WO2009018432A1

WO2009018432A1 - Method and apparatus for electronic depth and level sensing by using pressure sensors

Info

Publication number: WO2009018432A1
Application number: PCT/US2008/071739
Authority: WO
Inventors: Paul Gerard Mayer
Original assignee: Bristol, Inc.
Priority date: 2007-08-02
Filing date: 2008-07-31
Publication date: 2009-02-05

Abstract

A method for determining a depth within a fluid is provided. The method comprises positioning at least a first pressure sensor below a surface of the fluid. The first pressure sensor is provided to generate a first pressure measurement. Positioned above the surface of the fluid is a second pressure sensor. The second pressure sensor is provided to generate a second pressure measurement. A depth of the first pressure sensor below the surface of the fluid can be determined based on the difference between the first and second pressure measurements.

Description

METHOD AND APPARATUS FOR ELECTRONIC DEPTH AND LEVEL SENSING BY USING PRESSURE SENSORS

TECHNICAL FIELD The present invention relates to a depth sensor, and more particularly, to a depth sensor that utilizes multiple pressure sensors.

BACKGROUND OF THE INVENTION

Depth sensors have enjoyed success in a variety of industries including diving, drilling, and other scientific industries where the depth of a known or unknown fluid needs to be measured. It is important to note, that "fluid" should not be limited to liquids as the depth of a gas may also need to be calculated. This is particularly true in mining, for example.

One particularly well known device used for determining depth comprises a submersible pressure sensor that measures the pressure "head" of a fluid above the resting place of the submerged sensor. A prior art depth sensor of this kind is shown in FIG. 1. The depth sensor 100 shown in FIG. 1 includes a pressure sensor 101, a vent tube 102, cables 103, and interface 105.

As shown, the pressure sensor 101 is submersed in the fluid 107. Attached to the pressure sensor 101 is a vent tube 102. The vent tube 102 is attached to the pressure sensor 101 at one end and open to the atmosphere (or ambient pressure if in a closed system) at another end. The head of pressure of the fluid 107 is measured by the difference between the pressure acting at the end 104 of the vent tube 102 and the pressure acting at the pressure sensor 101. Thus, the pressure sensor 101 is often called a differential pressure sensor. This is because the pressure sensor 101 does not measure an absolute pressure, but rather can only determine a difference in pressure. Based on the difference in pressures at the surface and at the depth of the pressure sensor 101, the depth 106 can be determined. This information can be sent to an interface 105 via cable 103. A problem with this arrangement is that the vent tube 102 is typically integral to the cable 103. Because the vent tube 102 is incorporated into the cable 103 that connects the pressure sensor 101 to the interface 105, the cable 103 cannot use electrical connectors. Rather, the cable 103 is also integrated into the body of the pressure sensor 101. If the cable 103 is damaged, the entire depth sensor 100 may become unusable. Another problem with the depth sensor 100 is that a different depth sensor 100 may need to be used depending on the particular application. It is important for the end 104 of the vent tube 102 to be above the surface of the fluid in order to achieve an accurate measurement. Therefore, one important consideration in manufacturing the depth sensor 100 is the length of the vent tube 102. Additionally, because the vent tube 102 is integral with the cable 103, the cable 103 must also be a sufficient length to allow the end 104 of the vent tube 102 to extend above the surface of the fluid 107. However, a problem exists because the vent tube 102 and cable 103 can be costly and therefore, customers do not want to pay for more than they need. On the other hand, if the length of the vent tube 102 and cable 103 are produced with too short of a length, the depth sensor 100 becomes either unusable or inaccurate. Therefore, each depth sensor 100 is custom made based on the estimated depth that the unit will be subjected to. The life of the vent tube 102 may also be limited by external factors. Because the vent tube 102 is open to the environment at end 104, the vent tube 102 may become clogged with dirt and debris. As a result, routine cleaning may be required to keep the depth sensor 100 operational. Furthermore, one of the greatest problems encountered with traditional vent tubes is condensation of water within the vent tube. The weight of the condensed water will subtract from the measurement and if enough water collected within the vent tube, the sensor would produce a substantial error.

