PRESSURE MEASUREMENT
By Dr. Diew Koolpiruck
Revised by Wanchak 2008
INC 336 Industrial process measurement, 2005
TABLE OF CONTENTS
A.
INTRODUCTION
B.
MANOMETER
C.
ELASTIC-ELEMENT MECHANICAL PRESSURE GAGES
D.
ELECTRONIC PRESSURE SENSORS
E.
DIFFERENTIAL PRESSURE SENSORS
F.
HIGH PRESSURE MEASUREMENT
G.
LOW-PRESSURE (VACUUM) MEASUREMENT
INC 336 Industrial process measurement, 2005
INTRODUCTION
INC 336 Industrial process measurement, 2005
ความดัน
PRESSURE
• หมายถึง แรง (force; F) ตอ หนวยพื้นที่ (area; A)
•
•
•
•
ใน SI unit ความดันมีหนวย เปน ปาสคาล (Pa) หรือ
นิวตันตอตารางเมตร ( ) หรือ
กิโลกรัมตอเมตรตอวินาทีกําลังสอง (
)
สวนความดันในหนวย มิลลิเมตรปรอท (mmHg)
หนวยความดัน
• 760 mmHg = 101325 Pascal หรือ 1 atm = 101325 Pa = 101.325 kPa
แตอยางไรก็ตามความดันในหนวย mmHg ไมใช SI unit แตก็อนุโลมให
ใชคาความดันในหนวย mmHg หรือ ความดันบรรยากาศเปนหนวย
ความดันมาตรฐาน
• 1 atm = 101325 Pa = 101.325 kPa = 1.01325 bars = 760 mmHg
• 1 psi = 0.068947 bar
• 1 atm = 1.01325 bar
INTRODUCTION
INC 336 Industrial process measurement, 2005
ความดันเกจ ( Gage Pressure)
• คือเปนคาความดันที่อานจากมาตรวัดความดัน ในกรณีที่เปนความดันที่
สูงกวาความดันบรรยากาศ เนื่องจากมาตรวัดความดันสวนใหญจะตอง
เปรียบเทียบคาความดันที่วัดกับคาความดันบรรยากาศรอบๆขางขณะนั้น
ทําใหคาที่อานออกมาไดนั้นเปนคาความแตกตางระหวางความดันที่วัด
และความดันบรรยากาศขณะ
ความดันสุญญากาศ ( Vacuum Pressure, P vac )
• กรณีที่เครื่องวัดอานคาความดันที่ต่ํากวาความดันบรรยากาศจะไดวา
ความดันสุญญากาศ = ความดันบรรยากาศ - ความดันสัมบูรณหรือ
INTRODUCTION :Standards and Calibration
Deadweight testing
Pressure is defined as force per unit
area. Dead-Weight Pressure Testers use
these measurements of force and area
to produce a pressure to calibrate
instrument with great accuracy. The
force is derived from weights and the
area is that of a piston in a cylinder
INC 336 Industrial process measurement, 2005
INTRODUCTION :Standards and Calibration
Secondary pressure testing
Pressure Calibrator
INC 336 Industrial process measurement, 2005
MANOMETERS
INC 336 Industrial process measurement, 2005
MANOMETERS
INC 336 Industrial process measurement, 2005
ELASTIC TRANSDUCERS
INC 336 Industrial process measurement, 2005
ELASTIC TRANSDUCERS : Bourdon tubes
INC 336 Industrial process measurement, 2005
ELASTIC TRANSDUCERS : Bellows pressure gage
INC 336 Industrial process measurement, 2005
ELASTIC TRANSDUCERS : Diaphragm pressure gage
Diaphragm pressure gage
A diaphragm is a sheet of a semi-flexible material anchored at its periphery and most often round in
shape. It serves either as a barrier between two chambers, moving slightly up into one chamber or
down into the other depending on differences in pressure, or as a device that vibrates when certain
frequencies are applied to it
INC 336 Industrial process measurement, 2005
ELECTRONIC PRESSURE SENSORS : Strain gauge
Major types of resistance strain gages
• Bonded foil strain gages(metallic)
• Bonded semiconductor strain gages
• Thin film strain gages
• Diffused semiconductor strain gages
INC 336 Industrial process measurement, 2005
ELECTRONIC PRESSURE SENSORS : Strain gauge
Bonded foil strain gages
INC 336 Industrial process measurement, 2005
ELECTRONIC PRESSURE SENSORS : Strain gauge
Bonded foil strain-gage pressure transducer
INC 336 Industrial process measurement, 2005
ELECTRONIC PRESSURE SENSORS : Strain gauge
Wheatstone circuit
Strain gauge transducer with
diaphragm element
INC 336 Industrial process measurement, 2005
ELECTRONIC PRESSURE SENSORS : Strain gauge
Diaphragm-type strain-gage pressure pickup
INC 336 Industrial process measurement, 2005
ELECTRONIC PRESSURE SENSORS : Strain gauge
Diffused semiconductor strain-gage transducer
Diffused semiconductor strain
gages represent a further
improvement in strain gage
technology because they eliminate
the need for bonding agents.
INC 336 Industrial process measurement, 2005
ELECTRONIC PRESSURE SENSORS : Diffused sensor
Diffused sensor transducers and auto reference techniques
INC 336 Industrial process measurement, 2005
ELECTRONIC PRESSURE SENSORS : Strain gauge
Thin-film strain gages
• Sensors to be deposited directly onto conductive and non-conductive
components. No mounting cements or adhesives needed.
