Dual transducer

A pressure transducer system for developing accurate pressure measurements with a torque driven pressure transducer where the transducer has dual capacitor devices in tandem and mechanically coupled to permit relative displacement along a elongated axis in response to temperature and where the devices are displacable by angular torque about the elongated axis to obtain independent correlatable measurements from the dual capacitor devices.

RELATED APPLICATIONS 
This application is related to Ser. No. 770,352 filed Oct. 3, 1991 and 
entitled, CAITOR AND PRESSURE TRANSDUCER. 
FIELD OF THE INVENTION 
This invention relates to capacitors and pressure transducers and more 
particularly to relatively small capacitors utilizing micro-displacements 
for use in high pressure transducers requiring a wide pressure range of 
operations under changing temperature conditions and for producing 
pressure measurements with a high degree of accuracy. 
BACKGROUND OF THE INVENTION 
While the present invention finds particular usefulness in the oil 
industry, it has particular application in other hostile pressure and 
temperature environments where size of the transducer and accuracy of the 
measurements are important. 
In an oil well pressure transducer, it is common to size a sensor unit to 
go into a small diameter metal pressure housing for measurement of 
pressures up to 20,000 psi and even more and which can be subjected to 
downhole temperatures up to 400.degree. F. or more. The pressure housing 
must have a wall thickness sufficient to withstand the downhole pressures 
so that the O.D. of the internal pressure transducer or sensor is in the 
neighborhood of one inch in diameter. 
A downhole oil well transducer or pressure gauge can be transported by a 
wireline, cable or pipe string to one or more levels of interest in a well 
bore where both temperature and pressure are sensed over a period of time. 
Typically, pressure measurements are repeatedly sampled and recorded over 
a period of time at a sampling rate determined by down hole electronics 
and may be stored in a downhole memory for subsequent replay or sent to 
the surface of analysis. Alternately, sometime pressure gauges are 
attached to production strings or other downhole equipment for 
measurements over extended periods of time or "permanently". In both uses 
of a pressure gauge, sudden downhole pressure changes can also typically 
accompanied by a temperature change in a relatively short period of time. 
It is also important for the accuracy of the pressure sensor to not change 
its calibration over a period of time in the borehole in response to 
pressure or temperature effects in the boreholes. Thus, there is a need 
for a pressure gauge for high pressure measurements which is also 
insensitive to sudden changes in temperature or effects of pressure. The 
present invention relates to high pressure transducers which can 
accurately measure high pressure changes under transient temperature 
conditions independently of the temperature changes. 
Under the combined effect of high temperature and pressure conditions, the 
typical pressure sensor structure is subjected to high stress by the 
applied high pressure and subjected to high temperature both of which 
cause creep in the materials of the sensor structure. Creep in materials 
tends to be exponentially related to both temperature and stress levels. 
The effect of creep or permanent deformation in materials is to alter the 
calibration or measurement characteristics of a sensor and cause the 
sensor to obtain inaccurate measurements from its calibration standard 
over a period of time. 
Heretofore, capacitance type transducers have been utilized for pressure 
measurements where a downhole oil well pressure varies an electrical 
capacitance as a function of pressure. The pressure is translated to a 
capacitance measurement by a pressure diaphragm moving parallel arranged 
capacitance plates toward and away from one another. An example of the 
kind of device is shown in U. S. Pat. No. 4,322,775. 
I have also coupled a Bourdon tube to a capacitance type of sensor such as 
disclosed in U. S. Pat. No. 4,873,870 in which the pressure in the bourdon 
tube generates a directional linear force to displace quartz supported 
parallel arranged capacitor elements toward and away from one another. 
While this device is satisfactory for a number of applications, it is a 
difficult unit to manufacture. 
One of the major concerns in utilizing downhole pressure sensors is that a 
downhole transducer malfunction can lose data or require re-running the 
downhole testing program with a different transducer. This can involve 
considerable expense and lost time. Another concern with the type of 
transducer disclosed herein is temperature sensitivity of different 
materials which can cause a drift of the measured pressure values over a 
period of time. 
In the present invention, I have developed a capacitor device which has a 
unique relationship of capacitors that can be constructed from metals to 
respond to low force inputs and be relatively insensitive to temperature 
changes and has a redundancy feature to avoid abortion of a pressure 
testing program because of a malfunction. The capacitor device is 
particularly adaptable to measurement of high pressure with a high degree 
of accuracy and repeatability over a period of time. 