Therefore, there exists a need for a depth sensor that can be used without a standard vent tube. Additionally, there exists a need for a depth sensor that can be produced with more standardized and uniform components. The present invention overcomes these and other problems and an advance in the art is achieved.

SUMMARY OF THE INVENTION

A method for determining a depth within a fluid is provided according to an embodiment of the invention. The method comprises positioning at least a first pressure sensor below a surface of the fluid. The first pressure sensor generates a first pressure measurement. The method further comprises positioning a second pressure sensor above the surface of the fluid. The second pressure sensor generates a second pressure measurement. The method further comprises determining the depth of the first pressure sensor below the surface of the fluid based on the difference between the first and the second pressure measurements.

A depth sensor is provided according to an embodiment of the invention. The depth sensor comprises a first pressure sensor located below a surface of a fluid. The first pressure sensor generates a first pressure measurement. A second pressure sensor is located above the surface of the fluid. The second pressure sensor generates a second pressure measurement. The depth sensor further comprises electronic circuitry for determining a depth of the first pressure sensor below the surface based on the first and second pressure measurements.

A depth sensor for determining a depth within a vessel is provided according to an embodiment of the invention. The depth sensor comprises a vessel including a fluid contained therein. The depth sensor further comprises a first pressure sensor located below a surface of the fluid. The first pressure sensor generates a first pressure measurement. The depth sensor further comprises a second pressure sensor located above the surface of the fluid. The second pressure sensor generates a second pressure measurement. The second pressure sensor is formed integral with a portion of the vessel. The depth sensor further comprises electronic circuitry for determining a depth of the first pressure sensor based on the first and second pressure measurements.

ASPECTS

One aspect of the invention includes a method for determining a depth within a fluid, the method comprising: positioning at least a first pressure sensor below a surface of the fluid, wherein the first pressure sensor generates a first pressure measurement; positioning a second pressure sensor above the surface of the fluid, wherein the second pressure sensor generates a second pressure measurement; and determining the depth of the first pressure sensor below the surface based on the difference between first and second pressure measurements. Preferably, the method further comprises the first pressure sensor with an operating range different than the operating range of the second pressure sensor. Preferably, the method further comprises the first pressure sensor with an operating range substantially the same as the operating range of the second pressure sensor.

Preferably, the method further comprises providing electrical conductors that allow communication between the first pressure sensor and the second pressure sensor.

Preferably, the method further comprises providing at least a third pressure sensor.

Preferably, the method further comprises positioning the at least third pressure sensor a predetermined distance from the first pressure sensor. Preferably, the method further comprises: positioning the at least third pressure sensor below the surface of the fluid; generating a third pressure measurement from the at least third pressure sensor; and determining a density of the fluid based on the difference between the first pressure measurement and a third pressure measurement and the distance between the first pressure sensor and the at least third pressure sensor.

Another aspect of the invention comprises a depth sensor, comprising: a first pressure sensor located below a surface of a fluid, wherein the first pressure sensor generates a first pressure measurement; a second pressure sensor located above the surface of the fluid, wherein the second pressure sensor generates a second pressure measurement; electronic circuitry for determining a depth of the first pressure sensor below the surface based on the first and second pressure measurements.

Preferably, the electronic circuitry determines the depth of the first pressure sensor based on the difference between the first pressure measurement and the second pressure measurement.

Preferably, the depth sensor further comprises at least a third pressure sensor.

Preferably, the at least third pressure sensor is located a predetermined distance from the first pressure sensor. Preferably, the at least third pressure sensor is located below the surface of the fluid. Preferably, the depth sensor further comprises electronic circuitry for determining the density of the fluid.

Another aspect of the invention comprises a depth sensor for determining a depth within an enclosed vessel, comprising: a vessel including a fluid contained therein; a first pressure sensor located below a surface of the fluid, wherein the first pressure sensor generates a first pressure measurement; a second pressure sensor located above the surface of the fluid, wherein the second pressure sensor generates a second pressure measurement, wherein the second pressure sensor is formed integral with a portion of the vessel; and electronic circuitry for determining a depth of the first pressure sensor below the surface based on the first and second pressure measurements.

Preferably, the vessel comprises a substantially enclosed vessel.

