| Topic: |
Science > Physics |
| User: |
"Donalbane" |
| Date: |
15 Feb 2006 03:51:21 PM |
| Object: |
time constants in pitot static tubes |
I was recently looking for differential pressure sensors to use in
conjunction with a pitot tube to make dynamic pressure measurements.
In the course of my search, I found a sensor that used the same
principle as a thermal anemometer to measure the pressure, by letting a
small amount of the air from one channel flow across a thermal element
through to the other channel. I wondered if this "tiny leak" would
affect the dynamic pressure measurement in the event that the measured
value was very low (tenths of an inch of water). When I raised this
question to a colleague, I was informed that the flow rate, Q, of the
leak would need to be much much smaller than an "effective flow rate"
at the tip of the pitot tube.
This "effective flow rate" is essentially the rate at which the fluid
(in this case, air) molecules are being packed into the tube to
maintain the static pressure value. Aparently, this phenomenon is
well-known to intrumentation guys, but is never discussed in textbooks.
This packing rate results in a settling time for a given pitot tube in
a particular flow field. A similar effect might be observed with very
long fluid lines experiencing a sudden increase in pressure at one end
of the line. Supposedly, it takes some time before the pressure
equalizes throughout the line.
Can anyone explain this phenomenon to me and point me in the direction
of a reference where I might be able to derive the appropriate time
constants for particular systems?
Thank you,
Don
.
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|
| User: "Olin Perry Norton" |
|
| Title: Re: time constants in pitot static tubes |
16 Feb 2006 10:14:33 AM |
|
|
Donalbane wrote:
I was recently looking for differential pressure sensors to use in
conjunction with a pitot tube to make dynamic pressure measurements.
In the course of my search, I found a sensor that used the same
principle as a thermal anemometer to measure the pressure, by letting a
small amount of the air from one channel flow across a thermal element
through to the other channel. I wondered if this "tiny leak" would
affect the dynamic pressure measurement in the event that the measured
value was very low (tenths of an inch of water). When I raised this
question to a colleague, I was informed that the flow rate, Q, of the
leak would need to be much much smaller than an "effective flow rate"
at the tip of the pitot tube.
This "effective flow rate" is essentially the rate at which the fluid
(in this case, air) molecules are being packed into the tube to
maintain the static pressure value. Aparently, this phenomenon is
well-known to intrumentation guys, but is never discussed in textbooks.
This packing rate results in a settling time for a given pitot tube in
a particular flow field. A similar effect might be observed with very
long fluid lines experiencing a sudden increase in pressure at one end
of the line. Supposedly, it takes some time before the pressure
equalizes throughout the line.
Can anyone explain this phenomenon to me and point me in the direction
of a reference where I might be able to derive the appropriate time
constants for particular systems?
Thank you,
Don
Dear Don,
There may be two separate questions here. The first is
"What is the response time of a pitot tube?"
As others have pointed out, this is a function of the
sense lines connecting the pitot tube to your pressure
transducer. Good places to start are:
Doebelin, Ernest O., Measurement systems; application and design,
McGraw-Hill, fourth edition, 1990, pp. 123-131 and pp. 473-489.
Holman, J. P., Experimental methods for engineers, McGraw-Hill,
second edition, 1971, pp. 160-167.
The second question is "What is the effect of having a
nonzero mean flow in the sense lines and through the holes
in the pitot tube?" I have wondered about this myself -- I have thought
about bleeding a small amount of clean, dry gas through a pitot
to keep it from clogging in a dusty gas stream.
In my thinking, this question can be decomposed into
two parts, considering first the sense lines and then the
pitot tube itself.
1. A nonzero mean flow in a tube means that there is
a pressure drop, which will bias your pressure measurement,
i.e., the pressure at the transducer will not be equal to the
pressure at the pitot because of the pressure drop
in the line(s). Assuming that the flow rate is small, you should
be able to use the Poiseuille equation to calculate this pressure
difference.
2. What effect does the flow of air in or out of the hole in the
pitot tube have on the pressure there? (Normally, one assumes
that the velocity is zero at the tip, so that one measures the
stagnation pressure there.) I'm guessing that this effect should
be negligible as long as the velocity through the hole is small
compared to the freestream velocity, although it would be
good to have some data on this point. I think this may be
what your colleague was tlaking about.