As an example of probabilities of malfunctions, if one hundred single 
pressure sensors have a 5% failure rate then one unit out of every 20 
units would fail in use. On the other hand if you have one hundred tandem 
connected pressure sensors with the same failure rate per unit, the 
probability that both sensors of a tandem unit will simultaneously fail is 
one out of 400 units. Thus, redundancy by use of tandem transducers can 
build substantial reliability into a downhole pressure sensor. Redundancy, 
however, also creates temperature problems since a transducer is matched 
to a base support with respect temperature expansion characteristics and 
separately mounted transducers can have different temperature expansion 
characteristics. The present invention provides for both redundancy and 
temperature compensation. 
SUMMARY OF THE INVENTION 
In the present invention, a transducer has dual capacitors which are 
defined by spaced apart capacitor plates respectively located on capacitor 
base members. The capacitor base members are vertically arranged in a 
pressure housing. The capacitor plates for each capacitor are disposed at 
equally offset locations relative to a central vertical axis for the 
device so that a capacitor is located on either side of the central 
vertical axis. One of the capacitor base members can be angularly torqued 
(displaced) about the central vertical axis by an applied torque force to 
dependently vary the respective capacitance of the capacitors. The angular 
torque displacement is obtained by a spirally wound Bourdon tube which, 
when subjected to internal fluid pressure, produces a torque about the 
central vertical axis. 
The Bourdon tube is attached to the displaceable capacitor base member and 
to a reference base member and provides a torque force to capacitance 
torque beams in the angularly displaceable capacitor base member. Although 
the Bourdon tube can be subjected to high pressures, the stress levels in 
the metal Bourdon tube can be designed to be well within its elastic 
limits because only a low force is required to obtain a micro displacement 
of the torque beams and the capacitor base member and to obtain micro 
measurements by the capacitors. Micro displacements of the capacitors are 
easily measured. Thus, creep and permanent distortion in the materials, 
which are caused by stress, are minimized in the system. Further, the 
effect of Bourdon tube creep is reduced by the constraining effect of the 
torque beams as will be described more fully herein. 
A high capacitance sensitivity can be obtained with low angular deflection 
by arranging the capacitor base members so that related capacitance 
between capacitor plates are varied as a function of an angular 
relationship of the capacitor plates relative to a mid-plane. That is, a 
micro dimensional change in the capacitor gap produces a defined 
measurement parameter. The high sensitivity is obtained by measurement of 
a small displacement of the capacitor plates at a significant distance 
from the center of angular displacement (the displacement or central 
vertical axis). The elastic characteristics of a metal torque beam means 
coupled to the force end of the spirally wound Bourdon tube by a torque 
coupling become the primary determining element relative to elastic 
properties. 
The reason that the torsion beam means is a primary determining element is 
that the deflection of the Bourdon tube is restrained by the torsion beam 
means to be a small fraction of the unrestrained deflection of the Bourdon 
tube. Thus, the deflection of the Bourdon tube is controlled by the 
elastic characteristics of the torsion beam means and the Bourdon tube 
becomes essentially a pressure to force converter. By using a low driving 
torque force of a Bourdon tube (even for high pressure) and a minute 
angular deflection of the torsion beam means, the stress levels in the 
Bourdon tube and particularly in the torque beam means can be kept well 
within micro-elastic limits. High performance metal alloys can then be 
used to provide correspondingly high micro yield values so that near 
perfect elastic characteristics are attainable in the operating range of 
the transducer. 
Temperature is an important factor because it can affect the calibration of 
the sensor. While a pressure measuring device at an ambient temperature 
can be generally corrected by measured temperature, a change of 
temperature from an ambient value can thermally affect the response of the 
pressure measuring device to pressure which affects the accuracy of the 
pressure measurement. Changes in temperature often occur with changes in 
pressure so it is important to accurate pressure measurement for the 
pressure measuring device to be insensitive to changing or varying 
temperatures or to compensate for the changing temperatures. 
