Pressure transducer

A pressure transducer incorporates a tubular body symmetrical in two transverse directions, and two pairs of diametrically opposed active strain gages secured to a single surface of the body at equally spaced locations which maintain mechanical and electrical two-fold symmetry, oriented to respond to transverse tangential bending strain in the wall of the tubular body. The four strain gages are connected in a four-legged bridge circuit, which produces a large amplitude linear output signal which is compensated for first and second order errors, without hysteresis. The transducer has a very large safety factor against rupture and may be used in line with a conduit without impeding fluid flow. It is usable for all pressure ranges down to cryogenic temperatures, and may be welded in place with no seals as part of a conduit. It is highly sensitive, with improved resolution.

BACKGROUND 
1. Field of the Present Invention 
The present invention relates to pressure transducers and more particularly 
to pressure transducers employing strain gages. 
2. The Prior Art 
A variety of pressure transducers have been developed in the prior art, 
several of which employ plural active strain gages in a bridge circuit, 
along with passive elements which are responsive to temperature but not 
strain, in order to effect a temperature compensation. One such transducer 
is described in Swiss Pat. No. 549,794. While such prior art transducers 
are effective to reduce the effect of variation of temperature, they do 
not attempt to compensate for other error signals and display a large 
hysteresis, and so the attainable resolution is quite poor. They are not 
suitable for vacuum operation, and therefore cannot be leak-checked with 
the most accurate leak-checking techniques. They are also unsuitable for 
use in conditions of low temperature or high pressure. 
Some of the pressure transducers known in the prior art also suffer from 
one or more of a variety of other disadvantages, including the need for a 
feed through capability to permit a strain gage mounted on one side of a 
surface to transmit a signal to the other side of the surface, and the 
need for movable seals or sealing mechanisms for sealing against the 
pressure being measured. These disadvantages are especially critical when 
the pressure transducer is to be used with toxic or radioactive gas or 
fluids. Special care is required to prevent leakage when the seals wear 
out or otherwise fail. 
Another disadvantage of some pressure transducers is the tendency to 
produce output signals which vary in steps instead of continuously, which 
introduces hysteresis and limits the resolution of which such transducers 
are capable. Such hysteresis is generally caused by sliding seals or 
friction in mechanical linkages. 
Another disadvantage of prior art pressure transducers is that they produce 
a relatively weak or low amplitude signal for a given change in pressure, 
so that resolution is also limited because of the signal-to-noise ratio of 
the output of the transducer. 
Still another disadvantage of many prior art pressure transducers is that 
they must be connected to the vessel containing the fluid whose pressure 
is measured by means of flanges and fittings with seals or the like, and 
cannot be welded into place. 
Also, some pressure transducers are not capable of responding quickly and 
accurately to changes in pressure, but produce a ringing or oscillating 
output signal in response to sudden pressure changes. 
BRIEF SUMMARY OF THE PRESENT INVENTION 
It is the principal object of the present invention to provide a pressure 
transducer which is not subject to the above disadvantages. 
More specifically, one object of the present invention is to provide a 
pressure transducer design which compensates for most first and second 
order errors, which is generally insensitive to non-pressure related 
forces and disturbances. 
Another object of the present invention is to provide a pressure transducer 
which is usable over the entire range from very low to very high 
pressures. 
Another object of the present invention is to produce a pressure transducer 
in which there is a high degree of safety, with the burst strength of the 
transducer being many times greater than the stress which is normally 
placed on the pressure transducer during normal use. 
A further object of the present invention is to provide a pressure 
transducer which automatically compensates for errors and nonlinearities 
in the characteristics of the active strain gage elements and their 
installation. 
Another object of the present invention is to provide a pressure transducer 
which does not require any deformable seals or feed through devices. 
A further object of the present invention is to provide a pressure 
transducer having a relatively high amplitude output signal, with a 
correspondingly good signal-to-noise ratio. 
Another object of the present invention is to provide a pressure transducer 
which is capable of a high degree of resolution. 
A further object of the present invention is to provide a pressure 
transducer which is usable at very low temperature, to measure pressures 
of cryogenic liquids and gases directly. 
Another object of the present invention is to provide a combination strain 
gage which is relatively inexpensive to construct, and install. 
A further object of the present invention is to provide a pressure 
transducer which has a rapid response without producing hysteresis in the 
output signal. 
