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. Generally, the signal is detected and aligned by having an oscillator that is used to trigger a transmit transducer to launch an ultrasonic packet that is received by a receive transducer. The oscillator's frequency is adjusted so that transit time is an integer multiple of the period of the oscillator signal. This allows the arrival of the ultrasonic packet to be compared with a rising edge of the oscillator signal for adjusting the oscillator signal. Flow rate determination is facilitated by having an oscillator signal with a period that is proportional to the transit time. Unfortunately, noise obscures the arrival time of the ultrasonic packet. If the noise is large enough, it completely hides the signal. This results in false and missed detections. Smaller amounts of noise cause jitter in detecting when the ultrasonic packet arrived.
Thus there exist a need for a method and apparatus that can detect an ultrasonic packet in the presence of noise.