at:
http://www.tinaja.com/flut01.html
(see tech musings #131)
and there is a lot of data on this sensor on the net. I have never
worked with one but I plan to! :o)
-roman



Simon Nield wrote:
{Quote hidden}

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2000\10\17@121540 by Andy Shaw

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Thanks everyone for the input. I'm not sure which way to go at the moment I
think I may try measuring the pressure in the Air tank since I have most of
the parts for this handy. I guess I would also like to try some of the other
options (I really like the Perspex rod stuff). Also did anyone see the
article I saw with the capacitance method in it. I think it may have been on
Circuit Cellar (or possible Elektor), anyone any experience of using this?

Andy

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2000\10\17@122502 by Andrew Kunz

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Capacitance will change as a function of water purity.

Andy









Andy Shaw <TakeThisOuTandysEraseMEspamspam_OUTSCO.COM> on 10/17/2000 12:05:27 PM

Please respond to pic microcontroller discussion list <RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU>








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cc:      (bcc: Andrew Kunz/TDI_NOTES)



Subject: Re: [EE]: Help water level








Thanks everyone for the input. I'm not sure which way to go at the moment I
think I may try measuring the pressure in the Air tank since I have most of
the parts for this handy. I guess I would also like to try some of the other
options (I really like the Perspex rod stuff). Also did anyone see the
article I saw with the capacitance method in it. I think it may have been on
Circuit Cellar (or possible Elektor), anyone any experience of using this?

Andy

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2000\10\17@122705 by Alan B. Pearce

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>Also did anyone see the article I saw with the capacitance
>method in it.

I do not have any experience with it, but I do know that the spacecraft that is currently on its way to Saturn has a unit that is going to land on Saturn's moon Titan ahs one of these on it. I did not see it before it went, but I understand it is made out of a number of parallel plates which were dipped in a coating solution which thinned itself to a layer a handful of molecules thick to form a dielectric over the plates. This is necessary as no one knows what is on the surface of Titan, and if it is liquid they want to know the dielectric constant of it, but the plates need to be insulated in case the liquid is electrically conductive.

I imagine for your use you could use the capacitor as the capacitor in a 4046 PLL, with possibly another 4046 as the master oscillator. You could trim the master oscillator using a D/A or PWM from a PIC, by having a varicap as its capacitor, and then use a A/D to measure the control voltage of the other one to find how deep you are.

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2000\10\17@145903 by Alice Campbell

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> Capacitance will change as a function of water purity.
>
> Andy
>
...but this is not necessarily a bad thing or hard to deal
with:

http://209.146.243.45/product.htm

for capacitative liquid sensing

alice

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2000\10\17@173426 by David Minkler

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Hi,


> I seem to remember seeing a rain gauge project that used two (insulated)
> wires separated by a fixed gap as part pf some sort of oscillator. But guess
> what I've lost the mag. Anyone got the details of know if this would work
> for me?

Some jet aircraft use a system similar to what you describe as a fuel
level sensor.  My recollections of discussions with the fuel systems
guys was that the probes were non-linear and had to be characterized but
this should not be a problem with a system that you can get your hands
on, especially since you have a (PIC) to do the conversion /
linearization.  Contaminants in the water will change the dielectric
constant but a reference sensor (as suggested in the link provided by
Alice Campbell) in the same fluid should eliminate that problem.

Regards,
Dave

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2000\10\17@185258 by Peter L. Peres

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>3. Continuous range from say 1 cm to say 30 cm deep, with 1mm accuracy.
>4. Possibly best not to use anything that depends upon pressure (since
>this may vary depending upon sub depth).

So you want to try a wet resistance type of sensor with unknown (changing)
water of unknown temperature and make it accurate to 0.3% ? I hope that
your analog design and electrochemistry knowledge is in the range
between excellent and amazing, because you will need it.

Most subs do not stay on even keel most of the time so you actually need
two sensors and average the read heights. Given the size it sounds more
like a 1/2 caliber torpedo than a model sub ;-)

Now, a pair of capsulated potentiometers or torsion transducers and a pair
of small floats will probably get you near there (if 2% is near enough).
You can easily average the two readings in analog mode before converting
to digital. Don't forget to account for when the fish is standing on end
;-). And do not rely on gravity to keep the floats on the surface of the
liquid, use springs.

good luck,

Peter

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2000\10\17@185304 by Peter L. Peres

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>Given that it is for a sub, the water shouldn't slosh around all that
>much.

