Pressure transducer

A pressure transducer for in particular the measurement of relatively high pressures in the range of 10,000-50,000 psi and including an elongated frame having a capillary tube extending therethrough and employing a coupler at one end of the frame for sensing input pressure. A sensing member is provided disposed about the capillary tube at the other end of the frame and has, defined with the capillary tube, a sensing chamber in communication with the capillary tube. A recess is provided in the sensing member defining a relatively thin wall adjacent the annular sensing chamber. This wall has a pressure responsive sensing surface that extends substantially parallel to the capillary tube length and upon which strain gages are secured.

BACKGROUND OF THE INVENTION 
The present invention relates in general to pressure transducers and is 
concerned, more particularly, with a new and improved fluid-filled, 
direct-sensing pressure transducer. Even more particularly, the present 
invention relates to a pressure transducer adapted for high pressure 
measurements such as in the range of 10,000-50,000 psi. 
The following is a list of prior art patents that in general relate to 
pressure transducers: U.S. Pat. Nos. 3,349,623, 4,369,659, 3,678,753, 
3,349,623, 2,940,313, 2,627,749, 2,326,047, 3,336,555, 2,738,677. Some of 
these patents are owned by the assignee herein and show fluid filled 
pressure transducers. 
U.S. Pat. Nos. 2,949,313 and 2,627,749 both illustrate pressure indicators 
that employ strain tubes or the like for transmitting pressure to a 
diaphragm on which pressure is sensed by strain gages. 
U.S. Pat. Nos. 3,326,047 and 3,336,555 illustrate pressure transducers. 
U.S. Pat. No. 3,336,555 illustrates an unfilled pressure transducer with 
strain gage sensing. U.S. Pat. No. 3,326,047 on the other hand illustrates 
a fluid filled pressure transducer employing a pressure sensing capsule 
having inner and outer deformable cylindrical walls. The liquid filing in 
a transducer of this type has a relatively large volume and thus creates 
limitations upon the range of pressure measurements. 
U.S. Pat. No. 2,738,677 is actually directed to a pressure indicator 
particularly for combustion chambers such as in an internal combustion 
engine. 
Liquid filled pressure transducers owned by the assignee herein include 
U.S. Pat. Nos. 3,349,623; 3,678,753; and 4,369,659. The early U.S. Pat. 
No. 3,349,623 describes a device employing an annular sensing chamber with 
associated strain gages. U.S. Pat. No. 3,678,753 is believed to be an 
improved form of the earlier version employing a top cap member and 
associated disc-shaped compartment defined between the cap member and the 
body of the instrument. The transducer illustrated in U.S. Pat. No. 
3,678,753 provided an increased operating pressure range. U.S. Pat. No. 
4,369,659 describes a melt pressure transducer preferably for use in 
pressure measurements associated with an injection molding machine and 
employing a novel temperature compensating filler rod. 
Another prior art patent is U.S. Pat. No. 3,128,628 to Lebow. This patent 
illustrates a pressure transducer but does not employ any capillary tube. 
In all of the prior art, referred to hereinbefore one of the main 
limitations, is the inability to operate particularly at high pressure 
levels such as in a pressure range of 10,000-50,000 psi. 
Accordingly, one important object of the present invention is to provide a 
liquid filled pressure transducer operating at high pressure ranges. 
Another object of the present invention is to provide an improved pressure 
transducer that is fluid filled and that employs a reduced volume of 
fluid. This reduced volume of fluid provides for minimization of 
temperature effects upon pressure and furthermore makes for improved 
diaphragm constructions. 
Accordingly, a further object of the present invention is to provide an 
improved liquid filled pressure transducer in which diaphragm stresses are 
reduced. 
Still another object of the present invention is to provide an improved 
fluid filled pressure transducer that provides for reduced snout diaphragm 
stresses during applied pressure and upon exposure to elevated 
temperature. 
Still another object of the present invention is to provide an improved 
fluid filled pressure transducer provided with a snout piece of a 
different material, provided for the purpose of temperature compensation 
and for improving overall performance. The snout piece permits the liquid 
filled void to expand at the same rate as the liquid itself during 
temperature changes. 
