Multi-temperature heater for use with pressure transducers

Details Multi-temperature heater for use with pressure transducers Multi-temperature heater for use with pressure transducers

2000-10-10

2004-08-10

Hirshfield, Andrew H. (Department: 2854)

Measuring and testing

Fluid pressure gauge

Diaphragm

C073S724000, C073S708000, C073S756000

Reexamination Certificate

active

06772640

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an improved heater for use with heated pressure transducers. More particularly, the present invention relates to an improved method for converting a pressure transducer configured for operating at a first temperature to a pressure transducer configured for operating at a second temperature, or for configuring a pressure transducer to operate at one or more temperatures.
BACKGROUND OF THE INVENTION
FIG. 1
shows a sectional view of a prior art heated capacitive pressure transducer
100
. Transducer
100
includes several major components such as an external shell
110
, a heater shell
120
, a heater
130
, a capacitive pressure sensor
140
, a front end electronics assembly
160
, a heater control electronics assembly
170
, and an input/output (I/O) electronics assembly
180
. As will be discussed in greater detail below, 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
,
170
,
180
, have been omitted from FIG.
1
. However, heated capacitive pressure transducers such as transducer
100
are well known and are described for example in U.S. Pat. No. 5,625,152 (Pandorf); U.S. Pat. No. 5,911,162 (Denner); and U.S. Pat. No. 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. Heater shell
120
is disposed within the lower enclosure
112
and includes a lower enclosure or can
122
and a cover
124
. Heater
130
includes a barrel heater
132
and an end heater
134
. Barrel heater
132
is wrapped around the external cylindrical sidewall of can
122
and end heater
134
is disposed on the bottom of can
122
. Barrel heater
132
and end heater
134
are electrically connected via wires
136
so the two heaters
132
,
134
may be simultaneously controlled via a single electrical signal. Sensor
140
and front end electronics assembly
160
are disposed within heater shell
120
. Heater control electronics assembly
170
and I/O electronics assembly
180
are disposed within the upper electronics enclosure
114
. A temperature sensor (e.g., a thermistor)
190
is fixed to an internal surface of heater shell
120
.
Sensor
140
includes a metallic, flexible, diaphragm
142
and a pressure tube
144
. Tube
144
extends from an area proximal to the diaphragm through the heater shell
120
, and 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
.
Returning to
FIG. 1
, ideally, the output signal of transducer
100
varies only according to changes in the pressure of the fluid in tube
144
. However, changes in the temperature of transducer
100
, or temperature gradients within transducer
100
, can affect the output signal. This is primarily due to the different coefficients of thermal expansion of different materials used to construct the sensor
140
. A secondary effect relates to the temperature sensitive performance of front end electronics
160
. Accordingly, the accuracy of transducer
100
can be adversely affected by temperature changes in the ambient environment.
To minimize the adverse effect of changing ambient temperature, the temperature sensitive components of transducer
100
(i.e., sensor
140
and front end electronics
160
) are disposed within heater shell
120
, and in operation the heater
130
heats the heater shell
120
to a controlled, constant temperature. Heater
130
and heater shell
120
essentially form a temperature controlled oven that maintains the temperature of the temperature sensitive components at a constant preselected value.
Heater
130
is normally formed by placing wires or traces (e.g., copper) characterized by a selected electrical resistance onto a flexible, electrically insulating, thermally conductive shell. The traces are selected so that they will heat the heater shell
120
to a preselected temperature when a particular electrical signal is applied to the traces. The electrically insulating, thermally conductive shell is commonly made from thin layers of silicone rubber or Kapton (i.e., a polyimide high temperature film sold by Dupont under the trade name Kapton).
FIG. 1A
shows a front view of a prior art barrel heater
132
. As shown, the heater includes a heating element
132
A that has been placed on an electrically insulating, thermally conductive shell
132
B. Heater element
132
A includes a resistive wire
132
C disposed between two terminals
132
D,
132
E. The amount of heat produced by heating element
132
A is primarily determined by the electric resistance provided between terminals
132
D,
132
E and the electric signal applied to the terminals
132
D,
132
E. End heater
134
is constructed in a similar fashion as barrel heater
132
.
Heater
130
is normally permanently bonded to the external surface of beater shell
120
as indicated in FIG.
1
. Heaters made using Kapton shells are normally bonded to the external surface of shell
120
with pressure sensitive adhesive. Heaters made using rubber shells are normally bonded to the external surface of shell
120
using a high temperature “Vulcanization” pro

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