Ultrasonic flowmeter sensors

The field of ultrasonic measurement covers a variety of models and types of flowmeters. The term “ultrasonic” does not unequivocally define the type of flowmeters. “Ultrasonic” merely indicates that the flow rate is measured using ultrasound. Actual flow is measured by either of three fundamentally different methods:

• Transit time differential method (time-of-flight)
• Doppler method
• Cross-correlation method

The most applied and most accurate method is transit time differential (time of flight). It can be applied to liquid, gas, cryogenic (including LNG) and in high temperature applications.

Nowadays, a variety of ultrasonic sensor types are available for direct installation on the pipe or delivered preinstalled in a measuring tube. The sensors can be divided into three subgroups:

• Clamp-on sensors
• Insertion sensors
• Inline sensors

Sound waves can pass through materials without causing damage. Medical diagnosis, materials testing, and flow measurement all make extremely good uses of this property. As far as measuring flow is concerned, this means that the sensor can be outside of the pipe and does not necessarily have to be in contact with the fluid. Devices mounted in this way are normally referred to as “clamp-on” sensors (Fig. 1). A characteristic of clamp-on sensors is that ultrasonic waves always pass through the centre of the pipe.

Sensors that are in direct contact with the fluid are known as “insertion or wetted sensors”. They are also used with multipath transit time meters, which have sound paths offset from the mid-plane of the pipe. Dual-path systems are an example (Fig. 3). Multipath systems offer a better linearity and are less susceptible to disturbance in the flow profile. Some manufacturers also offer insertion sensors that are hot tapped.

Prefabricated measuring tubes (see Fig. 3) are ideal for transposing laboratory-determined measuring accuracies directly to the site. An in-line sensor consists of a prefabricated measuring tube complete with preinstalled insertion sensors – which can be wetted or isolated from the fluid. Configurations like this generally employ in-line sensors with a multipath system consisting of two, three, four or more pairs of sensors.