Uh oh. And how. Better use baffle walls with holes to subdivide the tank
inside, and break the sloshing. If the water can slosh at a frequency
near the oscillation frequency along some axis your craft will be a real
dolphin ;-) And this applies for any water height in the tank btw.

Peter

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2000\10\17@185309 by Peter L. Peres

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Andy, you assume that the tank is open to the water and that nothing
opposes its movement in and out. This is not the way it works on tiny
subs afaik (but maybe you know better ;-).

The only ACCURATE way to know the volume of water in the tank is to use a
volumetric pump to fill and empty it and count turns. Assuming there are
no leaks (hahaha) that will be accurate. But not to 0.3%.

If this is some sort of slow research sub then this will be adequate from
all points of view, excepting for the pressurized volumetric pump which is
a chapter for itself. If you want to go fast, use the navy way, blow air
into the tanks to raise, and vent air out the top to go down. And use
a valve to close off the water when not changing pressure. Andy, you
are into this afaik, what kind of ballast control do you use on model subs
?

Peter

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2000\10\18@010716 by Andy Shaw

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Ahhh, enough said on that one then....

Andy

----- Original Message -----
From: "Andrew Kunz" <KILLspamakunzspamBeGonespamtdipower.com>
To: <EraseMEPICLISTspamEraseMEmitvma.mit.edu>
Sent: Tuesday, October 17, 2000 5:24 PM
Subject: Re: [EE]: Help water level


{Quote hidden}

<spamBeGonePICLISTspamKILLspamMITVMA.MIT.EDU>
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of
> the parts for this handy. I guess I would also like to try some of the
other
> options (I really like the Perspex rod stuff). Also did anyone see the
> article I saw with the capacitance method in it. I think it may have been
on
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2000\10\18@010747 by Andy Shaw

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Thanks Peter,
I'm rapidly going off the cap. idea! Actually 20cm is probably more like the
actual size. I use a tank open to the water at the bottom with an air intake
at the top of the tank that goes via a small compressor into a sealed tank.
To dive the compressor pulls air out of the tank into the sealed tank. To
surface the compressed air is released back into the tank forcing out the
water (so long as you are not too deep!). So I'm currently looking at using
the pressure in the sealed tank as an indication of the amount of water
taken on... I could go with two float type devices but I don't think I will
have the space (that was why the cap. approach looked nice not much space
used).

Thanks

Andy

{Original Message removed}

2000\10\18@010800 by Andy Shaw

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Hi Again,
Yep Been there got the baffles... !

Andy

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Sent: Wednesday, October 18, 2000 9:47 PM
Subject: Re: [EE]: Help water level


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2000\10\18@020135 by Gennette, Bruce

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Baffles just got me thinking of parallel restrictions . . .

How about a window low in one side of the tank with a UV LED shining in at a
small forward angle so that it doesn't shine across the tank to a detector;
*UNLESS* there is higher refractive index water in the way which bends the
light beam more than the low refractive index air does (higher bends towards
the normal).  With the right filters over the window no visible light will
get in to confuse your detector.

You could use a small cluster of LEDs at the light inlet port, or a special
lens to shape the beam from a single LED into a narrow, vertical column that
paints a series of detectors.  Only the detectors that are under water pick
up the refracted beam.

Bye.

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2000\10\18@071804 by Andrew Kunz

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Peter,

Which Andy.  Andy Shaw gave the original posting for one of his model subs.  He
and I know each other through the list because I developed the (PIC-based)
electronics another guy sells (@spam@Skipspam_OUTspam.....rcboats.com), and I think Andy Shaw is one
of his regular customers.  Alas, I haven't gotten a sub (yet).

He is looking for a method of predicting depth based upon water volume, ie, pump
so much water in, arrive at (near) given depth.

