User`s guide
60
2. BACKGROUND
A thin walled cylinder has a wall thickness smaller than 1/10 of the cylinder’s radius. In this case, only
the membrane stresses are considered and the stresses are assumed to be constant throughout the wall
thickness.
The ASME boiler codes require continuous monitoring of pressure in thin walled pressure vessels. In
certain processes, use of mechanical pressure gauge or electrical pressure transducer to monitor the
pressure is unpractical, as the diaphragm can become encrusted with chemical products quickly.
Therefore, a new method is required.
2.1 Piezoresistive Pressure Sensor
As shown in figure below, a pressure transducer consists of a diaphragm and four strain gages installed on
the metal film attached to the diaphragm. Note that strain gauges
and
are in the radial direction and
strain gauges
and
are in the direction transversal to the radius. Therefore, when pressure increases,
the resistance of
and
increases and
and
decreases. The four strain gages form a Wheatstone
bridge, as shown in figure below. The change in output voltage of a pressure transducer is directly
proportional to the change in pressure. The relationship between output voltage (
) and the excitation
voltage (
) is shown equation 1.
Figure 15a Cross Section of a Pressure Transducer
Eq.1