Low cost pressure transducer particularly for medical applications

A pressure transducer (10) for measuring fluid pressure in a fluid path comprising a strain gauge circuit (37) of thick film piezoresistors formed on an alumina diaphragm (36) in a Wheatstone bridge configuration. Each resistance leg of the bridge typically includes a thick film measuring piezoresistor (R1A) to which selected thick film patch-in piezoresistors (R1B, R1C, R1D, R1E) are selectively connected to create a measuring resistance network (PN1). The measuring resistance network is adjusted to a predetermined resistance value to balance and optimize the electrical symmetry of the bridge. The pressure transducer (10) includes a mechanical stop member (46) located adjacent the alumina diaphragm (36) such that the fully deflected diaphragm contacts the stop thereby physically preventing the diaphragm from deflecting an amount that would cause the diaphragm to rupture.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to the area of pressure transducers 
and more particularly to pressure transducers for measuring a patient's 
blood pressure. 
2. Description of the Prior Art 
Pressure transducer sensor elements are commonly designed to have four 
resistor legs formed on a yieldable diaphragm area of a sensor element. 
Typically, a Wheatstone bridge is used in which the resistance in the 
bridge legs changes in response to pressure applied to the diaphragm. In 
order for the device to work in an acceptable manner, the resistance value 
of each of the legs must nominally be the same, and the input and output 
impedances of the bridge must also meet predetermined criteria. It is in 
the manufacture of the sensor element to satisfy these concerns that great 
cost is incurred. 
For example, commonly employed sensor elements may be semiconductor devices 
with the resistive bridge legs formed such as by appropriate doping of 
selected portions of material in the semiconductor material. In order to 
"balance" the bridge legs so that they have substantially the same 
resistance values, serial resistors are added into the bridge legs. Their 
size is physically trimmed so that the bridge legs have identical 
resistance values thereby balancing the bridge. Not only are additional 
manufacturing steps required, but these added balancing resistors 
introduce variability and instability into the system that is not 
desirable. 
Another form of sensor element may be made by utilizing thick film 
technology. One example of such a sensor element is made by depositing 
piezoresistive cermet inks onto a ceramic alumina substrate. The inks are 
then fired to form piezoresistors which change resistance in response to 
the alumina substrate being flexed by the pressure forces. These types of 
sensor elements are less expensive than the semiconductor type, however, 
there are drawbacks in their electrical characteristics that have 
heretofore limited their use. The greater unpredictability of the 
resistance values of the cermet ink materials ensures that significant 
adjustment of resistance values will be necessary to balance the bridge. 
Further, when the active measuring resistors of the bridge are physically 
trimmed, the piezoresistive characteristics of the fired cermet changes 
and/or deteriorates. To overcome that problem, it is known to connect the 
pressure transducer with the piezoresistor bridge to an external device 
which is used to balance and calibrate the bridge with external resistors. 
In addition to limitations in their electrical characteristics, the use of 
thick film piezoresistor sensor elements has been limited because of their 
more fragile mechanical structure. For example, the volumetric 
displacement of the alumina is greater than the volumetric displacement of 
a comparable silicon substrate. Therefore, to maintain the integrity of 
the mechanical structure of the substrate, the diaphragm flexure should be 
minimized, and that limitation generally requires that the diaphragm be 
relatively thick and/or that the diaphragm area be relatively small. 
In low pressure applications, such as medical applications, the above 
described limitations in the electrical and structural characteristics of 
cermet materials present significant problems. For example, blood pressure 
transducers are often tied into a fluid line inserted into the vascular 
system of a patient. The pressure in that line may be subject to very 
rapid changes which are equivalent to pressure shock waves, and the range 
of pressure magnitudes in the fluid line is large. For example, the line 
may be providing fluids intravenously by a gravity drip which normally 
presents a relatively low pressure, but a deadender may be inserted in the 
line which creates a relatively short pressure shock wave at a 
significantly higher pressure. Such a pressure change would normally 
dictate a smaller and thicker diaphragm. However, in medical applications, 
there is also a need to measure the fluid pressure with a high degree of 
accuracy and precision. Therefore, the area of the diaphragm must be 
sufficiently large to contain all the thick film resistors used in the 
Wheatstone bridge network; but it must be thin enough to flex sufficiently 
in order to obtain the necessary discrimination and precision. As a 
result, pressure shock waves can overflex the diaphragm and cause it to 
fracture or splinter rendering the pressure sensor useless. 
Also, in low pressure applications, the diaphragm of the ceramic alumina 
substrate is typically thinner and may be porous and may be in constant 
contact with a saline solution which is often used in medical 
applications. The fluid pressure may force the saline solution through the 
alumina diaphragm and into contact with the electric circuits on the 
opposite side of the diaphragm with undesirable consequences. Therefore, 
the rigorous demands placed on pressure transducers in medical 
applications in combination with the inherent electrical and structural 
limitations of thick film piezoresistor pressure sensor elements have 
tended to prevent that technology from being utilized for pressure sensors 
in the medical field. 
