What is Pressure: Definition and 1000 Discussions

Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled gage pressure) is the pressure relative to the ambient pressure.
Various units are used to express pressure. Some of these derive from a unit of force divided by a unit of area; the SI unit of pressure, the pascal (Pa), for example, is one newton per square metre (N/m2); similarly, the pound-force per square inch (psi) is the traditional unit of pressure in the imperial and U.S. customary systems. Pressure may also be expressed in terms of standard atmospheric pressure; the atmosphere (atm) is equal to this pressure, and the torr is defined as 1⁄760 of this. Manometric units such as the centimetre of water, millimetre of mercury, and inch of mercury are used to express pressures in terms of the height of column of a particular fluid in a manometer.

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  1. blinkz303

    How to calculate the leak rate based on pressure drop

    I am currently working on a project trying to calculate leak rate and fluid loss from pressurised vessels filled with water over a time period of 30 minutes. So far I have used the Boyles Law which assumes that PV/T = constant which gets us P1 * V1 = P2 * V2 and PdV+VdP = 0 I formed this over to...
  2. T

    Gas law problem (changing volume, temperature and pressure)

    I can’t quite work my head around this question, I am having a difficult time analyzing the question, I can’t seem to make out what the initial and final conditions are would appreciate all the help I could get cheers
  3. A

    Fluid mechanics - Additional liquid capacity due to compression

    A cylindrical tube (diameter = D, width = L) is completely filled with a liquid (density = ρ). A pump pressurizes the system with a pressure P. Consequently, 1) the solid tube is compressed and deformed according to Hooke's law (σ = ε.E), and 2) the liquid is compressed and deformed, following...
  4. E

    B Pressure through a curved hose - expected results?

    Hello, I am doing some quick experiments testing how pressurized air behaves when being ejected from a straight vs. curved object, and I am wondering if this is the expected behavior and a possibly quick explanation of why this would be behaving this way... I am using the model below and...
  5. K

    Is the pressure everywhere on the inflated fluid pad the same?

    I guess the pressure would reach equilibrium after the weight is stable, that is when points X and Y are stable. So, the answer should be A?
  6. soothsayer

    Gas conductance through tube at low pressure

    I have a vacuum chamber of volume V connected to a vacuum pump via a tube of radius R and length L, and I want to design this "tube" such that R can be controlled in order to provide a constant rate of pressure change inside of the chamber of 2.5 mbar/sec, from atmosphere down to ~0.5mbar...
  7. K

    Measuring Pressure on a Concrete Table

    Hi guys, I am not doing homework, it is just suddenly brainstormed this question. What if I place a very small mass like 10kg on the center of a concrete 10m*10m flat table supported by the ground, then how to measure the surface pressure acting on the center and right on the edge? Or the...
  8. T

    How does tire pressure affect distance traveled by a bicycle coasting to a stop?

    Hi, I have been wondering if there is some equation which will allow me to calculate the distance and, or time, necessary for my bike to come to a complete stop knowing only the initial speed and tire pressure, without applying brakes and with negligible air resistance?
  9. J

    Question regarding water pressure and slopes

    I have a question about water pressure PSI regarding the presents/lack of slopes. If two identical pressure sensors are set up on the ocean floor at the same depth. One being surrounded by a cone of ramps increasing in height moving outwards away from it. The second sitting on a flat plain...
  10. Jenq

    A Pressure balance in a sphere

    Hi A sphere with radius r is buried in an elastic ideal medium at pressure P1. Inside the sphere I use energy E to create a variation of pressure of dP. What variation dPx I would measure if the sphere was buried in a medium at pressure P2 using the same energy E? Is it possible to solve...
  11. theycallmevirgo

    Intro text on building a small fluid pressure chamber?

