Capacitive pressure transducer with isolated sensing diaphragm

A capacitance transducer has a central chamber with a conductive diaphragm disposed therein to separate the chamber into at least two portions, at least one portion having an electrical conductor disposed thereon to form in combination with the diaphragm at least one variable sensor capacitor, and wherein at least one portion of the chamber is coupled by a passageway to communicate with an isolator having an isolator chamber and an isolator diaphragm which in combination with the conductive diaphragm enclose a substantially incompressible fluid such that when pressure is applied to an exposed side of the isolation diaphragm, the conductive diaphragm is urged to deflect and thus change the sensor capacitance. Configuration of the isolators, spaced from the sensor capacitors, and other factors disclosed herein, eliminate unwanted mechanical and thermal stresses thereby improving the capacitive sensors' response to pressure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to isolator arrangements for capacitive 
pressure sensors and to improved diaphragm mounting structures. 
2. Prior Art 
U.S. Pat. No. 3,618,390, owned by the same assignee as the present 
invention teaches the use of a sensing diaphragm which bottoms out on 
excessive pressure to protect the isolating diaphragms. This invention 
provided great impetus to capacitance pressure measurement techniques, as 
manifested by substantial commercial exploitation and success. The present 
invention as taught herein may be used in cooperation with the structure 
claimed in U.S. Pat. No. 3,618,390. 
SUMMARY OF THE INVENTION 
The present invention includes the use of a capacitance type pressure 
sensor having a diaphragm disposed in a central chamber and having 
isolators so that the process fluid or other pressure is applied to the 
isolator and the pressure is communicated to the sensor by a substantially 
incompressible fluid via passageway means to the central chamber. The 
diaphragm is then urged by the incompressible fluid to move to a position 
which along with an electrically conductive surface disposed on an 
internal surface of a portion of the central chamber forms a variable 
capacitor, which when driven by a suitable circuit produces an electrical 
signal responsive to pressure. 
The invention envisions remote isolators, electrical isolation of the 
sensor from the isolators, and improved sensor mounting, material 
selection and configuration to reduce the effects of static line pressure 
and temperature. Such deleterious effects are substantially reduced, 
resulting in an improved capacitive signal which is representative of 
pressure.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A transducer which preferably is used for differential pressure, gauge 
pressure, flow, level or other such pressure measurement is shown 
generally at 10. The transducer includes a transducer housing or frame 12 
which supports a sensor housing 14 and a pair of isolator housings 16a and 
16b. It is envisioned that housings 14, 16a and 16b may be included in or 
spaced from housing 12. The pressures to be sensed are represented by 
arrows 18 and 20 at the transducer input ports. Pressures 18 and 20 act on 
isolator diaphragms 22 and 24, respectively. Diaphragms 22 and 24 
preferably are very flexible and are formed in a conventional manner. The 
corrugations 26 of diaphragms 22 and 24 represent a preferred isolator 
diaphragm construction having a plurality of convolutions, as desired. 
Chambers 27 and 29 are defined by diaphragms 22 and 24 in cooperation with 
the respective housings 16a and 16b. Chambers 27 and 29 are coupled to 
passageways 28 and 30, which preferably are formed by stainless steel 
tubing, but may be formed from other suitable materials. 
Sensor housing 14 is preferably machined and formed from a metal such as 
stainless steel, preferably an austenitic stainless steel such as 304 is 
used. Generally, housing 14 is formed from two portions 32 and 34 which 
preferably are substantially equal in size and when assembled are 
generally divided by a sensor diaphragm 36 which is held at its edges and 
will elastically deflect under differential pressures and which typically 
is subjected to a desired radial tension. A central cone shaped cavity 38 
with bores 42 and 44 is formed in portion 32 and, similarly a central cone 
shaped cavity 46 with bores 48 and 50 is formed in portion 34. Conduits 52 
and 54 are formed in portions 32 and 34 to communicate with passageways 28 
and 30, respectively. The interior openings of the conduits 52 and 54 form 
continuations of the passageways 28 and 30 to communicate with a chamber 
53 defined in portion 32 by diaphragm 36 and the central section of 
material 60a in portion 32 and a second chamber 55 defined in portion 34 
by diaphragm 36 and the central section of material 60b in portion 34. An 
electrical conductor 56 is inserted through bore 42 to chamber 38 and, 
similarly, a conductor 58 is inserted through bore 48 to chamber 46. 
Conductors 56 and 58 may be metal tubes to aid in filling the sensor 
chambers with a non-compressible fluid. 