Preferably, the second pressure sensor is formed integral with a cap for the vessel.

Preferably, the cap includes one or more electrical conductors on a first side, wherein the electrical conductors are provided to allow communication between the first pressure sensor with the second pressure sensor.

Preferably, the cap includes one or more electrical conductors on a second side, wherein the electrical conductors are provided to allow communication between the first and second pressure sensors with at least one interface.

Preferably, the at least third pressure sensor is positioned a predetermined distance from the first pressure sensor. Preferably, the at least third pressure sensor is positioned below the surface of the fluid and the depth sensor further comprises electronic circuitry for determining a density of the fluid based on the difference between the first pressure measurement and a third pressure measurement generated by the at least third pressure sensor and the difference between the first pressure sensor and the at least third pressure sensor. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a prior art depth sensor.

Figure 2 shows a depth sensor according to an embodiment of the invention. Figure 3 shows the depths sensor according to another embodiment of the invention.

Figure 4 shows the depth sensor being partially formed in a cap.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 2 - 4 and the following description depict specific examples to teach those skilled in the art how to make and use the best mode of the invention. For the purpose of teaching inventive principles, some conventional aspects have been simplified or omitted. Those skilled in the art will appreciate variations from these examples that fall within the scope of the invention. Those skilled in the art will appreciate that the features described below can be combined in various ways to form multiple variations of the invention. As a result, the invention is not limited to the specific examples described below, but only by the claims and their equivalents.

FIG. 2 schematically shows a depth sensor 200 according to an embodiment of the invention. The depth sensor 200 includes a first pressure sensor 201 and a second pressure sensor 202. The two pressure sensors are coupled via lines 203. While only two pressure sensors are shown, it should be understood that more than two pressure sensors may be used. According to an embodiment of the invention, the pressure sensors 201 and 202 are absolute pressure sensors. It should be understood, however, that other calibrated pressure sensors, such as a PSIS pressure sensor may be used and the invention should not be limited to absolute pressure sensors. Pressure sensors are generally known in the art, and the particular pressure sensor chosen should not limit the scope of the invention.

In the embodiment shown in FIG. 2, the two pressure sensors 201 and 202 are coupled to one another. See FIG. 3 for an alternate embodiment. In the embodiment shown in FIG. 2, the first pressure sensor 201 is positioned an unknown distance 206 below the surface of the fluid 207. The pressure sensor 202 is shown positioned substantially above the surface of the fluid 207. In the embodiment shown in FIG. 2, the first pressure sensor 201 generates an absolute pressure of the fluid. Additionally, the second pressure sensor 202 generates an absolute pressure above the fluid. However, it should be understood that the pressure sensors 201, 202 may generate other pressure measurements, for example a modified differential, or a gauge pressure. Gauge pressure should be understood to mean pressure measured relative to atmospheric pressure. The precise pressure measurement should not be limited to these examples, and other pressure measurements that are capable of operating without a vent tube, such as the vent tube 102, can be used.

Because the first pressure sensor 201 is located below the surface of the fluid 207, the two pressure sensors will produce different readings. The pressure measurements generated by the two pressure sensors 201, 202, can be sent to electronic circuitry (not shown). The electronic circuitry can subtract the two pressure measurements and calculate the depth of the first sensor 201 according to known methods. The electronic circuitry can include, but is not limited to, analog subtraction amplifiers or using digitally corrected pressure measurements subtracted using a microcontroller or other digital signal processing device. The precise circuitry used should not limit the scope of the present invention.

The depth sensor 200 according to an embodiment of the invention, provides a number of advantages over the prior art depth sensor 100. As shown, the depth sensor 200 eliminates the need for a vent tube. This is because the pressure sensor 201 generates an absolute pressure measurement rather than a differential pressure measurement. Therefore, the pressure sensor 201 can be calibrated with respect to a vacuum and once in use, does not need further calibration. Additionally, because a vent tube does not need to be integrated with the cable 203 and the pressure sensor 201, the cable 203 can utilize electrical conductors to connect to the pressure sensor 201. This advantage could not be realized in the past because both the cable and the vent tube are formed integral with the pressure sensor. The advantage of being capable of utilizing electrical conductors is substantial because a separate pressure sensor 201 does not need to be used for each application. Rather, the same pressure sensor 201 can be used and connected with different length cables depending on the anticipated depth of the fluid 207.