These and other objects and advantages of the present invention will become 
manifest by an inspection of the following description and the 
accompanying drawings. 
In one arrangement of the present invention there is provided a hollow 
deformable body which is symmetrical about two orthoganal directions, with 
two pairs of diametrically opposed active strain gages attached to the 
exterior surface of the body at spaced locations which are symmetrical 
relative to the body, oriented to respond to transverse tangential strain, 
and with all four of the strain gages being interconnected in a full 
bridge arrangement.

Referring first to FIG. 1, a pressure transducer incorporating an exemplary 
embodiment of the present invention has a hollow tube 10 with a 
noncircular cross-section, symmetrical in two orthogonal directions. The 
tube 10 is symmetrical about a vertical plane passing through its center 
line or axis, and is also symmetrical about a horizontal plane passing 
through the center line. This symmetrical relationship is sometimes 
hereinafter referred to as two-fold symmetry. Four strain gages 12, 14, 16 
and 18 are mounted on the exterior surface of the tube 10, with the strain 
gages 12 and 16 diametrically opposed, and the strain gages 14 and 18 also 
diametrically opposed. All four strain gages are secured to the tube 10 at 
positions which maintain the two-fold mechanical symmetry of the 
transducer. As described hereinafter, the strain gages are also 
symmetrical electrically. These positions are sometimes hereinafter 
referred to as points of symmetry. 
As shown in FIG. 2, the strain gages 12 and 16 are mounted on convex 
portions of the tube 10, while the strain gages 14 and 18 are mounted on 
concave portions thereof. When the pressure within the tube 10 is 
increased, the radius of curvature of the tube 10 at all portions tends to 
increase, with the convex portions becoming less convex and the concave 
portions becoming flatter or less concave, as the shape of the 
cross-section of the tube 10 tends toward a circle. Similarly, when the 
pressure decreases, the convex portions increase their convexity and the 
concave portions increase their concavity. The locations of the strain 
gages 12 and 16 are under compression for increased pressures, because of 
the bending strain in the wall of the tube 10, and the locations of the 
strain gages 14 and 18 are in tension. Accordingly, the two pairs of 
strain gages change their resistance in opposite directions for any change 
in pressure, giving outputs (viz, the resistance variations) which have 
opposite signs. In a bridge arrangement, as illustrated in FIG. 2, it will 
be seen that the different signs of the outputs of the strain gages 12 and 
16, relative to the outputs of the strain gages 14 and 18, produce an 
additive effect, so that as much as a fourfold larger signal is available 
at the output of the bridge. 
While the main signals (responsive to pressure) developed by the strain 
gages 12-18 are added in the bridge, most error signals are subtracted or 
cancelled out, because they induce equal and opposite effects in additive 
legs of the bridge. Such error signals result, for example, from outer 
forces, such as mechanical bending or twisting, pushing or pulling of the 
tube 10, and also nonlinearities such as differences in cementing the 
various strain gages to the tube 10, gage nonlinearities, material 
characteristics, etc. 
The following table illustrates the ability of the present invention to 
ignore first and second order errors. In the following chart, A indicates 
a positive error, B indicates a negative error, and O indicates no error, 
for each of the four strain gages. In all of the conditions shown in the 
chart, no output signal is developed by the error. 
__________________________________________________________________________ 
Gage 
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12 B O O A A O A B B O A B O A B B O A 
14 A B A O O A A O B O A B O A B O B B 
16 A O A B O O O A A A A B B B O O B B 
18 B A O O A B O O A A A B B B O B O A 
__________________________________________________________________________ 
In FIG. 2 the bridge arrangement has a battery 20 connected across one pair 
of opposite terminals of the bridge, with a meter 22 connected across the 
other pair of bridge terminals. Any other indicator may be substituted for 
the meter 22, such as a digital type indicator. Variations in the 
resistance of the gages 12-18 as a result of temperature, affect all of 
the gages equally and therefore produce no additional signal. Nonlinearity 
of the characteristic of the strain gages 14-18 is also compensated for, 
because a reduced output contributed by the strain gages 12 and 16, is 
compensated for by a greater output contribution from the strain gages 14 
and 18. In this way, linear operation of the assembly is extended into a 
much greater range than that attained by pressure transducers known in the 
prior art. Because of its error compensation characteristics, the pressure 
transducer of the present invention is also useful over an extremely wide 
range of temperatures, extending down to cryogenic temperatures, and an 
extremely wide range of pressures, from vacuum to ultra high pressures. 