The electrically conductive portions of the housing 14 are electrically 
insulated from the metal tubes forming passageways 28 and 30 and from 
conductors 56 and 58. An insulative nonporous material 60a and 60b such as 
glass or ceramic, is filled into cavities 38 and 46 and bores 42 and 48 
and is bonded to housing portions 32 and 34 along a surface forming an 
angle .theta. with respect to the plane formed by the joining of portions 
32 and 34. The central section of material 60a and 60b and the central 
area of housing portions 32 and 34, as well as inner ends of conductors 56 
and 58 are then contoured or recessed by grinding or machining, preferably 
to provide a suitable stop surface for sensor diaphragm 36 when the 
diaphragm 36 deflects under an overrange pressure applied to the isolator 
diaphragms. Conduits 52, 54 may be a single cylinder as shown, or a 
plurality of small cylinders in accord with the teaching of U.S. Pat. No. 
3,618,390 to provide diaphragm support in an overpressure condition. 
A suitable electrically conductive material is then deposited in a layer on 
the inner surface of material 60a and 60b in each housing portion as at 61 
or 63. The layers face opposite sides of the sensor diaphragm 36 and are 
electrically coupled to conductors 56 and 58, respectively. Sensor 
diaphragm 36 preferably is formed from a suitable electrically conductive 
material and is fixed into position between housing portions 32 and 34 and 
layers 61 and 63 by a continuous bead weld 62 thus forming a common plate 
for material 61 and 63, hence forming two capacitors C.sub.1 and C.sub.2. 
A suitable conductor 64 is then coupled to sensor housing 14 which is at 
the same electrical potential as diaphragm 36. Sensor diaphragm 36 can 
also be formed from a nonconductive material and have a conductive portion 
disposed in or on the diaphragm to form such common plate for a variable 
sensor capacitor. A suitable conductor 64 is then coupled to such 
conductive portion. Bolts 70 may then be added to take up the pressure 
forces on sensor housing 14. 
A suitable, substantially incompressible fluid, such as silicone oil, is 
then filled into each side of the transducer assembly through conductors 
56 and 58 to the sensor diaphragm chamber formed in housing portion 32 by 
diaphragm 36, and to isolating chamber 27, and similarly to the sensor 
chamber in housing portion 34, and isolating chamber 29. When such spaces 
are filled, conductors 56 and 58 are pinched off at their outer ends and 
suitable leadwires are attached thereto. 
The action of pressure on isolator diaphragms 22 and 24, the substantially 
incompressible fluid in chambers 27 and 29, passageways 28 and 30, and on 
sensing diaphragm 36 is fully explained, for example, in U.S. Pat. No. 
3,618,390. The invention of the sensing diaphragm 36 bottoming out in an 
overpressure condition as taught in U.S. Pat. No. 3,618,390 or the 
isolating diaphragm 22 or 24 bottoming out in an overpressure condition 
may, as desired, be used with the present invention. 
The physical location of isolator diaphragms 22 and 24 spaced from sensor 
diaphragm 36 is shown somewhat schematically, as the location of isolator 
diaphragms 22 and 24 is not critical, providing that such diaphragms are 
located so as not to apply undesired mechanical stress, other than the 
pressure through the incompressible fluid, to sensor housing 14. While 
sensor housing 14 preferably is fixedly mounted in housing 12 it is not 
required that it be rigidly mounted as by welding. As shown, it is 
retained by flexible straps 71, which are formed from an electrical 
insulative material to electrically isolate the sensor housing 14 from 
transducer housing 12 and to support sensor housing 14. 
With chambers 27, 29, passageways 28 and 30 (including openings in conduits 
52 and 54) and the chambers between layers 61 and 63 and diaphragm 36 
filled with incompressible fluid, differentials between pressures 
represented by arrows 18 and 20 will cause diaphragm 36 to deflect 
proportional to pressure differential and its capacitance relative to 
layers 61 and 63 changes. 
Another embodiment of the invention is shown in FIG. 2. In this embodiment 
the sensor housing 14A is somewhat wider than the embodiment of FIG. 1. 
While the numbering corresponds to FIG. 1, (with a capital letter forming 
a part of the alphanumeric designations thereof) it is observed that with 
the increased width of sensor housing 14A, bores 44A and 50A are somewhat 
deeper in FIG. 2 than bores 44 and 50, and material 60A and 60B, has been 
filled to include a portion of such bores. An angle .theta. is the 
included angle from the plane of diaphragm 36A at its rest position to the 
conical surface forming the recess in the respective housing portion in 
which material 60A and 60B is filled. This angle determines the effective 
depth of the material 60A,60B (or 60a, 60b in the first form of the 
invention) which backs the capacitor plates 61A and 63A (or 61 and 63). 
Although an angle .theta. of approximately 45.degree. is preferred for the 
embodiments of FIGS. 1 and 2, it has been found that angles from 
25.degree. to 70.degree. have resulted in improved stability and thus 
improved performance over known constructions which comprise, for example, 
a non-compressive bond between the insulating material and metal. The 
angle also can be measured with respect to the central axis of the sensor 
housing which is perpendicular to the plane of diaphragm 36A (or 36) when 
it is at rest. 
