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'[OT]: Measuring Depth of Submarine using Different'
2002\02\02@161409 by

Hello,

Here is the setup.  I have a submarine with a differential pressure sensor.
The high pressure end of the sensor will be placed in a 'ballon' that is in
the water and the low pressure end will be in the interior of the sub.  For
technical reasons I can't run a tub up to the surface of the water.

How do I can this differential pressure into a measurement of the subs
depth?  Since the submarnie was sealed at the water surface pressure (i.e. 1
atm) and is rigid I am assuming the pressure in the submarnie will remain 1
atm regardless of the depth of the submarnie.  Now, the pressure of the
'ballon' will change will change will depth as it is non-rigid, but what is
the conversion factor from differential pressure to depth?  I have a book
(which I don't have access to right now) that gives the relationship, but I
don't really trust the source and it is to 0 decimal places of accuracy!  I
will be using the sub in FRESH water.  (A search of the Internet has been
unfruitful - likely because I am not sure what I should be searching for.)

Regards,
Donovan

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Donovan Parks wrote...

>Here is the setup.  I have a submarine with a differential pressure sensor.
>The high pressure end of the sensor will be placed in a 'ballon' that is in
>the water and the low pressure end will be in the interior of the sub.  For
>technical reasons I can't run a tub up to the surface of the water.

>How do I can this differential pressure into a measurement of the subs
>depth?  Since the submarnie was sealed at the water surface pressure (i.e. 1
>atm) and is rigid I am assuming the pressure in the submarnie will remain 1
>atm regardless of the depth of the submarnie.  Now, the pressure of the
>'ballon' will change will change will depth as it is non-rigid, but what is
>the conversion factor from differential pressure to depth?  I have a book
>(which I don't have access to right now) that gives the relationship, but I
>don't really trust the source and it is to 0 decimal places of accuracy!  I
>will be using the sub in FRESH water.  (A search of the Internet has been
>unfruitful - likely because I am not sure what I should be searching for.)

Searching on "pressure conversion factors", google.com pointed me to a
bunch of online units converters.  According to:

http://www.processassociates.com/process/convert/cf_prs.htm ,one foot of
H2O is equivalent to 0.433527516 PSI.

Other than creating an inconvenience for you, the fact that you're using
a differential pressure sensor instead of an absolute sensor is of no
consequence; if the pressure inside your sub starts changing, you've got
more urgent problems than making accurate depth measurements- you're
sinking.

Hope this helps...

Dave

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All pressure gauges are differential by definition.
I understand that real, full scale subs are compressed a measurable amount
on deep descents. It is a small amount.

A pressure line outside the hull with the reference side of the gauge inside
the hull will indicate pressure.

The easiest popular source for pressure vs. depth would likely be a SCUBA
Diving text. Divers also wear a device like this to indicate depth.

John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"

{Original Message removed}
Donovan,

Is there a particular reason you're trying to measure your depth with a
pressure sensor instead of using, say, sonar?  Just curious.

Dale
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- Arnold Edinborough

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Hello Dale,

Expense - a pressure sensor is cheaper than a sonar unit.  Besides that, I
had a pressure sensor already.  Have you tried using a sonar sensor to
measure depth?  Do you receive a strong signal from the reflection off the
waters surface?

Regards,
Donovan

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On Sat, 2 Feb 2002, Donovan Parks wrote:

> Hello Dale,
>
> Expense - a pressure sensor is cheaper than a sonar unit.  Besides that, I

OK, I can see that would be a couple of excellent reasons.  Those are very
often the exact reasons I have for things I do!  8-)

> Have you tried using a sonar sensor to measure depth?  Do you receive
> a strong signal from the reflection off the waters surface?

No, I haven't.  I have done quite a bit of work over the past year with
sonar rangefinding, and the units I've been using are pretty sensitive --
I would be very surprised if they wouldn't see the surface.  Of course if
you went much below four or five fathoms the ones I have been using
probably wouldn't do much good, they seem to be reliable only out to about
30-something feet.