The relative sizes of the strain gages 12-18 are selected, in relation to 
the curvature of the concave and convex portions of the tube 10, so that 
the change in the levels of the outputs of all of the gages are equal for 
a given pressure change. Different length strain gages placed on the 
convex and concave portions of the tube 10 are preferable, where the radii 
of curvature of the convex and concave portions of the tube 10 are 
different, but an optional shape of the cross-section yields output 
signals which change equally, using gages of identical size and gage 
factor. Such a cross-sectional shape is one that has approximately the 
same radius of curvature for the concave and convex portions of the 
cross-section at the pressure being measured, or near the center of a 
range of pressure being measured. 
The shape of the cross-section of the tube 10 may be referred to as 
peanut-like, when it has convex and concave portions, as shown in FIGS. 1 
and 2. The principals of the present invention may be employed, however, 
with other cross-sectional shapes having two-fold symmetry, as long as 
they are noncircular, such as an elipse, a flattened oval, or the like. In 
each case, the strain resulting from increased pressure is in compression 
at the portions having the greatest convexity and in tension at the 
portions having the least convexity, so that the outputs of the two pairs 
of strain gages are of opposite sign, and equal in magnitude. 
If it is desired that the bridge be balanced, with a zero meter reading at 
any given (null) pressure, a pair of resistors are connected in parallel 
with the two opposite arms of the bridge containing the pair of strain 
gages to be balanced, so that the resistance in all four arms is equal at 
such pressure. The output is then equal to the difference between the 
actual pressure and the null pressure. It is also possible to employ the 
present invention in a servo type bridge system, in which the value of an 
extra resistor, included in the bridge in series with one of the gages, is 
varied until the bridge becomes balanced, with the pressure being read 
from an indicator driven by the servo system which varies the resistor. 
The tube 10 is preferably formed of steel or another metal with a high 
yield strength, and therefore a large range of elastic deformation. The 
wall thickness of the tube is chosen to supply ample burst strength, 
reached only after non-elastic deformation to a circular cross-section. 
The burst strength is considerably higher than the working range of the 
transducer. The ratio between the working pressure and the burst pressure 
is frequently called the safety factor. The safety factor of the present 
invention has been determined to exceed 100 for a peanut cross-section 
formed of stainless steel 304, working at a pressure of 150 p.s.i. This is 
much greater than the safety factors of existing transducers which hardly 
exceed 5. 
The length of the tube 10 is chosen to be at least equal to the 
circumference of the tube 10, so the transducer is relatively insensitive 
to end effects. The tube 10 may be welded in place, so that no seals are 
necessary, and when positioned as part of a conduit, offers little 
restriction to flow of fluid therethrough. 
The strain gages 14-18 are positioned on the tube 10 so as to be sensitive 
to tangential strain in a direction transverse to the center line of the 
tube. 
Referring now to FIG. 3 a ribbon strain gage 24 is illustrated on which are 
supported for conventional strain gages 12, 14, 16 and 18. The four strain 
gages are connected in a bridge circuit, with each of the bridge terminals 
being connected to one of four output terminals 26-29. The 
interconnections are made by conductors 25 which are also supported on the 
same ribbon 24. Both the conductors 25 and the four gages themselves are 
preferably formed on the ribbon 24 by using conventional techniques in a 
single process, such as photoetching. A voltage source as the battery 20, 
is connected to two opposite terminals of the bridge, and an indicating 
device such as the meter 22 is connected to the other two terminals. The 
ribbon 24 is preferably formed of a thin flexible material which is 
secured to the tube 10 by cementing or the like. The strain pages 12-18 
are evenly spaced apart by a distance equal to one quarter of the 
circumference of the tube with which it is to be used, so that, when 
secured to the tube, it forms the symmetrical arrangement illustrated in 
FIG. 2. The ribbon is preferably secured to the tube by adhesive means, as 
is customary in the art. 