In the present invention, upper and lower transducers with similar torque 
beam constructions are connected in a vertical and tandem arrangement 
where the upper and second transducer has one of the capacitor base 
members mechanically coupled to a similar one of the capacitor base 
members of the first and lower transducer. The other base members of the 
first and second transducers are connected to a common support base. The 
first and second transducers are in vertical alignment with one another. A 
coupling system is arranged to interconnect the upper and lower 
transducers and to permit relative vertical displacement between the 
coupled base members so that vertical displacement due to temperature 
effects on one of said base members is isolated from the other base 
member. The coupling system, however, is arranged to be rigid with respect 
to displacement of the coupled base members about the vertical 
displacement axis so that each base member is responsive to the angular 
displacement obtained by the spirally wound Bourdon tube. 
Each transducer has an independent electrical system and the measured 
parameters from each of the transducers is independently derived. Thus, if 
one transducer should fail the other transducer can be relied upon to 
obtain the measurements.

DESCRIPTION OF THE PRESENT INVENTION 
By way of background, as shown in FIG. 1, a downhole cylindrically shaped 
well tool 20 is sized for insertion through a small diameter well tubing 
and adapted for coupling to the end of a wireline cable 21. The cable 21 
extends to a surface located spooling reel or drum (not shown). The tool 
20 generally includes a DC battery pack section 22, as a source of 
electrical power, an electronic section 23 with electrical circuitry for 
electrically processing and for providing electrical power, a temperature 
sensor section 24 with a temperature probe for sensing temperature and a 
pressure sensor section 25 with a pressure transducer for sensing 
pressure. An opening 26 admits fluid under pressure to the pressure sensor 
or the transducer in the sensor section 25. For further reference 
purposes, see U.S. Pat. No. 4,763,259. 
In permanent gauge installations the temperature and sensor sections are 
incorporated with downhole equipment for permanent position or location in 
a well bore. 
Referring now to FIG. 2 and FIG. 3, the operating concept of the apparatus 
of the present invention for a single transducer is schematically (but 
disproportional) illustrated for descriptive purposes. In FIG. 2, separate 
electrical capacitors 29, 30 are illustrated where the capacitor 29 has 
parallel capacitor plates 29a, 29b which are separated by a capacitance 
gap 29c. The capacitor 30 has parallel capacitor plates 30a, 30b which are 
separated by a capacitance gap 30c. The plates 29a, 30a are fixed and in a 
common plane transverse to the plane of the drawing and the plates 29b, 
30b are in a parallel common plane. The plates 29b, 30b are attached (see 
dashed line) to a torsion beam means 32. The beam means 32 has a torque 
axis 34 in a transverse plane and can be torqued about its axis 34 to 
angularly displace the common plane for the plates 29b and 30b and 
dependently alter the capacitance of the capacitors 29, 30. 
As shown schematically in more detail in FIG. 3, the capacitor plates 29a, 
30a are on a fixed base member 38 and are located equidistant from a 
central horizontal axis 40. A support means 39 with a horizontal extension 
39a and a vertical extension 39b is fixed to a lower base member 41. 
The capacitor plates 29b and 30b are located on elements 42a, 42b of a base 
member 44. The elements 42a, 42b are connected by horizontal extensions 
44a, 44b to a vertical torque coupling element 46 are connected by a 
vertical torsion beam element 32 to the support means 39. The torque 
coupling element 46 is connected to the stub or closed end of a spirally 
would Bourdon tube 48. The open end of the Bourdon tube 48 passes through 
the base member 41 and is attached thereto. The axis 34 of the torsion 
beam element 32 and the coil axis of the Bourdon tube 48 are aligned on a 
common vertical axis and intersect the horizontal axis 40. The axes 34 and 
40 defined a vertical plane. 
It can be appreciated that the structure is arranged so that an applied 
pressure in the Bourdon tube causes the Bourdon tube to develop a torque 
force which is applied to the beam element 32 so that the plates 29b and 
30b are angularly displaced by torque about the vertical displacement axis 
34 and dependently change the capacitor gaps 29c, 30c. Thus, separate 
capacitors respectively utilizing the capacitance plates 29a, 29b and 30a, 
30b will have dependently related capacitance changes in response to 
rotative displacement. As will be discussed herein, the torque 
displacement is in micro dimensions which reduces the stress in the 
torsion beam element. 
