1083ch5 1 PDF
1083ch5 1 PDF
1083ch5 1 PDF
709
© 2003 by Béla Lipták
710 Pressure Measurement
5.6 5.8
DIFFERENTIAL PRESSURE INSTRUMENTS 743 HIGH-PRESSURE SENSORS 762
Introduction 744 Introduction 763
Measurement Error 744 Mechanical High Pressure Sensors 763
Example 745 Dead-Weight Piston Gauges 763
Smart Transmitters Reduce Error 745 Button-Type Pressure Repeater 764
D/P Instrument Designs 745 Helical Bourdon 764
Filter Status Indicators 745 Bulk Modulus Cells 764
D/P Switches 745 Pressure-Sensitive Wires 765
D/P Indicators 745 Change-of-State Detection 765
Temperature Compensation and Over-Range Dynamic Sensors 765
Protection 746 Reference 765
Ranges and Materials 746 Bibliography 765
Liquid Manometers 747
D/P Transmitters 747
Dry, Force Balance Design 747 5.9
Pneumatic Version 747 MANOMETERS 766
Suppression and Elevation 748
Introduction 767
Flat and Extended Diaphragms 748
Liquid-Sealed Designs 767
Ranges and Pressure/Temperature
Inverted Bells 767
Ratings 748
Cylindrical and Ledoux Bells 767
Wafer Elements 748
Double Bell Unit 768
Torque Tube Sensors 749
Ring Balance Manometers 768
Low-Differential Transmitters 750
Membrane Type Design 750 McLeod Vacuum Gauges 768
Bibliography 750 90 ° Rotation Type 768
Piston-Type McLeod Gauge 769
Micromanometer With Precision Needle 770
5.7 Visual Manometers 770
ELECTRONIC PRESSURE SENSORS 751 Indicator Fluids 770
Introduction 752 Liquid Barometers 770
Electrical Safety 752 Glass Tube Manometers 770
Strain Gauge Transducers 753 Well-Type Design 771
Historical Development 753 Vacuum Measurement 771
The Bonded Strain Gauge 754 Multitube and Interface Manometers 771
Temperature Compensation 754 Inclined Tube Designs 772
Detecting the Change in Resistance 754 Micromanometers 772
Transducer Designs 755 Float Manometers 772
Bonded Designs 755 Servomanometers 773
Force Balance 755 Conclusions 773
Capacitance Transducers 756 Bibliography 773
Potentiometric Transducers 757
Resonant Wire Transducers 757
Piezoelectric Pressure Sensors 757 5.10
Magnetic Transducers 758 MULTIPLE PRESSURE SCANNERS 774
Inductive Elements 758 Introduction 774
Linear Variable Differential Rotary Pressure Scanners and Distributors 775
Transformer 758 Scanning Frequency 775
Inductive Transmitters 758 Differential Pressure Scanning 775
Reluctive Elements 759 Digital Interface Units 776
Pressure detection devices can be classified on the basis of Silicon microchip technology continues steadily to reduce
the pressure ranges they can measure, on the basis of the the cost of advanced features, reducing the size and weight
design principle involved in their operation, or on the basis of hardware and improving their availability and accuracy,
of their application. In this chapter, the various categories are while extending the long-term stability their calibration.
not separated in any strict manner. Industrial instruments are Many of the sensors are available with digital communication
discussed in detail, with emphasis on the most commonly used capability, which can serve calibration, adjustment, and
devices; laboratory instruments are covered in less detail. reporting of process variables, allowing for complete plant-
wide integration.
With so many types of sensors, it might seem that making
the proper selection for a particular installation would be
INTRODUCTION
difficult and time consuming. Actually, this is not the case.
A multitude of devices are covered here for the purpose of
Each section starts with a brief summary of basic features
completeness, but for a typical industrial installation, the
applicable to the group of instruments discussed in that sec-
selection is fairly simple, and often repetitive.
tion. This information allows readers to quickly determine
whether that category of instrumentation is suitable for their
Orientation Table
application.