Olin Perry Norton
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| User: "Donalbane" |
|
| Title: Re: time constants in pitot static tubes |
16 Feb 2006 04:01:28 PM |
|
|
Olin, yes. This is what I'm trying to understand. I know that there
will be an effect if there is flow in the pitot tube, but I'm trying to
see how small this can be and be negligible relative to the pressure
measurement. I did a comparison of the velocities as you mentioned,
and found that for the low pressures we are measuring, the sensor was
not suitable, however the sensor claims to be ESPECIALLY good at low
pressures, so I'm not sure that the comparison was valid.
Don
.
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|
| User: "Olin Perry Norton" |
|
| Title: Re: time constants in pitot static tubes |
17 Feb 2006 09:17:13 AM |
|
|
Donalbane wrote:
Olin, yes. This is what I'm trying to understand. I know that there
will be an effect if there is flow in the pitot tube, but I'm trying to
see how small this can be and be negligible relative to the pressure
measurement. I did a comparison of the velocities as you mentioned,
and found that for the low pressures we are measuring, the sensor was
not suitable, however the sensor claims to be ESPECIALLY good at low
pressures, so I'm not sure that the comparison was valid.
Don
If you are looking for a good pressure sensor
to measure very small differential pressures,
you might try Validyne. ( http://www.validyne.com/ )
Their product line goes as low as 0.1 inch of water
full scale.
Olin Perry Norton
.
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|
| User: "Brian Whatcott" |
|
| Title: Re: time constants in pitot static tubes |
17 Feb 2006 06:25:15 PM |
|
|
On Fri, 17 Feb 2006 09:17:13 -0600, Olin Perry Norton
<norton@dial.msstate.invalid> wrote:
Donalbane wrote:
Olin, yes. This is what I'm trying to understand. I know that there
will be an effect if there is flow in the pitot tube, but I'm trying to
see how small this can be and be negligible relative to the pressure
measurement. I did a comparison of the velocities as you mentioned,
and found that for the low pressures we are measuring, the sensor was
not suitable, however the sensor claims to be ESPECIALLY good at low
pressures, so I'm not sure that the comparison was valid.
Don
If you are looking for a good pressure sensor
to measure very small differential pressures,
you might try Validyne. ( http://www.validyne.com/ )
Their product line goes as low as 0.1 inch of water
full scale.
Olin Perry Norton
But if you're looking for a sensor that will measure VERY small air
pressure differences, there is an almost no-cost method.
Two paint cans, half filled with water. Capped off. Stub pipes at
the top run to the sensing points.
Two pipes at the foot of the two cans connecting them together.
A thick pipe, with a stop-***** half way across..
The other, a thin pipe with a transparent wall - like glass.
Open the stop ***** to level the fluid in the cans. Ensure there is an
air bubble in the thin pipe. Close the stop-*****.
A 0.001 inch WG difference to the two cans (if they are say 6 inch
diameter) would move the bubble 2.3 inches sidewards if the sight
pipe is 1/8 inch diameter!
Brian Whatcott Altus OK
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| User: "Olin Perry Norton" |
|
| Title: Re: time constants in pitot static tubes |
20 Feb 2006 09:15:22 AM |
|
|
Brian Whatcott wrote:
But if you're looking for a sensor that will measure VERY small air
pressure differences, there is an almost no-cost method.
Two paint cans, half filled with water. Capped off. Stub pipes at
the top run to the sensing points.
Two pipes at the foot of the two cans connecting them together.
A thick pipe, with a stop-***** half way across..
The other, a thin pipe with a transparent wall - like glass.
Open the stop ***** to level the fluid in the cans. Ensure there is an
air bubble in the thin pipe. Close the stop-*****.
A 0.001 inch WG difference to the two cans (if they are say 6 inch
diameter) would move the bubble 2.3 inches sidewards if the sight
pipe is 1/8 inch diameter!
Brian Whatcott Altus OK
That sounds like a neat trick. I'll try it next time
I need to measure a very small delta P. It would be
a good way to check the calibration of an electronic
pressure transducer.
