Air and Water pressure measurements

[linnix]

I am trying to measure the pressure
above (0 to 10 meters) and
below (0 to 5 meters) water.
With the atmospheric pressure at sea level
of 101,000 Pascal, I estimated that
the minimum pressure is 10 meters
above water at 100,974 Pascal and
the maximum pressure is 5 meters
below water at 151,470 Pascal.

I am considering the Freescale
MPX2200 (upto 200KPa) for below water and
MPX2010 (upto 10KPa) for above water
measurements. The sensors will be attached
to four wires and buried underground.
The MPX2200 is easy to attach, with
0.1" spacing pins. The MPX2010 would
require a pcb.

First of all, are my calculations correct?
Are these sensors good for this experiment?
Are there alternatives to MPX2010 without
needing a pcb? Thanks.

---------------------------------------------

Original = 101,000 Pa
Minimum = 100,974 Pa
Maximum = 151,470 Pa

Density of water is 1030 kg/mmm
Density of air is 0.26 kg/mmm
Pressure at water level = P(zero) = 101,000 Pa

Below water level:
P = P(zero) + P(water)
= 101000 + 1030 kg/mmm * 9.8 m/ss * X m

Above water level:
P = P(zero) - P(air)
= 101000 - 0.26 kg/mmm * 9.8 m/ss * X m

 

[Ancient_Hacker]

I am trying to measure the pressure
above (0 to 10 meters) and
below (0 to 5 meters) water.

That's a bit puzzling. Do you want to measure the ambient pressure
from +10 to -5 meters, or do you want to ignore the ambient pressure
and measure the pressure at some sampling port? ? At the same time?
To what accuracy? Does the air sensor have to be able to tolerate the
-5 meter under water environment? And why are they buried? We've
got a lot of questons, and we're not even from New Jersey.

 

[linnix]

OK, the main goal is to map the contour and
measure the water level in a moving hillside.

That's a bit puzzling. Do you want to measure the ambient pressure
from +10 to -5 meters, or do you want to ignore the ambient pressure
and measure the pressure at some sampling port?

I need to measure air pressure for references at
several locations tracked by gps (within 1 meter or better)

? At the same time?

Yes, several water pressure sensors at the same time.

To what accuracy?

Fraction of a meter if possible.

Does the air sensor have to be able to tolerate the
-5 meter under water environment?

Yes, replacing them as needed.
Perhaps inside a pipe to be lowered during samplings.

And why are they buried?

For underground water level.

We've got a lot of questons, and we're not even from New Jersey.

No problem, thanks for asking anyway.

 

[Don Lancaster]

I am trying to measure the pressure
above (0 to 10 meters) and
below (0 to 5 meters) water.
With the atmospheric pressure at sea level
of 101,000 Pascal, I estimated that
the minimum pressure is 10 meters
above water at 100,974 Pascal and
the maximum pressure is 5 meters
below water at 151,470 Pascal.

I am considering the Freescale
MPX2200 (upto 200KPa) for below water and
MPX2010 (upto 10KPa) for above water
measurements. The sensors will be attached
to four wires and buried underground.
The MPX2200 is easy to attach, with
0.1" spacing pins. The MPX2010 would
require a pcb.

First of all, are my calculations correct?
Are these sensors good for this experiment?
Are there alternatives to MPX2010 without
needing a pcb? Thanks.

---------------------------------------------

Original = 101,000 Pa
Minimum = 100,974 Pa
Maximum = 151,470 Pa

Density of water is 1030 kg/mmm
Density of air is 0.26 kg/mmm
Pressure at water level = P(zero) = 101,000 Pa

Below water level:
P = P(zero) + P(water)
= 101000 + 1030 kg/mmm * 9.8 m/ss * X m

Above water level:
P = P(zero) - P(air)
= 101000 - 0.26 kg/mmm * 9.8 m/ss * X m

One foot of water is roughly half a psi or more exactly 0.434 psi

Easily derived by remembering that a cubic foot of water weighs 62.4
pounds (and holds 7.5 gallons).