There are several ballasting systems used in model subs.  Andy Shaw's uses a
sealed air tank which is compressed or expanded (worm drive usually) to vary
depth.   Other methods include a compressed-air system which blows the tanks, a
la US Navy.  Toy subs (as compared against "real" models <G>) use dynamic
diving, in which the sub uses wings to fly to a given depth.  Often "real"
models use more than one method (the only way to stabilize a moving sub, really,
is to use the dive planes).

Anyway, there is often very little resistance to water entering the ballast tank
- it usually has LOTS of holes in the bottom to allow a fast dive.

Andy









"Peter L. Peres" <spamBeGoneplpEraseMEspamACTCOM.CO.IL> on 10/18/2000 04:55:57 PM

Please respond to pic microcontroller discussion list <PICLISTspamBeGonespamMITVMA.MIT.EDU>








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cc:      (bcc: Andrew Kunz/TDI_NOTES)



Subject: Re: [EE]: Help water level








Andy, you assume that the tank is open to the water and that nothing
opposes its movement in and out. This is not the way it works on tiny
subs afaik (but maybe you know better ;-).

The only ACCURATE way to know the volume of water in the tank is to use a
volumetric pump to fill and empty it and count turns. Assuming there are
no leaks (hahaha) that will be accurate. But not to 0.3%.

If this is some sort of slow research sub then this will be adequate from
all points of view, excepting for the pressurized volumetric pump which is
a chapter for itself. If you want to go fast, use the navy way, blow air
into the tanks to raise, and vent air out the top to go down. And use
a valve to close off the water when not changing pressure. Andy, you
are into this afaik, what kind of ballast control do you use on model subs
?

Peter

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2000\10\18@082113 by Olin Lathrop

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> So I'm currently looking at using
> the pressure in the sealed tank as an indication of the amount of water
> taken on.

That means you will really be measuring the volume of air in the tank times
the temperature.


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2000\10\18@082526 by Andrew Kunz

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Andy (Shaw):

And temperature _will_ be a significant variable to consider.  Temp will change
as a result of the de/compression, and due to water temperature and ambient.

Andy Kunz









Olin Lathrop <RemoveMEolin_piclistspamspamBeGoneCOGNIVIS.COM> on 10/18/2000 08:10:50 AM

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Subject: Re: [EE]: Help water level








> So I'm currently looking at using
> the pressure in the sealed tank as an indication of the amount of water
> taken on.

That means you will really be measuring the volume of air in the tank times
the temperature.


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2000\10\18@083607 by Olin Lathrop

flavicon
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> > So I'm currently looking at using
> > the pressure in the sealed tank as an indication of the amount of water
> > taken on.
>
> That means you will really be measuring the volume of air in the tank
times
> the temperature.

Oops, I meant to say you will be measuring the MASS of air in the tank times
the temperature.  The volume is presumably held constant by the tank.

The overall point still is that you will need to temperature compensate the
pressure reading to get the accuracies you stated earlier.  Water
temperature can change quite a bit in just a meter or two of depth, and
sometimes just a few meters lateral distance.  I don't have any submarine
experience, but this comes from swimming, snorkeling, and the like.


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2000\10\18@090441 by Tom Handley

picon face
  Andy, this topic has come up before. I think it was Tjaart (Gawd I
wish he would come back to the this list...) where he was measuring
the level of a local river.

  The recent issue of Sensor Magazine has an article on the principles
of level measurement. Another good resource is the Omega book set which
is normally free. In particular, the "Flow and Level Handbook". I'm an
old Air Force guy so submarines, and ejecting from a perfectly good
aircraft, is a `foreign concept' to me ;-)

     http://www.sensormag.com
     http://www.omega.com

  - Tom

------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

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2000\10\18@090857 by Tom Handley

picon face
  Andy, sorry. The correct link to Sensors magazine is:

     http://www.sensorsmag.com/

  - Tom


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2000\10\18@092312 by staff

flavicon
face
I still like the idea of having no moving parts and the perspex
device inserted into the liquid, using the reflected infrared
and an analogue sensor. This should give 9 or 10 bits analogue
and that would be fine enough resolution for what you wanted.
I think this idea might even work with a tapered bar rather than
a stepped one?