Another advantage of the present invention is that the ranges of the two or more pressure sensors do not need to be the same. This is because the generated measurements may be scaled and compensated prior to being subtracted from one another. The subtracted signal may then be zeroed and amplified by the associated circuitry.

FIG. 3 shows a depth sensor 300 according to another embodiment of the invention. The depth sensor 300 shown in FIG. 3 includes a first pressure sensor 301, a second pressure sensor 302, and a third pressure sensor 313. All three pressure sensors can communicate with an interface 305.

As can be seen, the depth sensor 300 shown in FIG. 3 differs from the depth sensor 200 shown in FIG. 2. However, the determination of the depth of the first pressure sensor 301 can be performed in substantially the same manner as described above in relation to the depth sensor 200. One significant difference is that the first and second pressure sensors 301, 302 are not coupled to each other. Rather, they both independently communicate with an interface 305, which can compare the pressure measurements generated and compare them. In some embodiments, the second pressure sensor 302 can be directly coupled to the interface 305. In this embodiment, the second pressure sensor 302 would be formed integral with the interface 305. The advantage of this configuration is the reduction of wiring, cables, etc. This is only possible if the interface 305 is in a location that the pressure would not differ from the pressure substantially above the surface of the fluid 307. In many applications this can be an accurate approximation. It should be understood that in other embodiments, the first pressure sensor 301 can be directly coupled to the interface 305. In this embodiment, the first pressure sensor 301 would be formed integral with the interface 305. Additional embodiments allow for more than one interface 305, and both the first and second pressure sensor 301, 302 can be directly coupled to an interface. Additionally shown in FIG. 3 is a third pressure sensor 313. The third pressure sensor 313 is positioned a known distance 315 from the first pressure sensor 301. Providing the third pressure sensor 313 can be particularly advantageous in applications where the density of the fluid is unknown. With the use of the first pressure sensor 301 and the third pressure sensor 313 positioned at a known distance from one another, the density can be determined based on the assumption that the fluid 307 in which the pressure sensors are submersed is incompressible. "Incompressible" should be understood that some compression does occur and that there is a pressure differential with changes in depth. For example, water is typically considered to be incompressible; however, pressure does change with depth. Even for gases this can be a good assumption for relatively small changes in height. With at least two pressure sensors located at a known distance from each other the density of the fluid 307 can be calculated as follows.

A known equation for hydrostatic equilibrium is: p

— h gZ = constant ( 1 )

P where P = pressure p = density g = gravitational constant

Z = height

Therefore, if two pressures are measured at two different heights, a & b, the equation becomes:

^^L -Z^L = g(Z_b - Z_a) (2)

P P By solving for density, equation 2 becomes:

_{P =} ^(P» -^P-> (3) g(z, -z.)

