Talk:Static pressure

Talk Page Archive

Archive 1 has been created with a link at above right. It is an exact copy of the talk page as it was before this edit. Archive 2, when needed in the future, should be a new subpage (same as creating an article) titled "Talk:Static pressure/Archive 2" and the link added to the template on this page's code. For further information on archiving see Wikipedia:How to archive a talk page. See also User:5Q5 for the used archiving procedure. Thank you. Crowsnest (talk) 21:34, 20 May 2008 (UTC)

Incidental advertising

On 20 April Wiki-editor Djhnsn added links to the following two sites:

• Measurement of Static Pressure on Aircraft
• NASA's Measurement of Aircraft Speed and Altitude

On 22 April Wiki-editor Giuliopp removed the two links, commenting that a “sneaky advertisement” was being removed.

These sites contain valuable and relevant reports produced by the US National Aeronautics and Space Administration. However, these web-sites are operated by a commercial organisation (SpaceAge Control, Inc.) and each of the two web-sites contains one page of promotional material supporting the commercial organisation.

The above actions raise the question of how much advertising is tolerable at a site linked to Wikipedia? Wiki advice given here says:

If the link is to a relevant and informative site that should otherwise be included, please consider mentioning it on the talk page and let neutral and independent Wikipedia editors decide whether to add it. This is in line with the conflict of interest guidelines.

In the spirit of this advice, I am mentioning this matter on this Talk page. I believe these two NASA reports are highly valuable and relevant to the concept of static pressure. The reports were written when William Gracey was head of NASA’s Langley Research Center. Gracey subsequently incorporated this material in his book, and that book is quoted and cited in Static pressure. The reports are therefore particularly relevant because they give all Wiki readers access to information that was previously only accessible to those few readers who have access to Gracey's book. I also believe that one page of corporate promotional material is well within the bounds of what can be tolerated. What do other Wiki-editors think? If there is no suitably-substantiated objection to restoring these two links within a week or so I will be inclined to restore them.

If there is a rule that might prevent these two valuable NASA reports from being accessible via Wikipedia I consider that Wikipedia:Ignore all rules is relevant – “If a rule prevents you from improving or maintaining Wikipedia, ignore it.” It is true that each of the links to the two NASA reports contains a page of corporate promotional material but, on balance, I believe these two links improved Wikipedia (until they were deleted.) Dolphin51 (talk) 12:30, 22 April 2008 (UTC)

I find it a bit naive to think that the advertising in question was 'incidental'. User Djhnsn has clearly a much bigger interest in promoting SpaceAge Control Inc. than he has in improving Wikipedia, given that all of his twelve contributions have consisted simply in disseminating links to that company, even if they contain also some relevant information.

I question the authoritativeness of a commercial website that offers to download US governmental documents – even altered with the insertion of advertising pages – in exchange for user's registration, also claiming that such documents have a market value, when they are in fact publicly available, free of charge.

If Djhnsn – who didn't post any explanation on the talk page, as required by WP guidelines – had cared about improving this article, he would have added links to the original documents: 1, 2, which can be located in just a few minutes using Google or the NTRS search engine. There's no question in my mind that these mean attempts to exploit WP for commercial gain should be deleted without hesitation. Giuliopp (talk) 23:16, 22 April 2008 (UTC)

Well, his motives don't matter. But if the direct gov links have all the info and present it clearly, then I'd go ahead and delete the commercial ones. Voice-of-All 01:05, 23 April 2008 (UTC)

Static pressure in HVAC systems?

This article does not appear to at all address the common use of static pressure in HVAC systems. That seems strange, since I think it probably the most common usage. (E.g. a fan might produce 140 CFM of air blowing into a static pressure of 0 in h20, but only 110 CFM blowing into 0.5 in h20.)

I think that's part of what the first archived comment is about, as well... jhawkinson (talk) 18:04, 14 March 2009 (UTC)

Definition

It seems from my perspective you guys are going about this from completely the wrong standpoint. The article keeps making the statement comparing static pressure to Bernoulli's equation. This is not the origination of static pressure in any sense of the meaning. Approaching it from a energy conservation standpoint, all energy and momentum concerns should keep in mind that fluids have macro and micro properties. While dynamics pressure is a macro energy, the pressure term in bernoulli's equation measures the microscopic energy associated with the fluid. This makes more sense viewing the equation as such: P*dV + 1/2*m*v^2=total energy. Now I'm also going to give a derivation from statistical thermodynamics. Static pressure is most easy to conceptualize in that it is the pressure you would measure the fluid having while moving in the same inertial reference frame as the local fluid. From a control volume analysis within the reference frame of the fluid, the pressure times the velocity can actually be found to be 1/3 of the kinetic energy of the gas, directly relating it the density and temperature of the gas. Keep in mind that static temperature is entirely a measure of the kinetic energy of the molecules as observed from the reference frame of the fluid. My point in all this is to say that the static pressure of a fluid is much more than the article seems to be making it out to be. It is a direct measure of the force the fluid applies to its surroundings (whether that is other fluid or a surface). This defines all aerodynamics through Mach=5. It is equivalent to the temperature of the fluid and kinetic energy associated with the microscopic affects of the fluid. So yes, Bernoulli's usage of P_static is entirely 100% factual given the incompressible assumption and it is much more than simply something measured from some reference point in the fluid, as the article states. If anyone wishes to debate this, let me know. Iron_Engineer (talk) 19:54, 1 July 2009 (UTC)

