Pressure Transducer

The present invention relates in general to pressure transducers, and pertains more particularly to liquid-filled pressure transducers, such as pressure transducers of the type disclosed in U.S. Pat. No. 3,349,623 to Pastan issued Oct. 31, 1967 and U.S. Pat. No. 3,678,753 issued to Eggleston, et al July 25, 1972. Even more particularly, the present invention relates to relatively lower range melt pressure transducers.

As discussed in both the Pastan patent and the Eggleston, et al patent, fluid-filled pressure transducers are designed to be used in those systems in which it is undesirable for the medium whose pressure is being measured to enter into the instrument. In such cases, the instrument itself is filled with a fluid which is coupled by means of a diaphragm or some other device to the medium whose pressure is to be measured. The fluid which fills the instrument directly transmits the pressure of the medium to the sensing device. A typical fluid for the transducer is mercury.

The Eggleston, et al U.S. Pat. No. 3,678,753 described a pressure transducer that includes an elongated frame and a capillary tube extending through the frame and terminating at one end adjacent one end of the frame. A coupler closes that end of the frame and defines with the frame a chamber that is in communication with the capillary tube. The other end of the capillary tube communicates with a sensing device which has a small deflection throughout its full operative range. The sensing device is in the form of a deformable cap member having a recess therein, defining with a portion of the frame, a thin sensor compartment that is of disc-shape and is in communication with the other end of the capillary tube. A liquid, preferably mercury, fills the thin sensor compartment, chamber and capillary tube so as to transmit directly the pressure applied against the coupler to the sensing device to render a pressure measurement.

The Eggleston, et al pressure transducer is generally designed, however, for pressures down to about 1500 p.s.i. With this prior art transducer, pressures lower than 1500 p.s.i. are not effectively measured because the deflectable wall or diaphragm of the cap member has to be constructed too thin making the device quite susceptible to manufacturing and fracture problems.

Accordingly, it is an object of the present invention to provide an improved fluid-filled pressure transducer that is in particular adapted for making low pressure measurements such as those in the range of 250-1500 p.s.i. More particularly, the present invention carries out this low pressure measurement without requiring a large component design. In particular, the present invention permits low pressure measurements using the standard 1/2 inch - 20 threaded snout.

Another object of the present invention is to provide an improved low-range melt pressure transducer that is relatively simple in construction and that can be fabricated relatively inexpensively.

Still another object of the present invention is to provide an improved technique for constructing the sensing device.

To accomplish the foregoing and other objects, features and advantages of the invention, there is provided a pressure transducer, particularly for low pressure measurements and which comprises an elongated frame having means defining an elongated passage therethrough, a capillary tube extending through the frame and terminating at opposite ends of the frame and a coupler that closes one end of the frame and defines with the frame a chamber in communication with one end of the capillary tube. A liquid filled deformable sensor includes a cap member having an internal recess in a top wall. The recess defines a thin disc-shaped compartment at the other end of the frame which is in liquid communication with the other end of the capillary tube. A liquid fills the capillary tube, chamber and compartment for applying pressure exerted against the coupler to the sensor. In accordance with the invention, the cap member has an outer top surface defined by a cap member top wall forming a thin diaphragm having a diametrically disposed raised beam extending across the top surface and having a gage receiving surface thereon. Strain gage means are attached to the gage receiving surface of the beam. In a preferred embodiment, there is provided a plurality of strain gages longitudinally disposed along the beam. There are preferably four of these gages, two for measuring compressive strains and two for measuring tensile strains. The beam preferably extends across the full width of the cap member and preferably has a constant width therealong. In an alternate embodiment of the invention, the beam may have flared ends. The thickness of the beam is preferably at least on the order of twice the thickness of the thin diaphragm wall.

In accordance with the method of construction of the present invention, it has been found that in constructing the beam, one can begin with a flat diaphragm surface such as described in the Eggleston, et al patent and simply remove the surface in a controlled manner so as to define the beam. It has been found that the removal or cutting of a portion of the top wall of the cap member is preferably carried out so as to provide a beam of a thickness approximately twice that of the adjacent thin wall. It has furthermore been found that by starting with a cap of range R, once the beam is formed, the range of the device has then been altered to have a range of R/4.