From the foregoing basic illustration of a concept of the present 
invention, it can be appreciated that a Bourdon tube is utilized to 
develop a low torque force in response to high pressure which acts on a 
torsion beam member about a vertical displacement axis and produces a 
micro dimensional deflection of dependent capacitors. By maintaining the 
amount of deflection within the micro-elastic characteristics of the 
material for the beam member and utilizing low torque forces, the stress 
levels in the beam member can be kept low which permits high accuracy and 
repeatable measurements. The high accuracy measurements are obtainable 
because permanent distortion of the material stress does not appreciably 
occur and consequently does not affect the measurements. Material 
criterion for the torsion beam element is that the material should have a 
micro yield and micro creep point which is above the stress level produced 
by a torque force. 
Referring now to FIGS. 4, 5 and 6, a more detailed illustration is provided 
for the capacitor arrangement of the present invention. In the 
illustration, the structure of a capacitance sensor includes a first lower 
transducer 35. The lower transducer 35 (FIG. 4) has an elongated, 
cylindrically shaped central fastener rod 50 with a central longitudinal 
axis 40. In FIG. 4, the axis 40 is shown in a horizontal position. The 
axis 40 is normal to the planes of the capacitor plates on spaced apart 
first plate base member 38 and second plate base member 44 where the 
planes would be transverse to the plane of the drawing. The rod 50 is part 
of the support means which couples a first vertical capacitor plate base 
member 38 to a fixed center section 65 (See FIG. 6) in the second vertical 
capacitor plate base member 44. The center section 65 is fixed or attached 
to a vertical support member 110 as will be explained hereafter. 
The first base member 38 is formed from a cylindrically shaped member and 
is made of a material having inherently dimensional stable characteristics 
under changing environmental conditions such as temperature and time 
aging. Quartz is a suitable material. 
As shown in FIG. 5 and FIG. 6, the second capacitor base member 44 is 
formed from a cylindrically shaped metal plate member and has a first slot 
configuration of mirror arranged slot systems 62, 63 which are defined by 
spaced apart wall surfaces and which are tortuously located in the body of 
the second base member 44. The first slot configuration provides or 
defines the central plate section 65 and spaced apart torsion beam members 
or sections 66a, 66b which connect to outer plate sections 67a, 67b (See 
FIG. 5). The beam members 66a, 66b are rectangular shaped in transverse 
cross section with a narrow dimension in the plane of the drawing (FIG. 5) 
and a long dimension in a transverse plane (See FIG. 4). The beam members 
66a, 66b are adapted to be torqued about a central vertical torque or 
displacement axis 34 where the torque or displacement axis 34 is located 
centrally of the beam members 66a, 66b and on a vertical median plane 
extending through the second base member 44. The displacement axis 34 also 
intersects and defines a vertical plane with the axis 40. The displacement 
axis 34 is parallel to the parallel planes in which the capacitor plates 
are located. 
The central section 65 thus is a generally rectangularly shaped member 
defined between the slots systems 62 and 63 which are symmetrically 
arranged with respect to a central displacement axis 34. The outer 
sections 67a, 67b of the second base member 44 are attached by the torsion 
beam members 66a, 66b to the central section 65 along the central axis 34. 
As shown in FIG. 5, the slot system 62 has a central vertically walled 
portion 61a connected to parallel displaced walled end portions 62b and 
62c by transversely arranged wall slot portions 62d and 62e. The slot 63 
has similarly arranged portions 63a, 63b, 63c, 63d and 63e relative to the 
slot system 62. The spacing between the end portions 62b, 63b and 62c, 63c 
of the slots 62 and 63 define the narrow width dimension of the beam 
members 66a and 66b. The length of the slot portions 62b, 63b and 62c, 63c 
also defines the length of the beam members 66a and 66b. 
Referring to FIG. 5, in the center of the central section 65 and the base 
member 44 is a mounting bore 70 which is centered on the axis 40. The bore 
70 receives an annular outer tubular support ring 72 (see FIG. 4). The 
outer support ring 72 is welded about its periphery at its end surface to 
the central section 65. Disposed within the outer support ring 72 (See 
FIG. 4 & 5) is an annular inner tubular support member 76 which is 
attached to the rod SO. 
Referring to FIG. 4, the inwardly facing planar surface 77 of the outer 
plate sections of the second base member 44 is on the same vertical plane 
as the upper surface 77a of the center section 65. The facing surface 79 
of the first base member 38 is parallel to the surface 77 of the second 
base member 44. Between the inner support member 76 on the second base 
member 44 and the first base member 38 is a disc shaped spacer member 78. 
The spacer member 78 effectively defines the capacitance gap for capacitor 
plates on the surfaces 79 and 77. 
