Bubble Sensors – Tubing Selection

Strain Measurement Devices’ standard bubble sensors will function with almost any soft or semi-rigid plastic or rubber tube. However, SMD can also design sensors for almost any tubing material. The main considerations with any tube are: A630 Bubble Sensor

  • How well does the tube get acoustically coupled to the sensor?
  • How much ultrasonic energy gets reflected at the sensor/tube interface?
  • How well does the tube transmit ultrasound through the tubing material?

Plastic and Rubber Tubing (Shore A Durometer)

Plastic and rubber tubing on the Shore A Durometer scale will dry couple to a bubble sensor without the need for an acoustic couplant such as grease. Generally, pressing the tube into a slot in the bubble sensor housing with a width of about 80% of the outside diameter of the tube is enough to achieve good acoustic coupling. The tube walls must be thick and rigid enough to make sufficient acoustic contact. Reinforced (braided) tubing can sometimes be problematic in terms of ultrasound transmission.

Extremely thick wall tubing and/or tubing materials that transmit ultrasound exceptionally well can produce ultrasonic “crosstalk” where the ultrasonic receiver picks up a lot of ultrasonic energy that travels through the walls of the tube instead of through the fluid inside of the tube. Special considerations must be taken for these tubes.

Plastic Tubing (Shore D Durometer)

Generally, more rigid tubes on the Shore D hardness scale like PTFE, PFA, FEP, etc. will dry couple with our sensors as long as the walls are thin enough that they can be compressed by 20% when pressing it into the sensor without damaging the sensor or the tube. However these tubes do permanently deform with repeated reinsertions. This permanent deformation can cause reduced acoustic coupling and the sensor may not function with repeated insertions. If necessary, we can develop a sensor with custom semi-soft interface materials to mitigate or eliminate this effect.

Glass Tubing

Glass tubing is extremely brittle and a custom sensor must be designed to properly interface with it. Acoustic coupling can be achieved using a “wet” coupling material like grease or a soft elastomeric interface on the sensor.

Metal Tubing

Like glass tubing, metal tubing cannot be deformed and a custom sensor that uses “wet” coupling such as grease or a soft elastomeric interface must be used. Additionally, metal tubing has an extremely high acoustic impedance and sensor parameters must be finely tuned to the tubing wall thickness in order to properly transmit ultrasound into the tube.


To send a tubing sample for a compatibility check with our sensors:

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