Hi Iron Engineer. Thanks for your comments, and your invitation to debate the subject. Most of what is presently in Static pressure was written by me, and I am keen to remain involved in any improvements and developments of the article.

WP:Verifiability says The threshhold for inclusion in Wikipedia is verifiability, not truth. Consequently, what is important is not what we believe to be true, but what we can verify by posting suitable references and in-line citations. The material presently in Static pressure is fairly well covered by references and in-line citations, so when you write that the article is presently written from the wrong standpoint it doesn't sit comfortably with WP:Verifiability. What references do you propose to use to substantiate any additions you make to present the view of static pressure that you have described above?

I disagree that the article compares static pressure with Bernoulli's equation. The article presents static pressure, dynamic pressure and total pressure as three of the terms in the simplified form of Bernoulli's equation.

You have written that static pressure is the pressure you would measure while moving in the same inertial reference frame as the local fluid. In the case of an aircraft in flight, the fluid is the atmosphere and it is stationary relative to the Earth's surface. Would you say it is possible, or impossible, to have a system in the aircraft to measure static pressure? (The aircraft is moving at speed relative to the reference frame attached to the atmosphere.)

I look forward to discussing this further with you. I'm sure we both have the objective of making Static pressure as good an article as possible. Regards. Dolphin51 (talk) 07:30, 2 July 2009 (UTC)

I have not made any argument concerning a static pressure's measurability. It has long been known that a pitot tube oriented orthogonal to a fluid's motion will measure the static pressure at that point. My entire point rests that static pressure is so much more than Bernoulli's equation. The concept of static pressure has its roots in statistical thermodynamics and its application to Bernoulli's equation flows out of this. My statement about the reference frame of the fluid arises from this. The equations relating static pressure to temperature, kinetic energy of particles, and the interaction it has on the macro scale (ie aerodynamics) can all be directly derived from an atomic level control volume analysis in the reference plane of the fluid.

After rereading the article, I believe my prior wording was a bit harsh. It is a pretty well written article, I just view it as half-complete. From my perspective, this article gives many applications of static pressure without including a through definition of what it actually is and where it comes from. Since static pressure is inherently a measure of atomic level interactions, this can only be achieved by the inclusion of such a description.

Iron_Engineer (talk) 18:37, 2 July 2009 (UTC)

Hi IE. Thanks for your prompt reply. You have written It has long been known that a pitot tube oriented orthogonal to a fluid's motion will measure the static pressure at that point. No. A pitot tube orthogonal to a fluid's motion measures pitot pressure, also known as stagnation pressure, ram pressure or total pressure.

I can accept that you were introduced to static pressure via statistical thermodynamics. I doubt you can verify that statistical thermodynamics is the origin of the term, and that all other applications are less than correct. I was introduced to static pressure via fluid dynamics and aircraft design. (All aircraft have at least one static port. It is a simple hole in the surface of an aircraft's fuselage, and the hole is connected by conduits to the altimeter and airspeed indicator.)

I agree that many things can be written about static pressure, all of it correct. But that doesn't mean all these things have to be in the first sentence of the introductory paragraph. WP:MTAA instructs that technical articles should be accessible to as wide an audience as possible. The article should begin with a simple introduction that is widely accessible to all readers, including children. Later in the article is the place for the PhD candidates to add their information about more complex notions.

Let's keep the discussion going. The objective is to make this article as good as possible. Dolphin51 (talk) 02:52, 3 July 2009 (UTC)

Now we're just talking semantics regarding the pitot tube. If a flow is moving to the right and the pitot tube is oriented pointing upwards at a wall (orthogonal to the flow), it measures static pressure. I'm confident we both know the basic fluid mechanics surrounding pitot tubes and static pressure and I'm of the opinion any thought otherwise is arising from simple miscommunication (what you say is orthogonal I would have called in line or parallel with the flow).