One significant advantage of the present invention is improvement of the 
static pressure effect on the pressure span of the transducer. In prior 
art embodiments, the effect of static pressure on span error has been 
found to be approximately a one percent (1%) change in output across the 
instrument span per one thousand (1000) pounds per square inch (PSI) 
change of static pressure. In such known transducers, the pressure on the 
outside of the sensor housing, caused by the pressure being sensed acting 
on the isolating diaphragms, and pressure from the inside of the sensor 
chamber caused by the pressure being sensed on the incompressible fluid 
resulted in sensor housing deformation outwardly in a known manner, as per 
Poisson's ratio. 
Further, in known methods of manufacturing such capacitive transducers, the 
insulating material, upon which the conductive material is deposited to 
form the second plate of each of the variable capacitors, has been 
relatively thin in comparison to the insulative material thickness in the 
central cavity of the sensor housing as disclosed herein. When the 
insulative material is thin or when the insulating material to metal 
interface is somewhat parallel to the rest axis of the diaphragm 
(perpendicular to plane of the diaphragm), the insulating material-metal 
interfaces (bond) 65a, 65b, 65A, 65B are then subjected to a shear force 
which may cause the bond to weaken or fracture. When pressure is applied 
to a sensor having a fractured bond, such pressure causes the insulating 
material to move away from the diaphragm. The movement of the insulator 
material causes an undesirable change in capacitance not representative of 
the sensed pressure, which adds to the error effect caused by the static 
line pressure. When the sensor is formed in accordance with the present 
disclosure, bonds 65a, 65b, 65A, 65B are substantially in compression and, 
consequently, much less vulnerable to such fracture. 
By removing the isolators from the side of the sensor housing, the 
capacitor plate spacing on both sides of the diaphragm 36 increases with 
increasing static line pressure applied at 18, 20 due to slight outward 
motion of the sensor portions with respect to the sensor diaphragm. This 
static line pressure increase also causes portions 32 and 34 to warp 
slightly about their respective neutral axes (shown in FIGS. 2 and 5 at 
X--X) as the two housing portions tend to contract adjacent the diaphragm 
(as shown by arrows 70A in the FIGS. 2 & 5B). (The insulation material is 
not specifically shown in FIG. 5A or 5B since these Figures are 
illustrative only and apply to the configurations of FIGS. 1 and 2.) Such 
warping is perhaps best explained by reference to FIG. 5A which shows 
portions 32 and 34 at rest and in FIG. 5B which shows an exaggerated warp 
condition (for emphasis) as caused by increasing static line pressure. As 
the static line pressure is increased the capacitance spacing (d) of FIG. 
5A between the diaphragm 36 and capacitor plates 61 and 63 increases to d' 
(as shown in FIG. 5B) and such spacing change is not representative of the 
applied differential pressure. In accordance with the present invention, 
the change in capacitance caused by such warping is substantially 
compensated for by the decrease in diaphragm radial tension caused by the 
contraction adjacent the diaphragm. Radial tension or prestress applied to 
the diaphragm at time of construction along with suitable dimensions and 
materials results in the elastic stiffness of the diaphragm decreasing 
with increasing static pressure. Preferably the diaphragm material is high 
strength steel having good elastic characteristics. The compensation 
advantage is present at all static line pressures, but more fully realized 
at static line pressures above 500 psi. 
The following equations further explain the static line pressure 
compensation according to a preferred embodiment of the instant invention 
having a first capacitor C.sub.1 and a second capacitor C.sub.2 as 
described herein: 
EQU 0=CH-CL/CH+CL.varies.Xp/Xo.times.1/.delta..sub.o 
Where 
0=the output signal from the differential pressure capacitance cell. 
CH=the capacitance of the greater of C.sub.1 or C.sub.2. 
CL=the capacitance of the lesser of C.sub.1 or C.sub.2. 
Xp=diaphragm deflection with differential pressure. 
Xo=capacitance spacing at zero (0) static line gauge pressure. 
Xo'=capacitance spacing at elevated static line pressure. 
.delta..sub.o =diaphragm stretch at time of construction (initial 
prestretch). 
.delta..sub.o '=diaphragm stretch at elevated static line pressure. 
Simplifying: 
EQU 0.varies.Xp/Xo.delta..sub.o 
When the transducer is formed in accordance with the present invention as 
the static line pressure increases, the capacitance spacing Xo increases 
to Xo' and the diaphragm stretch (.delta..sub.o) decreases to 
.delta..sub.o '. By holding the product of Xo.multidot..delta. 
substantially equal to Xo'.multidot..delta..sub.o ' hence, substantially 
equal to a constant, the diaphragm deflection (Xp) is responsive to the 
differential pressure applied thereto and the output (0) is thus 
independent of static line pressure. 
