UDG1606 Process Instrumentation & ControlFaculty of Chemical & Natural Resources Engineering

CHAPTER 3Pressure Measurement

OBJECTIVES

After you study this chapter, you should be able to

1. Explain the pressure measurement.

2. Describe the importance of pressure measurement.

3. Explain the basic operation, list the advantages and disadvantages of each pressure measurement device.

Introduction

Introduction

• Pressure is used to measure other variables such as flow and level.

• Distillation column operated by manipulating pressure and temperature.

Static Pressure and Dynamic Pressure

Static Pressure

• Fluid is not moving

• Not be pumped through pipe or flow channel.

Dynamic Pressure

• Fluid is in motion

• Pressure of water: inside and outside nozzle.

Pressure-Measuring Devices

• U-Tube Manometer

• Bourdon Tube Sensor

• Strain Gauge Sensor

U-Tube Manometer

• Consists of a clear glass or plastic tubeshaped into the form of a 'U'. The tubeis partially filled with a liquid, such aswater, alcohol.

• Both ends of the tube are open.

• Mainly used in laboratories forexperimental work and demonstrationpurposes.

• It can be used to measure the pressureof flowing liquids as well as gases, butcannot be used remotely.

• If pressures fluctuate rapidly itsresponse may be poor and reading isdifficult.

Bourdon Tube Sensor• The bourdon tube is made from hard

metal, i.e., bronze or brass is flattened and one end is closed off.

• The tube is bent into a curve, or spiral.

• The open end is attached to a header by which pressure can be introduced to the inside of the tube.

• The tube will deflect when inside pressure is different from the outside pressure.

• The tube will tend to straighten out if the inside pressure is higher than outside pressure.

• The tube tend to curve more if the pressure inside less than that outside.

• Most common pressure sensor.

Bourdon Tube Sensor

Strain Gauge Sensor

• Used to convert the displacementsgenerated into electrical signals.

• The amount of imbalance isproportional to the applied pressure.

• The two ratio arms of the bridge (R1and R3) are set equal to each other.

• Thus, with no force applied to thestrain gauge, the bridge will besymmetrically balanced and thevoltmeter will indicate zero volts,representing zero force on the straingauge.

Strain Gauge Sensor

• As the strain gauge is eithercompressed or tensed, itsresistance will decrease orincrease, respectively, thusunbalancing the bridge andproducing an indication at thevoltmeter.

• This arrangement, with a singleelement of the bridge changingresistance in response to themeasured variable (mechanicalforce), is known as a quarter-bridge circuit.

Basic Operation

• Fluid – Force – Diaphragm Displacement (detected by strain gauge sensor) – resistance change (detected by Wheatstone bridge).

Criteria For Selecting A Suitable Pressure-Measuring Instrument

• Operating Range

• Sensing Element

Operating Range

The pressure ranges roughly fall into the followingcategories:

– Extra low range: below 6.35 mm of water column gauge(e.g. 5 – 6 mm H2O).

– Low range: from vacuum to atmospheric in mm or inchescolumn. Vacuum ranges can be only from atmosphericpressure down to full vacuum (-1barg).

– Medium range: 10 – hundreds bar, kg/cm, or lb/in2.

– High range: thousands of bar, kg/cm, or lb/in2.

Sensing Element

The sensor can be divided into the following categories.

– Extra low range: inclined manometer at the local area ~ nolong-distance transmission.

– Low range: low range differential pressure.

– Medium range: Bourdon tubes. Spiral and helicalelements provide greater physical movement.

– High range: thick wall of helical Bourdon tube SS316 iscommonly used.