I was never implying altering the lead paragraph. I completely agree this fundamental description I would like to create is of a overly technical information for lead paragraph of something that is used in so many various disciplines and areas of interest. It is just my opinion that giving the fundamentalist description of this fluid property somewhere in the article is essential for a completely thorough encyclopedia article. This like you say would be primarily aimed at people who have studied gas dynamics, a genre that I suspect most people who have not entered graduate school for engineering or physics would not have encountered. Like I said before, I really do like the tone of the article as is and have no intention of changing what is there, I simply envision creating a new section detailing a more rigorous description from a fundamentalist standpoint. An example of what I mean can be found in the article temperature under theoretical foundation.128.6.73.200 (talk) 18:22, 3 July 2009 (UTC)

I assume the above entry by 128.6.73.200 was made by Iron Engineer. (Don't forget to use four tildes to sign your additions to Talk pages.) If we both know the basic fluid mechanics surrounding pitot tubes let's agree they are not used to measure static pressure - they are used to measure pitot pressure. We both know the difference between these two pressures, and we know the difference is more than semantics.

I apologise for my comments about putting all things in the first sentence of the opening paragraph. I was mistakenly assuming you had edited the first sentence of Static pressure when in fact Iron Engineer edited the first sentence of Stagnation temperature. My mistake.

If you know of some valuable information about static pressure, and have suitable references to support your information, please add it to Static pressure. Choose the most suitable place in the article - if it is an advanced approach to the subject put it somewhere towards the end. Support it with in-line citations so it can independently verified and you will get no resistance from me. Happy editing. Dolphin51 (talk) 00:54, 4 July 2009 (UTC)

Static fluid pressure

This bears reference to the topic "Static Fluid Pressure". My question to the members is:

When we say Pitot-tube pressure we mean Dynamic Pressure and when we say Static pressure we are reffering to simple hole drilled on the surface of the conduit (say pipe) opening in to a tube open to atmosphere and long enough to hold the water thrown by STATIC Head!

Now if instead of the Water we assume Steam (560 deg.C, 160 bar(a)) as being the flowing media and the small hole be replaced with a "Tee" followed by a very long topsy turvy Piping closed with a valve at the other end (to visualise, assume simple By-pass connection of Main steam supply pipe)

My question arises here as follows :

1. Will the fluid, flowing into the branching pipe, have Static Pressure only i.e; to say Elevation Head plus the fluid pressure corresponding to its pressure at that temp?

2. Since the fluid is being stopped at the end of its travel (assuming branch run long enough than the Dynamic head), would the pressure and temp in the branch be termed as Stagnation Temp & Pressure? If so, the stagnation pressure and temp. should come in to picture in the branch line only after the Dynamic head length has passed. This again is perplexing as to how in a particular conduit one can have two pressures!!, though at a distance?

3. Also, if the steam is flowing in the main line (continuously consumed by Turbine), and the bypass line (branch) is closed at the other end. Wouldn't the Fast flowing steam in Main pipe try to SUCK-in the fluid (steam) in branch. (The principle on which Gas stove works viz; Fast flowing Gas sucks the air in through the holes provided in the gas supply tube)??

Kindly shed some light as to how to picturise the phenomenon.

regards, —Preceding unsigned comment added by Pipeyoga011 (talk • contribs) 19:21, 6 September 2010

When we say Pitot-tube pressure we mean Stagnation pressure. If changes in elevation are so small they can be ignored, stagnation pressure is equal to static pressure plus dynamic pressure.

1. Yes, the fluid flowing into the branching pipe will have static pressure only. Stagnation pressure only arises from a Pitot-tube (that is, a tube facing forwards into the oncoming flow.)

2. No, the fluid in the branch would not be at stagnation temperature or stagnation pressure. This is because the branch pipe joins the main steam line at a simple opening in the side of the steam line, not at a Pitot-tube.

3. No, the fast-flowing steam would not try to suck steam out of the branch line. That is because the steam in the main line, and in the branch, are both at static pressure.

You are welcome to ask questions of a scientific nature at the Science Reference Desk. Talk pages, like this one, are primarily for discussion aimed at improving the article.

Best regards, Dolphin (t) 00:41, 7 September 2010 (UTC)

Thanks Dolphin51 for your reply. I shall try to go through the contents and onwards put such queries over SIENCE REFERENCE DESK ! Regards, pipeyoga —Preceding unsigned comment added by Pipeyoga011 (talk • contribs) 18:12, 16 September 2010 (UTC)

Static pressure is for fluid not in motion?

