Fluid measuring device and method

A pressure transducer and method for measuring pressure of a fluid flow in a tube include use of a sensing tube through which the pressurized flow passes. The sensing tube deforms outward in response to the pressurized flow within. Deformation measuring devices, such as strain gages, measure the outward deformation and allow computation of the pressure of the flowing fluid. A housing surrounds the sensing tube to relieve stresses on the sensing tube, to prevent damage to the sensing tube, and to contain any rupture of the sensing tube. The sensing tube may have a round, rectangular, or other shape cross-section. The pressure transducer allows continuous and non-invasive measurement of pressure inside a tube. In addition, a flow restriction such as an orifice may be provided in the sensing tube to enable a flow rate to be determined from the pressure drop across the flow restriction. Further, measuring device for measuring flow rate may utilize a sensing tube that bent (strained) because of forces causes by a change of momentum of flowing fluid due to a direction change of the fluid.

TECHNICAL FIELD

The present invention relates to pressure and flow rate transducers and methods for measuring pressure and flow rate. More particularly, the present invention relates to transducers and methods for measuring pressure of fluid flowing in a tube.

BACKGROUND OF THE INVENTION

The general problem of measuring flow rates and pressures of flowing fluids and fluid-like substances occurs in a wide variety of circumstances. Methods of measuring flow rates in a pipe often involve insertion of a probe or diversion of a portion of the flowing fluid. Such methods of measuring flow rate may involve undesirable disturbances of the flow and/or possible flow contamination. In traditional installations, where a pressure probe is offset from the flow, the added volume may entrap the material flowing in the line and present additional problems, such as clogging or bacteria growth.

In a particular exemplary flow measurement problem, that of measuring the pressure of slurries such as liquid concrete flowing in a tube, one method of isolating the slurry flow from the pressure transducer is to inject grease into a fluid line running from the tube to the pressure transducer while the measurement is being made. This intervening grease blocks concrete from reaching the pressure transducer, transmits pressure to the pressure transducer, and prevents damage to the pressure transducer. However, such a method is not suitable for continuous pressure measurement since grease must be injected before and during measurement. Some of the grease may be picked up by the flowing slurry, whereby such a method may introduce grease as an impurity to the slurry flow. Such an impurity may be unacceptable, whether the slurry is a flow of concrete, foodstuffs, or some other material.

Another method of measuring pressure in slurry flows involves measuring pressure of a sensing fluid in an isolated annular region. Between the sensing fluid and the slurry flow is an elastomeric cylinder which is to deform and thereby to transmit pressure from the slurry flow to the sensing fluid. An example of such a system is shown in U.S. Pat. No. 4,218,926, issued to DeVisser, entitled “Isolating Pressure Sensor.” Such systems have the disadvantages of being complicated and costly, as well as using an elastomeric material such as Buna-N, which may be unsuitable for contact with a corrosive or abrasive fluid or slurry.

In addition to slurries, it will be appreciated that there are many other fluids that may cause damage to a pressure transducer that is exposed to them. Examples of such fluids and fluid-like mixtures are corrosive materials and slurries or other mixtures containing abrasive materials.

From the foregoing general discussion and particular example, it may be seen that a need exists for an improved, inexpensive, and/or durable means of continuously measuring pressure and/or flow rate in flowing fluids.

SUMMARY OF THE INVENTION

A pressure transducer and method for measuring pressure of a fluid flow in a tube include use of a sensing tube through which the pressurized flow passes. The sensing tube deforms outward in response to the pressurized flow within. Deformation measuring devices, such as strain gages, measure the outward deformation (axial and/or circumferential strains) and allow computation of the pressure of the flowing fluid. A housing surrounds the sensing tube to relieve extraneous stresses on the sensing tube, to prevent damage to the sensing tube, and to contain any rupture of the sensing tube. The sensing tube and housing may have a round, rectangular, or other shape cross-section. The pressure transducer allows continuous and non-invasive measurement of pressure inside a tube. In addition, a flow restriction such as an orifice may be provided in the sensing tube to enable a flow rate to be determined from the pressure drop across the flow restriction. Further, measuring device for measuring flow rate may utilize a sensing tube that bent (strained) because of forces causes by a change of momentum of flowing fluid due to a direction change of the fluid.

Broadly stated, the invention involves measuring outward changes in shape of at least a portion of a sensing tube which contains a pressurized fluid, such as a fluid flowing therethrough. The changes in shape are related to inner pressure, and measurements of the strains may be used to determine the pressure within the tube. Strain gages may be used for measuring outward deformations due to the shape changes. A pressure transducer embodying the invention may be installed in a pipeline through which the fluid flows.

According to an aspect of the invention, a pressure transducer includes a sensing tube at least a portion of which mechanically deforms in response to a pressure on one side thereof, and at least one strain gage on the sensing tube for measuring deformation of the at least a portion of the sensing tube.

According to another aspect of the invention, a pressure transducer includes a sensing tube at least a portion of which changes shape in response to a pressure on one side thereof, and a sensor for measuring the changes in shape of the at least a portion of the sensing tube.

According to yet another aspect of the invention, a method of measuring fluid pressure includes measuring strain in a flow passage due to fluid flowing therewithin, as a representation of the fluid pressure.

According to a further aspect of the invention, a flow measuring device includes a sensing tube having a flow restriction therein, the sensing tube having an upstream portion which changes shape in response to an upstream pressure upstream of the flow restriction, and a downstream portion which changes shape in response to a downstream pressure downstream of the flow restriction; an upstream sensor for measuring changes in shape of the upstream portion; and a downstream sensor for measuring changes in shape of the downstream portion.