    As stated in the summary, I'd like to design a chamber to store water under pressure. Nothing too powerful - basically at the level of a super soaker, maybe a little higher. The trick is, I want it to be powered and controlled electronically. But, I come from electronics land and know nothing...
  12. FEAnalyst

    Stress in rectangular vessel subjected to hydrostatic pressure

    Hi, it's not easy to find formulas for stress in rectangular pressure vessels. However, I've found some in a Polish book titled "Podstawy konstrukcji aparatury chemicznej" (Fundamentals of the design of chemical process equipment) by J. Pikon. The problem is that the book provides equations for...
  13. iQadmat

    Intensity of two sound pressure waves

  14. M

    A Model the pressure of a zero-gravity simple fluid system

    Hi PF! A circular cylinder contains an inviscid liquid, which is softly disturbed (i.e. velocities are small). There are no body forces, which implies the equilibrium free surface is that of a spherical cap with radius ##R## (see figure A). Inviscid implies we can assume potential flow, such...
  15. brotherbobby

    Pressure higher up in the atmosphere

    Attempt : 1. Pressure at height ##z## : ##P(z) = P_{\text{atm}}-\rho_0 gz ##, ignoring density variation. But actually, we have ##\rho(z)<\rho_0\;\forall z>0##. Hence, we are subtracting a bigger value from ##P_{\text{atm}}## than we actually should, meaning that we would end up what a smaller...
  16. jaumzaum

    I Why does hot air have low pressure? (wind formation)

    I was studying wind formation from the continent to the sea during day (direct circulation) and the explanation I found in almost all the textbooks was that: 1. In the continent during day the ground becomes hotter 2. The particles in the air increases their temperatures due to induction 3. When...
  17. D

    I Questions about Negative Pressure and Vacuum Energy

    Currently, dark energy is described as a being that exerts a negative pressure while having a positive energy density. {\rho _\Lambda } + 3{P_\Lambda } = {\rho _\Lambda } + 3( - {\rho _\Lambda }) = - 2{\rho _\Lambda } However, there seems to be a problem with the negative pressure assertion...
  18. H

    G-force to pressure relationship inside a shock wave

    I need some help understanding shock waves, particularly the units of measure related to their pressure. Shock waves are frequently quantified as multiples of G, the gravitational constant. I need to understand how the G measurements related to the instantaneous pressure within the wave as...
  19. Cmotaval

    Pressure of water exiting a hole in PVC

    I have a 1" PVC pipe that is 36" long, closed on one end and hooked up to a pump on the other. The pipe has 10 holes each measuring 1/32" in diameter and I have been trying to use Bernoullis equation to find the pressure of the water exiting those holes. I believe the volume of water that will...
  20. E

    I Filling a rigid tire from a pressure regulated compressed air line

    This thought came to me while I was trying to fill my MTB bike tire a while ago as an exercise in laziness to not "check" the tire pressure with a separate gauge! Mostly, my mind just wanders... If I have some very large tank filled with compressed air at gauge pressure Ptank. The hose runs...
  21. wruehl1

    Calculating fill rate if pressure delta, volume, and time are known?

    I am looking for an equation that I can use to compute L/min or mL/min for a 480cc vessel going from 150bar to 250bar with a fill time of 6min. Sensors for flow rate at these pressures are hard to find, but I thought there might be a way to work this out with the parameters known. An equation...
  22. N

    Pressure drop through volumes of compressible fluid

    Thanks for your response. I'm trying to estimate the pressure in the cavity, i.e. P2. I know the areas, A1 which in practicality is an annulus and A2 which is a 600m pipe with i/d 8mm. I would like to initially understand how to calculate the pressure drop across the annulus. At the moment I'm...
  23. S

    I Does pressure affect the thermal breakdown of water?

