Brooks Instrument SLA7810/20 - Manual (Page 8)

1-2
Section 1 Introduction
Model SLA7810 & SLA7820
Installation and Operation Manual
X-PR-SLA7800-PC-eng
Part Number: 541B047AAG
December, 2008
1-3 Principle of Operation
The pressure to be measured acts on a stainless steel diaphragm. This
pressure is transmitted to the actual sensing element, a silicon chip (refer
to Figure 1-1). The chip consists of an anisotropically etched diaphragm
with four piezo resistive resistors, configured in a Wheatstone bridge,
etched into its surface (refer to Figure 1-2). The mechanical deformation of
this diaphragm results in an imbalance of the resistors, which is
proportional to pressure. An amplifier provides output to the control circuit
as well as the electrical output signal.
In addition to the pressure transducer the Model SLA7810/SLA7820 has
an integral control valve and control circuitry. A difference between the
pressure and set point signals will cause the electronics to modulate the
control valve in order to maintain the desired pressure.
A system block diagram is shown in Figure 1-3.
The integration of pressure transducer, control electronics and control
valve into one unit results in a compact size. The mounting dimensions are
comparable to a mass flow controller. Refer to Figures 1-4 and 1-5.
The “upstream pressure regulation mode” is shown in Figure 1-6 places
the pressure controller at the outlet of the pressure vessel. The pressure in
the process upstream of the controller will be constant independent of
downstream variation. Pressure measurement and control takes place at
the outlet side of the pressure vessel.
Figure 1-1 Model SLA7810/SLA7820 Pressure Transducer
Reduced Cost of Ownership
The Models SLA7810 and SLA7820 allow multi-gas and multi-range
capabilities to reduce customer inventory. Storage and pre-programming
of up to 10 pressure applications easily permits users to switch between
different gases and ranges on a single device. Also, the greater control
range provided by the co-planar valve gives users the option to decrease
the number of parts needed to control their entire process.