According to a still further aspect of the invention, a measurement device includes a body defining a flow passage therethrough, the flow passage having an inlet direction, and an outlet direction different from the inlet direction; a sensing element attached to one end of the body, the sensing element having a strain gage thereupon for measuring deformation of at least a portion of the sensing element; and a flexible element attached to an opposite end of the body and allowing flow therethrough in communication with the flow passage.

DETAILED DESCRIPTION

Referring toFIGS. 1 and 2, a pressure transducer10is shown for measuring the pressure of a fluid flow within a pipe, a portion of which is shown generally as pipe segments12and14. The term “fluid,” as used herein, includes liquids, gases, slurries, and multi-phase colloidal and non-colloidal mixtures such as gels, emulsions, fogs, foams, smokes, and gas-liquid, liquid-solid, and gas-solid mixtures, as well as other things that flow. The present invention is particularly useful for measuring pressures of flows of abrasive materials, as well as for measuring pressures in fluid flows were disturbance of the flow is undesirable.

The pressure transducer10includes a sensing tube16which is surrounded or encircled by a housing20. The housing20includes a cover21which is attached to housing flanges22and23. The sensing tube16fits within the housing20and is connected to the housing flanges22and23.

The sensing tube16and the housing20are coupled to the pipe segments12and14via pipe flanges24and26which also connect the pipe segments12and14to the housing20. The flow from one of the pipe segments to the other thereby passes through the sensing tube16. Pressure within the sensing tube16causes the tube to bow outward. This bowing outward elastically changes the shape of the sensing tube—the tube deforms and portions of the sensing tube deflect outward. This change of shape, in particular the outward deformation, under internal pressure, is detected and measured by means of strain gages30mounted on the outside of the sensing tube16. The readings from the strain gages30are then used to determine the pressure within the sensing tube16, using well-known relationships between pressure and strain, such as those found in Roark, Raymond J., Formulas for Stress and Strain, 4th ed., McGraw-Hill, 1965, which is incorporated herein by reference in its entirety. In particular, useful formulas for the present purpose are shown on page 302 in Table XIII—Formulas for Stress and Deformation in Pressure Vessels.

The parts of the pressure transducer10are explained in greater detail below.

Referring toFIG. 3, the sensing tube16is shown as a hollow cylinder. At either end of the sensing tube16are end portions34and36of the tube16. The end portions34and36connect to the housing flanges22and23, respectively, fitting into respective holes38and40in the housing flanges22and23. The end portions34and36are connected to the housing flanges22and23by, for example, welding. It will be appreciated that other means, for example, soldering, braising, threading, or other techniques, may be employed to attach the end portions34and36to the housing flanges22and23.

Preferably the inner diameter D and cross-sectional shape of the sensing tube16are substantially the same as the inner diameter and shape of the pipe segments12and14. Such equal diameter and also a similar or identical cross-sectional shape allow the sensing tube and pipe segments12,14to appear to fluid therein as a continuous flow path, unimpeded in the area of sensing tube16and pipe segments12,14and avoiding creating turbulence, pressure drops and so forth. However, if desired, the inner diameter of the sensing tube may be greater or lesser than that of the pipe segments or of different cross-sectional shapes from those of the pipe segments12and14.

As the interior of the sensing tube16is pressurized by pressurized fluid flowing therethrough, a central portion44of the sensing tube16deforms outward due to the internal pressure force upon it. Some deformation also may occur in the end portions34and36of the sensing tube16, but such deformation usually is substantially less than occurs at the central portion44because the end portions34,36are restrained by the housing flanges22and23. It will be appreciated that the deformation of the central portion44will generally be axisymmetric since the central portion is axisymmetric.

A graph inFIG. 4shows a curve50representing an exemplary typical deformation of the central portion44. The horizontal axis ofFIG. 4represents the axial position along the sensing tube16, with the left side of the graph being the inner edge of one of the end portions34and36. This is where the respective housing flange22or23no longer contacts the end portion34or36. This position is indicated inFIG. 3by dotted lines52and54. The right end of the horizontal axis represents the middle of the sensing tube, indicated inFIG. 3by the dotted line58. The vertical axis ofFIG. 4is the radially-outward displacement of the sensing tube from the unstressed condition represented at zero on the vertical axis. It will be recognized that the vertical and horizontal axes are differently scaled, and that the apparent shape of the tube inFIG. 4greatly exaggerates the radial displacements.

The model used to produceFIG. 4treats the housing flanges22and23as completely rigid, and therefore the radial displacement50is zero at the left side ofFIG. 4, which is at the boundaries52and54of the end portions34and36. Moving rightward alongFIG. 4the displacement50reaches a peak value and then slowly declines, achieving a profile with substantially zero slope at the middle58of the sensing tube16. The middle58is shown as a dotted line inFIG. 3.

Referring toFIG. 5, a graph of the strain distribution along the sensing tube is shown as a function of axial position. As inFIG. 4, the horizontal axis ofFIG. 5is axial position, with the left side corresponding to the edges52and54of the end portions34and36, and the right side corresponding to the middle58of the sensing tube16. The vertical axis is the magnitude of the strains, with tensile strains positive and compressive strains negative.

Both axial strain60and circumferential strain62are plotted inFIG. 5. The axial strain60has its greatest magnitude closest to the housing flanges22and23, at the position indicated as64inFIG. 5. The greatest magnitude strain occurs at the position64due to the stain concentration from the reinforcing flange used for attachment, and from the local fillet radius. The axial strain60is negative at this location, indicating that the outside of the sensing tube16is being compressed in the axial direction. Moving rightward alongFIG. 5, toward the middle58of the sensing tube16, the axial strain60rises, becoming slightly positive before becoming negative again.

In contrast, the circumferential strain62is always positive. Thus the sensing tube16is circumferentially in tension at all places. This is to be expected since the radial displacement50shown inFIG. 4is always positive, indicating that there is a positive outward hoop stress on the sensing tube16at all axial locations. The circumferential strain62is nearly zero along the edges52and54, and increases to a peak value in a region68somewhat away from the middle58of the sensing tube16.