Blood pressure transducers in use today may be two component devices such 
as that shown in U.S. Pat. No. 4,920,922. As shown in that patent, one 
piece of the transducer is a disposable fluid dome that is placed into the 
fluid line. The other piece is a reusable sensor housing to which the 
fluid dome mounts to couple pressure of the fluid in the dome to the 
sensor element within the housing. To maintain isolation between the fluid 
and the sensor, the disposable dome and the reusable sensor housing each 
have a diaphragm thereacross. In use, the diaphragms are placed in a 
contiguous relationship to couple pressure from the dome to the sensor. 
After use, the dome is removed from the housing and disposed of, while the 
housing, with the expensive sensor component, is reassembled with a new 
dome for reuse. It would be desirable to reduce the cost of the sensor 
component containing the strain gauge so that the entire pressure 
transducer is disposable. 
SUMMARY OF THE INVENTION 
The present invention provides a pressure transducer utilizing thick film 
piezoresistor technology to take advantage of its lower cost but with 
modifications to the technology that overcome drawbacks previously 
encountered in utilizing thick film piezoresistor technology for pressure 
transducers. The invention is especially suited for use in medical 
applications where disposal of the pressure transducer after each use is 
desired. 
To this end, and in accordance with the principles of the present 
invention, a pressure transducer includes a transducer element having a 
strain gauge with at least one thick film pressure measuring resistor and 
at least one thick film patch-in resistor, both formed on the diaphragm of 
the transducer element. The diaphragm flexes in response to pressure 
forces which varies the resistance values of both the pressure measuring 
and patch-in resistors. The transducer includes output terminals coupled 
to at least the measuring resistor which provide an electrical output 
signal varying in relation to the changes in resistance of the pressure 
measuring resistor. The sensor includes a structure by which to 
selectively, electrically connect the patch-in resistor(s) to the 
measuring resistor to form a measuring resistance network such that the 
electrical pressure signal being monitored is modified without physically 
altering the resistance values of the measuring and patch-in resistors. 
The measuring resistance network comprises the active resistance of the 
pressure sensor and is electrically adjusted without physically altering 
the piezoresistive characteristics of the thick film measuring and 
patch-in resistors. The pressure transducer detects changes in pressure of 
a fluid mechanically coupled to the diaphragm by sensing changes in the 
resistance values of the thick film resistors in the measuring resistance 
network. A thick film trimming resistor may be located on a rigid support 
structure contiguous with the diaphragm and is electrically connected in 
series with the measuring resistance network to form a series resistance 
network. The trimming resistor provides for minor, physical trimming of 
the strain gauge. The pressure transducer may include combinations of the 
measuring resistance networks and series resistance networks connected in 
a Wheatstone bridge configuration. In the Wheatstone bridge configuration, 
everything needed to balance and adjust the sensitivity of the bridge is 
contained within the pressure transducer and is accomplished as a part of 
the manufacturing process. 
The transducer element is mounted in a fluid path housing thereby exposing 
the diaphragm and strain gauge formed thereon to fluid in the fluid path, 
such as, for medical applications. The fluid path housing includes a 
mechanical stop member having a stop surface extending substantially over 
the area of the diaphragm. The stop surface is spaced from the transducer 
element to permit the full range of transducer element deflection under 
normal pressure variations. But, if the pressure increases so that the 
transducer element would be caused to deflect beyond the normally expected 
range, the transducer element contacts the stop surface which mechanically 
prevents any further deflection of the transducer element to thereby 
prevent fracture or splitting thereof. One surface of the transducer 
element including the alumina diaphragm is covered with a flexible liquid 
barrier to prevent fluid from leaking through the diaphragm. 
The construction of the present invention has the advantage of utilizing 
lower cost thick film cermet resistor technology to make a strain gauge 
for a disposable pressure transducer that may be put to immediate use 
without having to adjust or balance the transducer. The pressure 
transducer has the further advantage of being able to electrically adjust 
the measuring resistors in the legs of the Wheatstone bridge by creating 
measuring resistance networks having approximately the same resistance 
values without physically trimming or changing the size of the measuring 
piezoresistors or associated patch-in piezoresistors. The invention has 
the advantage of minimizing the voltage drop across the non-pressure 
measuring trimming resistors. The invention has the further advantage of 
providing an electrically symmetrical bridge configuration that does not 
necessarily require null compensation to compensate for ambient 
temperature variations. The pressure transducer construction provides a 
mechanical stop to prohibit the transducer element from rupturing because 
of excessive fluid pressure. 