With the use of at least two pressure sensors located a known distance apart and within the same fluid, the only unknown is the density. Thus, by providing at least a third pressure sensor 313, the density of the fluid 307 can also be determined. The pressure measurements and the distance can be sent to electronic circuitry and the density can be determined. In some embodiments, the electronic circuitry used to determine the density can be the same as the electronic circuitry used to determine the depth of the first pressure sensor. In other embodiments, separate electronic circuitry can be used. This provides another advantage over using a depth sensor such as the prior art depth sensor 100, which is restricted by the use of a vent tube. While the density can be calculated using only two pressure sensors located below the surface of the fluid 307, it should be understood that more than two pressure sensors may be used. Providing more than two pressure sensors can provide a more accurate measurement of the density of the fluid 307. FIG. 4 shows the depth sensor 400 according to an embodiment of the invention. The depth sensor 400 includes a first pressure sensor 401, a second pressure sensor 402, and a cable 403 to communicate the first pressure sensor 401 with the second pressure sensor 402. The depth sensor 400 is similar to the depth sensor 200, with the exception that the depth sensor 400 is adapted for an enclosed environment, such as a well, pipe, or some other vessel, for example. The pressure sensor 401 is substantially the same as the pressure sensors 201 and 301 described above. However, the pressure sensor 402 differs from the pressure sensor 202 in that the pressure sensor 402 further comprises a portion of the vessel. In the embodiment shown, the pressure sensor 402 comprises a cap 411. However, the pressure sensor 402 can comprise other portions of the vessel and should not be limited to the cap. In embodiments where the pressure sensor 402 comprises a cap, the pressure sensor 402 not only provides a pressure measurement above the surface of the fluid, but also substantially encloses the vessel 410. As shown in FIG. 4, the first pressure sensor 401 is positioned below the surface of the fluid 407. In some embodiments, the first pressure sensor 401 may be positioned on the base of the vessel, in which case, the depth of the pressure sensor 401 would depend upon the fluid level. The first pressure sensor 401 generates a first pressure measurement. In some embodiments, the first pressure measurement comprises an absolute pressure measurement. However, it should be understood that the first pressure measurement could comprise a gauge pressure. The first pressure sensor 401 can be electrically coupled to the second pressure sensor 402 using an electrical cable 403. In some embodiments, the electrical cable 403 can use electrical conductors (not shown) to connect the first pressure sensor 401 with the second pressure sensor 402. The second pressure sensor 402 can be positioned substantially above the surface of the fluid 407. The second pressure sensor 402 generates a second pressure measurement. In some embodiments, the second pressure measurement comprises an absolute pressure measurement. However, it should be understood that the second pressure measurement could also comprise a gauge pressure. In the embodiment shown in FIG. 4, the second pressure sensor 402 is formed integral to a cap 411. The cap 411 is provided to enclose the vessel 410. In some embodiment, the cap 411 provides a substantially airtight seal with the vessel 410. However, it should be understood that the cap 411 can be provided in some embodiments to prevent dirt and debris from entering the vessel 410. In this embodiment, it is not necessary that the cap 411 form a substantially airtight seal with the vessel 410. On a first side of the cap 411, electrical conductors (not shown) may be provided for the attachment of the electrical cable 403. On a second side of the cap 411, additional electrical conductors may be provided for the attachment of the electrical cable 404. In some embodiments, the electrical cable 404 allows the first and second pressure sensors 401, 402 to communicate with an interface 405. The interface 405 can be provided to electrically subtract the second pressure measurement from the first pressure measurement. The interface 405 may comprise a microcontroller or other digital signal processing device. In other embodiments, the interface 405 may comprise an analog subtraction amplifier. In still further embodiments, the subtraction of the second pressure measurement from the first pressure measurement may take place within the cap 411. In these embodiments, the interface 405 may still be provided for an output from the cap 411.

In some embodiments, the depth sensor 400 may include at least a third pressure sensor (not shown). The third pressure sensor may be used to calculate the density of the fluid 407 according to the methods discussed above in relation to the depth sensor 200.

The depth sensors 200, 400 as described above provide a number of advantages over depth sensors that are restricted by the use of vent tubes. Without having to incorporate a vent tube, the electrical connections made to the submersible pressure sensor can be simplified and can utilize electrical connections rather than having to integrate the cables into the pressure sensor. Further, the depth sensor can be used in a number of different applications, as the use is not restricted by the length of the vent tube. Another advantage is that pressure sensors may be provided with different operating ranges. This can allow for improved accuracy as a greater range can sacrifice accuracy in some applications. The detailed descriptions of the above embodiments are not exhaustive descriptions of all embodiments contemplated by the inventors to be within the scope of the invention. Indeed, persons skilled in the art will recognize that certain elements of the above-described embodiments may variously be combined or eliminated to create further embodiments, and such further embodiments fall within the scope and teachings of the invention. It will also be apparent to those of ordinary skill in the art that the above-described embodiments may be combined in whole or in part to create additional embodiments within the scope and teachings of the invention.

Thus, although specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. The teachings provided herein can be applied to other depth sensors, and not just to the embodiments described above and shown in the accompanying figures. Accordingly, the scope of the invention should be determined from the following claims.