For reasons explained below, it is desirable to measure both axial and circumferential strains on the sensing tube16. In order to get the best response, strain gages70and71for measuring axial strain should be placed at position64(see graph ofFIG. 5), as close as possible to the end portion edges52and54. Strain gages72and73for measuring circumferential strain should preferably be placed in the region68where maximum circumferential strain occurs. Although placement of the strain gages70–73at the positions64and68result in the highest response to deformations of the sensing tube16, it will be appreciated that alternatively the strain gages may be placed in other positions on the sensing tube.

Since the sensing tube16deforms outward when containing a pressurized fluid, the sensing tube16will generally be made of a thinner material than the pipe segments12and14. The sensing tube may be made of the same material as the pipe segments, or alternatively may be made of a different material. The sensing tube may be made from a wide variety of generally rigid materials such as metals, plastics, and resins. By “generally rigid” it is meant that the material undergoes some deformation when stressed, but does not undergo gross deformations or changes of shape.

If desired, the sensing tube may be made of a material that withstands degradation from a corrosive or abrasive fluid flowing through the sensing tube.

It may be desirable to make the sensing tube from a higher strength material than that of the pipe segments, thus allowing the sensing tube to be even thinner.

The strain gages70–73may be solid state gages and/or foil strain gages. Such strain gages are commercially available and are well known. It will be appreciated that well-known methods of mounting strain gages to both electrically conducting and electrically non-conducting materials are available, and such methods will not be explained further here.

Wire leads74are used to connect the strain gages70–73to appropriate circuitry for measuring strains. It will be appreciated that there may be one or more intervening connections between the strain gages70–73and the appropriate circuitry.

Turning now toFIG. 6, a general wiring diagram for the strain gages70–73is shown. As can be seen, the strain gages70–73are configured as part of a Wheatstone Bridge. Specifically, strain gages70–73are connected together end-to-end to form the arms of the Wheatstone Bridge. The node between the gages70and72serves as the P+ terminal. The node between the gages70and73serves as the S−terminal. The node between the gages71and73serves as the P−terminal, and the node between the gages71and72serves as the S+ terminal. Wire leads74from the respective P+, P−, S+ and S− terminals are connected to the strain gages and extend from the sensing tube to an electrical connector80which is mounted on the housing20as shown inFIGS. 1 and 2. Suitable electrical connectors for this purpose are well known.

The leads74allow the gages70–73to be connected to appropriate external circuitry (not shown) for analyzing the signal from the Wheatstone Bridge formed by the strain gages70–73. Such external circuitry may include circuitry to balance the bridge and/or to provide power and calibration, as well as to provide amplification. Such circuitry is considered conventional and consequently further detail concerning the circuitry has been omitted.

The circuitry or a portion thereof may be on a circuit board which may be mounted in the housing20. It will be appreciated that the output signal from the circuit board may be transmitted to an external receiver without a hard-wired connection, for example by means of radio waves or the like.

Alternatively, it will be appreciated that the resistance of the strain gages may be measured other than by use of a Wheatstone Bridge. For example, the resistance of the strain gages may be measured directly by use of an ohmmeter.

It will be appreciated fromFIG. 6that the strain gages70–73are configured in the Wheatstone Bridge such that the outputs of the strain gages70,71which measure compressive (negative) axial strains are subtracted from the outputs of the strain gages72,73which measure tensile (positive) circumferential strains. With such a configuration, the output from the Wheatstone Bridge is an indication of twice the sum of the tensile and compressive strains. It will be appreciated that this configuration increases the signal from the transducer, for example doubling it. This increased signal results in greater sensitivity of the output from the pressure transducer.

The strain gages70,71used to measure axial compressive strain will preferably be oriented perpendicular to the strain gages72,73which measure the circumferential tensile strains, as shown inFIG. 3. More particularly, the strain gages70,71will preferably be oriented to capture the maximum strain in the axial direction, and the strain gages72,73will preferably be oriented to capture the maximum strain in the circumferential direction.

However, it will be appreciated that other configurations and orientations of the strain gages are possible. In addition, there may be more or fewer gages than those illustrated.

Referring toFIGS. 7 and 8, details of the housing flange22are shown. As discussed earlier, the central hole38of the housing flange22is sized to allow a close fit with the sensing tube16. An inner surface84of the central hole38abuts one of the end portions34or36of the sensing tube16. A welding flange86may be provided along the outer face of the central hole38in order to aid in welding the end portion34or36to the housing flange22.

The housing flange22has a pair of flat surfaces88and90provided thereon. The flat surfaces help support access doors94and96(FIGS. 1 and 2), which are described in greater detail below. It will be appreciated that there may be more or fewer doors than shown. It will be further appreciated that the housing flange may alternatively have no access doors.

The housing flange22also has a number of threaded holes100therein. The threaded holes100received threaded connectors such as bolts102(FIGS. 1 and 2) used to connect the housing flange22to the pipe flange24. Preferably the threaded holes100are all located at the same radial distance away from the center of the pipe flange22. Also preferably the threaded holes100are located symmetrically about the housing flange22. Further, preferably the threaded holes100are located symmetrically away from a line A—A′ which is perpendicular through the centers of the flat surfaces88and90. It will be appreciated there may be more or fewer threaded connectors and threaded holes than shown.

The bolts102are preferably securely attached to the housing flange22, such as by tack welding, soldering, or gluing them in place. It will be appreciated that the bolts may not necessarily be attached to the housing. In addition, threaded studs may be used in place of the bolts.

The outside of the housing flange22has a beveled edge106to aid in welding a circumferential surface108of the housing flange22to the cover21.