These and other objects and advantages of the present invention will become 
more readily apparent during the following detailed description in 
conjunction with the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to the cross-sectional view of FIG. 1 and the disassembled 
perspective view of FIG. 2, a pressure transducer 10 in accordance with 
the principles of the present invention is designed for use in medical 
applications to detect a bodily fluid pressure, for example, blood 
pressure. The transducer 10 is comprised of a fluid path housing 12 
containing a fluid path 14 which exits the fluid path housing 12 by means 
of connector stems 16 and 18. A Luer fitting, one of which is illustrated 
at 20, is attached to each of the connecting stems for connecting the 
pressure transducer 10 to associated tubing or other fluid circuit 
elements. Fluid flowing through the fluid path 14 of transducer 10 
impinges on a pressure sensor, for example, transducer element, 22 
containing a strain gauge made from a printed circuit of piezoresistive 
thick film cermet resistors in a Wheatstone bridge configuration. Power to 
the Wheatstone bridge is provided by electrical conductors at 24 having 
one end 25 in contact with circuit elements on the transducer element and 
another end 26 connected to wires 27 bundled within a cable 28. The cable 
28 terminates with an electrical connector 29. The electrical connector 29 
is adapted to interconnect with a standard commercial device (not shown), 
which is effective to supply power through the cable 28 and electrical 
conductors 27 and electrical contacts 24 to the Wheatstone bridge and 
receive output signals therefrom such as a voltage signal which is 
directly correlated to fluid pressure in the fluid path 14. In a manner 
well known, that external device converts the voltage signal into a 
representation of fluid pressure either as a numeric or graphic display. 
One feature of the present invention is that the construction of the 
pressure sensing transducer element 22 is comprised of a thin, flexible 
substrate 30, such as ceramic alumina, having one side 31 containing the 
strain gauge. The opposite side 32 of the alumina substrate 30 is 
connected to one side 33 of a thick rigid support member 34 which may also 
be ceramic alumina. The alumina support member 34 contains a centrally 
located hole 35 which exposes a circular center portion of the opposite 
side 32 of the alumina substrate 30 to fluid in the fluid path. Fluid 
pressure is effective to push or deflect the exposed circular center 
portion of the alumina substrate 30 away from the fluid path housing 14. 
The circular center portion of the substrate 30 which is bounded by the 
center hole 35 of the alumina support member 34 and which is subject to 
deflection by the fluid pressure is referred to as diaphragm 36. The 
portion of the substrate 30 contiguous with and contacting the support 
member 34 is also considered to be and is referenced as part of the 
support member 34. 
FIG. 3 is a bottom plan view of the transducer element of FIG. 2 
illustrating the physical layout of the electrical circuit of the 
transducer element 22 including the strain gauge circuit 37 within the 
diaphragm 36 which is shown within the phantom line 45 on the transducer 
element 22. FIG. 4 is a schematic electrical diagram of the circuit of the 
transducer element 22 shown in FIG. 3 including the strain gauge circuit 
37 which is connected in a Wheatstone bridge configuration. The transducer 
element 22 is manufactured by depositing (e.g., printing) on the alumina 
substrate 30 conductors 47 of a palladium silver cermet ink. The areas 55 
of the conductors 47 which border and lie under the measuring resistors 
and the contacts areas 57 of the conductors 47 are also printed with a 
gold cermet ink. Thereafter, a piezoresistive cermet ink is deposited to 
form the resistors in the circuit of the transducer element 22. The cermet 
inks are fired in accordance with known practices. The edges of the 
resistors lie over the gold areas 55 of the conductors 47. The strain 
gauge circuit 37 of the transducer element 22 includes four electrical 
strain gauges which are interconnected to form the four legs of a 
Wheatstone bridge. Each of the four bridge legs 1, 2, 3, 4 include at 
least one measuring piezoresistor R1A, R2A, R3A, R4A functioning as a 
strain gauge. Each bridge leg should desirably have the same predetermined 
resistance. Since the thick film resistors can only be manufactured to 
within .+-.20% of their desired nominal value, for example, after the 
resistors have been manufactured, the resistance of each of the measuring 
resistors is adjusted to as close as possible to the desired resistance 
value. 
To overcome the previously described problems resulting from physically 
trimming the thick film measuring resistors, the present invention 
provides at least one, but advantageously a group of, patch-in resistors 
associated with each measuring resistor to form a resistor measuring 
network. Each group of patch-in resistors is the same. For example, 
referring to bridge leg 1, the patch-in resistors include thick film 
piezoresistors R1B, R1C, R1D, R1E having one side 21 electrically 
connected by conductors 47 on the diaphragm 36 to one side 38 of measuring 
resistor R1A. The opposite side 23 of each of the patch-in resistors R1B, 
R1C, R1D, R1E, is connected by conductors 47 on the diaphragm 36 to a 
respective conductive contact 39 located over the support member 34 
outside the area of the diaphragm 36. An opposite side 51 of measuring 
resistor R1A is connected by a conductor on the diaphragm 36 to a 
conductive contact leg 40 located over the support member 34. By placing 
conductive jumpers 53 between selected ones of the contacts 39 and the 
contact leg 40, the patch-in resistors R1B, R1C, R1D, R1E, are 
selectively, electrically connected in parallel with the measuring 
resistor R1A. The conductive jumpers 53 may be a conductive polymer 
commercially available from Minico/Asahi Chemical, of Congers, N.Y. The 
conductive polymer is cured at a curing temperature below the firing 
temperature of the thick film resistors. By locating the contacts 39, 40 
over the support member 34, the jumpers 53 may be attached and removed 
without disturbing the electrical characteristics of the piezoresistors in 
the measuring resistance network. 