SUMMARY OF THE INVENTION 
To accomplish the foregoing and other objects features and advantages of 
the invention there is provided a pressure transducer which comprises an 
elongated frame having an elongated passage therethrough and a capillary 
tube extending through the frame passage and terminating at one end 
adjacent to one end of the frame. A coupler closes that one end of the 
frame and defines with the frame a chamber in communication with the 
capillary tube. A sensing member in accordance with the invention is 
disposed about the capillary tube at the other end of the frame and 
includes means defining an annular sensing chamber and fluid communication 
with the capillary tube. For this purpose there may be provided a passage 
transversely in the capillary tube to enable fluid communication from the 
capillary tube to this annular sensing chamber. The sensing member also 
includes means defining a recess therein forming a relatively thin wall 
adjacent the annular sensing chamber. This wall has a pressure responsive 
sensing surface that extends substantially parallel to the capillary tube 
length. The sensing of pressure may be accomplished by means of a strain 
gage arrangement disposed on the pressure responsive sensing surface of 
the relatively thin wall. The strain gage sensing means may be connected 
in a bridge arrangement. In accordance with one feature of the invention 
there is preferably also provided in the transducer at the diaphragm end 
thereof a snout piece of a different material than that of the frame of 
the transducer having a relatively low coefficient of expansion for 
providing temperature compensation.

DETAILED DESCRIPTION 
Reference is now made to FIGS. 1-4 which show complete details of one 
embodiment of a pressure transducer in accordance with the invention. FIG. 
5 shows the schematic diagram of the strain gage interconnections. FIG. 6 
is an alternate embodiment from the standpoint of illustrating the 
preferred use of a filler piece at the diaphragm end of the transducer. 
FIG. 7 is a graph of illustrating transducer performance. 
The pressure transducer includes a main frame 10, a sensing member 12 
provided at the top of the frame 10, a capillary tube 14 which extends 
through the frame, and a diaphragm coupler 16 secured to enclosing the 
bottom end of the frame. 
The lower section of the frame 10 is constructed in a similar manner to the 
construction illustrated in U.S. Pat. No. 3,678,753. Basically there is an 
elongated passage 18 that extends through the main frame and which is for 
accommodating the capillary tube 14. The capillary tube 14 at its bottom 
end terminates at a relatively small chamber 20 which is closed by the 
diaphragm 16. 
At the top end of the frame 10 there is included as part of the frame a top 
piece 22 through which the capillary tube 14 extends. The top piece 22 is 
for supporting the sensing member 12 in the position illustrated in the 
drawing. The sensing member 12 may be secured in position within the top 
piece 22 by being welded to the top piece. 
The capillary tube 14 as noted in, for example, FIG. 3, extends through a 
vertical passage in the sensing member 12. It is noted that in FIG. 3 that 
the capillary tube 14, in accordance with one embodiment of the invention, 
is provided with an annular recess 26 that defines an annular chamber 28 
essentially defined by the recess in the capillary tube and the inner bore 
of the passage that extends vertically through the sensing member 12. 
In order to provide fluid communication from the capillary tube to the 
chamber 28 there is provided a transverse passage 30 which is disposed in 
the position as illustrated in FIG. 3. 
The sensing member 12, as indicated previously, includes a vertical passage 
through which the capillary tube 14 extends. As illustrated in FIG. 1, the 
capillary tube 14 also extends beyond the top of the sensing member 12 and 
is provided at its top end with some type of a filler cap 32. The sensing 
member 12 is furthermore constructed by machining a single flat surface on 
the outside of its cylindrical body. This is illustrated in FIGS. 1-3 by 
the recess 34 that extends in the vertical direction in FIG. 3 somewhat 
less than one third of the total height of the cylindrical sensing member. 
As indicated previously, the cylindrical sensing member is through-drilled 
along its full length at its center axis and fitted closely to the 
capillary tubing which passes therethrough. The capillary tubing is 
preferably TIG welded at both ends as illustrated by the welds 38 in FIG. 
3. The ends of the sensing member 12, such as at surface 40 in FIG. 3, are 
machined to form weld-preparation surfaces including the cup shaped 
indentations so as to aid in maximizing weld penetration and strength 
between the capillary tube and the sensing member. 
Prior to insertion of the capillary tube, a 0.015 to 0.020 inch diameter 
lateral hole 30 is drilled through the center of the capillary to allow 
fluid communication between the capillary tube and the annular chamber 28. 
