WO1998014774A1

WO1998014774A1 - Method of determining a setback

Info

Publication number: WO1998014774A1
Application number: PCT/US1997/017842
Authority: WO
Inventors: Alvin E. Brown
Original assignee: Dietrich Technology Holding Corp.
Priority date: 1996-10-04
Filing date: 1997-10-01
Publication date: 1998-04-09
Also published as: AU4743997A

Abstract

A method of determining a transducer setback in an ultrasonic flow meter (10) determines an actual path length (20) of a flowing portion of a fluid in a conduit (12). Next the speed of sound in the fluid is determined (104). The flowing path transit time is calculated using the speed of sound in the fluid (106). A total path transit time is measured (108) and the flowing path transit time is substracted from it to find the setback (110).

Description

METHOD OF DETERMINING A SETBACK

Field of the Invention

The present invention relates generally to the field of ultrasonic flow meters and more particularly to a method of determining a transducer setback in an ultrasonic flow meter.

Background of the Invention

Ultrasonic flow meters have many advantages over other methods of determining flow rates. Ultrasonic flow meters can continuously measure the flow rate, while other methods generally measure average flow rates. In addition, ultrasonic flow meters are obstructionless and work with non- conductive fluids.

Ultrasonic flow meters have a pair of transducers that are placed on either side of the flow path of a fluid flowing through a pipe. The transducers are pointed at each other and placed on either side of the flow path of a fluid flowing through a pipe. The transducers are pointed at each other and the line between them has a component in the direction of the fluid flow. The principle used to detect flow rates is that the transit time of an ultrasonic packet will increase in the upstream and decrease in the downstream path. The amount by which the transit time changes is directly proportional to the flow rate. The measured transit time however includes a number of errors. The front window of a transducer provides a delay. Immersed transducers (fixed transducers see FIG. 1) have a stagnant volume between the transducer face and the flowing medium. In the case of clamp-on transducers (see FIG. 2) the pipe wall introduces a delay. There are also delays in the electronics. In the prior art these error producing delays are combined into a single correction factor. This has proved unsatisfactory, since the factors contributing to the error are not static and vary considerably in the same models. As a result a single correction factor does not provide satisfactory results.

Thus there exists a need for a method that accurately measures these delays and can correct for them continuously.

Summary of the Invention

A method of determining a transducer setback in an ultrasonic flow meter that overcomes these and other problems determines an actual path length of a flow portion of a fluid in a conduit. Next the speed of sound in the fluid is determined. The flowing path transit time is calculated using the speed of sound in the fluid. A total path transit time is measured and the flowing path transit time is subtracted from it to find the setback.

Brief Description of the Drawings FIG. 1 is a block diagram of an ultrasonic flow meter;

FIG. 2 is a cross section of a pair of clamp-on transducers attached to a conduit;

FIG. 3 is a flow chart of a process for determining a setback; and FIG. 4 is a flow chart of the process of determining an actual path length in a clamp-on transducer.

Detailed Description of the Drawings

The invention determines a setback by measuring a total transit time and subtracting a calculated flowing path transit time. The total transit time is measured by adding an upstream transit time and a downstream transit time and then dividing by two. In this way the effects of the flowing fluid are eliminated in determining the total transit time. The calculated flowing path transit time is determined by using trigonometry to calculate a flowing path length. The flowing path length is divided by a speed of sound in the fluid to determine the flowing path transit time. The speed of sound in the fluid is a function of the temperature of the fluid. By measuring the temperature of the fluid, the speed of sound in the fluid can be found in a look up table. FIG. 1 is a block diagram of an ultrasonic flow meter 10 attached to a conduit 12. A pair of transducers 14, 16 are immersed in the fluid and placed at angle to an inside diameter 18 of the conduit 12. The actual path length (flowing path length) 20 of a flowing portion of the fluid can be found knowing the diameter 18 and the angle of transmission ( ). The transducers (pair of immersed transducers) 14, 16 are coupled to a decoding electronics 22. The decoding electronics 22 are used to determine the transit times. A microprocessor 24 is coupled to the decoding electronics 22. The microprocessor 24 controls the decoding electronics 22 and performs a number of calculations necessary to determine the setback. The microprocessor (computer) 24 is coupled to a memory (computer readable device) 26 that stores the instructions executed by the microprocessor 24 to determine the setback. In addition, the memory stores look up tables (table plurality of fluid tables) on a variety of mediums (type of fluid). The look up tables relate the temperature of the medium to the speed of sound in the medium. A display 28 and an input device 30 are also connected to the microprocessor 24. The input device 30 can be used to input information such as the temperature of the medium, the diameter of the conduit and other information. Alternatively, the decoding electronics can include a temperature measurement device, such as a thermocouple. In another embodiment the setback is calculated at the factory and entered as a fixed offset in the equations used by the ultrasonic flow meter to calculate flow rates. FIG. 2 is a cross section of a pair of clamp-on transducers 40, 42 attached to a conduit 44. Determining the actual path length 46 is more complicated in the case of clamp-on transducers than immersed transducers.