Thanks, Olin Perry Norton
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| User: "Olin Perry Norton" |
|
| Title: Re: time constants in pitot static tubes |
17 Feb 2006 01:45:02 PM |
|
|
Donalbane wrote:
Olin, yes. This is what I'm trying to understand. I know that there
will be an effect if there is flow in the pitot tube, but I'm trying to
see how small this can be and be negligible relative to the pressure
measurement. I did a comparison of the velocities as you mentioned,
and found that for the low pressures we are measuring, the sensor was
not suitable, however the sensor claims to be ESPECIALLY good at low
pressures, so I'm not sure that the comparison was valid.
Don
NACA TN-803, "Some effects of rainfall on flight of airplanes and on
instrument indications," by Richard V. Rhode, April 1941, available at
http://naca.larc.nasa.gov/reports/1941/naca-tn-803/naca-tn-803.pdf
says, regarding water in pitot lines,
"The use of a hand-pressure pump in the pressure line to clear
it of water, which is an expedient adopted by some air lines,
cannot be considered a guaranty against serious errors if the pilot does
not recognize malfunctioning of the air-speed system. A continuously
operating mechanical pump, designed to provide a continuous slight
flow of air in the pressure line toward the pitot opening ("reverse leak"),
has been suggested as an alternative. Tests of a reverse leak, made
during the rain tests on pitot tubes previously described in this paper,
indicated that the method is successful in principal... "
Unfortunately, this report does not seem to give any details, such as
how much flow is a "slight flow."
Olin Perry Norton
.
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| User: "N:dlzc D:aol T:com \dlzc\ N: dlzc1 D:cox" |
|
| Title: Re: time constants in pitot static tubes |
17 Feb 2006 06:19:38 PM |
|
|
Dear Olin Perry Norton:
"Olin Perry Norton" <norton@dial.msstate.invalid> wrote in
message news:dt593v$9kh$1@nntp.msstate.edu...
....
Unfortunately, this report does not seem to give
any details, such as how much flow is a "slight flow."
I'd shoot for laminar flow (Re near 2000), if the purpose was to
keep the lines clear. (Or short bursts of turbulent, with
"signal hold" on the reading.) You could calibrate against
static pressure to get the flow-induced drop, and factor it out.
David A. Smith
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| User: "John C. Polasek" |
|
| Title: Re: time constants in pitot static tubes |
17 Feb 2006 11:05:02 AM |
|
|
On 15 Feb 2006 13:51:21 -0800, "Donalbane" <dtucker@arlut.utexas.edu>
wrote:
I was recently looking for differential pressure sensors to use in
conjunction with a pitot tube to make dynamic pressure measurements.
In the course of my search, I found a sensor that used the same
principle as a thermal anemometer to measure the pressure, by letting a
small amount of the air from one channel flow across a thermal element
through to the other channel. I wondered if this "tiny leak" would
affect the dynamic pressure measurement in the event that the measured
value was very low (tenths of an inch of water). When I raised this
question to a colleague, I was informed that the flow rate, Q, of the
leak would need to be much much smaller than an "effective flow rate"
at the tip of the pitot tube.
This "effective flow rate" is essentially the rate at which the fluid
(in this case, air) molecules are being packed into the tube to
maintain the static pressure value. Aparently, this phenomenon is
well-known to intrumentation guys, but is never discussed in textbooks.
This packing rate results in a settling time for a given pitot tube in
a particular flow field. A similar effect might be observed with very
long fluid lines experiencing a sudden increase in pressure at one end
of the line. Supposedly, it takes some time before the pressure
equalizes throughout the line.
Can anyone explain this phenomenon to me and point me in the direction
of a reference where I might be able to derive the appropriate time
constants for particular systems?
Thank you,
Don
This may be of help. A bulb with a restrictive orifice like the pitot
or your extra sensor that stores a gas or fluid has a sharp resonant
frequency. It is known as a Helmholtz resonator and is the basis of
the ocarina. That makes your search more difficult than just a time
constant. Working below resonance it might resemble a spring
coefficient.
I learned about this in the early days of ink jets when I was aksed to
find out why their design spattered ink when it got to 5 KC, and it
matched the Helmholtz criteria to within 10% or so. Jet tube .002" if
I recall. I wrote a paper on it but I can't find it.