    As per the title, I'm trying to research how the thermal breakdown (thermolysis) of water works. I gather that you need circa 3000°C to get it to break down, which is a lot, and I'm curious as to how pressure might affect this. I know that pressure affects the boiling point of water, and as...
  24. L

    Hazard levels for pressure vessels with internal bladders

    Hi , I am based in Australia and my question originates from some grey areas in the Australian system of classifying pressure vessels. I am guessing most users on this forum are US based . Does the US have a system for classifying pressure vessels such as air receivers , boilers, autoclaves...
  25. J

    I Newton's second law and pressure wave propagation

    Imagine a long deformable rod which has just been hammered on the top end (the bottom end is clamped to Earth). Consider a time interval $dt = t_{2} - t_{1}$ in which the pressure wave is traveling somewhere within the length of the rod (meaning some portion of the object has already "felt" the...
  26. S

    Hospital Air Handler, Discharge Airflow vs. Static Pressure

    We have a new AHU installed for a Compounding Pharmacy. The unit has two return fans and two supply fans. They are installed in parallel and are powered by VFDs. In the event of one fan failing, the other fan is supposed to speed up to maintain static pressure in the clean environment. This...
  27. PaxFinnica96

    Engineering Fluid Dynamics: Proof of the Static Pressure Head equation

    I am trying to mathematically prove the Static Pressure Head equation: H = p/ρg How can I prove this equation and thus determine the nature of the relationship between these variables?
  28. F

    Pressure in branching pipes when diameters are not equal

    Electrical engineer here hi! I'm little bit out of my comfort zone trying to figure out the following fluid mechanics problem. I have a branching pipe similar to schematic below... ...and I'm trying to find the pressures in branches 1 (p1) and 2 (p2). The d1 and d2 are not equal (d1 = 0.1*d...
  29. M

    Density versus Pressure -- Balancing the bouyancy of a floating capsule

    [Mentors’ note: no template as this thread had initially been misplaced in the technical forums and was moved here] Summary:: Enclosed Cubic foot capsule passing between two bodies of different densities but questioned pressure. You have a tub of fresh water 32 feet high sharing a wall with...
  30. H

    Engineering Hydrostatic Pressure of Ball Floating in Liquid

    Let's say there's an object floating in liquid. The depth of the object below the surface is 𝑑 and its height above the water is ℎ. The density of the ball is 𝑝𝑏 and the density of the liquid is 𝑝𝐿. The pressure above the water is 𝑃𝑎𝑡𝑚. I understand this situation when only the density of the...
  31. NTL2009

    In Search of: Cheap/common pressure switch

    I'm certain these must be common in automotive applications, but I'm just not sure what search terms to use. I'm looking for an on/off switch (to be electronically sensed, so low volt/current) that can be connected to a water pipe, and turn on/off at around 1~2 PSI. The specific application...
  32. G

    Choosing a Flexible hose for outlet air pressure vessel

    Hi everyone, What should be the considerations to choose the proper hose (max working press) installed at outlet air pressure vessels (11 bar) for industrial purposes? Please remark source\link. Also, I've found a table which lists the max sys operating press for 4 different fluids (fuel, oil...
  33. D

    Full Pressure release valve

    A mechanical pressure release valve opens at pre designed pressure and then automatically closes once pressure drops to a lower point generally around 5 to 10% of the designed release pressure. However I need a mechanical pressure release valve which opens automatically at pre designed pressure...
  34. J

    Fluid mechanics concept help please -- Pressure versus depth

    v
  35. sophiecentaur

    I The meaning and consequences of 'False Pressure'

    I like good espresso coffee and I've owned and used a Pavoni Europiccola lever machine for ages. Not surprisingly there are fora and Facebook groups all over the place, containing all levels of 'knowledge' about science and coffee making. There are two criteria which affect the coffee...
  36. jaumzaum

    Understanding Increase in Peripheral Resistance & Blood Pressure

    Hello guys, can anyone help me to understand why an increase in peripheral resistance causes an increase in the blood pressure? If we assume the equation Delta P = R times Q If we assume Q is constant, the greater the resistance the greater the the decrease in pressure, so the pressure should...
  37. samy4408

    Chemistry Confused by an exercise on the subject of chemical equilibrium

    Hello , i am a little confused about this exercise because we talk about gases reaction and we are asked about the concentrations P.S : i have other questions that depends on your answer . Thanks .
  38. A