Continuing with the reference to bridge leg 1, the group of patch-in 
resistors function as optional supplemental measuring piezoresistors that 
are selectively combined with measuring piezoresistor R1A to create a 
measuring resistance network PN1 which functions as a strain gauge. 
Therefore, the bridge leg 1 is electrically adjusted to be relatively 
close to its desired predetermined resistance value without physically 
altering the piezoresistive characteristics of the measuring and patch-in 
resistors. The resistance values of R1A, R1B, R2C, R1D, R1E are selected 
such that they can be electrically combined to provide a collective 
resistance for PN1 that is, for example, 90%.+-.5% of the design 
resistance value for the bridge leg. With the above network, thick film 
resistors that are individually manufactured to within .+-.20% of their 
nominal resistance value are adjusted to form a measuring resistance 
network that is within approximately .+-.5% of a desired network 
resistance value. To obtain the remaining desired resistance for bridge 
leg 1, a small trimming resistor R5 may be provided in series with the 
measuring resistance network PN1 to form a series resistance network. 
Resistor R5 is desirably formed of a deposited cermet ink and is located 
outside the area of the diaphragm so as to be generally insensitive to the 
measured pressure. In other words, the resistance value of the trimming 
resistor R5 remains relatively constant in response to flexure of the 
diaphragm. The design print resistance value for R5 is selected so that R5 
may be trimmed to, for example, 5.4% of the design resistance value of the 
bridge leg. Consequently, resistor R5 may then be used to adjust upwardly 
the resistance value of the series resistance network of bridge leg 1 to 
the desired predetermined resistance. Therefore, the desired resistance 
value of bridge leg 1 is achieved while ensuring that the piezoresistive 
characteristics of the measuring and patch-in resistors are not altered or 
deteriorated and further ensuring that a minimum voltage drop is desirably 
maintained across the series trimming resistor. 
With the above structure and electrical adjusting process, measuring 
resistance networks PN1, PN2, PN3, PN4 for each respective bridge legs 1, 
2, 3, 4 are created by selectively connecting the groups of patch-in 
resistors R1B, R1C, R1D, R1E; R2B, R2C, R2D, R2E; R3B, R3C, R3D, R3E; R4B, 
R4C, R4D, R4E; with respective ones of the measuring resistors R1A, R2A, 
R3A, R4A. Thereafter, the trimming resistors R5, R6, R7, R8 of each of the 
bridge legs 1, 2, 3, 4 are physically trimmed to adjust the resistance of 
the series resistance network of each of the bridge legs 1, 2, 3 4 to the 
desired predetermined resistance value. As described above, the resistance 
values of the measuring resistance networks are adjusted close to the 
desired bridge leg resistance;and therefore, each of the respective series 
trimming resistors R5, R6, R7, R8 can be made small with relatively little 
voltage drop. The trimming resistors R5, R6, R7, R8 are formed on a 
non-yieldable portion of the transducer element 22 outside the area of the 
diaphragm 36. Therefore, the trimming resistors R5, R6, R7, R8 are 
isolated from deflections of the diaphragm, and their resistance is 
constant during the operation of the strain gauge. 
The design impedance of the bridge is a function of many factors including 
the application, associated equipment, industry standards, etc. The bridge 
must be designed so that it has the sensitivity required by the 
application. Further, to maintain the desired sensitivity, the maximum 
voltage drop must be maintained across the measuring resistance network, 
therefore, the design impedance will dictate the values of the measuring, 
patch-in and series resistors. For the present invention, the design 
impedance, i.e., the desired predetermined resistance value, for each 
bridge leg network is 350 ohms. Given that the design resistance value for 
each measuring resistance network is, for example, 90%.+-.5% of the design 
resistance of the bridge leg, then each measuring resistance network has a 
resistance value of not less than about 298 ohms and not more than about 
333 ohms. Given that the trimming resistor has a design print resistance 
value of, for example, 5.4%, then each trimming resistor has a nominal 
resistance value of not more than about 19 ohms before trimming. 
Therefore, the total resistance value of the measuring, patch-in and 
trimming resistors of the series resistance network in each bridge leg is 
about 350 ohms. 
As illustrated in FIG. 4, the strain gauge circuit 37 has contacts 
comprised of test points TP1, TP2, TP3, TP4, contacts for jumpers J1, J2, 
input terminals +E, -E and output terminals +S, -S. Using the above 
contacts, various procedures may be used to calibrate each of the bridge 
legs. For example, in one procedure, the resistance values of the 
measuring, patch-in and trimming resistors are measured, and the potential 
measuring network resistance value in each bridge leg is calculated for 
every possible combination of patch-in resistors. Next, given the measured 
values of the series resistors, a combination of patch-in resistors is 
chosen which provides a bridge leg resistance which is closest to the 
design value and which, if required, can be brought closer to the desired 
value by physically trimming the series resistor. The bridge leg adjusted 
first is the one to which the resistance values of the other bridge legs 
can be most closely adjusted and trimmed. The above processes can .be 
automated wherein the resistance measurements, the calculations of the 
measuring resistance network and bridge leg resistance values, the 
selection of the patch-in resistors, the application of the conductive 
jumpers and the physical trimming of the series resistors with laser 
trimming or abrading techniques are accomplished under the control of a 
programmed computer. 