John Polasek
http://www.dualspace.net
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| User: "Ed Ruf REPLY to E-MAIL IN SIG!" |
|
| Title: Re: time constants in pitot static tubes |
15 Feb 2006 07:06:05 PM |
|
|
On 15 Feb 2006 13:51:21 -0800, in sci.engr.mech "Donalbane"
<dtucker@arlut.utexas.edu> wrote:
I was recently looking for differential pressure sensors to use in
conjunction with a pitot tube to make dynamic pressure measurements.
and
affect the dynamic pressure measurement in the event that the measured
value was very low (tenths of an inch of water).
What type of frequency response are you looking for?
--
Ed Ruf Lifetime AMA# 344007 (Usenet2@EdwardG.Ruf.com)
http://EdwardGRuf.com
.
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| User: "Donalbane" |
|
| Title: Re: time constants in pitot static tubes |
16 Feb 2006 03:04:45 PM |
|
|
Thanks very much for knowing what I was talking about Ed and Olin!
Ed, we aren't necessarily looking for a particular frequency response.
We just want to understand this problem more, so that we are smart
about what sensors we use and how long we wait before making our
measurements, etc. Thanks for the references!
Don
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| User: "tadchem" |
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| Title: Re: time constants in pitot static tubes |
15 Feb 2006 04:06:02 PM |
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|
Think of a pitot tube as a flute. The time constant should be on the
order of the period of the fundamental resonant tone of the flute.
That will vary with the speed of sound of the medium and the effective
length of the tube.
Tom Davidson
Richmond, VA
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| User: "Donalbane" |
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| Title: Re: time constants in pitot static tubes |
15 Feb 2006 04:16:10 PM |
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Can I use the same reasoning for long (not necessarily straight) lines,
by just treating them as straight cylinders?
Don
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| User: "tadchem" |
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| Title: Re: time constants in pitot static tubes |
15 Feb 2006 05:02:07 PM |
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|
If the radius of curvature is at a minimum several times the internal
diameter of the tube, (think of a tuba or French horn) then gradual
bends should not matter.
The background information that you would find most useful will be in
engineering references under the heading "fluid flow," subheadings
"compressible fluids", "pipes", and "gases."
HTH
Tom Davidson
Richmond, VA
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| User: "Donalbane" |
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| Title: Re: time constants in pitot static tubes |
15 Feb 2006 05:40:05 PM |
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|
It is my understanding that I was mixing two unrelated phenomena. The
speed of sound propagation of pressure is indeed the usual case, for
things like long lines, etc.
However, the case with the pitot tube is different, because you aren't
actually measuring pressure, but you're measuring flow and deriving a
pressure from it. The flow that you're measuring is the rate at which
molecules are being packed into the dynamic tube vs. the static tube.
So, the settling time isn't the usual speed of sound pressure
propagation time. This is why few people are familiar with this
phenomenon, apart from instrumentation gurus. Does anyone know any
more about this?
Don
.
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|
| User: "Spaceman" |
|
| Title: Re: time constants in pitot static tubes |
15 Feb 2006 05:52:37 PM |
|
|
"Donalbane" <dtucker@arlut.utexas.edu> wrote in message
news:1140046805.673352.283620@z14g2000cwz.googlegroups.com...
| It is my understanding that I was mixing two unrelated phenomena. The
| speed of sound propagation of pressure is indeed the usual case, for
| things like long lines, etc.
|
| However, the case with the pitot tube is different, because you aren't
| actually measuring pressure, but you're measuring flow and deriving a
| pressure from it. The flow that you're measuring is the rate at which
| molecules are being packed into the dynamic tube vs. the static tube.
| So, the settling time isn't the usual speed of sound pressure
| propagation time. This is why few people are familiar with this
| phenomenon, apart from instrumentation gurus. Does anyone know any
| more about this?
Hi Don,
I am not sure what you mean about time constants for such.
Have you seen this link?
http://www.grc.nasa.gov/WWW/K-12/airplane/pitot.html
maybe something there might help you.
:)
.
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| User: "Donalbane" |
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| Title: Re: time constants in pitot static tubes |
15 Feb 2006 06:40:03 PM |
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|
Thanks for the link.
As far as time constants, consider the pitot tube shown from the
referenced link. At time zero there is no flow, and both the static
pressure and total pressure are merely the result of random molecular
momenta, and hence equal. Now, insert the pitot tube into a flow.