While studying the forces on sails, I came across the term 'static pressure' without a proper explanation, so I looked it up here. I assumed that it is the pressure the fluid (in this case the air) would have if not in motion (so in this case atmospheric pressure). The article seems to want to say this too, but it doesn't. But is it is so simple, then why doesn't it simply say it? Or am I wrong? DirkvdM (talk) 17:17, 8 August 2011 (UTC)

You are wrong. It is not correct to say static pressure is the pressure a fluid would have if it were not in motion. Static pressure is a very valid concept even for fluids in high speed motion, or for bodies (such as aircraft) moving at high speed through the atmosphere. The static pressure of the atmosphere adjacent to an aircraft is fed to the aircraft's instruments via the static system and is used to drive the altimeter to indicate the aircraft's altitude. Static pressure and total pressure are also fed to the airspeed indicator to indicate the aircraft's airspeed.

In fluid dynamics it is necessary to have the concepts of dynamic pressure and total pressure. In fluid statics the concept of pressure is all that is necessary but in fluid dynamics it is necessary to distinguish between pressure and dynamic pressure and total pressure, so the word static is appended as a prefix. If the expression static pressure is unfamiliar, the easiest thing to do is to ignore the word static and read only the word pressure (which is familiar to most people.)

In fluid statics the pressure of a fluid can be measured using a gauge such as an aneroid, column of mercury, bourdon tube etc. Pressure increases with increasing depth in the fluid. In contrast, dynamic pressure can't be measured with a gauge. Dynamic pressure doesn't vary with depth in the body of a fluid. Dynamic pressure depends on the choice of reference frame, just as kinetic energy depends on the choice of reference frame. Total pressure is the arithmetic sum of static pressure and dynamic pressure so it also depends on choice of reference frame and can't be measured with a gauge.

In summary, static pressure is the pressure that can be measured using a gauge such as an aneroid, column of mercury, bourdon tube etc. It is different to dynamic pressure and total pressure which depend on the choice of reference frame and can't be measured using a gauge such as an aneroid etc. Dolphin (t) 22:24, 8 August 2011 (UTC)

DirkvdM, the question was debated at length a while ago. Your notion of static pressure is the most logical one and coincides with mine, learnt at uni, but authors appear to use almost universally static pressure as a misnomer for simply pressure (including possibly the author of your books on sails). My notion, coherent with the use of the term static, is:

• Pressure (or Fluid Pressure, if you want to use two words) = force per unit area exerted by a fluid (regardless of whether at rest or in motion).
• Static Pressure = pressure of the fluid (as above) far upstream of the body (a.k.a. free stream pressure), which coincides with the atmospheric pressure, or the fluid pressure exerted on the body at rest (hence static).

I don't write books though, so I don't establish usage rules on WP. --Giuliopp (talk) 02:34, 9 August 2011 (UTC)

Thanks for your responses. I really want to understand this because it is essential to my present study.
It seems to me that all three alternatives, 'static pressure', 'free stream pressure' and simply 'pressure' are confusing. The fluid doesn't have to be static. 'Free stream pressure' covers that, but it is incomplete because it also existst (and is then different) when the stream is no longer free. And simply 'pressure' I would interpret as 'total pressure'. Since 'static pressure' is the most widely used and doesn't have an alternative meaning (*), I suppose that is the best choice (the lesser of three evils :) ).
(*) That is, assuming that for fluid statics it doesn't matter if the fluid is in motion, just that a flow, if any, is homogenous and can therefore be considered internally static, even if in a larger scope it isn't. So the pressure of a fluid that is static (relative to what?) is a non-issue.
This is how I now understand it:

When a fluid, either static or in homogenous motion, is not influenced by the presence of an object in relative motion it only has static pressure. This is also called free stream pressure, but note that the stream velocity may be zero. If the fluid flows past an object or an object moves through it (or both), the fluid is deflected by it (up to some distance from the object, beside, in front or behind), which creates a dynamic pressure. Because the total pressure, which is the sum of static and dynamic pressure, has to remain constant, the static pressure then decreases.

Is that about right?
A few more issues:
1. A further explanation of what causes this dynamic pressure would be helpful. As I understand it, that is either, when the object moves through it, because it compresses the fluid, or, when it flows past an object, it has less space to move through, so it has to speed up. But the two effects are the same, right? But then I can't think of a good way to formulate it.
2. What if the flow is not homogenous? I suppose that is the case for the fluid some distance from the object, but still deflected by it. That would then be how the pressure propagates throuh the fluid, with the streamlines acting as deflecting objects upon each other (with the total force being spread over a larger area, so the dynamic pressure dropping and the static pressure rising). And what about colliding ocean currents? They will merge to some extent, complicating matters tremendously, I assume.
3. Is the fact that the total pressure is constant a case of conservation of energy?
4. Is the drop in static pressure the same as the underpressure created by a flowing fluid? That is a concept many people, including myself, will be more familiar with, so if correct it would be helpful addition to the article.
5. Can one say that static pressure works perpendicular to the object's surface (or rather the streamlines), and dynamic pressure tangential to it? I know pressure is not a vector (so should I say a force (potentially) caused by the pressure?), but looking at it this way would nicely explain how the transferral of static to dynamic pressure would create a lift-force on a sail or wing. And it would give us some nice terminology: normal pressure and tangential pressure. But of course we can't make up our own terminology, however good it may be. :) DirkvdM (talk) 09:21, 14 August 2011 (UTC)