After the resistance adjusting and trimming processes are completed, 
conductive jumpers are connected across the contacts of junctions J1 and 
J2 which are effective to complete the bridge circuit. At this point, the 
sensitivity of the pressure transducer is calibrated. The output signal 
from outputs +S, -S, must have a predetermined sensitivity relative to an 
input pressure so that the output signal may be used to produce accurate 
representations of pressure using commercial off-the-shelf equipment. An 
industry standard for the sensitivity of a pressure transducer output 
signal in medical applications is 5 microvolts/volt/millimeter of mercury 
(.mu.v/v/mm Hg). The sensitivity of the bridge is calibrated by connecting 
the bridge circuit to a reference source of fluid pressure, for example, 
100 mm Hg, and measuring changes in the output voltage signal in response 
to predetermined fluid pressure changes. If the sensitivity is too high, 
the value of a sensitivity resistance in series with the bridge is 
calculated which would provide the desired sensitivity. That sensitivity 
resistance value is divided in half; and the sensitivity resistors in each 
of the sensitivity resistor networks R9A through R9C and R10A through R10C 
are selectively connected together and/or physically trimmed so that each 
sensitivity network has a resistance value equal to half the sensitivity 
resistance value. The sensitivity resistance networks are designed to have 
a range of resistance values of from zero to about 340 ohms. Equal 
sensitivity resistance networks are connected to both sides of the bridge 
to maintain symmetry. 
The utilization of the sensitivity resistors to reduce bridge sensitivity 
also changes the resistance of the bridge as measured across the external 
power supply. To compensate for that, the resistors R12 and R13 are 
selectively connected in series and are connected in parallel across the 
power supply terminals. R12 and R13 are selectively physically trimmed to 
reduce the terminal impedance back to the design impedance value. The 
above sensitivity adjustments may also be automated. A 16,000 ohm 
calibration resistor R11 is connected in series with an external 150,000 
ohm resistor and connected to the +E and +S terminals to electrically 
simulate a pressure input. The calibration resistor R11 is trimmed so that 
the series load simulates an input pressure of 100 mm Hg. The calibration 
process is described in detail in U.S. Pat. No. 4,760,730 issued to the 
assignee of the present invention, and the disclosure of which is fully 
incorporated herein by reference. 
The above described processes create a strain gauge in a Wheatstone bridge 
configuration that is electrically symmetrical, that is, the resistance 
value of each of the legs is as close as possible to the same 
predetermined value. Electrical symmetry is also considered in adjusting 
the bridge sensitivity. Not only is electrical symmetry important, but 
physical symmetry of the thick film piezoresistors on the substrate 30 is 
also important. Referring to FIG. 3, the opposing measuring resistance 
networks PN1 and PN3 are located at the center of the diaphragm 36. Within 
each of the measuring resistance networks, the group of patch-in resistors 
is located on the diaphragm adjacent its respective measuring resistor. 
Those measuring resistance networks are identical in physical size and 
symmetrical about the diaphragm center. Consequently, those parallel 
resistance networks PN1 and PN3 are subject to theoretically identical 
stresses as the diaphragm deflects in response to fluid pressure; and the 
voltage drop across the series resistance networks of bridge legs 1 and 3 
should be identical. Further, since each bridge leg is as close as 
possible to the same resistance value, the voltage signal produced on the 
output terminals +S, -S will be an accurate representation of resistance 
changes in the bridge caused by changes in fluid pressure. 
The physical symmetry of the bridge is further enhanced by the resistance 
networks PN2 and PN4 of the opposing respective bridge legs 2 and 4, being 
located on the outer edge of the diaphragm adjacent the support member 34. 
Therefore, the bridge legs 2, 4 will experience theoretical identical 
resistance changes in respective measuring resistance networks PN2, PN4. 
The physical layout and location of the components of the transducer 
element on the outer edge and at the center of the diaphragm is also 
important to minimize thermal effects. The transducer element 22 is 
designed to minimize the effects of temperature gradients across its 
surface. If the steady state temperature gradient across the surface of 
transducer element 22 can be maintained relatively constant regardless of 
changes in ambient temperature and changes in current flow through the 
bridge, then the effects of a change of temperature are neutralized; and 
temperature compensation such as null compensation is not required. 
