Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Reexamination Certificate
1999-12-10
2001-01-30
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
C073S861280, C073S861220
Reexamination Certificate
active
06178827
ABSTRACT:
BACKGROUND OF THE INVENTION
TECHNICAL FIELD
The present invention relates to a simplified method for measuring the flow rate of a fluid whereby the propagation times of ultrasonic signals transmitted through the fluid can be detected to determine fluid flow rate. The invention further relates to improvement of a probe flow sensor configuration and to its installation, and similar sensing devices.
DISCUSSION
Transit-time ultrasonic flow sensors, also known as “time-of-flight” ultrasonic flow sensors, detect the acoustic propagation time difference between the upstream and downstream ultrasonic transmissions resulting from the movement of the flowing fluid and process this information to derive a fluid flow rate. Several different sensor configurations have been used including: 1) direct measurement of the propagation time of a pulse emitted by a first transducer and received by a second transducer where the change in time is a function of fluid flow rate; 2) dual “sing-around” sound velocimeters, where the difference in “sing-around” frequency between the velocimeters is a function of the fluid flow rate; 3) sensors producing continuous waves using two widely different high frequency carriers but commonly modulated with another much lower frequency signal where the phase difference of the modulated signal on the received carriers is a function of the fluid flow rate; and 4) sensors producing bursts of continuous waves using a single frequency on a pair of transducers, the burst duration being less than the acoustic propagation time between the transducers, where the time between the received transmissions is a function of flow rate.
The transducers of transit-time ultrasonic flow sensors are most often field mounted, whereby they are individually attached to the outside of a pipe, thereby offering the advantage of not having to break into the pipe as with other meters, in order to make the flow measurement. However, the uncertainty of the pipe wall integrity and the effects of its surface condition, and the uncertainties of locating, attaching and acoustically coupling the transducers to the pipe, and the uncertainties of the reflection from the interior of the pipe when it is used to complete the acoustic path between the transducers, can often lead to substantial measurement error. Even when the transducers are in contact with the fluid being measured (wetted), their mechanical location, which is again most often accomplished in the field, may result in misalignment or being disposed at the wrong distance or angle, again resulting in measurement error. As a result, these sensors are usually equipped with supporting electronics containing sophisticated diagnostic means for confirming proper installation and operation. Overall, these sensors are relatively expensive and have a reputation for sometimes producing erroneous measurements.
It is therefore an object of the present invention to provide means for reducing the complexity and cost of transit-time ultrasonic flow sensors, to improve their measurement reliability, and to make their installation easier.
It is a further object of the present invention to provide a cost effective means for enabling transit-time flow sensors to measure a fluid flow rate along two axes.
SUMMARY OF THE INVENTION
The above and other objects are satisfied with a transit-time flow sensor configured as a single modular unit as exemplified in accordance with preferred embodiments of the present invention. One of the preferred embodiments includes an insertion probe with two permanently mounted transducers which enters a relatively small opening in a pipe carrying a flowing fluid. A probe-mounted acoustically reflective surface is also provided to enable the acoustic path to be completed within the entity of the probe so that it does not depend upon any other reflective surface for its operation. This sensor is thus more simple in construction, easier to install correctly, and provides more reliable operation. The probe may of course be supplied factory mounted and calibrated in a short section of pipe, and thereby be considered a “full bore” sensor for replacement between two similar pipe sections.
The transducers are mounted one upstream and one downstream, in line with and at an angle to the fluid flow, and are directly wetted by the fluid. In this configuration the sensor is isolated from the attenuation and multipath problems which occur when the transducers are pipe mounted. The supporting electronics may be simplified in concept and incorporate cost effective components while still offering good measurement precision.
The method of flow rate sensing used with the present invention is different from the four methods listed previously in the “Discussion” hereof, in that it uses a variable frequency acoustic signal which is continuously transmitted by either one or the other transducers as they alternate between the transmitting and receiving functions. The relatively low alternation frequency is the exclusive modulation source and the primary detection of time difference occurs at the acoustic frequency without using an intermediate frequency. Since this method in its basic form is unstable because of both acoustic path and electronic related drifts and uncertainties, special provisions have been employed to correct these deficiencies so that high flow sensing sensitivity and zero stability, along with low noise level, is obtained.
In another preferred embodiment, two pairs of transducers are used within a single sensor housing and are located to determine fluid flow rate components along two orthogonal axes.
In a further preferred embodiment, an electric current is passed between surfaces of the sensor in the proximity of the acoustic path so that, by the process of electrolysis, those surfaces will be maintained clean.
REFERENCES:
patent: 4103551 (1978-08-01), Lynnworth
patent: 4221128 (1980-09-01), Lawson et al.
Kiewit David
Patel Harshad
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