Preferably the housing flanges22and23are substantially identical. However, it will be appreciated that the flanges may be different.

Details of the cover21are shown inFIGS. 9 and 10. The cover is preferably cylindrical in shape. The cover21has openings110and111therein to allow access to an interior region112. The opening110has flat surfaces116on either side. The flat surfaces116have threaded holes118therein for receiving and securing the access doors94to removably cover the openings110. The opening111has similar flat surfaces with threaded holes therein.

As noted earlier, an outer surface108of the housing flange22(and a similar outer surface of the housing flange23) are designed to be welded to an inner circumferential surface120of the cover21, thereby to form a housing20.

The housing20transmits external loads between the pipe segments12and14. For example, such loads are encountered when the pipe segments12and14are moved relative to each other by translation, bending, or twisting. It is desirable that such loads be transmitted through the housing20as opposed to through the sensing tube16. This is because the sensing tube16is relatively susceptible to damage, since as noted above it will generally be thinner in order that it may deform due to pressure within it. In addition, bending may induce undesirable strains in the sensing tube giving errors in the pressure readings.

The housing20also serves to protect the relatively fragile sensing tube16from other external forces. For example, the housing20protects the sensing tube16from having a dropped object directly impact it.

The housing20also protects from damage the strain gages70–73and the wire leads74.

Further, the housing20acts as a safety feature by containing any rupture that might occur in the sensing tube16. Since the sensing tube16is generally thinner and more liable to deform than the adjoining pipe segments12and14, the sensing tube16is also more liable to rupture. The housing20acts to contain the pipe contents that would escape the sensing tube16as the result of such a rupture. Thus loss of fluid due to a rupture is minimized, as is the possible safety risk from fluid escaping from a ruptured tube.

It will be appreciated that the flat surfaces for supporting the access doors94and96, such as flat surfaces88and90of the housing flanges22and23, and the flat surfaces116of the cover21, may be machined after the cover21and the housing flanges22and23are welded or otherwise joined together.

Referring toFIGS. 11 and 12, details of the access door94are shown. It will be appreciated that the access door96may include similar details, and may in fact be substantially identical to the access door94.

The access door94has a base122with flanges123attached thereto. The flanges123provide structural support for the access door94. They also provide grips to aid in installing or removing the access door94. The flanges and the base may formed from a single piece of material, such as by bending sheet metal or by cutting a piece of C-channel.

The base122has holes124therein. The holes124allow passage through the base122of threaded connectors such as screws126(FIGS. 1 and 2). The screws126secure the access door94to the cover21by mating with the threaded holes118. The base122also has a central hole130for mounting the electrical connector80(FIGS. 1 and 2) to the access door94.

It will be appreciated that the access door96need not also have an electrical connector, and therefore need not have a central hole corresponding to the central hole130. It will further be appreciated that the central hole need not be centrally located on the base of the access door, but may alternatively be located elsewhere on the base.

The access door94is preferably made of the same material as the rest of the housing20, or may be made of another material. For example, the access doors may be made of a stronger material than the rest of the housing, taking into account that the access door may be made of thinner material than the cover.

Turning now toFIGS. 13 and 14details of the pipe flange24are shown. The pipe flange24includes a flange portion130and a pipe connection portion132. The portions130and132may be formed as a single piece, or alternatively may be attached one to another by welding, for example.

The flange portion130has holes136therein corresponding in location to the holes100of the housing flange22. The holes136are large enough to allow the threaded portions138of the bolts102to pass therethrough. Nuts140may be screwed on the threaded portions138of the bolts102to secure the pipe flange24to the housing flange22. A flexible seal142is used to seal the connection between the pipe flange24and the housing flange22. The seal142is radially outboard of a bore146of the flange portion130, and radially inboard of the holes136. It will be appreciated that the material of the seal and the type of seal may be selected depending upon the fluid to be enclosed by the seal and upon the magnitude of the pressure to be maintained by the seal.

The pipe connection portion132has a central bore150and an exterior thread152. Preferably the central bore150of the pipe connection portion132has substantially the same diameter as, and is aligned with, the bore146of the flange portion130. Even more preferably, the bores146and150are the same diameter and are aligned with the bores of the sensing tube16and of the pipe segments12and14. With all of the bores aligned and having the same diameter, there are no steps or other obstructions that would unduly disrupt the flow of the fluid through the pressure transducer. Also, there are no obstructions which would serve to trap fluid passing through the pressure transducer10.

The exterior thread152on the pipe connection portion132is designed to mate with a corresponding interior thread on the pipe segments12and14. However, it will be appreciated that other ways of coupling the pipe flange to the pipe segment are possible. For example the pipe flange may have an internally threaded portion which mates with an external thread on the pipe segment. Alternatively, a fitting coupled to the pipe segment could be used to mate with a corresponding fitting on the pipe flange. It may alternatively be possible to weld or otherwise attach the pipe flange directly to the pipe segment. It will be appreciated that many methods of coupling pipes to fittings are known in the art and suitable of them may be employed in the present invention.

It will be appreciated that the pipe flange may have a different number of holes, for example eight, with a corresponding number of threaded connectors in the housing flange.

Further, it will be appreciated that the above-described use of pipe flanges to couple the housing to pipe segments is illustrative of a wide variety of possible pipe connectors and pipe connecting mechanisms for joining the pipe segments and the housing. Alternatively, for instance, one or both ends of the housing may be able to mate with a standard or non-standard fitting which is coupled to a pipe segment. As such a portion of the housing may be externally- or internally-threaded, for example. Alternatively, one or both ends of the housing may be mateable with a standard or non-standard quick disconnect coupling.

What follows are descriptions of a number of alternate embodiments of various parts of the transducer of the present invention. Features common to the above-described embodiment and various of the alternate embodiments are generally referred to only as needed to describe the particular features of the alternate embodiments.

