in the Quick Select module. As such, any Quick Select
module can be used in any base unit and the measure-
ment system will provide measurement accuracy in
conformance with the published specifi cation.
Section 1.4.1 The GQS-1 Quick Select
Pressure Module
GQS-1 Quick Select pressure modules provide special-
ized low pressure measurement capabilities. GQS-1
Quick Select pressure modules incorporate a micro-
machined silicon variable capacitance sensor.
Inside the sensor there is a micro-machined silicon
diaphragm. This diaphragm is between two non-moving
plates on which metal has been sputtered. The air
between the diaphragm and the non-moving plates acts
As pressure or vacuum is applied to the sensor, the
diaphragm moves changing the distance between the
diaphragm and the fi xed plates. This change in distance
changes the capacitance of the sensor. It is this variable
capacitance that is measured and correlated to pressure
or vacuum during the calibration process.
The sensor is connected to an Application Specifi c
Integrated Circuit (ASIC). This ASIC generates a linear
signal ramp and applies this signal to the top plate of the
sensor while an equal and opposite signal is applied to
When the measured pressure is balanced, for example,
when both ports are opened to atmosphere, the distance
between the diaphragm and both of the fi xed plates is the
same. When this is the case, the signal to the top plate
is capacitively coupled to the equal and opposite signal
applied to the bottom plate. As a result, no signal current
will fl ow through the center plate (diaphragm). When
the diaphragm is moved off center by the application of
pressure or vacuum, the excess current fl ows through
the center plate to an input differentiator in the ASIC.
The differentiator translates the frequency of the sensor
output into a voltage which is scaled over the full scale
GQS-1 modules are available in ranges from 0.25 inches
of water through 200 inches of water. These modules
can be confi gured to provide differential/gauge or
compound pressure measurement capabilities. They
are designed for use on clean, dry, noncorrosive and
nonconductive gases. A complete listing of the available
GQS-1 modules is provided in Appendix C.
Section 1.4.2 The GQS-2 Quick Select
Pressure Module
The GQS-2 Quick Select pressure module incorporates a
micro-machined piezoresistive strain gauge sensor. This
technology takes advantage of the fact that, when put
under stress, (as with the fl exing of a diaphragm under
pressure or vacuum) the resistive properties of a piece of
silicon will change. In the manufacturing process resis-
tors are deposited in a silicon substrate. The resistors
are typically confi gured in a wheatstone bridge orienta-
tion. When positioned in this fashion, the output will be
near zero when no pressure (stress) is applied and will
increase in a near linear fashion with the application of
pressure or vacuum. The reverse side of the substrate is
etched to provide the required diaphragm thickness for
the given pressure range.
When power is applied to the sensor the level of
resistance across the wheatstone bridge will change in
proportion to the level of pressure applied. The output
from the sensor is extremely repeatable and has mini-
mal hysteresis due to the fact that the sensor substrate is
silicon. The sensor output is then amplifi ed by circuitry
within the Quick Select pressure module. The amplifi ed
output is then calibrated over the operating range of the
module. Calibration coeffi cients for the module are stored
in Electrically Erasable Programmable Read Only Memory
(EEPROM) within the Quick Select pressure module.
GQS-2 modules are available in ranges from 5 through
10,000 psi. These modules can be confi gured to provide
gauge, compound or absolute pressure measurement,
as well as vacuum measurement capabilities. GQS-2
modules in range of 5 psi are designed for use on clean,
dry, noncorrosive and nonconductive gases. 316 stainless
steel sensor isolation is provided for ranges from 0/10
through 0/10,000 psi. A complete listing of the available
GQS-2 modules is provided in Appendix C.
Section 1.4.3 The GQS-RT Quick Select
RTD Interface Module
The GQS-RT1 allows the HHC base unit, when used
with RTD temperature probes, to provide precision
temperature measurement data. The GQS-RT1 plugs
directly into the Quick Select interface module bay in
the base unit. Any standard RTD probe with a Switch-
craft TA4M can be plugged into the Switchcraft TA4F
connector on the module. The GQS-RT1 module comes
factory programmed with the curves for Pt 100 (385 &
392), Cu 10 and Ni 120 RTD probes. It supports RTDs
with outputs in the range of 0/400 ohms. The GQS-RT2
is supplied factory programmed to support the Pt1000
( 385 & 392) RTD. This interface module will support
RTDs through an output up to 4000 ohms. The GQS-RT
module can be programmed with coeffi cients for other
RTD probes of interest and specifi c characteristics of a
probe already included in the on-board library can be