Maybe if you had to use moving parts I would have a single float
on an "arm" so you only have one pivot bearing, this would give
the most reliable movement and the least stiction. The pivot end
of the arm could have a analogue opto-interuptor going to your A/D
or maybe a reflective opto with reflection amount determined by
the position or angle of the arm. I think measuring pressure is
a recipe for disaster, ie if the sub goes under waves the pressure
will change, or unstable handling may cause the sub to go up/down
as it moves, causing pressure changes. Maybe your design might
later call for valves to close the tanks from the outside water?
I could see this being of use in a research sub.

Seems to me if you want to measure the water level, then measure
that in a way that is secure, if you want to measure pressure to
determine sub depth than by all means use a pressure sensor.
I just think the best way to measure liquid level is to measure
the liquid level! :o)
-Roman


Peter L. Peres wrote:
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2000\10\18@155558 by Stein Sem-Jacobsen

picon face
Again reffering to the F5. The capacitive probes has got a refference probe
all sorruonded by the fluid it is submerged into. This is used to correct
the measured tank level.

Regards
Sten Sem-Jacobsen

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2000\10\18@192214 by Gennette, Bruce

flavicon
face
Another idea - put an accordian shaped balloon (shaped like the boot on a
shock absorber) in the top of a (vertical) cylindrical tank to displace the
water.  Pump your air from the storage tank (compressed) back and forth to
the balloon in the ballast tube (expanded).  A string from the end of the
balloon and some pulleys then drives a rotary encoder (needs a spring loaded
take up reel at the other end).  [Could be mounted inside the balloon if
water is an electrical problem].

A completely closed (air) system should be more reliable than one where your
compressed air gets to contact the water.  Or, for more realism, use a
largish cylinder of compressed air (enough for several re-surfacings) and
just dump the air from the ballast balloon out some one-way valves when
diving - should look spectacular as the sub drops below the surface with
jets of air/water shooting out of the dive valves.

Bye.

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2000\10\19@092152 by Andy Shaw

flavicon
face
Ahhh you guys you spoil all of the fun! I could have spent ages putting all
of this together and making it work before I thought of that! Actually since
what I need to do is simply go from one fixed state (tank with no water in
it) to another (tank with set amount of water in it) do you think I could
get away with it... Probably not! Maybe I'll just go back to dead reckoning
based upon time and then do some sort of automatic trim....

Andy

{Original Message removed}

2000\10\19@105509 by stouchton

flavicon
face
How about this:  Looks like what is of concern is not the sub's depth from
the bottom, but its depth from the surface.  What about using an upward
firing depth sensor... look for the water air discontinuity.

When in college we used depth finder transducers to communicate to a
submerged sub to give it commands.  When we established a two way link, we
noticed a reflection that aligned with the surface of the pond, and timed
the reflection to judge the subs depth.

Now if we only had side-scan sonar we could find it again ........

{Original Message removed}

2000\10\19@111116 by Andy Shaw

flavicon
face
Actually the issue is not the depth of the sub (I already have depth sensors
using pressure on the sub), this issue is providing closed loop control over
the amount of water in the ballast tank. I'm rapidly coming to the
conclusion that this is heading towards one of those "If I wanted to get to
X I wouldn't start from here!" sort of conversations. I think that using a
different type of ballast tank (probably a piston style one as described by
Andy Kunz in an earlier mail and as often used by some of the sub builders
in Germany) would be a better way to do this. Shame I've already built the
thing (and to be honest it works really well - but I do like to try new
things!).

Anyway if anyone is interested the current configuration can be seen at
freespace.virgin.net/andy.shaw/subcomsep2000.htm
The bottom four pictures on the page show my sub. Click on the images to get
a bigger version. You can see the dive module with ballast system etc. If
you look closely you should be able to see the 16C71 which currently
controls the thing. More details are at:
http://freespace.virgin.net/andy.shaw/microcon.htm

Andy

----- Original Message -----
From: "Stouchton" <stouchtonspam_OUTspamcapitaresearch.com>
To: <RemoveMEPICLISTKILLspamspam@spam@mitvma.mit.edu>
Sent: Thursday, October 19, 2000 3:49 PM
Subject: Re: [EE]: Help water level


{Quote hidden}

> {Original Message removed}

2000\10\19@134542 by Olin Lathrop

flavicon
face
> Actually the issue is not the depth of the sub (I already have depth
sensors
> using pressure on the sub), this issue is providing closed loop control
over
> the amount of water in the ballast tank.