Fire science 101.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Ancient_Hacker]

Ancient_Hacker wrote:

OK, the main goal is to map the contour and
measure the water level in a moving hillside.

Okay, that's helpful. Those pressure sensors sound about right for the
job.

Of course there's plenty of environmental and packaging issues, so
you'd better build up one or two of these pipes and test them out in
the real world. You only get a few millivolts per psi, so you'll need
some pretty stable and low-offset op-amps to condition the signals.

The hardest problem might be to get the 1 meter vertical positioning
accuracy for the pipe's position-- hope you have some laser surveying
equipment. I don't think GPS is up to the task.
How do they measure hillside slump nowadays anyway?

 

[linnix]

Okay, that's helpful. Those pressure sensors sound about right for the
job.

Of course there's plenty of environmental and packaging issues, so
you'd better build up one or two of these pipes and test them out in
the real world. You only get a few millivolts per psi, so you'll need
some pretty stable and low-offset op-amps to condition the signals.

We might bury a microcontroller with the sensor,
so we only need to deal with digital interfaces.

There will be a mobile unit powering them up and collecting data.

The hardest problem might be to get the 1 meter vertical positioning
accuracy for the pipe's position-- hope you have some laser surveying
equipment. I don't think GPS is up to the task.

For horizontal positioning, we are using a WAAS enabled GPS and
statistical samplings of 100 points per location. Vertical positioning

is still up for suggestions.

 

[Rich Grise]

We might bury a microcontroller with the sensor, so we only need to deal
with digital interfaces.

There will be a mobile unit powering them up and collecting data.


For horizontal positioning, we are using a WAAS enabled GPS and
statistical samplings of 100 points per location. Vertical positioning

is still up for suggestions.

You go to the building superintendent, and offer to give him this nice
barometer if he'll tell you the building's height.

Oh, wait - wrong problem! ;-P

But seriously, how much is your budget here? I've seen a laser measuring
device in the shop (we hire the guy to bring it in) that apparently gets
0.001" precision at, say, 3 meters range. I'd think there's some kind of
laser surveying system - what do they use to measure the plate drift by
faults? I'm almost sure I've seen something like that on edjamacashunal
teevee. Don't volcanologists do something like this too?

But I'd have serious qualms about calibrating an altimeter to get within
a meter of the actual altitude - this is why I thought, "barometer"; but
there has to be some kind of reference. Where is the ground? ;-)

Is there a benchmark nearby, and maybe a surveyor in town?

Hey - here's a brainstorm - just anchor your posts, and put a little
corner reflector at the top of each one at a calibrated height above
ground, and track them with raster-scan LIDAR. ;-)

Good Luck!
Rich

 

[Rich Grise]

One foot of water is roughly half a psi or more exactly 0.434 psi

Easily derived by remembering that a cubic foot of water weighs 62.4
pounds (and holds 7.5 gallons).

Fire science 101.

I don't even know what a 'mmm' is. )-;

Thanks,
Rich

 

[Ecnerwal]

Is there a benchmark nearby, and maybe a surveyor in town?

Hey - here's a brainstorm - just anchor your posts, and put a little
corner reflector at the top of each one at a calibrated height above
ground, and track them with raster-scan LIDAR. ;-)

Not too far off what a modern surveyor does with his total station (no
raster scanning, but using corner reflectors and lasers). And that is
what someone I know used for tracking glacier motion a couple of years
ago. Transits are so passe that I've had one on loan for over 10 years
at this point. I don't forget that it's on loan, and even remind the guy
who loaned it to me that I have it from time to time, but it's old,
slow, out-of-date technology.

 

[linnix]

You go to the building superintendent, and offer to give him this nice
barometer if he'll tell you the building's height.

It's a moving (sliding) hillside of several acres,
directly above a community of expensive houses.

Oh, wait - wrong problem! ;-P

But seriously, how much is your budget here?