This chapter covers a wide range of pressure sensors, The reader should find Table 5.1a, the Orientation Table for
which can measure pressures from ultrahigh vacuums, such Pressure Detectors, of value in narrowing the choices. For each
−13
as 10 mmHg, to ultrahigh gauge pressures approaching category of sensors, this table indicates the overall pressure
400,000 PSIG (2,800 MPa). range that the category is capable of detecting. The table also
The range of the costs and inaccuracies of these instru- notes whether the unit is available for industrial on-line instal-
ments are equally broad. A simple, 1.5 in. diameter, 5% inac- lation or for laboratory use only. Although any transmitting
curate gauge might cost only $10, while a fused-quartz helix instrument can easily be provided with an inexpensive analog
sensor with an error of 0.01% and a digital readout could cost or digital local indicator, the table differentiates those sensor
$6000. The cost of pressure transmitters range from a few categories, which primarily serve as local gauges or indicators.
hundred dollars or less for the disposable models that have Also distinguished are the sensor categories, which are com-
limited features, to $2000 for smart models with built-in monly available in microprocessor-based smart configurations.
digital PID control algorithms and/or digital networking The table also indicates the type of pressure reference used.
capabilities. When the environmental pressure surrounding the instrument
712
© 2003 by Béla Lipták
TABLE 5.1a
Orientation Table for Pressure Detectors
Features Applicable Pressure Ranges
Laboratory or Pilot
Inline Device
Plant Device 10 -14 10 -10 10 -6 10 -3 10 -1 1 50 200 400 600 4 7 11 102 103 104 105 106
Type of Design −300 -200 -100 -10 -5 -1 ±0.1 +1 +5 +10 +100 +200 +300
C-Bourdon
Bourdon
Spiral Bourdon
Helical Bourdon
Quartz Helix
Strain Gauge
Capacitive Sensors
Electronic
Potentiometric
Resonant Wire
Piezoelectric
Magnetic
Optical
High-Press.
Sensors
Inverted Bell
Manometers
Ring Balance
Float Manometer
Barometers
Visual Manometers
Micromanometers
Repeaters.
D/P Cell
Pressure
Std. Diaphragm
Button Diaphragm
Ioniza-
Hot Cathode
tion
Cold Cathode
713
© 2003 by Béla Lipták
714
TABLE 5.1a Continued
Orientation Table for Pressure Detectors
Pressure Measurement
Features Applicable Pressure Ranges
Laboratory or Pilot
Inline Device
Plant Device
10 -14 10 -10 10 -6 10 -3 10 -1 1 50 200 400 600 4 7 11 10 2 10 3 10 4 10 5 10 6
Type of Design -300 -200 -100 -10 -5 -1 ±0.1 +1 +5 +10 +100 +200 +300
Thermocouple
Thermal
Thermopile
Resistance Wire-Pirani
Convectron
Quartz Helix
Mechanical
McLeod
Molecular Momentum
Capacitance
Spinning Ball
Reference Pressures
TABLE 5.1b
Pressure Conversion Reliable reference pressures are important, because they can
To Convert to Pascals (Pa) From Multiply by be a source of error just as much as an error on the measure-
atmosphere (standard) 1.01 × 10
5 ment side can. In the case of absolute pressure sensors, the
atmosphere (technical = 1 kgf/cm )
2
9.81 × 10
4 reference chamber cannot be fully evacuated to absolute zero
5 pressure, but full vacuum is only approached within a few
bar 1.00 × 10
3
thousands of a millimeter of mercury (torr). This means that
centimeter of mercury (0°C) 1.33 × 10 a nonzero value is being treated as zero, which, when mea-
centimeter of water (4°C) 98 suring higher vacuums, can cause significant errors. The other
2
dyne/cm 0.1 potential error source is the possibility of in-leakage of atmo-
foot of water (39.2°F) 2.98 × 10
3
spheric air into the vacuum reference chamber of absolute
gram-force/cm
2
98 pressure detector.
inch of mercury (32°F) 3.39 × 10
3 In case of positive pressure detectors, if the barometric
3 pressure is the reference, atmospheric pressure variations
inch of mercury (60°F) 3.37 × 10
cause a problem. As the atmospheric pressure can vary, by
inch of water (39.2°F) 249 about 1 in. of mercury (13.6 in. or 0.345 m of water), the
inch of water (60°F) 248 resulting error can be significant if the process pressure is
2 4
kgf/cm 9.81 × 10 near atmospheric. In addition, the output signal of the sensor
kgf/m
2
9.81 can change even when the process pressure is constant. The
kgf/mm
2
9.81 × 10
6 resulting error might not be significant when detecting high
2 6 gauge pressures, but it can be a problem with compound
kip/in (ksi) 6.89 × 10
detectors.
millibar 100
A compound pressure sensor is one that operates at near
millimeter of mercury (0°C) 133 atmospheric pressures and can detect the pressures on both
2
lbf/ft 47.8 above and below atmospheric. In controlling the pressure in
2 3
lbf/in (psi) 6.89 × 10 sealed rooms (clean rooms, biohazard containment chambers,
psi 6.89 × 10
3 ordinary tight buildings, etc.) where the ventilation systems
torr (mmHg, 0°C) 133 may purposely hold the pressure above or below atmospheric
pressure, this variable reference can be a source of problems.