So why do you need to know the amount of water in the tank?  Isn't this
something the servo controls implicitly in an attempt to maintain the
desired depth?

> Anyway if anyone is interested the current configuration can be seen at
> http://freespace.virgin.net/andy.shaw/subcomsep2000.htm

Pretty cool, Andy!  So when will this be fully operational torpedoing those
annoying jet skis?


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2000\10\19@152031 by hard Prosser

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face
Since you are trying to fill the tank to the point where the sub starts to
sink, why not look at the external water level? A simple float inside the
sail or on the top deck could be used to indicate the onset of submersion.
Of course, you will need to filter the signal to reduce wave effects & may
need them fore & aft  but I imagine they could be made very small (opto
interrupters or reed switches?)


Richard P

wrote :-
> Actually the issue is not the depth of the sub (I already have depth
sensors
> using pressure on the sub), this issue is providing closed loop control
over
> the amount of water in the ballast tank.

So why do you need to know the amount of water in the tank?  Isn't this
something the servo controls implicitly in an attempt to maintain the
desired depth?

> Anyway if anyone is interested the current configuration can be seen at
> http://freespace.virgin.net/andy.shaw/subcomsep2000.htm

Pretty cool, Andy!  So when will this be fully operational torpedoing those
annoying jet skis?


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(978) 772-3129, KILLspamolinspam.....cognivis.com, http://www.cognivis.com

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2000\10\19@211030 by Gennette, Bruce

flavicon
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Yet another thought.
Calculate the amount of air/water required for various depths and build
several tanks.
Calculate the amount of air required to push the water out of each tank at
the maximum depth.
Flood different combinations of tanks for different depths.
Over blow the tanks with compressed air from a storage cylinder to rise

kiss

by.
> {Original Message removed}

2000\10\19@231546 by McMeikan, Andrew

flavicon
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-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

If you only want to check for a full or empty tank you could have a float
that blocks the entry or exit port then just sense the overcurrent (or
magnetic sensor for float) then just have a smaller trim tank that you set
by hand.

No worries then about measuring, in fact you could do the measuring by
several small tanks and fill the bladders digitally, you would then have a
T-T2 digital to flotation converter.

hmm, sounds good to me, each T-T2 tank uses a bladder and has electronic
valves to switch between filling with air or going to an air recovery
pump, other side of bladder to ambient water.

still need the trim tank (or ballast) to get neutral at a digital
midpoint.  Four tanks gives 7 rates of ascent or descent, could probably
get away with less.

Space looks tight, but would depend on the buoyancy you have in the rest
of the sub as to the practicality of subdividing you current tank

       cya,    Andrew...

- {Original Message removed}

2000\10\20@055308 by Peter L. Peres

picon face
Hi Andy (Kunz),

I knew there were two Andys but I did not know that both were involved
with model subs. It is clearer now.

Thank you for the useful info. I know of one more (?) method to control
the tanks, it involves using a piston that is actually a tank wall and a
screw (is that a worm?) to move it by servo. There is a small venting
electrovalve at the top and holes in the bottom to allow the dead space
over (under) the piston (in fully evacuated position) to fill when the
model is put on the water. This type of variable volume tank is precise
enough for what the original poster wants and scales well. The smallest
I've tried used a 20cc plastic syringe and a high power (windlass type) RC
servo (and no venting magnet) in a 'bathtub' sub. It works surprisingly
well. Mention my name on the patent or I'll sue you ! ;-) I know someone
who built a large model using the mechanism of a car jack (modified), PVC
drain pipe and a piston salvaged from a dead motorcycle engine I think. It
used window washer servos and 12V electrics. Other cheap tank candidates
are bicycle and car tyre pumps. Use lots of grease for sealing. The air
pressure inside the hull will change with operation so differential
pressure sensors will be upset. You need to take care of this and of
possible fog when pressure changes inside ;-) (heaters on parts that
should not fog).