The rod 50 is threadedly attached to the support member 76 and extends 
through a centrally located opening on the first base member 38. A 
clamping means 80 threadedly attaches to the rod 50 so that the first and 
second base members 38 and 44 are assembled in a unitary assembly. A 
portion of the rod 50 extends outwardly of the support member 76 and is 
threadedly attached and welded to a balance mass 81. The balance mass 81 
overcomes gravity effects when the device is in a horizontal position. The 
spacer member 78, the rod 50, the clamping means 80, and the support 
member 76 can be made from a material which is selected to have similar 
temperature expansion characteristics to the selected material for the 
base member 38. A metal material such as Invar or the like is suitable. 
The base member 44 is made from a high strength material, such as a 
maraging stainless steel with good elastic characteristics for the torsion 
beam members. The steel base member 44 also will provide an electrical 
ground for the capacitor electrical system. By way of illustration, the 
coefficient of expansion for various materials averages (at room 
temperature) as follows: 
______________________________________ 
Invar 0.2 parts/million/.degree.F. 
Maraging Steel 6 parts/million/.degree.F. 
Quartz 0.3 parts/million/.degree.F. 
______________________________________ 
The first capacitor base member 38, as noted before, made from a 
cylindrically shaped member preferably constructed from a quartz material 
and has first and second independent capacitance plate films 79a and 79b 
(See FIG. 7) which are sputtered in separate locations onto a surface 79 
of the base member 38. The capacitance plate film 79a is arranged in 
spacial alignment with the planar surface 77 on the second capacitance 
base member 44. Connection is made to the edge of the quartz plate. 
Electrical wire conductors are then connected to each capacitor film plate 
for separate capacitor measurements. On the surface 77, facing capacitance 
plate films 77a, 77b on an insulator base (See FIG. 4) are provided, if 
desired, or the metal can be used as a ground surface in a grounded 
electrical capacitance system. 
As shown in FIG. 4, the planar surface 79 on the first base member 38 is 
arranged normally parallel to the planar surface 77 on the second 
capacitor base member 44 and is normally separated therefrom by a 
capacitor spacing distance or gap. The capacitance plate films 79a and 79b 
which are offset to either side of the central horizontal axis 40 and to 
either side of the vertical plane through the displacement axis 34. The 
plate films 79a and 79b are parallel to the planar surface 77 on the base 
member 44. The widths of the capacitor gaps between the respective plate 
films 79a, 79b and the surface 77 is basically defined by the width of the 
spacer member 78. It can thus be appreciated that the clamping means 80 on 
the fastener rod 50 attach the first capacitor base member 38 to the inner 
support ring 76 and, in turn, to the central section 65 of the second 
capacitor base member 44. 
Referring again to FIGS. 5 and 6, the second capacitor base member 44 is 
also provided with a second vertical wall slot system comprised of angular 
"L" shaped slots 90 and 91 which are symmetrically arranged with respect 
to the axis 34. The sidewalls of the slot portion 90a of the sidewalls of 
slot 90 align with the sidewalls of the slot portion 62a of the slot 62. 
The sidewalls of the slot portion 91a of the slot 91 align in vertical 
planes with the sidewalls of the slot portion 63a of the slot 63. The 
sidewalls of the slot portions 90b and 91b of the slots 90, 91 are aligned 
in horizontal planes with one another and are perpendicularly arranged 
relative to the displacement axis 34. It can be seen that the slot 
portions 90b and the slot portions 91b, respectively, define transverse 
beam portions 95, 96 about an axis perpendicular to the displacement axis 
34. The purpose of this arrangement is to minimize temperature effects by 
providing an equal and accurately controlled heat conduction path to each 
capacitance side. 
The torquing of the beam members 66a, 66b on the central section 65 of the 
base member 44 is accomplished by a spirally wound Bourdon tube 48 (FIGS. 
4 & 5). The Bourdon tube 48 has a closed stub end 101 (FIG. 5) which is 
aligned with the displacement axis 34 and is attached to the torque 
coupling element 44a of the base member 44. The spirally wound Bourdon 
tube 48 has a central vertical coil axis 107 which aligns with the 
displacement axis 34. When the Bourdon tube is subjected to internal 
pressure it will produce a torque force about the axis 34 and the axis 
107. As shown in FIG. 5, the base member 44 has a weight portion 93 to one 
side which provides for balancing of the unit to the effects of gravity. 