Immediately, particles in the air stream impact the particles at the
tip of the dynamic pressure channel in the tube and impart their
momentum. It takes some time before the air molecules in the dynamic
pressure channel of the tube have collectively acquired sufficient
momentum to produce the new total pressure at equilibrium. This time
to come to equilibrium is what I mean by the time constant.
Intuitively, it seems like if you were to (for some wacky reason) make
the volume of the dynamic pressure channel much larger than that of the
static pressure channel, you would see a longer response time (to come
to a new equilibrium) when the tube was placed into a flow field that
if both channels were small.
Don
.
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| User: "Spaceman" |
|
| Title: Re: time constants in pitot static tubes |
15 Feb 2006 06:58:15 PM |
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|
"Donalbane" <dtucker@arlut.utexas.edu> wrote in message
news:1140050403.006562.159460@z14g2000cwz.googlegroups.com...
| Thanks for the link.
|
| As far as time constants, consider the pitot tube shown from the
| referenced link. At time zero there is no flow, and both the static
| pressure and total pressure are merely the result of random molecular
| momenta, and hence equal. Now, insert the pitot tube into a flow.
| Immediately, particles in the air stream impact the particles at the
| tip of the dynamic pressure channel in the tube and impart their
| momentum.
By dynamic pressure I am thinking
you must mean the total pressure tube. (center tube)
| It takes some time before the air molecules in the dynamic
| pressure channel of the tube have collectively acquired sufficient
| momentum to produce the new total pressure at equilibrium.
If you are talking about the total pressure tube,
I would guess the time response would be at about
the speed of sound.
(basically at the rate the molecules can compress at.)
| This time
| to come to equilibrium is what I mean by the time constant.
| Intuitively, it seems like if you were to (for some wacky reason) make
| the volume of the dynamic pressure channel much larger than that of the
| static pressure channel, you would see a longer response time (to come
| to a new equilibrium) when the tube was placed into a flow field that
| if both channels were small.
Not really sure, but I think it would not take too much of a time
difference.
If any at all since the pressure will still build just about
at the speed of sound.
And of course sound seems to be a good factor for such
since they also state....
For higher than sound speeds.
There are corrections for the shock wave that can be applied
to allow us to use pitot tubes for high speed aircraft.
( I would gather they then use different compression factors)
:)
It sure does make one think about airplanes and all that
simple, but fancy, stuff they use.
:)
.
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| User: "Ed Ruf REPLY to E-MAIL IN SIG!" |
|
| Title: Re: time constants in pitot static tubes |
15 Feb 2006 07:10:45 PM |
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|
On Wed, 15 Feb 2006 19:58:15 -0500, in sci.engr.mech "Spaceman"
<Realspace@comcast.not> wrote:
If you are talking about the total pressure tube,
I would guess the time response would be at about
the speed of sound.
(basically at the rate the molecules can compress at.)
No, it is much much slower. It's a Poiseuille Flow problem. Not only is
tube length important, but so is sensor volume. I've got a paper I can dig
p at work on the classical method to estimate the lag. For tenths of an
inch of water the driving pressure difference is fractions of this, so the
response will be very slow.
--
Ed Ruf Lifetime AMA# 344007 (Usenet2@EdwardG.Ruf.com)
http://EdwardGRuf.com
.
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| User: "Spaceman" |
|
| Title: Re: time constants in pitot static tubes |
15 Feb 2006 07:15:55 PM |
|
|
"Ed Ruf (REPLY to E-MAIL IN SIG!)" <egruf_usenet2@cox.net> wrote in message
news:a2k7v1tpkj2l49kg4lbisp4ci099cb2t9v@4ax.com...
| On Wed, 15 Feb 2006 19:58:15 -0500, in sci.engr.mech "Spaceman"
| <Realspace@comcast.not> wrote:
|
|
| >If you are talking about the total pressure tube,
| >I would guess the time response would be at about
| >the speed of sound.
| >(basically at the rate the molecules can compress at.)
|
| No, it is much much slower. It's a Poiseuille Flow problem. Not only is
| tube length important, but so is sensor volume. I've got a paper I can dig
| p at work on the classical method to estimate the lag. For tenths of an
| inch of water the driving pressure difference is fractions of this, so the
| response will be very slow.
Wow,
Cool stuff.
thanks.
Off to look up Poiseuille Flow problem now.
