Measuring and testing – Fluid pressure gauge – With pressure and/or temperature compensation
Reexamination Certificate
2002-04-22
2003-07-08
Oen, William (Department: 2855)
Measuring and testing
Fluid pressure gauge
With pressure and/or temperature compensation
Reexamination Certificate
active
06588280
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to pressure transducers. More specifically, the present invention relates to pressure transducers with compensation for thermal transients.
FIG. 1
shows a sectional view of a prior art unheated capacitive pressure transducer
100
. Transducer
100
includes several major components such as an external shell
110
, a capacitive pressure sensor
140
, a front end electronics assembly
160
, and an input/output (I/O) electronics assembly
180
. In operation, transducer
100
generates an output signal indicative of a pressure measured by sensor
140
.
For convenience of illustration, many mechanical details of transducer
100
, such as the construction of sensor
140
and the mounting of sensor
140
and electronics assemblies
160
,
180
, have been omitted from FIG.
1
. However, capacitive pressure transducers such as transducer
100
are well known and are described for example in U.S. Pat. Nos. 5,625,152 (Pandorf); 5,911,162 (Denner); and 6,029,525 (Grudzien).
Briefly, external shell
110
includes a lower enclosure
112
, an upper electronics enclosure
114
, and a joiner
116
that holds enclosures
112
,
114
together. Sensor
140
and front end electronics assembly
160
are disposed in the space defined by lower enclosure
112
and joiner
116
. Mounting posts
162
support front end electronics assembly
160
over sensor
140
and wires
164
electrically connect front end electronics assembly
160
and sensor
140
. The I/O electronics assembly
180
is disposed in the space defined by upper electronics enclosure
114
and joiner
116
and is electrically connected to front end electronics assembly
160
.
Sensor
140
includes a metallic, flexible, diaphragm
142
and a pressure, or inlet, tube
144
. Tube
144
extends from an area proximal to the diaphragm through the lower sensor enclosure
112
. The lower, or external, end of tube
144
is generally coupled to a source of fluid (not shown). Pressure of fluid in the source is communicated via tube
144
to the lower surface of diaphragm
142
and the diaphragm
142
flexes up or down in response to changes in pressure within tube
144
. Diaphragm
142
and a reference conductive plate of sensor
140
form a capacitor, and the capacitance of that capacitor varies in accordance with movement or flexion of the diaphragm. Accordingly, that capacitance is indicative of the pressure within tube
144
. Front end electronics assembly
160
and I/O electronics assembly
180
cooperatively generate an output signal representative of the capacitance of sensor
140
which is, of course, also representative of the pressure within tube
144
. I/O electronics assembly
180
makes that output signal available to the environment external to transducer
100
via an electronic connector
182
.
FIG. 2
shows one example of how a capacitive pressure sensor
140
can be constructed. Capacitive pressure sensors of the type shown in
FIG. 2
are discussed in greater detail in U.S. Pat. No. 6,029,525 (Grudzien). The sensor
140
shown in
FIG. 2
includes a circular, conductive, metallic, flexible diaphragm
142
, a pressure tube
144
, and an electrode
246
. Electrode
246
and diaphragm
142
are mounted within a housing
248
. Electrode
246
includes a ceramic block
250
and a conductive plate
252
. The ceramic block
250
is rigidly mounted to the housing
248
so that a bottom face of block
250
is generally parallel to, and spaced apart from, the diaphragm. The bottom face of block
250
is normally planar and circular. The conductive plate
252
is deposited onto the bottom face of block
250
and is also generally parallel to, and spaced apart from, the diaphragm. Conductive plate
252
and diaphragm
142
form two plates of a variable capacitor
254
. The capacitance of capacitor
254
is determined in part by the gap, or spacing, between the diaphragm
142
and the conductive plate
252
. Since the diaphragm flexes up and down (thereby changing the spacing between diaphragm
142
and conductive plate
252
) in response to pressure changes in tube
144
, the capacitance of capacitor
254
is indicative of the pressure within tube
144
.
FIG. 2
shows only one of the many known ways of configuring a capacitive pressure sensor
140
. However, capacitive pressure sensors
140
generally include one or more conductors that are held in spaced relation to a flexible, conductive, diaphragm. The diaphragm and the conductors form plates of one or more variable capacitors and the capacitance of those capacitors varies according to a function of the pressure in tube
144
.
Capacitive pressure sensors often include additional features such as a tube
260
and a getter
262
as shown in FIG.
2
. When sensor
140
is being constructed, tube
260
is initially open and is used to establish a reference pressure (e.g., vacuum) in the portion of housing
248
above diaphragm
142
. Once the desired reference pressure is established (e.g., by attaching a vacuum pump to tube
260
), the upper portion of tube
260
is closed, or “pinched off”, so as to maintain the desired reference pressure inside the upper portion of housing
248
. Getter
262
is often included to absorb gas molecules that get into the upper portion of housing
248
after tube
260
has been pinched off (e.g., via outgasing of electrode
250
).
Returning to
FIG. 1
, in operation, front end electronics assembly
160
measures the capacitance (or capacitances) of sensor
140
and generates a voltage signal representative of the pressure in inlet tube
144
(e.g., the voltage signal may vary linearly with changes in pressure). I/O electronics assembly
180
typically amplifies and further conditions that voltage signal to generate the output signal of transducer
100
.
As shown, the distance between I/O electronics assembly
180
and sensor
140
is greater than the distance between front end electronics assembly
160
and sensor
140
. The front end electronics assembly
160
is mounted close to sensor
140
so as to reduce the length of the wires
164
. Reducing the length of wires
164
reduces the capacitance of those wires and in effect reduces a stray capacitance associated with the capacitance of sensor
140
and thereby improves the accuracy of transducer
100
. I/O electronics assembly
180
is normally separated from sensor
140
by a greater distance so that heat generated by assembly
180
does not adversely affect operation of sensor
140
.
Ideally, the output signal of transducer
100
varies only in response to changes in the pressure of the fluid in tube
144
. However, changes in temperature of the ambient environment can also affect the output signal. This is primarily due to the different coefficients of thermal expansion of different materials used to construct the sensor
140
. Many transducers incorporate heaters and heater shells to provide a controlled thermal environment for the pressure sensor and thereby minimize the affect of changes in the ambient temperature. However, unheated transducers such as transducer
100
also often provide mechanisms for compensating for temperature changes in the ambient environment.
Transducer
100
includes a temperature sensor S
1
, which is mounted on the I/O electronics assembly
180
. In operation, temperature sensor S
1
generates an output signal T
S1
, which is representative of the temperature of sensor S
1
. The sensor S
1
is positioned so that its temperature (and its output signal T
S1
) is representative of the temperature of the ambient environment of transducer
100
. In operation, transducer
100
uses the temperature sensor S
1
to provide compensation for ambient temperature changes. To provide this temperature compensation, I/O electronics assembly
180
generates the output signal of the transducer
100
according to the following Equation (1).
O
T
=O
U
+ƒ
1
(
T
S1
) (1)
In Equation (1), O
T
is the output signal of transducer
100
(a voltage representative of the pressure of the fluid in tube
144
); O
U
is an
Mathew Santhi E.
Mindlin Leonid
Poulin James M.
Quigley Claudia J.
Hale and Dorr LLP
MKS Instruments Inc.
Oen William
LandOfFree
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