The assembly of the base members 38 and 44 is supported on a vertical 
support member 110 and the mounting beam 73 welded to the support member 
110. The support member 110 is attached to a cylindrically shaped base 
member 112 which couples to a pressure inlet. The open end of the Bourdon 
tube 48 extends through an opening in the base member 112. The tube 48 is 
welded to the support member 110, 112 so that the Bourdon tube is fixed in 
position between the base member 110 and the base member 44. 
As may be appreciated from FIGS. 4 and 6, the base member 38 and the 
support member 110 constitute a first expansion unit assembly and the base 
member 44, and the Bourdon tube 48 constitute a second expansion unit 
assembly. Temperature changes produce equal displacement of the second 
expansion unit assembly relative to the first expansion unit assembly and 
compensate for changes in dimensions due to temperature. Even if not 
exactly equal, the difference in displacement is absorbed by the coil of 
the Bourdon tube without producing a significant torque effect. In 
practice, a metal cylindrical enclosure housing 138 encloses the 
capacitance at a vacuum or contains inert gas. 
One of the features of the present invention is the arrangement which 
enables use of micro-elastic characteristics of metals. By way of 
definition, the macro yield point of a metal can be defined as the point 
where the metal has a set or plastic strain (permanent deformation) of 
0.2% or two parts per thousand. The micro yield point of a metal is 
defined as the point where the metal has a set in a range of 0.01% to 
0.0001% or one part per ten thousand to one part per million. In utilizing 
micro-elastic characteristics, a low or small force produces a small 
deflection. As an example, a 0.3 inch pound torque is used to produce a 
capacitance deflection of 0.001 radians. This arrangement permits 
measurement of high pressure 10,000-15,000 psi or more by utilizing a 
Bourdon tube coupled to a capacitor transducer. The capacitor transducer 
utilizes a relatively small deflection so that the primary determining 
element is the torque beams which have very low stress levels. The Bourdon 
tube then operates in an essentially constrained mode as a pressure to 
force converter. Additionally since the stress levels in the torsion beam 
members are in the micro-elastic range, the elastic characteristics of the 
torsion beams can approach nearly ideal performance. Ideal performance is 
approached by the diminishing effect of hysteresis creep, and no-linear 
response as stress levels are reduced. 
The stress levels in obtaining micro-elastic characteristics are low 
because the deflection required for the capacitor sensor can be small, for 
example 0.001 radians. The beam members providing the displacement axis 
are stiff or rigid and the torque force applied is low, for example 0.3 
inch pounds. 
By way of example, the diameter of the base member 38 is about 0.850 
inches. The diameter of the base member 44 is about 0.900 inches and 0.125 
inches thick. The width of the slots is about 0.020 inches. The spacer 78 
is 0.001 inches thick. 
While the preferred embodiment is to exploit micro yield characteristics to 
produce accuracy and repeatability, macro yield materials may be suitable 
for some applications. 
A Bourdon tube as contrasted to a circular tube has a flattened or ovular 
cross section as compared to a circular cross section. In high pressure 
applications a flat oval cross section is commonly employed. In a 
flattened cross section, internal pressure produces higher stress in the 
wall because the member tends to move toward a circular cross sectional 
form. When a tube member with a flattened cross section is spirally wound, 
internal pressure tends to uncurl the spiral. The flatness of the tube, 
the coil diameter and the wall thickness also have a bearing on the 
stress. 
In the present invention, the spiral closed end 101 of a spirally wound 
flattened Bourdon tube is connected to a metal base member 44 which is 
constructed to enable torsional deflection of a beam member as a function 
of the applied pressure in the Bourdon tube. As shown in FIGS. 4 and 6, 
there are two complete turns of the Bourdon tube which have a non-circular 
cross-section between the circular end pieces 101a, 101b. It is preferable 
to have an even number of turns in the Bourdon tube. Bourdon tube design 
is well known, and the design should minimize the stress in the Bourdon 
tube to develop a low force for the beam members. There is zero force on 
the capacitance plates. In short, the capacitance members are moved 
relative to one another by angular torque deflection of the capacitor base 
members. Because the force moment of the Bourdon tube is small, the stress 
level in the torque beam can be kept low. The use of high performance 
metal alloys can then provide near perfect elastic and stability 
characteristics of the torsion beam. 