:)
.
|
|
|
| User: "Ed Ruf REPLY to E-MAIL IN SIG!" |
|
| Title: Re: time constants in pitot static tubes |
15 Feb 2006 07:29:44 PM |
|
|
On Wed, 15 Feb 2006 20:15:55 -0500, in sci.engr.mech "Spaceman"
<Realspace@comcast.not> wrote:
Off to look up Poiseuille Flow problem now.
Simple transient fluid flow problem. If you are interested in measuring the
fluctuation is a total head/pitot probe, what is driving the flow from the
probe to the sensor is the fluctuations themselves. Say you have a total
head or pitot pressure of 10 psia with fluctuations of 0.25 psia. For the
transient response of the measurement it is the 0.25 psia differential
driving the flow in the tube.
--
Ed Ruf Lifetime AMA# 344007 (Usenet2@EdwardG.Ruf.com)
http://EdwardGRuf.com
.
|
|
|
| User: "Ed Ruf" |
|
| Title: Re: time constants in pitot static tubes |
16 Feb 2006 06:58:23 AM |
|
|
On Wed, 15 Feb 2006 20:29:44 -0500, in sci.engr.mech "Ed Ruf (REPLY
to E-MAIL IN SIG!)" <egruf_usenet2@cox.net> wrote:
On Wed, 15 Feb 2006 20:15:55 -0500, in sci.engr.mech "Spaceman"
<Realspace@comcast.not> wrote:
Off to look up Poiseuille Flow problem now.
Simple transient fluid flow problem. If you are interested in measuring the
fluctuation is a total head/pitot probe, what is driving the flow from the
probe to the sensor is the fluctuations themselves. Say you have a total
head or pitot pressure of 10 psia with fluctuations of 0.25 psia. For the
transient response of the measurement it is the 0.25 psia differential
driving the flow in the tube.
For application see:
AFWAL-TM-85-247-FIMN
The Design and Testing of Pneumatic Systems for Measuring Low
pressures in Hypersonic Wind Tunnels, by M. J. Wagner and G.A. Dale,
Flight Dynamics Laboratory, Wright-Patt AFB, November 1985.
which references,
Optimized Design of Systems for Measure Low Pressures in Supersonic
Wind Tunnels, by J.M. Kendall, NATO, AGARD Report 174, March 1958.
and
Prediction of Pressure Response in Low Pressure Flow Regimes, by M.R.
Cain, TM68-9, AFFDL, Wright-Patt AFB, October 1968.
.
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|
| User: "Spaceman" |
|
| Title: Re: time constants in pitot static tubes |
16 Feb 2006 12:01:19 PM |
|
|
"Ed Ruf" <egruf_usenet@cox.net> wrote in message
news:dus8v1t4m58r7evge5mcpja1jgmvi34h1t@4ax.com...
| On Wed, 15 Feb 2006 20:29:44 -0500, in sci.engr.mech "Ed Ruf (REPLY
| to E-MAIL IN SIG!)" <egruf_usenet2@cox.net> wrote:
|
| >On Wed, 15 Feb 2006 20:15:55 -0500, in sci.engr.mech "Spaceman"
| ><Realspace@comcast.not> wrote:
| >
| >>Off to look up Poiseuille Flow problem now.
| >
| >Simple transient fluid flow problem. If you are interested in measuring
the
| >fluctuation is a total head/pitot probe, what is driving the flow from
the
| >probe to the sensor is the fluctuations themselves. Say you have a total
| >head or pitot pressure of 10 psia with fluctuations of 0.25 psia. For the
| >transient response of the measurement it is the 0.25 psia differential
| >driving the flow in the tube.
|
| For application see:
| AFWAL-TM-85-247-FIMN
| The Design and Testing of Pneumatic Systems for Measuring Low
| pressures in Hypersonic Wind Tunnels, by M. J. Wagner and G.A. Dale,
| Flight Dynamics Laboratory, Wright-Patt AFB, November 1985.
|
| which references,
|
| Optimized Design of Systems for Measure Low Pressures in Supersonic
| Wind Tunnels, by J.M. Kendall, NATO, AGARD Report 174, March 1958.
|
| and
|
| Prediction of Pressure Response in Low Pressure Flow Regimes, by M.R.
| Cain, TM68-9, AFFDL, Wright-Patt AFB, October 1968.
Thank you very much Ed.
:)
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