**Pressure**measurement history was primarily based on the pioneering work of Evangelista Torricelli,who, for a short time,was a student of Galileo. During his experiments with

**mercury**-filled dishes, in 1643, he realized that the atmosphere exerts

**pressure**on Earth. Another great experimenter, Blaise Pascal, in 1647, conducted an experiment, with the help of his brother-in-law Perier, on the top of the mountain Puyde Dome and at its base. He observed that

**pressure**exerted on the column of

**mercury**depends on elevation. He named the

**mercury**-in-vacuum instrument they used in the experiment a

**barometer**.

In 1660, Robert Boyle stated his famous relationship: The product of the measures of

**pressure**and volume is constant for a given mass of air at fixed temperature. In 1738, Daniel Bernoulli developed an impact theory of**gas pressure**to the point where Boyle’s law could be deducted analytically. Bernoulli also anticipated the Charles–Gay–Lussac law by stating that**pressure**is increased by heating gas at a constant volume.In general terms, matter can be classified into solids and fluids. The word fluid describes something which can flow. That includes liquids and gases. The distinction between liquids and gases are not quite definite. By varying

**pressure**, it is possible to change liquid into gas and vice versa.**Concepts of Pressure**

For a ﬂuid at rest,

**pressure**can be deﬁned as the force F exerted perpendicularly on a unit area A of a boundary surface.

p = dF/dA

**Pressure**vary with elevation as:

dp = − w.dh

where w is the speciﬁc weight of the medium and h represents the vertical height.

The kinetic theory of gases states that

**pressure**can be viewed as a measure of the total kinetic energy of the molecules:p = (2/3).KE/V = (1/3) ρ.C² = NRT

where KE is the kinetic energy, V is the volume, C² is an average value of the square of the molecular velocities, ρ is the density, N is the number of molecules per unit volume, R is a speciﬁc gas constant, and T is the absolute temperature.

**Units of Pressure**

The SI unit of **pressure**is the

**pascal**:

1 Pa = 1 N/m²

that is, one

**pascal**is equal to one newton of force uniformly distributed over 1 squaremeter of surface.Sometimes, in technical systems,

**atmosphere**is used, which is denoted 1**atm**. One**atmosphere**is the**pressure**exerted on 1 square centimeter by a column of water having a height of 1 meter at a temperature of +4^{o}C and normal gravitational acceleration.A

**pascal**can be converted into other units by the use of the following relationships:

1

**Pa**= 1.45 × 10

^{-4}

**lb/in**

**²**= 9.869 × 10

^{-6}atm = 7.5 × 10

^{-4}

**cmHg**

For practical estimation, it is useful to remember that 0.1 mm H

_{2}O is roughly equal to 1**Pa**. In industry, another unit of**pressure**is often used. It is deﬁned as**pressure**exerted by a 1-mm column of**mercury**at 0^{o}C at normal atmospheric**pressure**and normal gravity. This unit is named after Torricelli and is called the**torr**.The ideal

**pressure**of the Earth’s

**atmosphere**is 760

**torr**and is called the physical

**atmosphere**:

1

**atm**= 760

**torr**= 101,325

**Pa**

The U.S. Customary System of units deﬁnes pressure as a pound per square inch

(lb/sq in.) or

**psi**

Conversion into SI systems is the following:

1

**psi**= 6.89 × 103

**Pa**= 0.0703

**atm**

A simple yet efﬁcient

**sensor**is based on the communicating vessels principle. Its prime use is for the measurement of**gas pressure**. A U-shaped wire is immersed into**mercury**, which shorts its resistance in proportion with the height of**mercury**in each column. The resistors are connected into a Wheatstone bridge circuit, which remains in balance as long as the differential pressure in the tube is zero.**Pressure**is applied to one of the arms of the tube and disbalances the bridge, which results in the output signal. The higher the**pressure**in the left tube, the higher the resistance of the corresponding arm is and the lower the resistance of the opposite arm is. The output voltage is proportional to a difference in resistances ΔR of the wire arms which are not shunted by**mercury**:Vout = V.(ΔR/R) = V.β.Δp

The

**sensor**can be directly calibrated in units of**torr**. Although simple, this**sensor**suffers from several drawbacks, such as necessity of precision leveling, susceptibility to shocks and vibration, large size, and contamination of gas by**mercury**vapors.
## 2 comments:

thanks for u're indo... we like it

February 24, 2011 at 12:29 AMikut nimbrung gan...

February 24, 2011 at 12:37 AM## Post a Comment