Ultrasonic flow meters

Transit Time Flow Meters measuring principle

Transit Time - How it works

A pair of ultrasonic flow transducers are used to send ultrasonic sound signals through the pipe wall and then through the fluid itself. Ultrasonic energy bursts are transmitted by one transducer and received by the second transducer. The process is then reversed so that the transit time difference (Δt = t 2 – t 1) determines the average flow velocity in the fluid. The velocity of the flow is accurately measured by the difference in the arrival times of signals from the upstream and downstream transducers. The configuration of pipe and process details are input into the flow meter so that the correct transducer spacing can be determined to provide the ultrasonic flow path across the flow stream. Transit time flow meters can be used with clean fluids with a small amount of suspended solids or aeration.

Dependant on the pipe size the correct flow transducer must be selected for the process. Typically pipes which are ½" to 2" in size are determined as a small pipe applications. These types of applications will require flow transducers which operate a 2 MHz.

Pipes which are above 2" can use transducers which operate at 1 MHz and for large pipe applications greater than 24" then transducers operating at 500Hz maybe used.

Transit time flow meter meters can be used with rigid pipe work, typically pipes would be constructed from stainless steel, PVC, PVDF, LD & HD polyethylene, copper, iron (cast and ductile).

Advantages of a transit time flow meter:

• Non-invasive, non contacting flow monitoring
• Measurement of non-conductive fluids
• Easy to install with no down time or interruption to your flow or process
• Reduced installation costs
• No moving parts to maintain

Transit Time Flow Meters common features:

• Virtually all clean, sonically conductive single phase homogenous liquids
• Typical accuracy ±1% of reading, better in many cases
• Bi-directional measurement to 12 MPS
• Pipe and tubing sizes from ½” (12mm) and higher:
• Large turndown ratio
• Positive, Negative, or Net totalizer function

Doppler Flow Meters measuring principle

Doppler - How it works

Ultrasonic frequencies in the range of 500,000 to 1,000,000 Hertz (500 kHz to 1 MHz) are generated by the flow meter and transmitted through the pipe wall into the liquid. The part of the flow meter that contacts the pipe is called the transducer. Sound signals are reflected off of suspended solids or air bubbles carried by the liquid. It is assumed that these reflectors are carried at the same speed as the liquid flow. Sound signals bounced off a moving reflector will be of a slightly altered frequency than the one generated by the flow meter. Sound signals bounced off of a non-moving reflector will be of the same frequency as the one generated by the flow meter. The flow meter transmits a fixed frequency sound signal into the pipeline. The flow meter collects the reflected signals, which typically have a number of different frequencies, when the fluid is moving. The transmit and average received frequencies are compared; this is the Doppler shift.

In typical Doppler meters, 260 Hertz of Doppler shift corresponds with roughly 1 MPS velocity

Advantages of a Doppler flow meter:

• Can be used with dirty fluids, ideal for most slurries or aerated liquids
• Non-intrusive, clamp-on sensors
• Low installation costs
• No moving parts to maintain

Doppler flow meters common features:

• Liquids containing 35 micron sonic reflectors, 100 ppm
• Measuring range from 0.15 to 30 FPS (0.05 to 9 MPS)
• Accuracy ±2% full scale, over calibrated span
• 115 / 230 VAC or 12-28 VDC power supply