An alternate embodiment sensing tube216is shown inFIG. 15. The sensing tube216has end portions234and236which are thicker than a central portion238of the sensing tube216. Transition regions242and244link the respective end portions234and236with the central portion238.

The end portions234and236each attach to a suitable housing flange. The end portions234and236have respective beveled edges248and250to facilitate welding of the end portions to the housing flanges. It will be appreciated that the length of the end portions may be greater than, the same as, or less than the thickness of the housing flanges.

The end portion234has a slightly smaller diameter than the end portion236. The mating housing flanges also have slightly different central bores to accommodate this difference in end portion diameters. Accordingly, the sensing tube216is designed to be inserted in one direction only, as indicated by arrow251inFIG. 15. During assembly, the end portion234, being slightly narrower, passes easily through the housing flange which has a hole sized to enable press fitting of the end portion236. Then both end portions are press fit simultaneously into their respective housing flanges. To further facilitate insertion in the direction251, the end portions234and236are provided with respective slightly tapered lead portions252and253.

It will be appreciated that alternatively the end portions may have identical diameters, thus allowing the sensing tube to be inserted in either direction. Further, one or both end portions may alternatively be mechanically attached to their respective housing flanges.

Referring toFIG. 16, a detailed view of the transition region244is shown. In the transition from the end portion236to the central portion238there is a radially inward surface254which smoothly transitions into a curved portion256of constant radius. The curved portion256then transitions smoothly into the central portion238.

As will be recalled from the discussion above in regard toFIG. 5, for a sensing tube16with uniform wall thickness the magnitude of the axial strain is greatest closest to where the sensing tube16is attached to the housing flanges22and23. This is the location where it is most advantageous to place the strain gages which measure axial stress. Moreover, for the sensing tube16placement as close as possible to the ideal location is further desirable because the axial strain decays quickly as one moves away from the ideal location, as shown inFIG. 5.

In contrast, for the sensing tube216, the ideal location for placing a strain gage to measure axial strains is at the boundary where the transition region244transitions to the middle region238. This is shown inFIG. 16as location260.

It is preferable that the transition region have a curved shape. This is because a sharp corner between the end portions and the middle portion would cause a stress concentration at the sharp corner that could result in cracking or fracture of the tube.

An additional advantage of the sensing tube216is that the surface of the central portion238does not slide against the housing flanges when the sensing tube216is installed in the housing flanges. Therefore strain gages may be installed on the sensing tube216prior to its installation in the pressure transducer housing. It will be appreciated that if the difference in diameters between the central portion238and the end portions234and236is great enough, the wires attaching the strain gages to each other and the wire leads for connecting the strain gages to external circuitry may also be installed on the sensing tube216prior to its installation in the pressure transducer housing. Such mounting prior to installation in the housing facilitates manufacture of the pressure transducer by avoiding the need to access the sensing tube through access doors to mount the strain gages and make connections between the strain gages.

FIG. 17shows an additional alternate embodiment sensing tube216′ which has a transition region244′ having a curved region256′ which has a non-constant radius of curvature. The transition region244′ is tapered with the curved region256′ having a radius of curvature which is at a minimum close to an end portion236′ and which gradually increases closer to a central portion238′. The tapered shape shown inFIG. 17may serve to further spread the region of stress concentration, thereby reducing the slope of the strain vs. axial location curve. This spreading of the stress concentration region makes the positioning of a strain gage for measuring axial strain less critical. The preferred position for positioning such a strain gage is indicated inFIG. 17as location260′, which is the location where the transition region244′ ends and the central portion238′ begins.

It will be appreciated that other shapes may be employed for the transition region between the end portions and the central portion. Such shapes may include curved and/or tapered portions, and need not necessarily be axisymmetric.

Referring toFIG. 18another alternate embodiment of sensing tube216″ is shown. The sensing tube216″ is similar to the sensing tubes216and216′; however, the sensing tube216″ has a structural isolation261between its attachment to a housing flange223and the portion of the sensing tube216″ where strain is measured. An end portion236″ of the sensing tube216″ has a rib262and a connecting member264. Between the rib262and the connecting member264is a circumferential groove266around the circumference of the end portion236″.

The connecting member264is attached to the housing flange223, for example, via welding, as evidenced by weld material270. The rib262is not directly attached to the housing flange223. The groove266serves to at least partially structurally isolate the rib262from the connecting member264. In essence, the part of the sensing tube216″ that is axially inboard of the connecting member264(particularly a central portion238″ where strains are measured) is mechanically isolated at least to some extent from loads transmitted from the housing flange223to the connecting member264. This mechanical isolation means that the output of the sensing tube216″ is less susceptible to errors resulting from loads which might otherwise be transmitted from the pressure transducer housing to the sensing tube216″.

It will be appreciated that both end portions of the sensing tube preferably have a mechanical isolation mechanism such as the one described above. However, alternatively a sensing tube may have mechanical isolation only on one of its end portions, and either no mechanical isolation or a different isolation mechanism on the other end portion.

FIG. 19shows an alternate embodiment pressure transducer310which has a sensing tube316which is slidably mounted in one of the housing flanges. In addition, the pressure transducer310includes a housing320which has a cover321. The cover321is preferably cylindrical. Ends324and326of the cover321are attached to housing flanges322and323, by welding or other suitable means.

One end330of the sensing tube316is securely attached to the housing flange322. An opposite end332of the sensing tube316is slidably mounted within the housing flange323. As shown inFIG. 19, the opposite end332has a circumferential groove336with a ring (sometimes referred to as an O-ring)338therein. The ring338may be made of a material which slides easily along the housing flange323and/or cushions the opposite end332from bouncing around within the opening of the housing flange323. Thus, for example, the ring may be made of a resilient material, or may be made of a low-friction material. The ring338seals the connection between the opposite end332and the housing323, preventing entry of fluid into the housing of the connection. Only minimal pressure is expected at this connection. It will be appreciated that many other well-known mechanisms for providing a sliding interface may alternatively be employed in the present invention.