• Minimal process intrusion. Structural modifications of components
typically not necessary. Will not disrupt gas flows.
• Fast time response. Adds neglible amount of mass to system and made
with thermally conductive materials.
• Harsh environment capability. Can be made to withstand temperatures
in excess of 1000oC.
• Sensors to be placed on 3 dimensional parts. Thin film conductive
traces can route leads around corners and edges.
INC 336 Industrial process measurement, 2005
ELECTRONIC PRESSURE SENSORS : Capacitive
INC 336 Industrial process measurement, 2005
ELECTRONIC PRESSURE SENSORS : Potentiometric
INC 336 Industrial process measurement, 2005
ELECTRONIC PRESSURE SENSORS : Resonant wire
A wire is gripped by a static
member at one end, and by the
sensing diaphragm at the
other. An oscillator circuit
causes the wire to oscillate at
its resonant frequency. A
change in process pressure
changes the wire tension,
which in turn changes the
resonant frequency of the wire.
A digital counter circuit detects
the shift. Because this change
in frequency can be detected
quite precisely, this type of
transducer can be used for low
differential pressure
applications as well as to
detect absolute and gauge
pressures.
INC 336 Industrial process measurement, 2005
ELECTRONIC PRESSURE SENSORS : Piezoelectric
A piezoelectric sensor is a device that uses the piezoelectric effect to measure pressure,
acceleration, strain or force by converting them to an electrical signal. It measures dynamic
pressures. They are generally not suited for static pressure measurements
INC 336 Industrial process measurement, 2005
ELECTRONIC PRESSURE SENSORS : Magnetic
Linear variable differential transformer
Linear reluctance transducer
INC 336 Industrial process measurement, 2005
ELECTRONIC PRESSURE SENSORS : Electro-Optical
INC 336 Industrial process measurement, 2005
DIFERENTIAL PRESSURE TRANSDUCERS
INC 336 Industrial process measurement, 2005
DIFERENTIAL PRESSURE TRANSDUCERS
INC 336 Industrial process measurement, 2005
DIFERENTIAL PRESSURE TRANSDUCERS
Diffused strain-gage differential-pressure
Transmitter (Honeywell Inc.)
INC 336 Industrial process measurement, 2005
HIGH PRESSURE MEASUREMENT
Very-high-pressure transducer
INC 336 Industrial process measurement, 2005
LOW PRESSURE MEASUREMENT
Vacuum gage types and ranges
INC 336 Industrial process measurement, 2005
LOW PRESSURE MEASUREMENT
McLeod gage
A McLeod gauge is a scientific instrument to measure very low pressures, down to 10-7 Torr.
The torr (symbol: Torr), defined as 1/760 atm.
1 Torr; ≈ 1 mmHg
Today, these gauges have largely been replaced by electronic vacuum gauges. McLeod gauges
continue to be used as a calibration standard for electronic gauges
INC 336 Industrial process measurement, 2005
LOW PRESSURE MEASUREMENT
Thermocouple gage
Pirani gage
Thermal Conductivity gauges rely on the fact that the ability of a gas to conduct heat
decreases with pressure*. In this type of gauge, a wire filament is heated by running current
through it. A thermocouple or Resistance Temperature Detector (RTD) can then be used to
measure the temperature of the filament. This temperature is dependent on the rate at which
the filament loses heat to the surrounding gas, and therefore on the thermal conductivity. A
common variant is the Pirani gauge which uses a single platinum filament as both the heated
element and RTD. These gauges are accurate from 10 Torr to 10−3 Torr.
*If the gas pressure is reduced, the cooling effect will decrease
DYNAMIC EFFECTS OF VOLUMES AND TUBING
Transducer installation types
INC 336 Industrial process measurement, 2005
DYNAMIC EFFECTS OF VOLUMES AND TUBING
Fill Fluid : The fill fluid will affect the safety of process, accuracy
and response time of the measurement. The following four points
need to be considered:
• Temperature limitations
• Expansion coefficient
• Process compatibilty
• Viscoscity
Temperature Effects : Three variables can be changed to
reduce the errors caused by temperature effects
• Diameter of capillary
• Choice of fill fluid
• Diaphragm size
INC 336 Industrial process measurement, 2005
ACCESSORIES
Gauge cock and gauge valves are
applied to the temporary stoppage of
measuring fluid for the purposes of
pressure gauge maintenance,inspection
Gauge cock
Gauge valve
and repair
These joints are applied to the connection
between diipe diameters.
Connecting joint
This dampener is of variable type being
able to adjust pressure amplitude and
applied to the decrease in pressure
pulsation.
Dampener
INC 336 Industrial process measurement, 2005
ACCESSORIES
This pipe siphon is applied to
measuring steam pressure or
the case where the
temperature of measuring fluid
is high
This tank siphon is applied
to the sulstitution
measuring fluid and the
case where oil is forbidden.
INC 336 Industrial process measurement, 2005
INSTALLATION GUIDELINES
INC 336 Industrial process measurement, 2005
SMART PRESSURE TRANSMITTERS
INC 336 Industrial process measurement, 2005
SUMMARY
INC 336 Industrial process measurement, 2005