Dale

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This is similar to a scuba computer that I did recently. The vessel will
deform and change volumetrically at fairly shallow depths - this is
something one learns
during the earliest scuba cert training. I wouldn't want to even try to
calibrate a system
with a differential sensor inside the vessel.
What max depth will your sub run at? for shallow depths (<100 ft.)I think
the most convenient,
reliable, and simplest solution for this may be a
Motorola gauge type pressure sensor. According to the data sheets they are
only for use in air,
but since they have a fluorosilicone gel covering the chip, I have found
that they have a
pretty long life in fresh water, even in chlorinated pool water. These are
very small devices,
approx 20mm round by 6 thick. 5V analog output models are very easily
scaled. The device can be
inside the sub with it's barbed fitting through the outer hull with a small
o-ring.
They are fairly linear. The hardest part of working with them is
themperature compensation.
f your water temp is pretty regular, say within =/- 10C, it will be no
problem getting accurate readings
to within about 1/2 ft, down to about 100 ft.
You might want to check out the MPS5700 family.

Chris

Alternately

{Original Message removed}
Hello Chris,

Thanks for the info.  I'm only looking at a depth of only 10 feet.  Right
now I am using the Motorola MPX5050 (a differential pressure sensor) and am
*hoping* that the sub will not change volumetrically at shallow depths so I
can get an accurate differential pressure.  Was your scuba computer is a
rigid vessel?  Why was it deforming in shallow water?

Regards,
Donovan

{Quote hidden}

are
> very small devices,
> approx 20mm round by 6 thick. 5V analog output models are very easily
> scaled. The device can be
> inside the sub with it's barbed fitting through the outer hull with a
small
{Quote hidden}

> {Original Message removed}
Donovan,
Since you ask, my dive computer wound up with a custom molded enclosure made
from UHMWPE,
or 'Hypact' This was used as a balance between durability/impact resistance
& rigidity.
More to the point, unless your sub is of extraordinary structural design, it
too will be
compressed some degree, even in 10 ft of water. If the 5050 is also an
integrated silicon chip
type sensor, consider that the hard silicon needs to deform in order for the
sensor
to function, and that it will deform in inches of water. I am inclined to
think that a structure that is fairly strong and well designed
will not compress enough to inject significant error, however, since you are
working in such
shallow water, it is more likely that you want fairly high depth
resolution. If you want
to be able to measure depth to say, =/- .5 in., you may find yourself
needing to compensate.
Of course, you could always cheat and use values verified at 10 ft, and
nobody would ever know,
unless they were actually checking your readings.

Anyway, you originally asked about converting DP to Depth. I am not certain
of what you mean, but see if this helps, if you don't already have this
info: Pascals to psi = 1.45 X 10^4. For ATM to psi, use 1.47 X 10^1. For
fresh water use 34 ft per additional ATM (of course, that's assuming 1 ATM
at surface). This gives you something like .432 lbs./ft. of depth. If your
water has lots of suspended or dissolved solids, such as salt water, adjust
toward 33 ft./ATM, as is commonly used for salt water. At 10 ft. depth in
fresh water you should see something like an absolute pressure of 19.02 psi.
If you assume your sub is perfectly unchanged at that depth, you are really
working with gauge pressure, since your enclosed rigid sub actually
converting your sensor from a differential type to one with a fixed
back-side pressure of 1 ATM. So you should see about a 4.32 psi increase
(psig) at 10 ft. The sensor doesn't care if the medium is water or air - it
should work the same way in both cases. Boyle's law says that the volume of
air in your balloon will be inversely proportional to the absolute pressure
exerted upon it - as long as the temperature remains constant.
Personally, I would ditch the balloon for fresh water use.....and I would be
thinking about how I was going to compensate for temperature changes. At
least that's how I would do it if I were relying on success in exchange for
a paycheck.

Chris

Chris, even

{Original Message removed}
Donovan Parks wrote...

>I'm only looking at a depth of only 10 feet.  Right
>now I am using the Motorola MPX5050 (a differential pressure sensor) and am
>*hoping* that the sub will not change volumetrically at shallow depths so I
>can get an accurate differential pressure.