The diameter 48 of the conduit 44 is known, but the angle β varies with the temperature of the fluid. This is because the speed of sound in the fluid is different from the speed of sound in the transducer 40, 42 and different from the speed of sound in a wall 50 of the conduit. The sound waves propagate through these various mediums according to Snells Law. It can be shown that the wall has no effect on the angle β (angle of transmission), since the sound passes through the wall. Similarly the angle at which the sound is propagating in the receive transducer 40 is equal to an initial angle of transmission a. The angle β can be found using the equation below:

SIN(a) ₌ SIN(β) SS, SS_f

Where SSt - is the Speed of Sound in the transducer, and SSf - is the Speed of Sound in the fluid.

Unfortunately, the speed of sound in the fluid and in the transducer is a function of temperature. As a result the angle β is a function of the temperatures of the mediums and varies with these temperatures.

The clamp-on transducers 40, 42 are filled with an index matching (i.e., same sound speed) material 52, 54 that makes contact with the pipe wall 50. Generally, the transducers 40, 42 are placed at a forty five degree angle (i.e., = 45 ° ) to the diameter 48 of the pipe 50. Shallower angles can result in total internal reflection at the transducer wall boundary. The angle β is in the neighborhood of twenty six degrees for many fluids at standard temperatures. In the prior art placement of the transducers 40, 42 is considered critical and specially designed guides are used to hold the transducers 40, 42 in place. These specially designed guides allow the setback to be determined based on the geometry and ignoring that the temperature affects the setback. Using this invention it is no longer necessary to use the specially designed guides.

FIG. 3 is a flow chart of the process used to calculate the setback. This process can be implemented by a computer (microprocessor) executing a set of instructions. The set of instructions can be stored on any computer- readable storage medium (e.g., diskette, CD-ROM, ROM, RAM). The process starts, step 100, by determining the actual path length of the flowing portion of the fluid at step 102. The calculation of the actual path length depends on whether the transducers are immersed or are clamp-on transducers. As a result a computer program may include a meter indicator variable that indicates whether the transducers are clamp-on or immersed. Next the speed of sound in the fluid is determined at step 104. The speed of sound in the fluid is found by measuring the temperature and using a look up table for the fluid to determine the speed of sound. Next, the flowing path transit time is determined at step 106. The flowing path transit time is calculated by dividing the actual path length by the speed of sound in the fluid. The total path transit time is measured at step 108. This measurement is done by launching an ultrasonic pulse and measuring the time from launch until detection at the receive transducer. Generally both the upstream and the downstream transit time are measured and added together to find a round trip time. By dividing the round trip time by two the total transit time is found. The increase in the upstream transit time over a no-flow situation is equal to the decrease in the downstream transit time. The setback is found by subtracting the calculated flowing path transit time from the total transit time at step 110, which ends the process at step 112. In the preferred embodiment half the setback time is attributed to each transducer and the setback is converted to an effective distance.

FIG. 4 is a flow chart of a process for determining the actual path length in the case of a clamp-on transducer. The actual path length in the clamp-on case will vary with temperature. As a result it is necessary to periodically recalculate the actual path length and the setback for clamp-on transducers. In the case of fixed, immersed transducers the actual path length does not vary and the setback can be calculated once at the factory. The setback is then a fixed correction factor. The process starts, step 150, by inputting the inside diameter of the conduit and the initial angle of transmission (a.) at step 152. The fluid temperature is measured at step 154. In the preferred embodiment the temperature of the transducer is assumed to be equal to the temperature of the fluid. Using the temperature, the speed of sound in the transducer is found in a look-up table at step 156. Similarly the speed of sound in the fluid is found in a look-up table at step 158. Using Snells Law the angle of transmission (β) is calculated at step 160. The calculated actual path length is determined at step 162, which ends the process at step 164.

Once the setback has been determined initially, any changes in the total transit time can be attributed to changes in temperature that changed the speed of sound. As a result, we can work backward to determine the temperature and the speed of sound that result in the measured change in the total transit time. This allows the setback to be adjusted on the fly without measuring the temperature of the fluid again. Thus there has been described a process that can accurately determine the setback for both fixed and clamp-on transducers. The process can be performed while the fluid is flowing and therefore used to recalculating the setback in the field. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alterations, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alterations, modifications, and variations in the appended claims.