DirkvdM, you are making a big deal of confusion, I'm afraid. In loose order: the idea of dynamic pressure acting as a "tangential pressure" makes no sense. Pressure is one thing, shear stress (from viscosity) is another. As Dolphin said, dynamic pressure is strictly the quantity 1/2 ρ v² and cannot be measured; the body doesn't sense it on its surface. What the body senses is simply the pressure (called by most authors static pressure). A body moving into a fluid does not "create a dynamic pressure", it alters the pressure field, which would otherwise have the same value of pressure everywhere (the free stream pressure or, as I call it, static pressure - neglecting the vertical variation of pressure due to gravity). This is just off the top of my head, I might come back later. --Giuliopp (talk) 10:26, 14 August 2011 (UTC)

@DirkvdM: You asked Is the fact that the total pressure is constant a case of conservation of energy? Answer: Yes!

In mechanics we have the principle of conservation of mechanical energy. (Mechanical energy is defined as the sum of the kinetic energy and the gravitational potential energy.) Providing the only forces acting on a body are conservative forces - gravity or springs - the total mechanical energy remains constant. For example, if you throw a ball into the air, as it goes up its kinetic energy falls but its potential energy increases so the total mechanical energy remains constant. The same thing happens as it falls. The principle of conservation of mechanical energy also applies to the flow of inviscid fluids but when applied to a fluid that principle is called Bernoulli's theorem. The kinetic energy of one unit of volume of fluid is given by its dynamic pressure. The potential energy of one unit of volume is given by its pressure, or static pressure. The total mechanical energy of one unit of volume is given by its total pressure.

Principle of conservation of mechanical energy:

potential energy + kinetic energy = mechanical energy

Bernoulli's theorem:

static pressure + dynamic pressure = total pressure

As you can see, it would be confusing to call the potential energy simply the pressure because that fails to distinguish between the three pressures mentioned in Bernoulli's theorem. So most authors avoid the ambiguity by using the expression static pressure to mean pressure, the one that can be measured with a gauge.

In fluid dynamics the concept of static pressure exists so we can make sense of Bernoulli's theorem. When we are not dealing with Bernoulli's theorem, such as when studying a book on sailing boats, it is probably unnecessary to use the expression static pressure - just use pressure instead.

Giuliopp has given an alternative explanation of the meaning of static pressure - that it is the pressure far upstream of a body, or the pressure that would surround a body if the body was not moving. I think this explanation may come from the aeronautical application of the expression. Aircraft have a static pressure system that is open to the atmosphere and conveys this pressure to the altimeter (and the airspeed indicator). The objective of an aircraft's static system is to sense the pressure of the atmosphere at the altitude at which the aircraft is flying. In the aeronautical context, static pressure is indeed an attempt to measure the pressure far upstream of the aircraft. However, there is a pressure field around any aircraft in flight and that pressure field distorts the local pressures so that the pressure in the aircraft's static system is not exactly the same as the pressure far upstream of the aircraft. The fact that these two pressures are not the same means the aircraft's altimeter and airspeed indicator are subject to an error - the position error - due to the position of the static port in the pressure field surrounding the aircraft. The position error exists because the aircraft is moving, is supporting its weight in flight, and is creating a pressure field around the aircraft. I can imagine people trying to clarify the situation by saying the pressure in the static system would be equal to the atmospheric pressure at the altitude at which the aircraft is flying if the aircraft was stationary and position error was zero. Dolphin (t) 11:33, 14 August 2011 (UTC)

Ah, that analogy between the components of mechanical energy and total pressure is really hepful. Thanks Dolphin! That conservation of energy is the reason for total pressure being constant should definitely go into the article. But also the analogy? How far can one take that? In the book I'm reading ('Sail Performance' by C.A. Marchaj), the author writes "... kinetic energy [...] is usually called the dynamic pressure ...", which I thought was such nonsense (energy is not pressure) that I sort of skipped that bit. Maybe I should have given it more thought.
That dynamic pressure can't be measured still bugs me. Well, of course it can, by using a pitot tube and substracting the static pressure from the total pressure. But if there is a pressure, it should be possible for it to work on something, and thus be measured. Giuliopp, you say the body doesn't sense it on its surface. Anywhere? Then the pressure doesn't work in any direction? (Whereas the static pressure is equally strong in all directions, I assume.) This sounds a bit like dark energy in cosmology, which can't be measured and is thought up to explain the accelerating expansion of the universe. Likewise, you know the total pressure has to remain constant, but still you see a drop in pressure. so there has to be some pressure you can't measure. 'Dark pressure'? :) DirkvdM (talk) 17:42, 14 August 2011 (UTC)

