E1C09 09/14/2010
15:4:53 Page 388
Bellows and Capsule Elements
A bellows sensing element is a thin-walled, flexible metal tube formed into deep convolutions and
sealed at one end (Fig. 9.9). One end is held fixed and pressure is applied internally. A difference
between the internal and external pressures causes the bellows to change in length. The bellows is
housed within a chamber that can be sealed and evacuated for absolute measurements, vented
through a reference pressure port for differential measurements, or opened to atmosphere for gauge
pressure measurements. A similar design, the capsule sensing element, is also a thin-walled, flexible
metal tube whose length changes with pressure, but its shape tends to be wider in diameter and
shorter in length (Fig. 9.9).
A mechanical linkage is used to convert the translational displacement of the bellows or capsule
sensors into a measurable form. A common transducer is the sliding arm potentiometer (voltagedivider, Chapter 6) found in the potentiometric pressure transducer shown in Figure 9.11. Another
type uses a linear variable displacement transducer (LVDT; see Chapter 12) to measure the bellows
Spring
Bourdon
tube
Scale
Applied
pressure
Open
to applied
pressure
Pinion
Mechanical
linkage
Closed end
A –– A
A
A
Sector
Figure 9.10 Bourdon tube pressure
gauge.
R L
E i
p 1
p 2
R x
E o
x
L
Figure 9.11 Potentiometer pressure
transducer.
388 Chapter 9 Pressure and Velocity Measurements
15:4:53 Page 388
Bellows and Capsule Elements
A bellows sensing element is a thin-walled, flexible metal tube formed into deep convolutions and
sealed at one end (Fig. 9.9). One end is held fixed and pressure is applied internally. A difference
between the internal and external pressures causes the bellows to change in length. The bellows is
housed within a chamber that can be sealed and evacuated for absolute measurements, vented
through a reference pressure port for differential measurements, or opened to atmosphere for gauge
pressure measurements. A similar design, the capsule sensing element, is also a thin-walled, flexible
metal tube whose length changes with pressure, but its shape tends to be wider in diameter and
shorter in length (Fig. 9.9).
A mechanical linkage is used to convert the translational displacement of the bellows or capsule
sensors into a measurable form. A common transducer is the sliding arm potentiometer (voltagedivider, Chapter 6) found in the potentiometric pressure transducer shown in Figure 9.11. Another
type uses a linear variable displacement transducer (LVDT; see Chapter 12) to measure the bellows
Spring
Bourdon
tube
Scale
Applied
pressure
Open
to applied
pressure
Pinion
Mechanical
linkage
Closed end
A –– A
A
A
Sector
Figure 9.10 Bourdon tube pressure
gauge.
R L
E i
p 1
p 2
R x
E o
x
L
Figure 9.11 Potentiometer pressure
transducer.
388 Chapter 9 Pressure and Velocity Measurements