1 - 5 Physics SBAs for the Primary FRCA

1 - 5 Physics SBAs for the Primary FRCA
Photo by Unseen Studio / Unsplash

Question 1

You are preoxygenating a 32 year old man for a laparoscopic appendicectomy, and your consultant decides that now is the perfect time to quiz you about rotameters.

Which of the following is correct regarding rotameters?

  • They are cone shaped with the diameter decreasing further up the tube
  • At low flows gas flow is laminar, and at high flows it is turbulent
  • The bobbin should not spin in the rotameter while oxygen is flowing
  • The gas flow rate is measured from the bottom of the bobbin
  • At high flows the viscosity of the gas is more important than density


  • At low flows gas flow is laminar, and at high flows it is turbulent

Rotameters are rather old fashioned these days, with most modern machines using electronic flow controls, but the FRCA will continue to examine your knowledge of these whistling wind pipes for years to come.

Rotameters are a variable orifice, constant pressure flow meter.

This means they get wider towards the top. The point of this is to ensure the gap between the side wall of the tube and the floating bobbin increases as you increase the flow rate.

At low flows, gas flow is largely laminar because of the small gap between bobbin and side wall. At higher flows, the larger gap allows turbulent flow to predominate.

Laminar flow is more influenced by the viscosity of the gas, while turbulent flow is largely determined by the density.

Readings are usually taken from the top of the bobbin on an anaesthetic machine (unless it has a designated marker) and the middle of the ball on a wall oxygen supply.

  • Gas flow through the rotameter is controlled by a needle valve at the base
  • Calibration depends on the size and shape of the tube, the weight and dimensions of the bobbin, and the properties of the gas
  • The bobbin has vanes that cause it to spin, demonstrating that it has not got stuck in the tube and the flow reading is accurate
  • Each rotameter is unique to a specific gas


    • No power supply required
    • Reliable
    • Easy to read


    • Unique to each gas at a set temperature and pressure
    • Clunky

You can check out our dedicated post on rotameters and gas flow here