3
Example: 1 PSI = 6.89 × 10 Pa/PSI = 6890 Pa In selecting pressure measurement devices for such applica-
= 27.7 in. water tions, the design engineer must not ignore this and must either
= 2.31 ft. water determine that the effect of barometric pressure variation can
= 2.04 in. Hg
2
be safely neglected, or must measure and correct for this
= 0.07 kgf/cm
variation in the measurement and control systems.
TABLE 5.1c
Pressure Detector Errors, Ranges, and Costs
Type Range Inaccuracy Approx. Cost
General-purpose Bourdon-tube 15–10,000 PSIG 2% $100
indicator (1–690 bars)
High-accuracy test gauge Low vacuum to 3000 PSIG 0.1% to 0.01% $300–$6,000
(Low vacuum to 207 bars)
Bourdon/spiral case-mounted Low vacuum to 50,000 PSIG 0.50% $1,200
indicator/recorder (Low vacuum to 3450 bars)
Spring-and-bellow case-mounted Low vacuum to 50 PSIG 0.50% $1,600
recorder (Low vacuum to 3.5 bars)
Nested capsular case-mounted 10–90 PSIG, 0.50% $1,600
recorder (0.7–6.2 bars)
Low-pressure bell case-mounted −0.1 to 0.1 in. H2O 2% $2,200
indicator (−3 to 3 mm H2O)
Beam-mounted strain gauge 0–1000 PSIG 0.25% $800
(sensor only) 4–20-mA DC output (0–69 bars)
Piezoresistive transducer 0–5000 PSIG 0.50% $500
4–20-mA DC output (0–365 bars)
“Smart” piezoresistive transmitter 0–6000 PSIG 0.10% $1,200–$2,000
4–20-mA DC output (0–414 bars)
“Smart” field communicator for remote — $1000–$3,000
calibration and configuring of “smart”
transmitter
Capacitive sensor/transmitter 1 in. H2O–6000 PSIG 0.2% 1000
(25 mm H2O–414 bars)
SELECTING THE PRESSURE DETECTOR transmitted, the differential pressure or the electronic sensors
described in Sections 5.6 and 5.7 should be considered. The
When local pressure indication is required and the process high-pressure sensors, described in Section 5.8, are the rec-
pressure range is between 0 to 10 in. H2O (2.6 kPa) and 0 to ommended choices for pressures from 20,000 PSIG (140
100,000 PSIG (690 MPa), the conventional pressure sensors, MPa) up to 400,000 PSIG (2,800 MPa).
which are described in Sections 5.3, 5.4, 5.5, and 5.11 can Multiple pressure sensors, including scanners and
be considered. The local pressure gauges, described in Section multiplexers, are discussed in Section 5.10. Pressure rep-
5.11, can have ranges from 10 in. H2O (2.6 kPa) up to 100,000 eaters capable of repeating pressures from full vacuum to
PSIG (690 MPa). 10,000 PSIG (69 MPa) are described in Section 5.12.
For the measurement of near-atmospheric pressures, Pressure and differential pressure switches for applications
the bellows diaphragm sensors and manometers (Sections at up to 20,000 PSIG (138 MPa) pressures can be found
5.3, 5.4 and 5.9) are the most likely choices. Similarly, for in Section 5.13.
local vacuum measurement down to 1 mmHg (0.13 kPa),
the diaphragm, the bellows-type, and the vacuum manom-
eters (Sections 5.3, 5.5, and 5.9) will give satisfactory
−12
performance. Vacuum sensors, ranging from 10 to 760 Accessories
mmHg, are discussed in Section 5.14. See Figure 5.14a for
a summary of all the available vacuum sensors and their The pressure detectors are often provided with various acces-
ranges. sory items (discussed in Section 5.2), which serve to protect
Where remote transmission is required, the force balance them from process conditions and environmental effects, are
or motion balance transmitters (Sections 5.3, 5.4, 5.5, and provided to reduce maintenance. The most common causes of
5.7) will handle most applications. They can detect vacuums failure or maintenance problems include plugging, vibration,
down to 1 mmHg (0.13 kPa) absolute and gauge pressures freezing, corrosion, excessive temperatures, and hard-to-
up to 100,000 PSIG (690 MPa). When small, near-atmospheric handle process materials. The various protection devices dis-
or high pressures up to 200,000 PSIG (1,400 MPa) are to be cussed in Section 5.2 can assist in making the installation
less sensitive to such effects and can reduce the required tasks Bibliography
of periodic servicing, testing, calibration, and maintenance.
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interchangeable.
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