The hole 30 may be drilled approximately 3 inches from one end of the 
sensing member and the relative position of the capillary tube and the 
sensing member are arranged so that the hole is placed as indicated in 
FIG. 3 at about the mid distance of the recess 34. The capillary is, of 
course, welded at both ends of the sensing element as illustrated in FIG. 
3 to form essentially upper and lower liquid leak-tight joints. 
Hydraulic pressure from inside of the capillary tube bore is transmitted to 
the annular sensing element by way of the hole 30. This fluid 
communication enables the inside surface of the sensing member to be 
pressurized. 
The machined recess 34 defines a relatively thin wall 46, the thickness of 
which is perhaps somewhat exaggerated in FIG. 3. The wall 46 has a 
pressure responsive sensing surface 48 to which the strain gages are 
secured. Hydraulic pressure under the machined flat essentially at the 
annular chamber 28 creates high bending stresses across the thin flat wall 
46 where the active strain gages are attached. This action senses the 
strained surface along the center line axis. In this regard, refer to FIG. 
2 which illustrates, along the center line of the capillary 14, the active 
strain gages 51 and 53. Also note in FIG. 2 the other strain gages 52 and 
54 which, with the strain gages 51 and 53, provide the total strain gage 
circuit. The strain gages 52 and 54 may be considered as the inactive 
strain gages but do sense some compression strains to add to the 
electrical sensitivity. However, the strain gages 52 and 54 are used 
primarily to complete the Wheatstone bridge and to provide thermal 
compensation. 
FIG. 2 also shows, associated with the strain gages 51-54, electrical 
interconnection tabs. These include tabs 58 and 59 to the left in FIG. 2 
and tabs 60, 61 and 62 to the right in FIG. 2. These connection tabs are 
connected with the strain gages so as to connect the strain gages in the 
pattern illustrated in FIG. 5. In FIG. 5 the same reference characters are 
employed to identify the same strain gages illustrated in FIG. 2. Thus, 
the circuit interconnection of FIG. 5 illustrates the strain gages 51-54 
schematically represented as variable resistances, varying with applied 
pressure. These resistances are interconnected in a bridge arrangement as 
illustrated in FIG. 5 having excitation inputs at terminals 64 and 65 and 
having a signal output at terminals 66 and 67. 
An electrical input signal is usually applied across the input terminals 64 
and 65 and the pressure responsive voltage is measured across the output 
terminals 66 and 67. Once again, the active gates are gages 51 and 53 and 
thus the majority of the pressure responsive signal is generated by these 
legs of the bridge. The gages 52 and 54 primarily provide for temperature 
compensation so that if there is a change in temperature at the sensing 
member, the bridge is automatically adjusted by virtue of the temperature 
of both of the nonactive gages causing essentially a nulling of the 
bridge. 
One of the improved features in accordance with the present invention is 
the reduced amount of liquid fill that is employed in the transducer. This 
drastically reduces the fill displacement and thus controls pressure 
induced deflections. This is thus successful in minimizing diaphragm 
stresses and in enabling the use of thicker diaphragms. The reduction in 
fill is carried out by virtue of the use of the sensing member 12 with its 
substantially minimal fill volume. The volume is also minimized by virtue 
of other structural elements that are employed including the relatively 
small snout chamber and capillary tube. 
Reference is now also made to FIG. 6 which shows an enlarged fragmentary 
view illustrating a preferred embodiment for the snout end of the device 
illustrating the snout filler piece 70 which is primarily used for 
temperature compensation reasons. This piece may be constructed of Kovar 
or Invar. Alternatively, this may be constructed of any very low 
coefficient of expansion material. It is preferred that it have a 
coefficient of expansion much less than that of the stainless steel frame. 
This fill piece 70 allows the void in chamber 20 to expand at the same 
rate as the liquid during temperature changes. The filler piece 70 
illustrated in FIG. 6, functions to reduce internal fill pressure as the 
tip is heated with no applied pressure. 
There have been at least two different liquid fills that have been 
employed, one being mercury and the other sodium potassium (NaK). The NaK 
filled transducer is a lower pressure range transducer while the mercury 
transducer has a pressure range of 10,000-50,000 psi. The range of 
pressure operation when using a NaK fill is 10,000-15,000 psi. 
Thus, the filler piece, in combination with the chamber 20 illustrated in 
FIG. 6, provides a thermally compensated internal volume which is desired. 