A different arrangement is illustrated in FIG. 4, where a ribbon of 
material 30 supports a thin fiber of semiconducting material 35 having 
electrically conductive taps along its length which are connected to four 
output terminals 31-34. The apparatus of FIG. 4 constitutes also a ribbon 
having four equally-spaced strain gages, as shown in FIG. 3, except that 
each strain gage is composed of a section of the same strand of 
semiconductor material 35. The conductors 36, to which the taps are 
connected, are also supported on the ribbon 30. Opposite ends of the 
semiconductor material 35 are connected together by the conductor 36, and 
this and three intermediate equally-spaced taps are connected with four 
terminals 31-34. The lengths of semiconductor material between adjacent 
taps function as separate strain gages, which permits the use of a single 
semiconductor strip to perform the same function as the four gages 12-18 
illustrated in FIG. 3. In use, the ribbon 30 is cemented in place around 
the periphery of a tube 10, which has a circumference equal to the length 
of the ribbon. It is installed so that the completed assembly is 
symmetrically arranged, with the midpoints of each strain gage length 
between adjacent taps aligned with the points of symmetry of the tube 10. 
Such an assembly can be fabricated by known techniques, with the positions 
of the strain gage lengths fixed, relative to each other, so that with 
only cementing the assembly in place, the assembly is ready for use, with 
the heretofore required steps of pre-balancing and pre-testing, completed 
before installation. 
The pressure transducer of the present invention can be employed for 
measurement of a variety of different pressures. Preferably relatively 
thick walls are provided for the tube 10 when large pressure differentials 
between the inside and outside of the tube 10 are to be measured, and 
relatively thinner walls are employed for the tube 10 when smaller 
pressure differentials are to be measured. It is also apparent that there 
is no need for any flexible seals or feed-throughs in the present 
invention. 
When it is not desired to mount the tube 10 as a conduit for supporting 
fluid flow, one end may be closed by welding or the like, with the 
remaining end welded in place against a wall of the vessel containing the 
fluid whose pressure is to be measured, with an aperture in the wall of 
the vessel communicating to the interior of the tube 10, so that the same 
fluid pressure exists in both the vessel and the tube 10. 
The present invention may be used for elevated temperature applications by 
using, for the strain gages 12-18, units which are capable of withstanding 
relatively higher temperatures, and soldering, braising or welding the 
strain gages, or their supporting ribbon 24, in place on the surface of 
the tube 10. Such special techniques for high temperature uses of strain 
gages are well known and need not be described in any greater detail. 
Although the arrangement illustrated in FIG. 1 has all of the strain gages 
on the exterior of the tube 10, it is equally feasible to mount all of the 
strain gages on the interior thereof, when the voltage source 20 and the 
indicating device 22 are located within the tube 10, or when the interior 
of the tube 10 is at atmospheric pressure. Such an application arises when 
the tubular tube 10 is closed at one end and is inserted into the interior 
of a vessel through an aperture in the outside wall thereof. 
The present invention is useful for measuring pressures inside and outside 
the body 10 relative to atmospheric pressure by the methods described 
above, and is also useful for measuring the differential pressure between 
two fluids acting on the interior and the exterior of the body 10. 
It is apparent from the foregoing that the pressure transducer of the 
present invention is adapted for measuring pressure under a variety of 
conditions. The linear range of indication of the transducer is extended 
substantially beyond that available in the prior art, and a considerably 
larger continuously variable, high resolution output signal is produced. 
The resolution attainably with the present invention is on the order of 
one part in one million. 
In an alternative embodiment of the present invention, the tube 10 is bowed 
so that it is curved along its length. One end of the tube is connected to 
the fluid whose pressure is to be measured, and the other end is closed by 
a suitable cap or seal. At least the center portion of the length of the 
curved tube has a noncircular, preferably peanut, cross-section, and the 
four strain gages are secured symmetrically to this portion, as described 
above. The curved tube tends to straighten out with increased pressure, 
and behaves exactly like the well known Bourdon tube, and can be used to 
give a mechanical indication of the pressure within the tube, with 
reference to a fixed scale which may be calibrated in units of pressure, 
as well known in the art. In this way, both a mechanical and an electrical 
indication of pressure is available. The bending of the tube does not 
affect the electrical indication developed by the present invention, 
because the error signals resulting from the bending of the tube cancel 
out, as described above. 
It will be apparent to those skilled in the art that various modifications 
and additions may be made in the present invention, without departing from 
the essential features of novelty thereof, which are intended to be 
defined and secured by the appended claims.