How much the internal volume of your sub changes with depth will depend
on its geometry.
If it has a circular cross-section the hull material will end up
experiencing purely compressive forces, and I doubt whether you'd be
able to detect any change in the sub's internal volume (and thus its
internal air pressure).  On the other hand, if it has an elliptical
cross-section it's going to experience forces which will tend to flatten
the ellipse, and which may make it deform very significantly indeed if
the hull material isn't quite rigid.  So, shape matters.

As for temperature errors, looking at the Motorola datasheet for the
MPX5050 it appears the sensor and its integrated electronics will have a
temperature-induced error of  1% of full-scale or less from 0-60 deg. C
(i.e., +/- 0.075 PSI maximum error, equivalent to a depth error of 2"
H2O).  And you're hardly likely to encounter this much temperature
change.

A much bigger error source will be the change in air pressure inside the
hull caused by changes in the air temperature.  With a fixed mass of air
trapped in a fixed volume, the pressure will be proportional to absolute
temperature.  To compensate for this you can use a PTAT temperature
sensor like National's LM335 and measure its output with the PIC A/D,
and do the compensation in software.

Frankly, I think it would be a whole lot easier to just use an AP sensor
like Motorola's MPX4200A (3-29 PSI absolute) instead; no fuss, no muss.
I'm using these things in a product and have found them to be very
stable with temperature.  Great little devices.

Hope this helps a bit...

Dave

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Hello Dave, Chris, Dale:

Upon hearing the potential nightmare I am facing using the MPX5050, I belive
I will take Dave and Chris suggest and move to an absolute pressure sensor
(I'll look into the Motorola's MPX4200A).  Thanks for the help (I'll
probably have more questions yet, so stick around) :) .

Regards,
Donovan

> Donovan Parks wrote...
>
> >I'm only looking at a depth of only 10 feet.  Right
> >now I am using the Motorola MPX5050 (a differential pressure sensor) and
am
{Quote hidden}

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I have questioned the data sheet for the MPX5700 recently and Motorola is at
a loss as to how the temp info should be interpreted. I found significant
variation (>2.5%) when going across a threshold at somewhere around 70
degrees F. in a number of units tested so far. In my app, the error amounted
to about 3 ft. of depth, in fact my system would turn off, thinking it was
no longer submerged when used in colder water. Initially I submerged the
unit from 70 degree F ambient into 53 degree water. It turned on as soon as
it went below 1 ft, as it was supposed to. After cooling from the new
environment, it would shut down. This drove me crazy searching in all the
wrong directions, because I first trusted the data sheet. Several ap
engineers have responded, and apparently can't verify the data sheet info
yet. It seems they can't give an exact interpretation of the listed error
figure. They have me waiting as they investigate further. It is most likely
that their full-scale error percentage figure means that at a single
temperature, there should be no more than that amount of error across the
full range, sort of a linearity figure. I'll wait to see what they come up
with before re-writing my algorithm.

I appreciate the additional thought on the temp of the in-hull air and
pressure on a differential sensor. You can tell I'm stuck in absolute and
gauge modes. Good point.

Dave, what temperature range have you tested your design in?

Chris
As for temperature errors, looking at the Motorola datasheet for the
MPX5050 it appears the sensor and its integrated electronics will have a
temperature-induced error of  1% of full-scale or less from 0-60 deg. C
(i.e., +/- 0.075 PSI maximum error, equivalent to a depth error of 2"
H2O).  And you're hardly likely to encounter this much temperature
change.

A much bigger error source will be the change in air pressure inside the
hull caused by changes in the air temperature.  With a fixed mass of air
trapped in a fixed volume, the pressure will be proportional to absolute
temperature.  To compensate for this you can use a PTAT temperature
sensor like National's LM335 and measure its output with the PIC A/D,
and do the compensation in software.

Frankly, I think it would be a whole lot easier to just use an AP sensor
like Motorola's MPX4200A (3-29 PSI absolute) instead; no fuss, no muss.
I'm using these things in a product and have found them to be very
stable with temperature.  Great little devices.

Hope this helps a bit...

Dave

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Just one more thought on this for me today...

I have seen commercial dive computers that have no port, tube or balloons
visible, and yet they are amazingly accurate in sensing depth. I have
searched and phoned sensor makers, foguring that someone must have a
chip-scale sensing element, but have found nothing of the sort. Has anyone
ever seen what I'm talking about? Know a way to sense pressure or depth
without porting water or bagging air? This thread has me thinking of
re-inventing my own rather small wheel...