    Fluid Heights in an Open U-Tube

    Pressure on side with liquid 1 = ##p_0 + \rho_1gh_1## Pressure on side with liquid 2 = ##p_0 + \rho_2gh_2## The solution sets the two pressures equal. I am confused how the two sides have the same amount of pressure, one would have more than the other.
  39. P

    I Calculate the internal pressure of a tube based on flow rate

    I have pump of unknown flowrate pumping water through a tube of length L and diameter D. I know that the flowrate of water exiting the tube is X mL/min. I would like to know the internal pressure of the tubing, how can I calculate it? This all occurs at sea level elevation.
  40. T

    What is the Real-Life Example of a De Laval Nozzle with Low Exit Pressure?

    In case of De Laval nozzle, the static pressure at the exit can be as low as 1/3rd of the ambient/release pressure. Kindly look at the conditions of operation part of the page. At present, I want to know any real life example (in as much detail as possible) of a De Laval nozzle where the exit...
  41. T

    Low pressure, high velocity flow vs static pressure

    In this scenario, we have a compressible flow at 0.99 barA pressure and flowing at 300 m/s velocity and is released into 1 barA pressure. Point is, whether the flow can be released at 1 barA or not at the release pressure is higher than the pressure of the compressible fluid inside the tube. And...
  42. W

    Connection between Pressure, Surface Density and Height?

    [Mentors' note: moved from technical forums so no template] Hi All, Working on a lab write-up, and I need background equations to support the reasoning for my experiment. To outline briefly, two-part experiment, first part was finding the ideal pressure for a basketball, where I inflated it...
  43. S

    I Ceiling fan pressure increase or decrease

    Would air pressure increase or decrease due to a ceiling fan in a room?
  44. H

    How to calculate water pressure drop in cooling tower piping system?

    When calculating water pressure drop in chilled water pipisystem (closed circuit) we always use Hazen Williams equation as follow:- but in case of calculating water pressure drop in cooling tower piping system (open circuit), shall we modify something in Hazen Williams equation or shall we use...
  45. Leo Liu

    [Aerodynamics] Bernoulli's equation and Pressure Coefficient

    We know that the definition of the pressure coefficient is $$C_p=\frac{p-p_\infty}{q_\infty}$$, where ##p## is the pressure at a point, ##p_\infty## is the ambient pressure (free-stream), and ##q_\infty## is the free-stream dynamic pressure. We also know that the Bernoulli's equation is...
  46. NTL2009

    Please help explain this Blood Pressure Monitor 'calibration' system

    I've wanted to check the accuracy of my Blood Pressure Monitor (BPM) (an automated Omron unit), and finally found this old (2016) thread on another site (linked below). Prior to this, I only found the less helpful 'compare the reading to your Doctor's office' (BP varies with every reading, not...
  47. D

    Problem understanding liquid pressure

    Good morning. I joined this forum to improve my self and give back what i will learn here for the members of this forum. I m confused about the notion of pressure and i will explain the point that i don't understand. since we know that the pressure is the force over surace. and since we know...
  48. W

    Ideal gas law- Find the pressure

    Question: Answer: In the third last line of working, I do not understand why the pressure variable is changing? Shouldn't pressure remain constant and only the Volume change?
  49. P

    Calculating Work Done by Gas at Constant Pressure

    A gas with a volume of 8m^3 with a temperature of 400K gets warmed up to 550K with a constant pressure of 200Kpa. How much work has the gas done to the environment? I think i might need to use the ideal gas law for this which is: ( P * V / T = N * K ) Where V is volume, T is temperature and N...
  50. S

    Plumbing Question about Pressure Reducing Valves

    I’ve asked this question in a Plumbing Forum and got no answer. So I thought maybe the Physics Forum might be able to help. My home has a high water supply pressure (>5bar) so I’m considering using a pressure reducing valve (PRV). This is basically to help protect the plumbing, as the house...
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