The measuring resistors and parallel patch-in resistors in all of the 
bridge legs are made from a relatively high gauge factor ink. Gauge factor 
is a measure of the change in resistance per unit change in the area of 
the resistor caused by strain in the substrate on which resistor is 
formed. In other words, gauge factor is a measure of the sensitivity or 
change of resistance of the piezoresistor in response to pressure causing 
the diaphragm to flex, thereby changing the physical size, or area, of the 
piezoresistor. Therefore, using a higher gauge factor will increase the 
sensitivity; however, as the gauge factor increases, the thermal stability 
of the resulting thick film resistor decreases. Reducing the thermal 
stability of the device may require the use of thermal compensation 
resistances which adds complexity and cost to the bridge circuit. In 
addition, the gauge factor of the measuring and patch-in resistors should 
be higher than the gauge factor of the series and other resistors in the 
bridge circuit which should be designed to experience less change in 
resistance in response to the fluid pressure. Therefore, a gauge factor 
must be chosen that provides the highest sensitivity without unreasonable 
reductions in the thermal stability of the bridge. With the present 
invention, the gauge factor of the measuring resistors and the patch-in 
resistors formed on the diaphragm 36 is approximately 19.+-.1. The gauge 
factor of the other resistors in the bridge circuit should be as small as 
possible; and with the present invention, the gauge factor of the trimming 
resistors, the sensitivity and calibration resistors all of which are 
located outside the area of the diaphragm 36 on the support member 34 is 
approximately 4.+-.1. Those resistors have very stable resistance values; 
and to the greatest extent possible, they represent constant resistance 
values. In contrast, the measuring resistors and associated plurality of 
patch-in resistors are very sensitive to fluid pressure changes and 
produce the major portion of the voltage drop in each of the serial bridge 
leg circuits. 
The measuring resistors R1A, R2A, R3A, R4A have a nominal print resistance 
value of 440 ohms. The patch-in resistors R1B, R2B, R3B, R4B have a 
nominal print resistance value of 1,600 ohms; patch-in resistors R1C, R2C, 
R3C, R4C have a nominal print resistance of 2,700 ohms; patch-in resistors 
R1D, R2D, R3D, R4D have a nominal print resistance of 5,600 ohms; and 
patch-in resistors R1E, R2E, R3E, R4E have a nominal print resistance 
value of 10,200 ohms. All of the measuring resistors and all of the 
patch-in resistors are manufactured from a cermet ink having a sheet 
resistivity of 4,000 ohms per square. The series trimming resistors R5, 
R6, R7, R8 have a nominal print resistance value of 19 ohms. The 
sensitivity resistors R9A, R10A have a nominal print resistance value of 
12 ohms, the sensitivity resistors R9B, R10B have a nominal print 
resistance value of 24 ohms; and the sensitivity resistors R9C, R10C have 
a nominal print resistance value of 77 ohms. All of the series trimming 
resistors and all of the sensitivity resistors are manufactured from a 
cermet ink having a sheet resistivity of 30 ohms per square. The impedance 
matching resistors R12, R13 have a nominal print resistance value of 475 
ohms and are manufactured from a cermet ink providing a sheet resistivity 
of 1000 ohms per square. The calibration resistor has a nominal print 
resistance value of 16,000 ohms and is manufactured from a cermet ink 
producing a sheet resistivity of 10,000 ohms per square. 
As previously generally described, the above strain gauge circuit is 
printed on the one side 31 of the alumina substrate 30 which is connected 
to an alumina support member 34. The substrate 30 is approximately a one 
inch square of alumina which is 0.009 inches thick. The support member 34 
is a one inch square of alumina which is 0.100 inches thick with a center 
hole 35 having a diameter of 0.800 inches, thereby providing a diameter of 
the diaphragm of 0.800 inches. The substrate 30 and support member 34, 
shown in FIG. 2, may be bonded together by adhesives or the like. One such 
adhesive may be a thixotropic organic paste material such as 
PYRO-PUTTY.TM. adhesive, type 656, available from Aremco Products, Inc., 
of Ossining, N.Y. The adhesive is cured at a temperature below the firing 
temperature of the thick film resistors. The adhesive should minimize 
extraneous stress forces on the substrate arising from the bonding 
material adhering to the substrate 30 and the support member 34. 
Alternatively, the components shown as the substrate 30 and support member 
34 may be molded, pressed and fired as a monolithic part. 
Referring to FIGS. 1 and 2, after the substrate 30 containing the strain 
gauge circuit 37 and the support member 34 are assembled into a transducer 
element 22, the transducer element 22 is mounted in the fluid path housing 
12. During use, the pressure transducer will not only measure forces 
applied by the fluid pressure, but the output signal from the strain gauge 
will reflect forces created by internal stresses that are mechanically 
transmitted from the housing members to the transducer element 22. For 
example, handling of the pressure transducer 10 during use should not 
result in forces being transmitted to the transducer element 22. 
Therefore, the pressure transducer 10 must be isolated from external 
forces that are transmitted through the pressure transducer housing 
members. Complete physical and force isolation is not reasonably possible; 
however, to minimize the exposure of the transducer element to external 
forces, a spacer 41 comprised of a closed cell acrylic foam with pressure 
sensitive adhesive on both sides is used to mount the transducer element 
22 to the fluid path housing member 12. A SCOTCH.RTM. VHB 4930 series 
product available from 3M is a suitable force absorbing material for the 
spacer 41. 