Well, the community spent a million dollars previously to patch things
up.
They are about to spend another million again. I am trying to convince
them to fix it right, but first I need some data.

I've seen a laser measuring
device in the shop (we hire the guy to bring it in) that apparently gets
0.001" precision at, say, 3 meters range. I'd think there's some kind of
laser surveying system - what do they use to measure the plate drift by
faults? I'm almost sure I've seen something like that on edjamacashunal
teevee. Don't volcanologists do something like this too?

The system has to be in place and working continuously.
Data exchanges/downloads can be scheduled in bursts.
GPS can get within a meter horizontally.
We can probably help correct the vertical data with landmarks or
features.
Some part of the hill moved 10 to 20 meters over the past 10 years.

But I'd have serious qualms about calibrating an altimeter to get within
a meter of the actual altitude - this is why I thought, "barometer"; but
there has to be some kind of reference. Where is the ground? ;-)

The ground is subject to movement.
Computer readable barometer would be acceptable.

Is there a benchmark nearby, and maybe a surveyor in town?

Yes, I think they charge $500 (or more) per visit.
We can't ask them to come everyday.
They surely wouldn't come after a heavy rain storm,
when we need the measurements.

Hey - here's a brainstorm - just anchor your posts, and put a little
corner reflector at the top of each one at a calibrated height above
ground, and track them with raster-scan LIDAR. ;-)

Good Luck!
Rich

PS: In response to your other post, mmm is my short hand for m * m * m
or cubic meters.

 

[Phil Allison]

**Groper alert !

OK, the main goal is to map the contour and
measure the water level in a moving hillside.

For underground water level.

** You must be totally nuts !!

Variations in air pressure due to WEATHER will completely SWAMP the
tiny varition due to altitude you mention.

PLUS - you cannot equate wet soil and water.

Wet soil is much denser than water.




........ Phil

 

[Rich Grise]

Rich Grise wrote:

The ground is subject to movement.
Computer readable barometer would be acceptable.


Yes, I think they charge $500 (or more) per visit. We can't ask them to
come everyday.
They surely wouldn't come after a heavy rain storm, when we need the
measurements.


PS: In response to your other post, mmm is my short hand for m * m * m or
cubic meters.

Ah! Thanks. In the US, I've seen m^3 or such.

But, if you find a benchmark, you'd only need the surveyor once, to set
your reference point. (I hope the thing I'm thinking of is called a
"benchmark" - it's a reference point of some kind that surveyors use
so they know where stuff is at, and it's supposed to be accurate to
some ridiculously tight tolerances. If there are expensive houses
there, there is very likely one nearby.)

But, I'm also thinking; isn't there differential GPS, that can get
within centimeters? How hard would it be to get Z axis information from
their signals?

Good Luck!
Rich

 

[linnix]

Ah! Thanks. In the US, I've seen m^3 or such.

But, if you find a benchmark, you'd only need the surveyor once, to set
your reference point. (I hope the thing I'm thinking of is called a
"benchmark" - it's a reference point of some kind that surveyors use
so they know where stuff is at, and it's supposed to be accurate to
some ridiculously tight tolerances. If there are expensive houses
there, there is very likely one nearby.)

But, I'm also thinking; isn't there differential GPS, that can get
within centimeters? How hard would it be to get Z axis information from
their signals?

Using a Garmin C320 and sampling hundred points (while eliminating some
points) , I can position horizontal locations within a meter. So,
given enough samplings, horizontal positioning is not a problem. In
theory, vertical positioning is at best 10% of horizontal. I don't
even bother to find out how to get that from my C320. By the way, you
have to poll the GPS from a PC. Their great software is supposed to
hide that detail from the user.

 

[John Perry]

DGPS can get millimeter precision within small areas. Horizontal, at
least. I know that GPS favors horizontal accuracy over vertical, but I
don't really know how much it can be corrected. You could make use of
it by using two GPS receivers and calibrating them at a reference
location, then moving one around to make your measurements. That's how
public DGPS works.