Claims

ClaimsWhat is claimed is:

1. A method of determining a setback in an ultrasonic flow meter, comprising the steps of: (a) determining an actual path length of a flowing portion of a fluid in a conduit; (b) determining a speed of sound in the fluid; (c) calculating a flowing path transit time; (d) measuring a total path transit time; (e) subtracting the flowing path transit time from the total path transit time to find the setback.

2. The method of claim 1, wherein step (a) further includes the steps of: (al) receiving an inside diameter of the conduit; (a2) determining an angle of transmission; (a3) calculating the actual path length using the inside diameter and the angle of transmission.

3. The method of claim 2, wherein step (a2) further includes the steps of: (i) when the ultrasonic flow meter is using a pair of clamp-on transducers, determining a speed of sound in the pair of clamp-on transducers; (ii) measuring an initial angle of transmission from one of the pair of clamp-on transducers; (iii) determining a speed of sound in the fluid; (iv) determining the angle of transmission, using the speed of sound in the pair of clamp-on transducers, the initial angle of transmission and the speed of sound in the fluid.

4. The method of claim 3, further including the step of: (v) when the ultrasonic flow meter is using a pair of immersed transducers, measuring the angle of transmission.

5. The method of claim 1, wherein step (b) further includes the steps of: (bl) measuring the temperature of the fluid; (b2) determining the speed of sound in the fluid using the temperature of the fluid.

6. The method of claim 1 , wherein step (d) further includes the steps of: (dl) measuring an upstream transit time; (d2) measuring a downstream transit time; (d3) determining the total path transit time using the upstream transit time and the downstream transit time.

7. A computer-readable storage medium containing computer readable instructions that when executed by a computer performs the following steps: (a) calculating an actual path length of a flowing portion of a fluid in a conduit; (b) retrieving a speed of sound in the fluid from a memory; (c) calculating a flowing path transit time; (d) receiving a total path transit time; (e) subtracting the flowing path transit time from the total path transit time to find a setback.

8. The computer-readable storage medium of claim 7, wherein step (a) further includes the steps of: (al) receiving an inside diameter of the conduit; (a2) retrieving an angle of transmission; (a3) calculating the actual path length.

9. The computer-readable storage medium of claim 8, wherein step (a2) further includes the steps of: (i) receiving a meter indicator variable; (ii) when the meter indicator variable indicates a clamp-on transducer, determining a speed of sound in the clamp-on transducer; (iii) receiving an initial angle of transmission; (iv) retrieving a speed of sound in the fluid; (v) calculating the angle of transmission, using the speed of sound in the clamp-on transducer, the initial angle of transmission and the speed of sound in the fluid.

10. The computer-readable storage medium of claim 9, further including the step of: (vi) when the meter indicator variable indicates that transducer is immersed, receiving the angle of transmission.

11. The computer-readable storage medium of claim 7, wherein step (b) further includes the steps of: (bl) receiving a temperature of the fluid; (b2) finding the speed of sound of the fluid in a table in the memory using the temperature of the fluid.

12. The computer-readable storage medium of claim 11, wherein step (b2) further includes selecting the table from a plurality of fluid tables according to a type of fluid.

13. The computer-readable storage medium of claim 7, wherein step (d) further includes the steps of: (dl) receiving an upstream transit time; (d2) receiving a downstream transit time; (d3) calculating the total path transit time, using the upstream transit time and the downstream transit time.

14. A method of determining a setback in an ultrasonic flow meter, comprising the steps of: (a) determining a flowing transit time; (b) measuring a total transit time; (c) subtracting the flowing transit time from the total transit time to determine the setback.

15. The method of claim 14, wherein step (a) further includes the steps of: (al) determining a flowing path length; (a2) determining a speed of sound in a fluid; (a3) calculating a flowing transit time using the flowing path length and the speed of sound in the fluid.

16. The method of claim 15, wherein step (al) further includes the steps of: (i) receiving an inside diameter of a conduit in which the fluid is flowing; (ii) determining an angle of transmission through the fluid; (iii) determining the flowing path length using the inside diameter and the angle of transmission.

17. The method of claim 15, wherein step (a2) further includes the steps of: (i) measuring a temperature of the fluid; (ii) looking up the speed of sound in the fluid using the temperature.

18. The method of claim 14, wherein step (b) further includes the steps of: (bl) measuring a downstream transit time; (b2) measuring an upstream transit time; (b3) determining the total transit time by adding the downstream transit time and the upstream transit time to form a round trip time and dividing the round trip time by two to determine the total transit time.