The effect of temperature on the torque output of a Bourdon tube is 
minimized because the differences in linear expansions produce a very 
small corresponding change in torque and the capacitance plate structure 
which significantly rejects any displacement other than torque. 
Temperature can also affect the capacitance structure. The top base member 
of the capacitor is preferably a low expansion material which is 
dimensional stable, such as quartz. The lower capacitance member is 
preferably made of the same material as the torque beam to avoid welds. 
The two capacitance plates are respectively mounted by aligned connections 
to a base member which, in turn, is attached to another mass. Since the 
structure is mounted in a vacuum, temperature change of the capacitance 
plates is affected primarily by thermal conduction through the mountings. 
The electronics used for this sensor can be as described in U.S. Pat. No. 
4,091,693. A ratio metric measurement is made using the relationship 
(C.sub.1 -C.sub.2)/(C.sub.1 +C.sub.2) so that the oscillator factor 
cancels out the reading. One of the important features of the present 
invention is that the capacitor can operate with minute deflection changes 
and produce measurable signals. 
Referring to FIG. 8, a schematic illustration shows the capacitor plates 
79a, 79b (base member 38) spaced parallel from plate surfaces 77a, 77b 
(base member 44). A dashed line 120 indicates a mechanical coupling to a 
vertical torque displacement axis 121. A second upper sensor 130, as shown 
in FIG. 5, is constructed and arranged with an identical arrangement of 
torsion beams 132, 134 and a central section 136 connected to a connector 
rod 138. The upper sensor 130 has capacitor plates 130a, 130b which are 
spaced apart from capacitor plates 140a, 140b on an outer base member 140 
(see FIG. 8). A dashed line 145 indicates a mechanical coupling to a 
vertical displacement axis 147. 
The upper base member 140 has a necked down connector portion 150 which 
connects to an elongated transverse bar member 152. The bar member 152 in 
cross section has a relatively long width dimension "W" as compared to 
it's depth dimension "D". The length "L" of the bar member 152 is 
sufficient to permit flexibility and to allow relative vertical movement 
of the base member 140 of the upper transducer should the temperature 
effects on the support member 110 cause a relative vertical movement of 
the upper transducer relative to the lower transducer. 
The lower base member 44 has a necked down portion 154 which connects to an 
elongated transverse bar member 156. The elongated bar member 156 (similar 
to bar member 152) in cross section has a relatively long transverse width 
dimension "W" as compared to it's depth dimension "D". The length of the 
bar member 156 is sufficient to permit flexibility and allow relative 
vertical movement of the base member 44 of the lower transducer should the 
temperature effects on the support member 110 cause a relative vertical 
movement of the upper transduced relative to the lower transducer. 
It can thus be appreciated that while temperature effects on the upper and 
lower transducers can move the transducers vertically relative to one 
another, the effects do not adversely impact the transducers. The bar 
members 152, 165 are electron beam welded at their ends 158 and the 
cross-sectional configuration provides rigidity in the plane normal to the 
axis 147, 121 so that torque effects on the lower base member are 
transmitted without loss to the upper base member. 
As shown in FIGS. 8 & 9, the capacitor plates 140a, 140b, 77a, 77b are 
connected (connection "X") to a common electrical ground or other 
electrical reference. The capacitor plates 79a, 79b are connected to a 
switch circuit means 160 which alternately samples the capacitances of the 
respective capacitors. The capacitance outputs are amplified and 
transmitted to an output circuit means 164 which derives a ratio signal 
output for transmittal and storage in a recorder or memory bank 166. 
Similarly, the capacitor plates 130a, 130b are connected in an independent 
electrical channel system to a switch 160', amplifier, output circuit 164' 
and recorder or memory bank 166' which are duplicate to the other 
electrical channel system. Thus two independent measurements are derived 
by independent systems. 
It can be appreciated that the independent systems provide significant 
reliability against malfunctions and derive independent measurements which 
provide significant reliability against electrical malfunctions and also 
provide for cross checking of measurements. All of the forgoing is 
accomplished with provisions for temperature compensation so that 
temperature effects are isolated between each of the transducers. It will 
be apparent to those skilled in the art that various changes may be made 
in the invention without departing from the spirit and scope thereof and 
therefore the invention is not limited by that which is enclosed in the 
drawings and specifications but only as indicated in the appended claims.