Dive tanks that have air over the open water in them always risk to sink
the ship because if the ship lists too much the air will escape and it
will start going down. The problem is particularly bad with fast subs
which can do vertical maneuvers under water using the dive planes. Also
you can never completely fill or empty them, depending on the attitude of
the vessel.

hope this helps,

Peter

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2000\10\20@060930 by Dirk R. Kuhlmann

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Hello,
is there a briefly description (link) how to use a PICSTART PLUS Development
Programmer
for incircuit programming ?

regards
Dirk

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2000\10\20@064144 by Andy Shaw

flavicon
face
Hi Olin,
Here are a few comments on how I (and others often use ballast tanks....), I
sent it to some-one that asked off list...

Basically the way I use the ballast tanks in my sub is to flood the tank
until the sub is essentially neutral in terms of buoyancy. Once this has
been done actual depth control is performed using the control surfaces on
the sub (rear and forward planes). I sense the actual depth by using a
pressure sensor. So in terms of the ballast tanks what I need to do is to
ensure that the correct amount of water is taken on to give me the neutral
state. The easiest(!) way to do this would be to add water (slowly)to the
tanks until the correct state is obtained, then measure how much water was
in the tank. Then whenever I want to dive I simply command the tank control
stuff to take on this exact amount of water. Now because of issues like type
of water (and water temp) the amount of water needed to be taken on will
vary from day to day. So what I want is a simple setup routine that I can
use whenever I sail. Simply put the boat in the water, run the tanks until
the tower is just under the surface, then mark the water level and store it
in eeprom. Then away we go. Only problem is the measuring how much water
bit! Sigh

As I think I mentioned in another post there are better tank designs (piston
tanks) that make the closed loop stuff much easier. What I was (I'm going
off the whole idea!) looking for was a way to used closed loop with my
current design!

Oh one other bit of information. Simply monitoring the state of the water
level outside of the sub does not work very well due to the relatively long
time it takes the hull to actually flood and settle (hence adding water very
slowly in the section above). But once you know how much to take on you want
to do this as quickly a possible....

Andy

{Original Message removed}

2000\10\21@005538 by Donald L Burdette

picon face
Andy -

Someone suggested closing the inlet to your ballast tank, and that got me
thinking.  If you have air in the ballast tank, and the tank is open to
the outside water, then whenever you dive, the air will be compressed and
you will take on more ballast water.

Is this a problem?  Do you need an active ballasting system that will
constantly adjust the ballast level?  Would it be easier to close some
intake valves and ensure that once you reach neutral bouyancy, you stay
there?

Don

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2000\10\21@011856 by Donald L Burdette

picon face
I can't imagnine why nobody has suggested this yet--come to think of it,
I can't imagine why it took me so long to think of it either :-)

A differential pressure transducer with one inlet at the bottom of the
ballast tank and the other at the top would be perfect.  You can get 1PSI
transducers, which is about 15 cm of water.  Maybe you need a 2PSI
transducer, but I doubt it.

I looked at your pix--looks nice!  Quite a lot of work, I bet.   But it's
a lot smaller than I imagined when you said 30 cm of ballast tank depth!
It looks like its only about 13 cm in diameter. How do you get 20 cm (was
30) of water in the tank?

Don

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2000\10\21@011922 by Donald L Burdette

picon face
I thought I'd correct a slight misunderstanding that some folks seem to
have.

The amount of  ballast does not determine the depth that the sub will
reach.  An object that is neutrally bouyant at the surface will also be
neutrally bouyant at 30 feet, 30 meters, or 1000 meters (assuming it
maintains constant mass and volume).  Varying the amount of ballast will
change ascent/descent RATE, not the depth.

Assuming constant mass and volume is a big assumption.  Unfortunately,
there's a positive feedback system working here.  As the depth increases,
the sub tends to be compressed.  This means it is more dense, and has
more of a tendency to sink.  As it rises, it expands, and has more of a
tendency to rise.  I deal with this effect all the time as a diver.
Various gas pockets within the body as well as the air bubbles in the
neoprene of my wetsuit compress as I descend.  Almost all divers wear a
Bouyancy Compensator Device which allows adjustment to neutral bouyancy
at any depth.  Of course, the BCD is really just a big bag of air, so it
makes the problem worse as you ascend/descend.