Using a Garmin C320 and sampling hundred points (while eliminating some
points) , I can position horizontal locations within a meter. So,
given enough samplings, horizontal positioning is not a problem. In
theory, vertical positioning is at best 10% of horizontal. I don't
even bother to find out how to get that from my C320.

But if vertical DGPS is comparable to horizontal DGPS, you can get
sub-meter precision at least. Maybe even millimeter precision. Does
anyone here have enough knowledge to say?

By the way, you

have to poll the GPS from a PC. Their great software is supposed to
hide that detail from the user.

So get a couple of handhelds that you can read directly .

John Perry

 

[linnix]

DGPS can get millimeter precision within small areas. Horizontal, at
least. I know that GPS favors horizontal accuracy over vertical, but I
don't really know how much it can be corrected. You could make use of
it by using two GPS receivers and calibrating them at a reference
location, then moving one around to make your measurements. That's how
public DGPS works.


But if vertical DGPS is comparable to horizontal DGPS, you can get
sub-meter precision at least. Maybe even millimeter precision. Does
anyone here have enough knowledge to say?

I believe that the signal precision is 10:1 (horizontal:vertical).

By the way, you

So get a couple of handhelds that you can read directly .

I can pull a 9 digits radians (in double precision floating point) out
of the C320.
But the number tends to drift and jump. It could be some government
requirements to degrade the signal. I am still working on filtering
the signals.
Actually, I am looking for GPS modules with direct radian output, so I
don't need
to hog my wife's C320. I would need to bury several of them and expose
the
antenna only. The problem is that anything above ground is subject to
lost or damage from man or nature.

 

[Terran Melconian]

I am considering the Freescale
MPX2200 (upto 200KPa) for below water and
MPX2010 (upto 10KPa) for above water
measurements. The sensors will be attached
to four wires and buried underground.
The MPX2200 is easy to attach, with
0.1" spacing pins. The MPX2010 would
require a pcb.

It appears that the MPX2010 is available in many packages, including
some SIP ones like the MPX2200. You might want to double-check and see
if you can get those instead; some are in-stock at Digikey so I doubt
they're especially rare.

Assuming for whatever reason you're stuck with the SO, and you just want
wires going down to the sensor, your best option is probably to get a
standard SO8 to DIP or SIP adapter for two or three bucks and ignore
half of it. "Surf Board" is one brand; there are others.

I would be concerned about noise on the long wires if you're putting
just the sensor underground and everything else up top; it is not
necessarily unworkable, but if this were my project, I would try to
confirm the viability of that approach early on.

 

[John Perry]

I believe that the signal precision is 10:1 (horizontal:vertical).

Yes, I understand that. But is that inherent somehow that can't be
overcome by differential GPS? My application didn't require vertical
reading, so I'm not as well informed there.

I can pull a 9 digits radians (in double precision floating point) out
of the C320.
But the number tends to drift and jump. It could be some government
requirements to degrade the signal. I am still working on filtering
the signals.

They shut off Selective Availability in the mid-90's. To get better,
you need special processing like they use in the (expensive) surveying
receivers. Filtering won't help; the fluctuation that remains is due to
atmospheric interference. That's why DGPS is still useful, even after SA.

The point of DGPS is that a reference receiver with exactly known
location receives the satellite signals with roughly the same
atmospheric interference experienced by all receivers within a few dozen
to a few hundred miles, depending on conditions. The reference station
then broadcasts corrections so that an appropriately equipped receiver
can use the broadcast corrections to correct its readings.

But if you have decent receivers very close together, you can do the
corrections based on your own reference receiver. Simply subtract the
GPS errors measured by your stationary reference receiver from the
remote receiver that you carry around, or the planted remotes that you
want to monitor for movement, and you should be able to get very precise
_relative_ locations.