Chris

{Original Message removed}
Chris Loiacono wrote...

>Dave, what temperature range have you tested your design in?

The product using the MPX4200A is specified for operation between -20C
and +70C, but the sensor itself is rated for -40C to +125C (Motorola
designed it for automotive use in turbocharger systems).  During
development, I tested from -80C to +130C with good results.

The product is an analog design that re-scales the MPX4200A output to a
different offset and span, and adds a second level of temperature
compensation for both offset and span shift to keep the final output to
within +/- 0.25% of span over the full temperature range.  I calibrate
the units to within 0.05% of span, giving some margin for aging, supply
variations, and so forth.  I've had no trouble doing this, which
indicates how well-behaved the basic sensor is.

The error behavior you're seeing in the MPX5700 is certainly odd, to say
the least; I've seen nothing like that on the MPX4200A- only a gradual
change in output scale factor over temperature, mostly at low
temperatures.  There appears to be almost no zero shift with
temperature, less than 10 millivolts.  Makes me wonder if the MPX5700
has some sort of design flaw.

Dave

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Chris Loiacono wrote...

>I have seen commercial dive computers that have no port, tube or balloons
>visible, and yet they are amazingly accurate in sensing depth. I have
>searched and phoned sensor makers, foguring that someone must have a
>chip-scale sensing element, but have found nothing of the sort. Has anyone
>ever seen what I'm talking about? Know a way to sense pressure or depth
>without porting water or bagging air? This thread has me thinking of
>re-inventing my own rather small wheel...

If they're coupling the pressure sensor to the outside with a fluid, the
port may be extremely small- even a tiny capillary would do.  Maybe it's
there, but hidden or disguised somehow?

You could always rip one apart to see...

Dave

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Chris Loiacono wrote:
>
> Just one more thought on this for me today...
>
> I have seen commercial dive computers that have no port, tube or balloons
> visible, and yet they are amazingly accurate in sensing depth. I have
> searched and phoned sensor makers, foguring that someone must have a
> chip-scale sensing element, but have found nothing of the sort. Has anyone
> ever seen what I'm talking about? Know a way to sense pressure or depth
> without porting water or bagging air? This thread has me thinking of
> re-inventing my own rather small wheel...

Probably a slightly flexible casing and a small
strain gauge to measure casing compression.
You could do the same on the sub, have a
diaphragm (or special hull section) etc that
has known compression at depth, you can sense
deformation with something as simple as a
trans/receive opto interrupter. Aim for 2mm
max deflection and run the opto in the linear
range, with software compensation. Should
give 8 bit accuracy, total depends on mechanical
error. A quality spring attached to your
diaphragm will help repeatability and I think
you could chuck one together pretty quick and
cheap.
-Roman

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It's more likely that I should look at my hardware design once more....
C

The error behavior you're seeing in the MPX5700 is certainly odd, to say
the least; I've seen nothing like that on the MPX4200A- only a gradual
change in output scale factor over temperature, mostly at low
temperatures.  There appears to be almost no zero shift with
temperature, less than 10 millivolts.  Makes me wonder if the MPX5700
has some sort of design flaw.

Dave

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That would be a quick \$500 to \$1000...
You wouldn't happen to have a spare laying around, would you?

Ooops, I shouldn't have said that. Now there will be 100's of new
competitors..

Good point. Perhaps I will try a smaller opening in my next update. I hate

C

If they're coupling the pressure sensor to the outside with a fluid, the
port may be extremely small- even a tiny capillary would do.  Maybe it's
there, but hidden or disguised somehow?

You could always rip one apart to see...

Dave

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Chris Loiacono wrote...

>That would be a quick \$500 to \$1000...
>You wouldn't happen to have a spare laying around, would you?

Ouch!  Well, then, maybe "take apart VERY carefully" would be better
than "ripping" one apart...

>Ooops, I shouldn't have said that. Now there will be 100's of new
>competitors..

\$500 to \$1000 is a lotta bucks.  These people need competition!