Numerous other objects, features and advantages of the invention should now become apparent upon a reading of the following detailed description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a cross-sectional view of one form of fluid-filled pressure transducer constructed in accordance with the principles of the present invention;

FIG. 1A is an enlarged cross-sectional view of a portion of the pressure transducer illustrating the further use of a snout filler piece;

FIG. 2 is a perspective view of the sensing element illustrated in FIG. 1;

FIG. 3 is a somewhat enlarged cross-sectional view of the sensing element also illustrated in FIG. 1;

FIG. 4 is a further cross-sectional view taken along line 4--4 of FIG. 1 showing further details of the sensing element;

FIGS. 5A and 5B show respective plan and cross-sectional views of the cap member with integral beam;

FIG. 6 is a diagram that is a plan view of the sensing element showing strain gage position and wiring terminals;

FIG. 7 is a schematic diagram illustrating the strain gage circuit of FIG. 6;

FIG. 8 is a graph of input pressure versus non-linearity, illustrating the improved linearity that is provided with the transducer of this invention; and

FIG. 9 is a plan view of an alternate beam construction in which the beam has flared ends.

Reference is now made to the drawings and in particular to FIG. 1 which is a cross-sectional view of a pressure transducer in accordance with the invention including a main frame 10, a sensing element 12 provided at the top end of the frame, a capillary tube 14 which extends through the frame, and a diaphragm coupler 16 secured to and enclosing the bottom end of the frame. The lower section of the frame 10 may be 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 element 12 in the position illustrated in the drawing. The sensing element 12 may be secured in position within the top piece 22 by being welded to the top piece.

In FIG. 1 the sensing element 12 is illustrated as comprising a base 26 and a cap member 30. The cap member 30 is welded to the base 26 and there is defined therebetween a thin disc-shaped compartment 32. FIG. 1 also illustrates the fill tube 34 and associated fill plug 36. The fill tube 32 communicates to the thin disc-shaped compartment 32. The liquid fill within the device including the fill within the chamber 20, compartment 32 and capillary tube 14, is introduced through the fill tube 34 under pressure so that all areas are completely filled with the mercury. The tube 34 is then capped off by the plug 36.

Reference is now made to FIGS. 2-4 which show further details of the sensing element 12 as indicated previously, the sensing element 12 includes a base 26, and a cap member 30. The cap member 30 is welded to the base 26 such as illustrated in FIGS. 3 and 4 at the weld 35. In this connection also note FIG. 2 that shows the annular weld line at 35. FIG. 3 shows the thin disc-shaped compartment, as does FIG. 4. It is noted also in FIGS. 3 and 4 that the top end of the capillary tube is illustrated as terminating directly at the thin disc-shaped compartment 32. FIGS. 3 and 4 also illustrate a weld at 37 where the top of the capillary tube 14 is welded to the very top surface 33 of the base 26.

Now, the cap member 30 has an outer top surface defined by a cap member top wall 40 forming a thin diaphragm. Integral with the wall 40 is a diametrically disposed raised beam 42 that extends across the top surface and has an upper gage receiving surface 44. The diaphragm functions as a force-collector of internal pressure to increase the force carried by the beam. This allows the beam to be thicker and to thus display proper linearity as described hereinafter in connection with FIG. 8.

The sensing diaphragm that is formed by the cap member with the beam 30 provides a structure that is in particular operable over lower pressure ranges such as ranges of 250, 500, 750, 1000, and 1500 p.s.i. This construction tends to reduce the stiffness of the diaphragm 40 permitting the beam section thereof to be thicker than a flat diaphragm of the same range. For example, for a range of 750 p.s.i. the thickness of the beam may be 0.0177 inch while, the thickness of a flat diaphragm for the same pressure range would have to be 0.0100 inch which is too thin and susceptible to fracture and non linearity. In this regard, reference will also be made hereinafter to Table I.

Reference is also now made to FIGS. 5A and 5B which illustrate the cap member 30 alone. In FIG. 5B, in particular, it is noted that there are provided the dimensions T1 and T2. The dimension T1 is the thickness of the beam 42 while the dimension T1 is the thickness of the diaphragm 40. It has been found that the interrelationship between these dimensions preferably falls within a certain range. It is desired to have the diaphragm 40 thickness as thin as possible, but this cannot be made too thin or fracture problems occur. The thickness of the beam 32 is preferably as thick as possible, but if it becomes too thick, sensitivity of the device is lost. It has been found that the ratio of T1/T2 should preferably be on the order of 2.0. This is illustrated in FIGS. 3 and 5B, in which it is noted that the cut away portion shown in FIG. 3 cuts away the original cap member configuration, which was originally totally flat on the top, to an essentially one-half depth so that the thickness of the diaphragm 40 is approximately one-half of the thickness of the beam 42.

With regard to the dimensions T1 and T2, reference is also now made to Table I which is set forth herein.

By way of example, for the pressure range of 250 p.s.i., the dimension T1 is in the range of 0.0099/0.0104 inch. The dimension T2 is in the range of 0.0044/0.0050. Taking the mean for each range, this would provide a ratio of T1/T2 of 2.18.