The cushioning function mentioned above provides some structural isolation between the sensing tube316and the housing320.

In the pressure transducer310shown inFIG. 19, the ends324and326of the cover321are welded to the housing flanges322and323. This is in contrast to the configuration shown inFIGS. 1–2, which has a circumferential surface of each of the housing flanges being welded to the inner surface of the cover. The arrangement shown inFIG. 19is advantageous in that it provides a stiffer housing which better withstands external loads.

Referring toFIG. 20, an alternate embodiment housing320′ is shown wherein ends324′ and326′ of a cylindrical cover321′ are attached to housing flanges322′ and323′ radially inward of respective circumferential edges342and343of the housing flanges322′ and323′. The housing320′ provides an even stiffer structure when compared with the housing320shown inFIG. 19. It will be recognized, however, that there is a limit to how far radially inward the cover321′ may be located, since the cover321′ must be radially outward of threaded holes349and350of the respective housing flanges322′ and323′.

Referring toFIG. 21, an alternate embodiment housing420is shown wherein housing flanges422and423have respective outer raised portions426and428to better support respective seals between the housing flanges and corresponding pipe flanges. The outer raised portions426and428preferably extend all the way around respective bores430and432of the housing flanges422and423. Similar corresponding raised portions (not shown) may be provided on the pipe flanges.

In addition, outer raised portion428of the housing flange423has a beveled edge436and the housing flange422has a beveled edge438about the bore432. The beveled edges436and438facilitate installation of a sensing tube through the bores430and432in the direction indicated by the arrow440. It will be appreciated that the beveled edges facilitating installation of a sensing tube in one direction may be employed in conjunction with the sensing tube having end portions with different diameters, shown inFIG. 15and described above.

FIG. 22shows an alternate embodiment sensing tube516which has a rectangular cross-section. Strain gages518are placed on one or more sides520of the sensing tube516. The sides may all have the same length in which case the cross-section of the sensing tube is a square. Alternatively, the sides may have different lengths, making the cross-section of the tube a non-square rectangle.

A tube with a rectangular cross-section is advantageous in that it produces larger strains as compared with a tube of a circular cross-section. This advantage comes from the fact that the rectangular cross-section tube deforms or expands in the longitudinal (or axial) direction as well as in the equivalent of a circumferential or latitudinal (or lateral) direction. This bulging in an extra direction makes for higher strains and therefore a higher gain signal output by the strain gages.

It will be appreciated that the deformation may be greater in a side having greater width. Therefore it may be advantageous to locate strain gages on a long side of a sensing tube with a non-square rectangular cross-section.

The sensing tube516may be mounted in housing flanges similar to those described above, with suitable modifications being made for the rectangular shape.

It will be appreciated that the features described above with regard to the circular cross-section sensing tube embodiments shown inFIGS. 15–19may be employed with a rectangular cross-section sensing tube, with suitable modifications being made.

It will further be appreciated that sensing tubes with other cross-sectional shapes, for example that of a triangle or other polygon, may alternatively be employed.

A further additional embodiment is illustrated inFIG. 23, wherein a sensing tube616has a flow restriction such as an orifice618therein. The orifice618causes a pressure drop across along flow direction620, such that the pressure in an upstream region622is greater than the pressure in an downstream region624. It will be appreciated that the strains in the sensing tube616upstream of the orifice618will be different from the strains downstream of the orifice. It is to be expected, therefore, that strains downstream of the orifice will be different from strains upstream of the orifice. By locating strain gages on the sensing tube616both upstream and downstream of the orifice618, the pressure drop across the orifice618may be determined. Since there is generally a known relationship between the pressure drop across the orifice618and the flow through the orifice618, the sensing tube616may be used as a flow meter to calculate the flow therethrough.

The sensing tube616may have a thicker wall portion630in the region of the orifice618thereby to hinder interaction between the straining of the sensing tube upstream and downstream of the orifice618.

The sensing tube616includes a temperature measuring device632to measure temperature in the fluid or in the sensing tube near the fluids. Exemplary temperature measuring devices include thermocouples and thermistors.

The flow restriction618is described above as an orifice. It will be appreciated alternatively other flow restrictions may be employed which cause a pressure drop which is dependant on the rate of flow therethrough.

The strain gages of the sensing tube616may be wired in a Wheatstone Bridge to provide a direct measure the flow rate. Referring toFIG. 24, one possible wiring configuration is given, where εHUand εLUare the circumferential (hoop) and axial (longitudinal) strains, respectively, on the upstream side of the flow restrictions, and εHDand εLDare the circumferential and axial strains on the downstream side of the flow restriction. When so configured the output ε0of the bridge is equal to εHU+εLD−εHD−εLU. The pressure drop across the flow restriction is a function of the output ε0. Since the flow rate is a function of the pressure drop across the flow restriction, the flow rate is a function of ε0.

The present invention has hitherto been described as involving a sensing tube which completely surrounds a fluid flow. The present invention also embraces embodiments where a portion or part of a tube deforms a relatively greater amount in response to internal pressure. For example, a rectangular cross section sensing tube may have one its sides, or a portion of that side, which has a thinner wall than the rest of the sensing tube, such that only that side or portion of a side deforms significantly in response to internal pressure. Strain gages mounted on that side or portion of a side may measure the deformation, and the measurements may be used to determine pressure.

Similarly, as shown inFIG. 25, a sensing tube716with a circular bore has a flat wall portion720which is thinner than the remainder722of the tube wall. Since the tube wall is thinner at the flat portion720, the flat portion of the tube716will deform more than the remainder722of the tube wall.