Dave

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> This is similar to a scuba computer that I did recently.

Just out of curiosity - What sort of hoops did you have jump
through to satisfy insurers that your algorithm was valid, tested and
wouldn't cost them a fortune ?

Steve.

======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: stevebtla.co.nz      fax +64 9 820-1929
======================================================

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What about filling the vessel with a less compressible material. Then
you could calculate the difference between that and 1atm, and go from
there. I was thinking of a liquid...check the archives about the
submerged computer :)

Josh
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Chris Loiacono wrote:
{Quote hidden}

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At 03:04 AM 2/4/2002, you wrote:
>Just one more thought on this for me today...
>
>I have seen commercial dive computers that have no port, tube or balloons
>visible, and yet they are amazingly accurate in sensing depth. I have
>searched and phoned sensor makers, foguring that someone must have a
>chip-scale sensing element, but have found nothing of the sort. Has anyone
>ever seen what I'm talking about? Know a way to sense pressure or depth
>without porting water or bagging air? This thread has me thinking of
>re-inventing my own rather small wheel...

I know of no commercial dive computer that does not have an open port to a
sensor element. The port may be (very) small, and is almost always
covered/protected so that it doesn't get fouled with debris. It is usually
filled with a silicone gel over which a thin rubber 'sheet' is laid to
prevent the sticky silicone from gathering junk. Many dive computers are
completely filled with oil so that the designers needn't even consider the
effect of pressure on the case.

Also, I replied just a day or two ago to someone looking for a sensor for
barometric pressure. My reply may be of interest to you too, so I've pasted
it below.

The 0-14 bar sensor has a stainless steel cap which is (relatively) simple
to seal with an o-ring. If you're like me and prefer not to deal with
analog design if it can be reasonably avoided, this is a great solution. A
complete digital pressure + temperature sensor module in < 0.5
cm^3...pretty damn good if you ask me.

Kris Wilk
ReefNet Inc.
http://www.reefnet.on.ca

PREVIOUS POST******************

An absolutely BEAUTIFUL sensor module is made by Intersema
(http://www.intersema.com). They have a 0-15 psi surface mount package that's
completely temperature compensated, has a digital interface (i.e. talk to
it directly from your uC), and is also a temperature sensor. Literally
plug-and-play. And it's *VERY* power efficient...designed for building into
wristwatches and other portable devices.

They also make an identical module with 0-14 bar range, which is what we
have been using in our marine data logging equipment to measure depth.

What's really amazing is that they're not expensive (probably \$20 in
singles, and about 11 bucks in 1000s). If you've seen the new Swiss Army
knife with the built-in altimeter, that's one place this module has been
used...

I have no ties to Intersema other than being a very happy customer of theirs.

******************

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Thanks Kris, and sorry I missed your previous posting. I have never had more
of an opportunity to look at anyone else's product other than having had a
few different models in my hand for a quick look. That' a sure clue that
more research is needed on my part. The  experience you have shared is much
appreciated. The Intersema sensor seems to be much more like what I was
hoping to find. I will let you know how it works out.

Chris

{Original Message removed}
Hello,

Dave or anyone: Do you have a source for the MPX4200A.  A search with
http://www.findchips.com brought up nothing.  A Canadian source would be VERY
preferable, but any source would be welcomed.  What did you pay for it?

Regards,
Donovan

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Donovan Parks wrote...

>Dave or anyone: Do you have a source for the MPX4200A.  A search with
>http://www.findchips.com brought up nothing.  A Canadian source would be VERY
>preferable, but any source would be welcomed.  What did you pay for it?

It was a year ago, but I'm pretty sure we got ours from Newark
Electronics (http://www.newark.com).  I don't see any entry for it there now
but they do have the MPX4250AP, which goes to 36 PSI instead of 29 PSI
as does the MPX4200.  Should work just as well.

Newark lists the MPX4250AP at \$22.58 ea., stock number is 07F9899.
Looks like they have about a thousand in stock.

Avnet lists them also, but they're out of stock.

Hope this helps...

Dave

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I get MPX5700GOP's from Newark Electronics. Don't know if they have the
4200A in stock. Most likely they can get it if they don't.

CL

{Original Message removed}

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