To facilitate assembly of the transducer element 22 into the fluid path 
housing member 12, a common corner of the substrate 30, the support member 
34 and the spacer 41 is chamfered. The fluid path housing 12 contains a 
fluid path cavity in the fluid path 14 having a cross-sectional area 
matching the cross-sectional area of the diaphragm 36. Further, the fluid 
path housing 12 has an internal peripheral shoulder adjacent a second 
internal cavity having a shape matching the periphery of the transducer 
element 22. The second cavity and shoulder receive and support the 
transducer element 22 in proper alignment with respect to the fluid path 
14 in the fluid path housing 12. Proper alignment is required so that the 
contacts 57 on the transducer element are oriented to contact the 
electrical conductors 24. 
During use, the opposite side 32 of the substrate 30 is exposed to fluid in 
the fluid path 14. The transducer element 22 is Al.sub.2 O.sub.3 alumina 
substrate commercially available from Kyocera America Inc. of DesPlaines, 
Ill. Depending on the techniques used to manufacture the diaphragm and the 
diaphragm design, the alumina diaphragm may be porous; and therefore, 
there is some potential that the pressure of the fluid in the fluid path 
14 may force fluid through the alumina substrate 30. Fluid forced through 
the alumina diaphragm 36 could interfere with the electrical connections 
and circuits of the strain gauge printed on the one side 31 of the 
substrate 30. To prevent that potential for fluid seepage, the surfaces 42 
on the spacer 41, the support member 34 and substrate 30 are coated with a 
flexible liquid barrier. For example, a thin impermeable layer of a 
medical silicone gel system, Part No. Q7-2218 available from Dow Corning 
which is then cured, or a vapor deposition layer of YLENE.RTM., or a UV 
curable silicone based elastomer. 
In order to accommodate the bridge circuit of the present invention and 
provide the desired sensitivity, the alumina diaphragm on the substrate 30 
is relatively large and thin; and therefore, in responding to fluid 
pressures, the diaphragm 36 has a relatively large volumetric 
displacement. When used as a blood pressure monitor, the pressure 
transducer 10 is designed to operate in a pressure range of from -30 mm Hg 
to +300 mm Hg; however, the application of a deadender to the line may 
result in a pressure in excess of +4,000 mm Hg. To protect the diaphragm 
36 from excessive deflections caused by overpressure conditions during use 
which would normally result in the diaphragm rupturing, a mechanical stop 
member 43 is connected to the fluid path housing member 12. Referring to 
FIG. 5, the mechanical stop member 43 has one side 44 which is adjacent an 
opposite side of the substrate 30 of transducer element 22 when the 
mechanical stop member 43 is connected to the fluid path housing member 
12. The one side 44 has a circular recessed area, or well, 46 which has a 
depth permitting the diaphragm to deflect through its full range of normal 
operation, for example, 0.001 to 0.00.2 inches. However, the recessed area 
46, which is nominally 0.002 inches deep, functions as a mechanical stop, 
or stop surface to limit flexure of the diaphragm 36 of the transducer 
element 22. Surface 46 has a slightly concave shape to conform to the 
shape of the deflected diaphragm 36. Therefore, pressure shock waves which 
would normally cause an unprotected diaphragm to fracture from excess 
deflection, push the diaphragm 36 of transducer element 22 into contact 
with the mechanical stop member 43 prior to the point of fracture of the 
transducer element 22, thereby protecting the transducer element from 
damage. 
In assembling the mechanical stop member 43 to the fluid path housing 
member 12, an adhesive is applied along the perimeter of the mechanical 
stop member as shown by the glue line 48 (FIG. 2). The sidewalls 50 have a 
peripheral lower edge 52 with a peripheral step or notch 54 on an inner 
directed side of the peripheral lower edge 52. As the fluid path housing 
member 12 is slidingly engaged over the outside walls 47 of the mechanical 
stop member 43, the inner peripheral edge with notch 54 is effective to 
capture and retain the adhesive applied along the line 48. That adhesive 
connection is effective to keep adhesive away from and off of the one side 
31 of the substrate 30. Any adhesive touching the transducer element 22 
could mechanically couple the transducer element 22 to the pressure 
transducer housing and make the transducer element 22 susceptible to 
housing stresses caused by external forces. 
When the mechanical stop 43 is fully engaged with the fluid path housing 
12, the one side 44 of the mechanical stop 43 is in contact with the one 
side 31 of the substrate 30 of the transducer element 22. To minimize the 
probability that such contact will interfere with the electrical bridge 
circuit on the one side 31 of the substrate 30, the one side 44 of the 
mechanical stop member 43 has a number of recesses, or reliefs, 76 shown 
in FIG. 5. The recesses 76 are adjacent to more sensitive elements of the 
electrical bridge circuit, for example, areas in the circuit where 
conductive polymer jumpers 53 are used to selectively connect resistors on 
the transducer element 22. 