Furthermore, it will understood that mechanisms may be employed for locally enhancing (amplifying) strain in the vicinity of a strain gage (increasing the sensitivity of the strain gage). For example, the sensing tube716has thinner portions such as channels or grooves724. One or more of the strain gages placed on, adjacent to, or near such thinner portions may encounter larger strains than at the rest of the sensing tube.

Thinner portions for locally enhancing strain may enhance the strain for only one direction (axial or circumferential) or may enhance strain for both directions.

The invention has been described hitherto as involving strain gages mounted on the outside of a sensing tube. Alternatively, the strain gages may be mounted on the inside of a sensing tube in a suitable application, such as in measuring pressure in a flow of a non-corrosive, non-abrasive fluid.

It will be understood that other methods of measuring deformation and/or deflection of a sensing tube may be employed. For example, a piezoelectric material may be attached to a wall of a sensing tube to measure deformation of the tube.

Referring toFIG. 26, an alternate embodiment pressure transducer810is shown which uses changes in capacitance to measure deflection of a sensing tube816. The sensing tube816is surrounded by a close-fitting cylinder818, which in turn is secured to a housing820. The housing820a cover821which is attached to end flanges822and823.

The transducer810includes a capacitance probe which measures capacitance between a central portion834of the tube816, and the cylinder818. The capacitance probe may measure capacitance between a pair of points, as indicated by the arrows840inFIG. 26. Alternatively, the capacitance probe may measure capacitance along a region between the central portion and the cylinder. A capacitor bridge may be used with the change in capacitance correlated to pressure within the tube. Suitable capacitance probes includes those marketed by Capacitec, Inc. of Massachusetts.

As the sensing tube816deforms because of internal pressure and the central portion834bows outward, the capacitance between the cylinder818and the central portion834decreases. Thus the capacitance probe may be used to determine the amount of deflection of the central portion834. This measured deflection may be used to determine the pressure of the flow within the sensing tube.

The cylinder818also provides additional structural support for the housing820. The additional support provided by the cylinder818may enable the cover821to be reduced in thickness. The effect may be an overall reduction in weight of the housing. It will be appreciated that this structural support benefit of an inner cylinder may be obtained with other embodiments of the inventions not utilizing capacitance probes. Such inner cylinders for structural support may be located at any of a variety of locations between the sensing tube and the cover.

The cylinder818may also mechanically isolate the sensing tube816, rendering the sensing tube less susceptible to errors resulting from loads which might otherwise be transmitted from the housing to the sensing tube.

FIG. 27shows another alternate embodiment, a pressure transducer910which has a laser light source912, a light detector914, and a sensing tube916. The light source912and the light detector914may be suitably mounted to housing, such as those described above, which encloses the sensing tube916. The visible and/or non-visible light output from the light source912is directed-towards a reflective outer surface932of a central portion934of the sensing tube916. The output reflects off the outer surface932and is received by the detector140as shown by light path944. The light may be directed where the central portion undergoes maximum deflection in use, but the light may be directed elsewhere, if desired. As the central portion934deflects and deforms, the location at which the light from the light source912is incident on the detector914will vary. By configuring the light detector914such that the output therefrom varies as a function of where the light strikes the detector, an indication of the deformation of the sensing tube may be obtained. Exemplary light detectors include photodiode arrays and CCD arrays, with the output of the array varying as a function of the spatial location at which the light from the light source912is incident on the detector914.

Although the pressure transducer910is described above in terms of detecting reflected light, it will be appreciated that the same principle may employed in a pressure transducer reflecting and detecting other types of continuum waves or electromagnetic energy, such as sound waves, radio waves, microwaves, or other sorts of radiation, by substituting suitable sources and detectors for the light source912and the light detector914.

FIG. 28shows yet another alternate embodiment, a pressure transducer960which has an optical displacement sensor962for detecting displacement of the sensing tube966. The optical displacement sensor962may include a bundle of optical fibers. On some of the fiber strands, light is emitted and on other of the fiber strands, reflected light is received. The bundle of fibers is substantially perpendicular to the deflecting surface of the sensing tube966. Deflection is measured by the amount of light from the emitting fibers that reflect back into the receiving fibers. The amount of light received by the receiving fibers is a function of the distance between the optical displacement sensor962and the sensing tube966. Suitable fiber bundle optical displacement sensors include those marketed by Philtec, Inc. of Annapolis, Md.

Thus the embodiments described above involve a variety of sensors, such as strain gages, piezoelectric devices, capacitance probes, optical devices, or continuum wave detectors, for measuring deformation and/or deflection of a sensing tube.

Referring toFIG. 29, yet another embodiment, a sensing tube1016, is shown. The sensing tube1016includes short, wide strain gages1020for measuring axial strain and long, narrow strain gages1030for measuring circumferential strain. The gages1020and1030encompass a sizable portion, approximately half, of the perimeter of the sensing tube1016.

By encompassing a sizable portion of the perimeter, the error due to any local nonuniformity in the area of the sensing tube covered by the gages (e.g., a nonuniformity in wall thickness) will be reduced. This is because the strain in the area with the nonuniformity will be “averaged” with many other areas that do not have the nonuniformity.

It will be appreciated that the gages may be such so as to encompass substantially all of the perimeter of the sensing tube, if desired.