@DirkvdM: It is fairly simple to convince yourself that dynamic pressure is the kinetic energy of one unit of volume. Energy, including kinetic energy, is specified in joules, and volume is specified in cubic metres. So the kinetic energy of each unit of volume is specified in joules per cubic metre. One joule is equivalent to one newton.metre so the kinetic energy of each unit of volume can be specified in newton.metres per cubic metre or newtons per square metre. One newton per square metre is a measure of pressure (or stress) and is equivalent to one pascal which is the SI unit of pressure. You can see that dynamic pressure is the kinetic energy of each unit of volume of fluid; static pressure is the potential energy of each unit of volume, and total pressure is the mechanical energy of each unit of volume.

The reason I say dynamic pressure and total pressure can’t be measured is probably rather obscure. Kinetic energy, dynamic pressure, mechanical energy and total pressure are all dependent on the choice of reference frame. (I am sitting at my computer and I can take the view that my speed is zero so my kinetic energy is also zero. Alternatively I can choose an inertial reference frame fixed to the center of the Earth, take the view that my speed is 465 m.s^-1 times the cosine of my latitude and calculate my kinetic energy with the usual formula. In doing so I haven’t measured my kinetic energy – I have selected an inertial reference frame, measured my speed and calculated my kinetic energy.) It is possible to measure a unique value for forces, masses and accelerations; but displacements and speeds (and velocities) are dependent on the choice of reference frame so it isn't possible to measure a unique value for any displacement or speed. See Galilean invariance. (Prior to Galileo, people tried to determine the absolute velocity of the Earth. Galileo showed that there is no such thing as an absolute velocity. If we knew the mass of a body or the density of a fluid, and we could measure the kinetic energy of that body, or the dynamic pressure of that fluid, we could determine its absolute velocity. Galileo convinced the scientific community it couldn't be done.)

For the same reasoning I say we can’t measure dynamic pressure directly. First we select a reference frame and measure the speed of the fluid relative to that reference frame. Then we use the half rho speed squared formula to calculate dynamic pressure. If we select a different reference frame we measure a different speed and end up with a different dynamic pressure.

Strictly speaking, a pitot tube doesn’t measure total pressure – it brings the fluid to a stop and measures the stagnation pressure. Stagnation pressure is the static pressure in a region where the fluid speed is zero - such as inside a pitot tube. It is equal to total pressure in one reference frame only.

We measure static pressure using a gauge, and we measure speed using a pitot-static system or any one of a number of devices. Once we have selected a reference frame we can determine the speed and calculate dynamic pressure and total pressure. The important conclusion for the purpose of this discussion is that if we plug a mercury column or aneroid or bourdon tube into a body of fluid we see the pressure (static pressure) of that fluid. We don’t see dynamic pressure or total pressure. In the special case of a reference frame attached to the body of fluid, the speed of the fluid is zero, its dynamic pressure is zero and total pressure is equal to static pressure. You can argue that in that situation total pressure can be measured using the mercury column, aneroid or bourdon tube, but that is a coincidence that only occurs with that one reference frame. Dolphin (t) 23:32, 14 August 2011 (UTC)

Ah yes, the depence on the reference frame I get, it just had to sink in. If you measure total, static, and as a result dynamic pressure, the latter two have the pitot tube as their frame of reference. But Giuliopp said that the body doesn't sense the dynamic pressure on its surface. Which I interpreted as meaning the dynamic pressure doesn't excert a force in any direction. But of course it does, namely on the tip of the pitot tube (for example), as a component of the total pressure.

You say "Stagnation pressure is the static pressure in a region where the fluid speed is zero". Zero with the pitot tube as a reference frame, of course. And because the fluid stagnates within that reference frame, you only get static pressure. Right, you can only measure static pressure, and to 'measure' dynamic pressure you have to convert it to static pressure by stopping the flow.