Actually, I am looking for GPS modules with direct radian output, so I
don't need
to hog my wife's C320. I would need to bury several of them and expose
the
antenna only. The problem is that anything above ground is subject to
lost or damage from man or nature.

That seems to make for a straightforward application of differential
techniques as I described above. Several C320's should do a very good
job for you, I would think.

My one and only GPS project ended just before SA was shut off, so I'm
not current. Good luck!

jp

 

[Iwo Mergler]

DGPS can get millimeter precision within small areas. Horizontal, at
least. I know that GPS favors horizontal accuracy over vertical, but I
don't really know how much it can be corrected. You could make use of
it by using two GPS receivers and calibrating them at a reference
location, then moving one around to make your measurements. That's how
public DGPS works.


But if vertical DGPS is comparable to horizontal DGPS, you can get
sub-meter precision at least. Maybe even millimeter precision. Does
anyone here have enough knowledge to say?

Very roughly, with full view of the sky you can expect vertical accuracy
to be about 2-3 times worse than horizontal - given by the geometry.

If you have a DGPS system which can do sub-cm accuracy, you'll get
sub-3-cm vertical accuracy.

To the OP, please don't expect 1m accuracy by "sampling 100 points and
eliminating some". GPS is not that predictable. The ionospheric errors
have a tendency to 'wander', slowly. In other words, you may have a
whole hour where your handheld is consistently off by 5m or more.

Some handheld GPS devices are cheating - if they think you're not moving,
they "nail down" whatever position they have, to look more accurate. If
you can, switch off filtering. If you can't, power cycle the device for
each measurement.

You can indeed get good positions out of many samples, but you need to
sample over a few days.

Samples can be eliminated if you have access to the residuals from
the position solution.

Kind regards,

Iwo

 

[linnix]

Is DGPS same as WAAS? If not, how would you compare the two?
The Garmin C320 is WAAS enabled.

Very roughly, with full view of the sky you can expect vertical accuracy
to be about 2-3 times worse than horizontal - given by the geometry.

If you have a DGPS system which can do sub-cm accuracy, you'll get
sub-3-cm vertical accuracy.

To the OP, please don't expect 1m accuracy by "sampling 100 points and
eliminating some". GPS is not that predictable. The ionospheric errors
have a tendency to 'wander', slowly. In other words, you may have a
whole hour where your handheld is consistently off by 5m or more.

Yes, that's what I am seeing.
I just need to space out the 100 points over days.

Some handheld GPS devices are cheating - if they think you're not moving,
they "nail down" whatever position they have, to look more accurate. If
you can, switch off filtering. If you can't, power cycle the device for
each measurement.

You can indeed get good positions out of many samples, but you need to
sample over a few days.

That's not a problem. We can sample for days or weeks.

Samples can be eliminated if you have access to the residuals from
the position solution.

I get radians in double precision floating points.
I am currently looking at 9 digits precisions.

Kind regards,

Iwo

Thanks.

 

[Ecnerwal]

Is DGPS same as WAAS? If not, how would you compare the two?

WAAS appears to be a rather coarse, distributed DGPS that negates you
needing to have a stationary reference receiver, as they have some and
broadcast the corrections. Presumably this means that you can get the
corrections "in real time", as well. Per Garmin, it is apparently
limited to the US, though other, similar systems are being implemented
elsewhere.

DGPS is a pretty simple concept, and doing it locally to your project
gets you the best possible resolution. You have a stationary reference
receiver, which sits there and stores its location as reported over time
into a computer. You park this somewhere that isn't thought to be
moving, yet is near the area of interest that is moving.

You have other receivers, and you store where they say they are over
time as you take measurements with them and move them around, or in this
case, as the hillside they are fixed to moves with them.

Then, you take the one set of data and compare it with the other set of
data to get a more precise idea of where the moving ones really were, by
removing the relative recorded motions of the one that was not moving,
at the time of each measurement. You don't really get a precise idea of
what is happening until you do the data-crunching step, but when you do,
it's quite precise.