Also, the filler piece as indicated previously is selected to have thermal 
properties that temperature compensate the transducer for differential 
thermal expansion coefficients between the internal fluid which is 
preferably mercury and the main body of the transducer which is usually 
stainless steel. As indicated previously, it is preferred to have a low 
temperature coefficient of expansion particularly in comparison with that 
of the frame material. 
One of the advantages that has been realized with the improved sensing 
technique of the present invention is the capability of now being able to 
increase the thickness of the diaphragm 16 while reducing the stresses 
thereof. It has also been possible in accordance with this construction to 
minimize overall thermal characteristics. Reducing fill displacement under 
pressure reduces the diaphragm deflection and the resulting stresses 
allows the diaphragm to be increased in thickness within certain 
limitations to improve insitu durability. 
In connection with the above, the maximum diaphragm thickness is related to 
two independent effects including internal fluid displacement under 
applied pressure and secondly internal pressure generated by thermal 
expansion of the contained fluid fill. In accordance with the invention, 
the sensor's small internal volume reduces the amount of fill and its 
resulting compression. Moreover, the sensor's low displacement or 
deflection under pressure further reduces total fluid displacement and 
resulting stresses. Moreover, the filler piece is adapted to control 
thermally induced internal pressure that also further reduces stresses. 
The combined improvements outlined above allow diaphragm thickness to be 
increased without increasing stresses and controls internal pressure 
effects versus temperature. The snout filler piece 70 is used in essence 
to replace some of the stainless steel at the tip of the instrument with a 
low temperature coefficient of expansion material thus temperature 
compensating the void 20. This preferred material as indicated previously, 
is Kovar or Invar. 
With regard to the reduction in volume, it is noted that the internal 
volume of the sensing member 12 is very small because the capillary tube 
fills the passage through the member with very close tolerance fit. As a 
matter of fact, in the illustration of FIG. 3, there is shown a relatively 
predominant recess 26 in the capillary tube. However, in another 
embodiment that may be employed, the capillary tube need not be recessed 
at all but instead one can rely upon the slight difference in diameter 
between the bore of the member 12 and the outer diameter of the capillary 
tube. In such case, the annular sensing chamber about the capillary tube 
actually extends between top and bottom ends of the sensing member from 
weld-to-weld. 
By way of example, the amount of volume of liquid in the capillary tube may 
be approximately 75 percent of the volume in the annular space about the 
capillary tube. The total volume within the entire transducer including 
the capillary tube may be in a range from 1.0.times.10.sup.-3 to 
2.0.times.10.sup.-3 " cube. This very small volume compares with a 
transducer volume on the order of 3.2.times.10.sup.-3 " cube for a 
transducer which is of the type described in U.S. Pat. No. 3,678,753. It 
can be readily seen that there is an improvement in the reduction of 
volume fill by at least 2 to 16 between the volumes of the transducer in 
the prior art and that in the present construction. With regard to the 
capillary tube, the inner diameter thereof can range from 0.005 to 0.010", 
and the outer diameter thereof can range from 0.060" to 0.25". 
With regard to the filling of the transducer, this is accomplished at the 
top end of the capillary tube. In FIG. 1 the capillary tube is shown 
capped off but before this occurs the capillary tube and the rest of the 
voids communicating therewith in the device are filled with say mercury 
under forced pressure so that all of the void areas are filled with the 
mercury. The capillary tube is then sealed off to retain the mercury in 
the transducer. 
Reference is now made to FIG. 7 which is a graph of input pressure being 
sensed at the diaphragm versus the pressure lost at the snout diaphragm. 
There are actually two curves illustrated in FIG. 7. The curve C1 is a 
pressure curve for a prior art transducer such as the one described in 
U.S. Pat. No. 3,678,753. The curve C2 is the pressure curve for the 
transducer in accordance with the present invention. It is noted that even 
at high pressure ranges in the area of 40,000 psi that the absorbed 
pressure is only on the order of about 150 psi; consequently, diaphragm 
thickness can be increased. Increased thickness creates higher absorbed 
pressures, but its increased thickness will accommodate the higher 
pressures without generating higher stresses than those illustrated in 
curve C1 of FIG. 7. 
Having now described a limited number of embodiments of the present 
invention, it should now be apparent to those skilled in the art that 
numerous other embodiments and modifications are contemplated as falling 
within the scope of the present invention as defined by the appended 
claims.