It's still sinking in. :) Being a bit of a philosopher of science (see my user page) I'm working on an alternative view. Still very crude, but here it is: There's just pressure, just one kind, which is a foce that at any point works equally strong in all directions and therefore has no direction and is not a force really. If there is an object in it, that creates a boundary to the fluid and therefore a force. But that force is again equal in all directions. Unless the body has pysical dimensions (as all real bodies do) and there is a pressure gradient (isn't there always?), caused by a force working on it. Such as the earth pulling on the air. Then, with the air as 'middleman', that force is transferred to the object, pushing it upward (this is where Archimedes comes in). But if the object moves, it causes an extra gradient (fore-aft). And its shape (eg wing) may cause another gradient (up-down). Now all these forces excert pressure. But this pressure has a direction and is therefore not really a pressure, but a force. Still working on this ... :) But I now find myself calling static pressure simply pressure, so maybe this is sort of what you mean to tell me? DirkvdM (talk) 11:02, 15 August 2011 (UTC)

I agree that pressure applies equally in all directions.

Here is another idea. Imagine you want to write an introductory text book on fluid dynamics. You want to present Bernoulli's theorem without the added complexity of dynamic pressure and total pressure. That can be done simply by using the following equation:

pressure + ${\tfrac {1}{2}}\rho v^{2}+\rho gh$ = constant

or, if changes in elevation can be ignored the ρgh term can be omitted. Dolphin (t) 12:32, 15 August 2011 (UTC)

Ah, but there the problem is back. ½ρv² in units is kg/m³ x m²/s² = kg/ms², which is the unit of pressure. That is, I work on the assumption that when you have the same (derived) unit you are dealing with the same thing (although I have found that not everyone agrees with that). So this is pressure. So my idea falls flat on its face. Or does it? Work in progress. :) DirkvdM (talk) 16:50, 15 August 2011 (UTC)

DirkvdM, you lost me on your "alternative view", the 'traditional view' sounds much simpler to me. Your assumption that "when you have the same (derived) unit you are dealing with the same thing" is incorrect. ½ρv² is dimensionally a pressure but - unlike the fluid pressure - does not physically represent any force acting on any surface. It really is just that: the product of density and square of velocity, which happens to have the same dimensions as the fluid pressure (= the force inside a fluid per unit area) and be a term of Bernoulli's equation (which in turn allows you to deduce - not measure - the value of dynamic pressure itself). --Giuliopp (talk) 18:32, 15 August 2011 (UTC)

Well there is the problem. If it looks like cheese and smells like cheese and tastes like cheese ... it's not necessarily cheese. Maybe processed cheese, which some claim is not really cheese. But we're dealing with physics here, hard science. That's more like people claiming synthetic vitamins aren't real. But if the chemical formula is the same then it's the same stuff - assuming chemistry is as hard a science as it claims to be, because if it were different then there would be something missing in chemistry. Same here. The formula fully describes what it is, right? So if the formula comes out wiht the same unit it has to be the same thing. It has to be pressure.

But then there is the problem that it doesn't excert any force when it hits a surface. Or is that the problem? That it doesn't hit a surface? Maybe because by its nature it works parallel to the surface. But then it would have to have a direction, and pressure is not supposed to have direction. Or maybe that is usually the case, but not here. After all, it only occurs when there is (relative) movement. If it stems from something that inherently has a direction, then it also must have direction?

Or what about this. Conservation of energy doesn't necessarily mean conservation of pressure. Pressure is one way energy can manifest itself (is that right?). And one form of energy can change to another. Maybe the formula just describes the pressure that is lost. To something else. A force that works in a direction parallel to the surface that moves through the fluid (or vice versa). And then you can't measure it (directly) because any measuring device will itself have a surface that deflects the fluid and makes the force work parallel to that surface. (Maybe if you move it into the flow really really fast ... :) )

New snag: if something exists you have to be able to measure it, because how else can you claim it exists? Well, you can't measure it directly (and does direct measurement even exist in the first place?). But you can change it into something you can measure, namely pressure, by stopping the flow (the thing that makes it unmeasurable). Such as at the tip of a pitot tube. All you need to do then is measure the pressure in the flow (the other hole) and the difference is the lost pressure. To find the force you simply have to multiply it by the area (which involves integration because there will be a gradient, but that's no problem).

In short:

Relative movement between a fluid and an object results in underpressure because some pressure is lost because its energy has been transferred to a force that by its nature works in a direction parallel to the object's surface.

That sounds satisfactory to me (for now). DirkvdM (talk) 07:29, 16 August 2011 (UTC)

If that satisfies you that's good, as long as you don't put it in the article :) You say that "if the formula comes out wiht the same unit it has to be the same thing. It has to be pressure". That conclusion is completely arbitrary and I'll leave this discussion giving you another example: work and torque have both in the same unit measure (Newtons times meters) but express two completely different physical quantities. One represents energy exchanged by a system, the other one expresses how strongly a force can make a body rotate around an axis (even when no energy is exchanged at all). How about that? Regards. --Giuliopp (talk) 13:20, 16 August 2011 (UTC)

Ah yes, I was aware of that problem. But I haven't figured it out satisfactorily yet and it's not relevant here, so I conveniently forgot about it. The thing is that the relationship between the force and the distance is different. In the case of work (which is just plain energy, except that it says something about the energy) it is a distance in the direction of the force and in case of moment (or torque) it's a distance perpendicular to the force.