In another example, for the range of 1000 p.s.i., the dimension T1 is in the range of 0.0220/0.0226. The dimension T2 is in the range of 0.0100/0.0110. Again, taking the mean for each dimension, this provides a ratio of T1/T2 of 1.92. The range of the ratio of T1/T2 is preferably from 1.5 to 4.0.

As indicated previously, the beam 42 has an upper gage receiving surface 44. In this regard, note FIGS. 2 and 4 that illustrate four strain gages G1-G4. Each of these strain gages may be of conventional design and may be of "D" alloy foil material type. The gage resistance is 350 ohms. With regard to the placement of the strain gages, reference is now made to FIG. 6 which shows these four gages disposed along the beam 42 of the cap member 30. The gages G1 and G4 measure compression while the gages G2 an G3 measure tension. Thus, the gages G1 and G4 measure negative-induced strain while the gages G2 and G3 measure positive-induced strain. It is noted that the gages G2 and G3 are spaced closely together symmetrically about the center of the cap member. The gages G1 and G4 are disposed longitudinally in line with the gages G2 and G3. The gage G1 is spaced from the gage G2 and the gage G4 is spaced from the gage G3 as noted in FIG. 6.

The gages G1-G4 are connected in a circuit as illustrated in FIG. 7. In FIG. 7 the resistors R1.varies.R4 correspond, respectively, to the gages G1-G4.

In FIG. 6, the gages G1-G4 are interconnected in a manner corresponding to the circuit of FIG. 7. In this connection, there are provided conductive contacts C1-C3 on one side of the beam 42 and contacts C4-C6 on the other side of the beam 42. Conductive runs interconnect the strain gages to the electrical contacts C1-C6. Again, the interconnections form a circuit as illustrated in FIG. 7.

FIG. 7 shows the gages as connected in a bridge construction having a pair of input terminals connected at one side of the bridge and a pair of output terminals taken from the other side of the bridge. An input signal would be applied across the input terminals and a pressure responsive voltage is measured across the output terminals.

FIG. 1 previously illustrated the capillary tube 14 at its bottom end terminating at a relatively small chamber 20 which is closed by the diaphragm 16. However, in a preferred embodiment of the invention there is provided a filler piece as illustrated in FIG. 1A by the filler 70. In this regard, FIG. 1A 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 filler 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. 1A functions to reduce internal fill pressure as the tip is heated or with no applied pressure.

Thus, the filler piece, in combination with the chamber 20 illustrated in FIG. 1A, 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.

Reference is now made to FIG. 8 which is a diagram of input pressure in percent of full scale versus non-linearity in percent of full scale. There are two curves illustrated in FIG. 8, namely curves C1 and C2. Curve C1 shows a standard flat diaphragm and it is noted that there is typically a substantial non-linearity in operation. The curve C2 represents the construction of the present invention. It is noted in the curve C2 that this deviates only a small amount from the desired linearity (zero access).

It is noted in the preferred embodiment of the present invention, that the beam 42 extends diametrically, symmetrically, and from side-to-side of the sensing element. FIG. 9 shows an alternate embodiment in which the beam 42 has flared ends and thus includes a center section 42A that has a substantially constant width therealong and then opposite flared ends 42B. This construction also provides operation similar to that depicted in the curve C2 in FIG. 8. In the embodiment of FIG. 9, the view that is illustrated is only a plan view, it being understood that the walls forming the beam are basically vertical upstanding walls.

With regard to the coupler diaphragm 16, it is preferred that in addition to the Kovar insert for temperature compensation, that one also employ a substantially thick flat machined coupler diaphragm. The thicker machined diaphragm, in comparison with an older version employing a thinner formed and welded diaphragm, is more rugged and presents a flat surface to the measuring media. A thicker diaphragm is possible because of reduced displacement due to the use of the mercury fill.

When the transducer described herein is used in a temperature range having a maximum of about 750.degree. F., mercury is the preferred fluid to fill the device. Mercury has a low compressibility and a boiling point in excess of 700.degree. F., so that within that operative range, no vapor pressures are generated to cause secondary expansion of the chamber 32 to distort the wall 40 and beam 42 of the cap member 30. An alternative fill that may be used with with the device of the present invention is one of sodium-potassium referred to as a NaK fill. This of course also is in liquid form.

From the foregoing description, those skilled in the art will appreciate that numerous modifications may be made to this invention without departing from its spirit. Therefore, it is not intended to limit the breadth of this invention to the embodiments illustrated and described. Rather, it is intended that the breadth of this invention be determined by the appended claims.