Claims

CLAIMS We claim:

1. A method for determining a depth within a fluid, the method comprising: positioning at least a first pressure sensor below a surface of the fluid, wherein the first pressure sensor generates a first pressure measurement; positioning a second pressure sensor above the surface of the fluid, wherein the second pressure sensor generates a second pressure measurement; and determining the depth of the first pressure sensor below the surface based on the difference between first and second pressure measurements.

2. The method of claim 1, further providing the first pressure sensor with an operating range different than the operating range of the second pressure sensor.

3. The method of claim 1, further providing the first pressure sensor with an operating range substantially the same as the operating range of the second pressure sensor.

4. The method of claim 1, further comprising providing electrical conductors that allow communication between the first pressure sensor and the second pressure sensor.

5. The method of claim 1, further comprising providing at least a third pressure sensor.

6. The method of claim 5, further comprising positioning the at least third pressure sensor a predetermined distance from the first pressure sensor.

7. The method of claim 6, further comprising: positioning the at least third pressure sensor below the surface of the fluid; generating a third pressure measurement from the at least third pressure sensor; and determining a density of the fluid based on the difference between the first pressure measurement and a third pressure measurement and the distance between the first pressure sensor and the at least third pressure sensor.

8. A depth sensor (200), comprising: a first pressure sensor (201) located below a surface of a fluid (207), wherein the first pressure sensor (201) generates a first pressure measurement; a second pressure sensor (202) located above the surface of the fluid (207), wherein the second pressure sensor (202) generates a second pressure measurement; electronic circuitry (205) for determining a depth (206) of the first pressure sensor (201) below the surface based on the first and second pressure measurements.

9. The depth sensor (200) of claim 8, wherein the electronic circuitry (205) determines the depth (206) of the first pressure sensor (201) based on the difference between the first pressure measurement and the second pressure measurement.

10. The depth sensor (200) of claim 8, further comprising at least a third pressure sensor (313).

11. The depth sensor (200) of claim 10, wherein the at least third pressure sensor (313) is located a predetermined distance (315) from the first pressure sensor (301).

12. The depth sensor (200) of claim 11 , wherein the at least third pressure sensor (313) is located below the surface of the fluid (207).

13. The depth sensor (200) of claim 12, further comprising electronic circuitry (205) for determining the density of the fluid (207).

14. A depth sensor (400) for determining a depth within an enclosed vessel (410), comprising: a vessel (410) including a fluid (407) contained therein; a first pressure sensor (401) located below a surface of the fluid (407), wherein the first pressure sensor (401) generates a first pressure measurement; a second pressure sensor (402) located above the surface of the fluid (407), wherein the second pressure sensor (402) generates a second pressure measurement, wherein the second pressure sensor (402) is formed integral with a portion of the vessel (410); and electronic circuitry (405) for determining a depth of the first pressure sensor

(401) below the surface based on the first and second pressure measurements.

15. The depth sensor (400) of claim 14, wherein the vessel (410) comprises a substantially enclosed vessel (410).

16. The depth sensor (400) of claim 14, wherein the second pressure sensor (402) is formed integral with a cap (411) for the vessel (410).

17. The depth sensor (400) of claim 16, wherein the cap (411) includes one or more electrical conductors (403) on a first side, wherein the electrical conductors (403) are provided to allow communication between the first pressure sensor (401) with the second pressure sensor (402).

18. The depth sensor (400) of claim 16, wherein the cap (411) includes one or more electrical conductors (404) on a second side, wherein the electrical conductors (404) are provided to allow communication between the first and second pressure sensors (401, 402) with at least one interface (405).

19. The depth sensor (400) of claim 14, further comprising at least a third pressure sensor (313).

20. The depth sensor (400) of claim 18, wherein the at least third pressure sensor (313) is positioned a predetermined distance (315) from the first pressure sensor (401).

21. The depth sensor (400) of claim 20, wherein the at least third pressure sensor (313) is positioned below the surface of the fluid (407) and the depth sensor (400) further comprises electronic circuitry (405) for determining a density of the fluid (407) based on the difference between the first pressure measurement and a third pressure measurement generated by the at least third pressure sensor (313) and the difference between the first pressure sensor (401) and the at least third pressure sensor (313).