A transducer, made in accordance with the embodiments of FIG. 1 and FIG. 2, 
but not having an angle .theta. between 25.degree. and 70.degree. rather 
having a cylindrical, metal-insulating material interface bond; that is, 
the bond interface was first generally perpendicular (.theta.=90.degree.) 
from diaphragm 36, then generally parallel (.theta.=0.degree.) from 
diaphragm 36 generally as shown in U.S. Pat. No. 3,618,390, was tested 
under actual loading conditions. This early form of the invention did not 
then include the compressive bond taught herein, but rather had the prior 
art shear bond. The improved bond is helpful as taught to avoid bond 
fracture and it is believed from analyzation and evaluation that such 
fracture did not occur and, therefore the nature of the bond did not 
affect test results. In the embodiment tested, the other principles of the 
invention were followed, such as separating the isolator 16a, 16b from 
sensor housing 14 and compensating for sensor housing 14 warp with a 
suitable sensor diaphragm 36 prestress. Sensor diaphragm 36 was 1.8 mils 
thick and approximately 1.12 inch in diameter and had approximately 
105,000 PSI in prestress applied (though prestress from 50,000 to 200,000 
PSI may be acceptable) and was formed from NiSpan C; insulative material 
60a, 60b, 60A, 60B was Owens 0120 glass; and, sensor housing 14 was NiSpan 
C material approximately 1.250 inches in diameter. Capacitance spacing 
(Xo) in the center was approximately 0.0075 inches. Isolators 16a, 16b 
were formed from stainless steel (304SST) and were approximately 3 inches 
in diameter and were coupled to chambers 53, 55 by passageways 28,30 
formed from 1/16 inch O.D. stainless steel tubing. The results of such 
testing are shown in FIG. 3. As shown, all test points deviations due to 
static line pressure effect from zero (0) PSIG to two thousand (2000) PSIG 
are less than 0.2% across the differential pressure span of 0 to 240 
inches of water. The curves of FIG. 3 show a very small mechanical 
hysteresis. Such mechanical hysteresis is not uncommon and depends not 
only on the instantaneous value of stress as caused by differential 
pressure and static line pressure but also on the previous history of such 
stress. 
Yet a further improvement is attained by the present invention, as the zero 
stability of the transducer, which in known transducers varies with both 
temperature and static pressure, is improved because the isolator housings 
are not in direct physical contact with the sensor housing. Only the tubes 
forming passageways 28 and 30 are in direct contact with sensor housing 14 
and these tubes yield to accommodate loads or changes due to temperature 
on the isolators without stressing the sensor housing 14. 
A test also was conducted to demonstrate the improved uncompensated 
temperature effect with respect to the stability of the output capacitance 
signal of the present invention of the embodiment described above; such 
results are shown in FIG. 4. The "uncompensated" effect is the error 
present before any compensation of an electrical signal is applied. 
Electrical signal compensation is commonly used to reduce errors further, 
but it is highly advantageous to provide a structure having a low 
uncompensated error. Each curve of FIG. 4 represents a separate 
calibration. Several such calibrations were accomplished, seven of which 
are shown on FIG. 4, one at 100.degree. F., then again at 100.degree. F., 
then at 200.degree. F., then 100.degree. F., then 0.degree. F., then 
100.degree. F., again 200.degree. F., and finally at 100.degree. F. The 
curves show that the configuration resulted in excellent stability and in 
a very low thermal hysteresis as the capacitance deviation at 100.degree. 
F. for three calibrations at that temperature was less than .+-.0.18%. 
Thermal hysteresis refers to the difference in calibration results at a 
specific temperature after coming to that calibration temperature from 
higher and lower temperatures respectively. 
Many embodiments formed of different materials and having different 
dimensions have been successfully tested; in successful tests, the sensor 
diaphragm 36 was formed from Havar.RTM. steel of Hamilton Industries (or 
Elgiloy.RTM. alloy of Elgiloy Co.), insulative material 60 was alkali lead 
glass, specifically Corning 1990 glass and the sensor housing 14 was 
austenitic stainless steel. 
One further advantage of the present invention is that since the isolator 
diaphragms are no longer an integral part of the sensor housing 14, the 
size of the isolator diaphragms may be increased relative to the sensor 
housing. This increase in size is important in some instances to reduce 
the effects of temperature and other factors on overall transducer 
performance. 
Further, sensor housing 14 preferably is electrically isolated from 
transducer housing 12 resulting in a simplification of the transducer 
circuitry when electrical isolation is desired which is often the case for 
industrial pressure measurements. 
While the invention has been described using a variable capacitance sensor, 
those skilled in the art understand that a variable impedance, that is a 
variable impedance variable reactance sensor, can be used with the 
invention as described herein. 
In summary, the several listed advantages as well as those apparent to 
those skilled in the art are realized from the improvements of the present 
invention.