A base housing member 58 contains the electrical conductors 24 which are 
secured to the base housing member 58 in a known manner. Further, the 
insulated wires 27 contained in cable 28 are pressed into insulation 
separating grooves on tile lower ends 26 of the electrical conductors 24 
thereby creating an electrical contact between the conductor in the wires 
27 and their respective electrical conductor 24. The mechanical stop 
member 43 contains a number of holes 60 which are dimensioned and 
positioned to receive the upper ends 25 of the electrical conductors 24 as 
the base housing member 58 is moved into alignment and contact with the 
mechanical stop member 43, as best shown in FIGS. 1 and 2. The base 
housing member 58 contains four alignment pins 64 which are received by 
mating alignment sockets 66 in the mechanical stop member 43. As the 
alignment pins 64 are pushed into engagement with the alignment sockets 
66, that engagement is effective to connect the base housing member 58 to 
the mechanical stop member 43. Connecting the base housing member 58 to 
the mechanical stop member 43, pushes the upper ends 25 of the electrical 
conductors 24 into a contacting relationship with contact pads on the 
lower side of the substrate 30. 
As the fluid pressure causes the diaphragm 36 to deflect toward the one 
side 44 of the mechanical stop member 43, it is important that an air 
pressure force not build up between the one side 31 of the diaphragm 30 
and the one side 44 of the mechanical stop member 43. Any change in air 
pressure over ambient air pressure could adversely impact the accuracy 
with which the strain gauge 37 is measuring the fluid pressure force being 
exerted on the opposite side 32 of the diaphragm 36. Therefore, the holes 
60 in the mechanical stop member 43 are of sufficient size to permit the 
passage of air from the depression 46 through the holes 60 and into a 
space between the mechanical stop member 43 and the base housing member 
58. Further, the peripheral lower edge of the mechanical stop member 43 
has a tongue 70 which engages a peripheral groove 72 on the upper 
peripheral edge 74 of the base housing member 58. The engagement of the 
peripheral tongue 70 into the peripheral groove 72 is not air tight; and 
therefore, a pressure differential from ambient air pressure will not 
occur within the space between the base housing member 58 and the 
mechanical stop member 43 because of air flow through the joint created by 
the peripheral tongue 70 and the peripheral groove 72. 
To further protect the pressure transducer from excessive pressures during 
use, a supplementary fluid pressure relief valve is provided on the stem 
of the housing member 12 which has a radial through-hole 78 covered by an 
elastic band 80. Excessive fluid pressures will push the elastic band 80 
away from the radial hole 78 thereby permitting fluid to leak out of the 
fluid path housing member and relieving the excessive fluid pressure. 
In use, the pressure transducer 10 may be used to continuously monitor 
blood pressure by connecting the pressure transducer to a catheter 
inserted into the vascular system of a patient. The catheter is filled 
with a saline solution to form a static column by which blood pressure is 
transmitted through the catheter line. The pressure transducer 10 detects 
blood pressure and transduces the blood pressure into an electric signal. 
The electric signal is used by other equipment to provide numeric and/or 
graphic representations of the blood pressure. Normal slight handling or 
slight motion of the pressure transducer during use does not result in 
excessive spurious pressure readings because the sensor element is 
mechanically isolated from the housing with the spacer 41. Further, the 
mechanical stop provided by the mechanical stop member 43 protects the 
sensor element from overdeflecting in response to pressure shock waves 
that may be introduced into the fluid system. Fluid in the system is 
prevented from migrating through the alumina sensor element by the thin 
layer of silicone gel applied in the area of the diaphragm. The above 
features permit the use of less expensive thick film technology to provide 
a blood pressure monitor that is disposable. Further, when a new 
transducer is required, because all of the balancing and sensitivity 
adjustments are self contained within the pressure transducer and preset, 
no balancing or sensitivity adjustments are necessary before the 
transducer is used. 
While the present invention has been set forth by a description of the 
preferred embodiment in considerable detail, it is not intended to 
restrict or in any way limit the claims to such detail. Additional 
advantages and modifications will readily appear to those who are skilled 
in the art. For example, while alumina was chosen for the substrate 30 and 
support member 34 comprising the transducer element 22, zirconia or other 
material may also be used to provide a substrate material supporting the 
deposited resistive inks to define a strain gauge. The strain gauge may be 
designed such that the patch-in resistors formed on the diaphragm with the 
measuring resistors are sized to be connected in series with the measuring 
resistor. Alternatively, the patch-in resistors may be connected to the 
measuring resistor in a combination of parallel and serial connections. 
The invention may be used to provide a strain gauge that is comprised of 
only a single measuring resistance network. The pressure transducer 
housing members are made of a polycarbonate material, but other materials 
may be used. Further, the present invention illustrates all of the 
resistances on a single substrate, however, the resistances could be put 
on multiple substrates, thereby separating them and making individual 
substrates more compatible with performance requirements. The mechanical 
stop member may be flat instead of concave, and the stop member may be 
integrated into the structure of the pressure transducer in various 
different ways. The invention in its broadest aspects is therefore not 
limited to the specific details representative and illustrative examples 
shown and described. Accordingly, departures may be made from such detail 
without departing from the spirit and scope of applicant's general 
inventive concept.