[276]6._Pressure_measurements_UPD

# [276]6._Pressure_measurements_UPD - Languages for...

This preview shows pages 1–4. Sign up to view the full content.

Languages for engineering communication - 6 Pressure measurement 6.1 Introduction Pressure is represented as a force per unit area and has the same units as parameter discussed previously in lecture No.4 as stress, F A σ = The new aspect of the same parameter which will be discussed in this lecture is the force exerted by a fluid per unit area on a containing wall. The pressure measurement devices for fluid are usually measure pressure in three different forms: absolute pressure, vacuum and gage pressure. Absolute pressure is the absolute value of the force per unit area exerted on the containing wall by a fluid. Gage pressure is the positive difference between the absolute pressure and the local atmospheric pressure. Vacuum is the negative difference between the local atmospheric pressure and the absolute pressure. Fig. 1 Relationship between pressure terms From these definitions in Fig. 1 we see that: 1. The absolute pressure is usually positive. 2. The gage pressure may be positive or negative (vacuum) 3. The vacuum is usually smaller than the local atmospheric pressure. 1

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Some common units of pressure Because a fluid pressure is result of a momentum exchange between the molecules of the fluid and containing wall, it depends on the total number of molecules striking the wall per unit time and on the average velocity v of the molecules. For an ideal gas the pressure is: 2 2 3 2 2 1 1 [ ] [ ] 3 sec m N p nmv kg Pascal m m = = = (1) where n is molecular density, molecules/unit volume; m is molecular mass and velocity v is root mean square velocity: 2 2 2 2 1 2 3 ....... 3 n rms v v v v kT v v n m + + + = = = (2) where T 0 K is the absolute temperature of the gas, k =1.3803 23 10 / J K - × is Boltzmann’s constant, molecular mass, velocity is: 2 2 2 [ /sec] sec sec J K J Nm kgmm m m K kg kg kg kg = = = = = Since the pressure depends on collisions between the molecules, it should be dependent on the average distance passed by a molecule of radius r between collisions. The mean free path λ in an ideal gas is 2
2 2 8 r n λ π = [ 3 2 m m m = ] (3) More n smaller is the mean free path. For air the Eq. (3) reduced to 5 2.27 10 [ ] T m p - = × (4) Where 0 T K and pressure p must be in Pascal [ 2 / N m ].The mean free path decreases with increase in the gas pressure p (or the density n ) and with decrease in temperature T. The typical examples of mean free path in air at 20 C were calculated using Eq.(4): 1atm = 1.0132x 5 10 Pa - ( 29 8 1 6.564 10 atm m - = × 1torr = 133.32 Pa ( 29 5 1 4.989 10 torr m - = × 1 m μ = 0.13332 Pa ( 29 1 0.04989 m m = 0.01 m = 1.332 3 10 Pa - × ( 29 0.01 4.989 m m = At a standard conditions (p=1 atm) is quit small (about 0.1A), while in vacuum it reaches several meters length. 6.2 Vacuum devices with electrical output

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

## This note was uploaded on 11/15/2010 for the course CHEE 638 taught by Professor Hampton during the Spring '10 term at MO Southern.

### Page1 / 15

[276]6._Pressure_measurements_UPD - Languages for...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online