However, that is not relevant here, because there is only one direction, of the wind (or whatever).

Getting back on topic, near a solid surface the wind can only be parallel to it, and wind can only excert a force in the direction it's going, so it can never excert a force on anything. Ehm ... my brain is telling me it's too late to think, so I'll leave it at that for now. :) DirkvdM (talk) 18:32, 16 August 2011 (UTC)

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The article now provides an actual definition

Hey dolphin51 and Giuliopp, judging from now-archived entries in this talk page, both of you at least were interested in this article. So I wanted to let you know I've now stuck my own toe into the water. The lead really needed a definition, so I've ventured one. Then, while I was there, I also did some stylistic twiddling to improve clarity and ease of use. I hope the heavens don't come crashing down on me.—PaulTanenbaum (talk) 18:30, 3 May 2022 (UTC)

Hi Paul. Thanks for the ping. I have looked at your new version of the lead paragraph but I disapprove for a few reasons. Static pressure is a specialist sub-set of the concept of pressure so if we present a definition of static pressure it must be consistent with the definition of pressure; your definition doesn’t do that. You define static pressure in terms of random motion of molecules. Our lead in the article “Pressure” doesn’t mention molecules so the two leads aren’t consistent.

The concepts of pressure and static pressure are simple enough that they can be understood by most people, even those who don’t yet have an understanding of molecules. MOS:LEAD explains that the lead should provide a gentle introduction to the greatest number of readers; it shouldn’t deter people simply because they don’t recognise the concept of molecules.

WP:Make technical articles understandable is relevant and must be reflected in this article, and especially its lead. Dolphin (t) 23:08, 3 May 2022 (UTC)

A week has passed without reply or acknowledgement of my post so I will revert the lead to the status it has had for the past 14 years.

Deeday-UK and I found three different applications of the expression static pressure. Each application has a meaning sufficiently different to the other two that a single definition, common to all three, is not possible. Physical concepts, including static pressure, can be described and explained but are rarely defined in the same way as mathematical concepts. Our article should clarify and demystify the three concepts of static pressure but applying a formal definition doesn’t seem to offer much clarification of the three different applications. Dolphin (t) 09:38, 11 May 2022 (UTC)

I have made the change. See my diff. Dolphin (t) 10:01, 11 May 2022 (UTC)

(Sorry Dolphin, I didn't mean to ghost you: somehow I hadn't tracked that you'd been editing either the talk page or the article.)

Anyway, I agree that leads are not the place for detailed technical definitions—indeed, when I made my edits I hesitated to use the word definition. And I'm certainly not wedded to the version of the lead that I created. But I do have serious problems with the (long-standing) version that you restored:

• The first of its three bullets is circular, reading much like, "In the design of computer networks, intrusion detection is the detection in the network's intrusion detection system."
• The other two bullets are commentary not about static pressure, but about the term "static pressure": who uses it and how.
• Because it has no other content than that bullet list, it has something of the feeling of a disambiguation page.
• In particular, it contains zero material that applies to all three of those bulleted uses of the term.

Here are some characteristics that I think a good lead for this article would have.

1. In at least its first sentence there should be discussion of the phenomenon, not of the term for it.
2. There should be some explanation of what it is about the phenomenon that's so static, perhaps even with an explicit contrasting with any related phenomena that are usefully seen as dynamic.

I'm confident that we can develop something that scratches your aims and mine.—PaulTanenbaum (talk) 17:07, 11 May 2022 (UTC)

Static pressure is a sub-set of pressure for use by certain specialists and authors in specialist fields. A link to pressure is provided in the second bullet; perhaps the link should be elevated to the opening sentence?

The expression static pressure is used to distinguish between it and other pressures including dynamic pressure, total pressure, stagnation pressure. All are subsets of pressure and must conform to any explanation and description provided for pressure. My expectation is that any person reading any of the articles about these 3 specialist pressures will already be familiar with the over-arching concept of pressure. Dolphin (t) 22:59, 11 May 2022 (UTC)

I got it, static pressure is a variety of pressure. But the article should specify which variety. I got it, the term is used to distinguish this variety of pressure from various other varieties. But the article should explain precisely what static pressure is.— PaulTanenbaum (talk) 21:37, 16 May 2022 (UTC)