Explanation of the Transit-time measurement Principal
The flow rate is calculated from the speed of propagation of the ultrasound signal. This signal travels faster in the direction of fluid flow than in the opposite direction (like a swimmer taking longer to swim a certain distance in an upstream direction, compared to their downstream time). Two ultrasound pulses are emitted by the sensors, which act alternately as transmitters and receivers: One pulse from Sensor A to Sensor B in the direction of fluid flow (t1). The other pulse from Sensor A to Sensor B in the opposite direction of the fluid flow (t2). The mean velocity of the fluid is obtained from the difference in transit times Δt (t2-t1).
The flow rate is calculated according to the internal cross-section of the pipe and other internally calculated factors.
Notice how the sensors can be installed on the same side of the pipe, this sensor orientation is referred to as ‘V’ mode, wherein the ultrasonic signals bounce off the far side of the pipe. The other common mode is direct, or ‘Z’ mode where there is no bounce and the signal travels as shown.
In practice we suggest you set up in V mode on smaller pipes, perhaps up to 400 mm, and then consider using Z mode on the larger pipe sizes. V mode is easier to set up, but the signal is less.
The transit time equation is:
Dt is the transit-time difference
c – Fluid sound speed (calculated by the flow meter)
K – Flow profile correction factor based on the Reynolds number, fluid viscosity & density
D – Internal pipe diameter
q – Angle from the pipe normal.
The flowmeter works best on single phase fluids, but will tolerate entrained particulates and air. The device has powerful transmit and receive electronics and advanced sensors that recover very small signals. The overall accuracy tends to be 2-3 % in most installations, but results of 1% are possible with in-situ calibration. Sensors are handcrafted and built into pairs that have excellent long term stability and zero error has been minimised through careful design and manufacture. The best materials are used throughout, starting with the PEEK transducer bodies. This material is exceptionally rugged, having one of the highest melting points of any thermoplastic.
The sensors are best applied on the sides of the pipe, never on the top or bottom as particulates accumulate and there is almost always a small amount of air/gas that can really reduce the signal. Couplant is applied to the sensors, we either supply water based high performance couplant, but silicone grease is adequate and good at high temperatures, we recommend this for permanently installed sensors.