Andy wants to carefully trim for neutral bouyancy once, then remember the
ballast level so he can quickly and automatically reach that same
(neutral) bouyancy the next time he puts the sub in the water, or wants
to dive from a surfaced (highly bouyant) position.

Don

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2000\10\22@230157 by Randy Lilly

picon face
Sorry, I am new here and ended up in the middle of this submarine ballast
tank controller thread.  But I did just read about level sensors:

Check Don Lancaster's Tinaja.com

Download HACKAR1.PDF (note spelling)
Page 12 (Hacker2) ULN2429 Capacitive Liquid Level Sensor
Page 45 (Hacker10) Pressure Measurements
Page 101 (hacker21) Capacitive Humidity Sensor

It may be neat to look at electrolysis to generate hydrogen on demand and use
it to control the submarine depth.  Or drop some NiCad cells in the tank for
emergency surfacing.  Add a spark plug for extra thrust!

Randy Lilly -(Building a MPPT controller for solar panels using 16F877)

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2000\10\23@051454 by Alan B. Pearce

face picon face
>It may be neat to look at electrolysis to generate hydrogen on demand and use
>it to control the submarine depth.  Or drop some NiCad cells in the tank for
>emergency surfacing.  Add a spark plug for extra thrust!

A spark plug??? are you trying to emulate the Korsk?

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2000\10\23@094009 by Andy Shaw

flavicon
face
Hi Donald,
Yes the compression of the air in the tank can be a bit of a problem. But
does not really impact the problem I'm trying to solve just now. At the
depths I tend to use the sub at (no more than 6-8 feet max, any deeper and
you tend to have R/C problems). It is not really a big deal (since the
dynamic control from the planes etc. can easily deal with it). However it
can be an issue if you go too deep since like a real sub you may not have
enough pressure to blow the tank!

Andy

{Original Message removed}

2000\10\23@141137 by Randy Lilly

picon face
>>It may be neat to look at electrolysis to generate hydrogen on demand and
use
>>it to control the submarine depth.  Or drop some NiCad cells in the tank
for
>>emergency surfacing.  Add a spark plug for extra thrust!

>  A spark plug??? are you trying to emulate the Korsk?

Use palladium for the cathode in the electrolysis to make a minature P&F
nuclear reactor.  Use the helium ash to fill the ballast tanks. Flush the
neutrons into a copper tube for magnetohydrodynamic propulsion.  Presto, a
model atomic powered submarine.  No moving parts. Run silent, run deep.

But then a simpler approach would be to use Alkaseltzer tablets in
compartments that could be flooded.

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2000\10\23@170953 by Peter L. Peres

picon face
Hi Andy,

I have meditated over your problem and have come to an interesting
solution (patch) to your current system. Maybe. It relies on using a
volumetric method on the water that fills the tank.

Briefly, if your tank fills and empties through a piece of transparent
tubing and you have a small leak bubbling air into this tube all the time
(syringe needle + needle valve fed from your emergency recovery air or CO2
tank - the air consumption will be very low), then you can measure the
speed of the bubbles (regardless of their size and frequency) using two
photocell + LED assemblies that look 'through' the tube. You probably want
a reflective arrangement for bubbles (LED and detector at 90 or less
degrees). On top of it, this is a PIC project ;-). Of course I assume that
water is incompressible, that the tube diameter is known and that the
amount of air bubbled is small wrt. the water volume in the volume between
the photocells. You can probably run some tests using an aquarium air pump
and some transparent tubing for same.

You can further refine this by using a central electrovalve to shut off
bubble air to all tank bubblers when not required. This way you will not
run out of blow air by mistake, and there will be no slow draining by the
air bubbling into the tanks.

The system can be implemented in a few hours using 1/2" aquarium tubing
etc, and will take some work to make it practicable on the software side
(but the PIClist is here) ;-).

I hope that this helps,

Peter

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