It will be appreciated that the sensing elements described above may be made from a hardened material to resist wear, for example from an abrasive slurry. Alternatively, a suitable energy-absorbing liner may be used to resist wear. Shown inFIG. 30is a sensor1060which includes a sensing tube1066and a liner1070. The liner may be made of an energy-absorbing material, for example rubber. The liner1070either may be either attached to the sensing tube1066or may be an insert placed within the sensing tube. An exemplary liner material for a specific application is natural latex rubber with a Shore “A” hardness of 40, a rubber content of 90% or greater, a specific gravity of 1.0 or less, and a tensile strength of 3000 psi. It will be appreciated that other suitable materials, for example natural gum rubber, may alternatively be used. It will further be appreciated that the liner material may be selected based on compatibility with the fluid flowing through the sensing element. The thickness of the liner may be selected based on the size of particles in the fluid, for example on the size of abrasive particles in a slurry. The liner thickness may be selected to absorb the energy of impacting particles and deflect the particles back into the flow field.

FIG. 31shows a measurement device1100which allows direct measurement of dynamic flow rates and pressures of fluids such as gasses, liquids, slurries, and the like, without the use of any probe or other obstruction in the flow. The measurement device1100includes a central body1114having a flow passage therethrough, the body including a shaped conduit1116and conduit extensions1118and1120coupled to the shaped conduit at opposite, respective ends. The inlet for the flow passage through the central body1116is in a different direction from the outlet for the flow passage.

A sensing element1122and a flexible coupling1124are attached to respective opposite end of the central body. The sensing element1122has strain gages1132,1134,1136, and1138thereupon for measuring deformations due to pressure and/or flow within the measuring device1100. The sensing element1122may be similar to one or more of the various sensing tubes described above. A flexible seal or enclosure1140may be provided around the straining element1122as a pressure-containment. An exemplary flexible seal is a metal bellows.

The flexible coupling1124, as explained below, is able to increase its length along the flow direction in order to concentrate strains in the sensing element1122which arise due to the force on the central body1116as a result of the change in fluid flow direction through the central body. The flexible coupling1124may be, for example, a bellows such as a length of corrugated pipe, or a fiber-reinforced hose.

The sensing element1122and the flexible coupling1124may have suitable couplings at respective inlet and outlet ends1142and1144of the device1100, for coupling the device to a pipeline for carrying flowing fluid, for example. It will be appreciated that a wide variety of suitable attachment mechanisms may be employed, for example bolted flanges, threaded fittings, or frangible fittings. A structural element such a channel1146is attached to both of the ends1142and1144, on opposite sides of the central body1116, outboard of (farther from the central body than) the sensing element1122and the flexible coupling1124. The channel1146has support points such as eyelets1150for mounting or otherwise securing the measuring device1100to an external structure or fixed object, for example. The channel1146carries loads external to the flow measuring device1100, thus preventing these external loads from affecting the strain in the sensing device1122.

As a fluid flow passes through the central body1116, the fluid flow changes direction and thus momentum. A force on the fluid is required to be exerted on the fluid to effect this change in momentum. The force on the fluid produces a reaction force1154on the central body1116which is equal in magnitude and opposite in direction. The magnitude of the reaction force1154is a function of the mass flow rate of the fluid, among other factors.

Since the measurement device1100is securely mounted to an external structure, the reaction force1154causes appreciable deflection only within the channel1146. The flexible coupling1124provides relatively little resistance, when compared with the sensing element1122, to movement of the central body1116by the reaction force1154. Therefore, as shown inFIG. 31, the sensing element1122essentially becomes part of a cantilevered beam, and undergoes strain as a result of the movement of the central body1116by the reactive force. This strain is at its greatest on the sensing element1122farthest from the sensing device's neutral plane of bending1158. Therefore, a first set of strain gages1160, including the strain gages1132and1134, those farthest from the neutral plane of bending1158, are the gages used in measuring strains caused by the reaction force1154. A second set of strain gages1162, the strain gages1136and1138, those along the neutral plane of bending1158, may be used in measuring pressure within the sensing element, in a manner similar to that described above with regard to other embodiments.

It will be appreciated that output from the sensing element1122may be suitably calibrated to allow the readings from the first set of strain gages1160to be converted into flow rates. It will be further appreciated that lengths of the conduit extensions1118and1120may be adjusted in order to adjust the output signal from the sensing element1122.

A wide variety of variations of the above-described design may be employed. For example, the central body1116shown inFIG. 31is a single bend covering substantially 90°. However, the direction change may alternatively include multiple bends, or even a sharp corner, if desired. In addition, the total change of direction may be other than a right angle. For example, a measurement device1200(FIG. 32) includes a central body1216between a sensing element1222and a flexible coupling1224, the central body substantially reversing the flow direction, making a substantially 180° bend. The measurement device1200includes a straight structural element1246directly coupling ends1242and1244of the measurement device. The structural element may be a bar, beam, rod, or the like, and may be a single, solid piece. It will appreciated that alternatively the structural element1246may be differently shaped or may be made of multiple pieces, if desired.

In another variation, a measurement device1300(FIG. 33) includes a central body1316between an sensing element1322and a flexible coupling1324, the central body having a total amount of bend that is neither 90° nor 180°. A structural element such as the ones described above may used with the measurement device1300. Alternatively, the desired effect of concentrating the strains due to a reaction force1354at a sensing element1322may be accomplished by clamping or otherwise securing inlet and outlet pipes1366and1368which coupled to the measurement device1300.

Referring now toFIG. 34, an in-line measurement device1400is shown which has a central body1416between a sensing element1422and a flexible coupling1424. The measurement device1400is suitable for measuring flow in a break between substantially-parallel inlet and outlet pipes1466and1468. Elbows1470and1472turn the flow coming into and exiting out of the measurement device1400. A structural element1446is attached inboard of the elbows1470and1472, although alternatively it may be attached outboard of the elbows.

The measurement devices shown inFIGS. 31–34and described above may be enclosed in suitable housings, if desired. It will be appreciated that the devices may be coupled to suitable external components for acquiring, processing, storing, and/or displaying flow rate and/or pressure information. The measurement devices described may generally be placed in either direction in the flow, i.e., with the sensing element either upstream or downstream of the central body.