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Veterinary Anaesthetic Machine Made Simple - Flowmeters

Posted in Operations @ Mar 30th 2017 - By Dr Jen Davis BVMS Masters Vet Anaes. MANZCVS (Anaesthesia & Critical Care) Dip ECVAA
Veterinary Anaesthetic Machine - Flowmeters Part 6

Part 6 in the series – The Veterinary Anaesthetic Machine Made Simple, thanks to Dr Gas Vet

Originally published on the Vet Anaesthesia Tips blog. Scroll down for links to Parts 1 – 5 in the series

What do they do?

Flowmeters provide a constant indication, and allow adjustment,  of the flow rate of gases that are supplied to the anaesthetic machine from cylinders or pipelines (e.g. not vaporisers) such as oxygen, nitrous oxide and medical air.

Where are they found?

Flowmeters are found downstream (closer to the animal) of the gas cylinders/pipelines and oxygen flush valve, and upstream to the anaesthetic vaporisers.

Each gas has its own flowmeter, usually colour coded to match the colour of the pipeline/cylinder.

How do they work?

Flowmeters are a vertical glass or plastic tube containing a float or bobbin. At the bottom of the tube is a knob which operates a valve to allow gas to enter the tube.

When the knob is turned, gas flows up the tube then continues along the backbar of the anaesthetic machine on its way to mix with other gases and eventually reach the patient.

As the flow of gas increases (as the knob is progressively turned) the float or bobbin rises in the tube. Gas flow can then be estimated by observing where the bobbin lies on a scale painted on the tube. The units referred to on the scale are usually L/minute.

Some floats are shaped like small balls – gas flow should be read from the middle of the ball.

Other floats are bobbin shaped: 

With these, flow should be read from the top of the bobbin. (Note that the dot in the middle of the bobbin is to allow visualisation that the bobbin is rotating, it does not mean that the flow should be read from that mark.)

Usually each gas has only one flowmeter. However, some machines feature two flowmeters for certain gases, each with their own float and scale.

The first scale will have smaller graduations so that it is easy to set very low flows – when the highest flow indicated on that flowmeter is reached, gas automatically flows into the next flowmeter and the rate can then be read from the second flowmeter scale.

Safety features

Modern flowmeters contain several safety features to help ensure that a dangerous hypoxic gas mix (e.g. < 21% oxygen) is not delivered to the patient.

  • The oxygen flowmeter usually has an easily distinguishable knob so that it can be identified by colour, size, and feel. It is usually larger, and sticks out more than the other flowmeter knobs.
  • Flowmeters should be laid out in a series that ensures oxygen is the last gas to enter the mix as it travels towards the patient. This means that if there is a leak anywhere in, or between, the flowmeters, oxygen should not be lost. This may be achieved by having the oxygen flowmeter on the right of all others. However, some machines keep the oxygen on the left (as this in a familiar position for many anaesthetists) but modifications within the backbar of the anaesthetic machine mean it still enters the final gas mix last.
  • Bobbin flowmeters are often designed to rotate (also called rotameters) during gas flow, a small mark on one side of these bobbins allows you to visualise that they are rotating. This allows you to check that gas is definitely flowing, and that the bobbin is not just ‘stuck’ in the tube.
  • Some newer anaesthetic machines have a mechanism that links the oxygen flowmeter to the nitrous oxide and/or medical air flowmeters on the anaesthetic machine. This mechanism prevents any mixture that contains less than 21% oxygen from being delivered to the backbar. For example, if nitrous is turned on without any oxygen flowing the link will automatically turn on oxygen flow. However, be aware that many older machines used in veterinary practice do not have this safety feature.

Important care / maintenance notes

Flowmeters must be kept vertical – this is how they are designed to be used. Any other positioning may lead to sticking of the float/bobbin against the wall of the tube.

Dirt and static within the tube can also cause sticking.

Qualified service personnel should regularly check your anaesthetic machine, ensuring that your flowmeters are calibrated and working properly. These technicians should be contacted if you are concerned that flowmeters are sticking or not working properly.

Cracks in flowmeters may lead to leakage of gas from the anaesthetic machine. Potentially this might mean you are providing less oxygen to a patient that you think you are. Also leakage of nitrous oxygen is an occupational hazard for those working in the area. Leak checks of your anaesthetic machine should be performed daily (I will cover these in another blog post), and flowmeters should be on your list of areas to examine if a leak is suspected.

Any questions for Jen? Ask them in the Comments section below...

Click here to read The Veterinary Anaesthetic Machine Made Simple: Part 1

Click here to read The Veterinary Anaesthetic Machine Made Simple Part 2: Gas Cylinders

Click here to read The Veterinary Anaesthetic Machine Made Simple Part 3: Pipeline Gas Supply

Click here to read The Veterinary Anaesthetic Machine Made Simple Part 4: Pressure Gauges & Regulators

Click here to read The Veterinary Anaesthetic Machine Made Simple Part 5: The Oxygen Flush Valve

About Jen

Dr Jen Davis (@Dr GasVet) is a European Specialist in Veterinary Anaesthesia and Analgesia. She is currently undertaking a PhD at Murdoch University, investigating the early diagnosis of acute kidney injury induced by anaesthesia-related hypotension.

Jen also works part-time as registrar in veterinary anaesthesia at The Animal Hospital at Murdoch University, where she administers sedation, anaesthesia, and analgesia to all species of animal, as well as teaching undergraduate students and resident vets studying to become anaesthesia specialists.

A summary of Jen’s research, and open access to her published work can be found on ResearchGate.

For more excellent posts on veterinary anaesthesia vist Jen's blog: Vet Anaesthesia Tips and register to receive notifications of new posts by email

You can also follow Jen on:

Twitter: Vet Anaesthesia Tips  |  Facebook: Veterinary Anaesthesia Tips


Jay Jorgenson @ Sep 26th 2017 11:26am
I didn't know anything about anesthesia systems before this article. It's pretty impressive the machine can regulate the amount of anesthesia that is needed and maintains the flow of the medication. This will make me feel more comfortable taking my dog to the Vet. Thanks for sharing!
Judy @ Sep 26th 2017 11:38am
Thanks Jay, I'm pleased to hear our blog post has helped to make you feel more comfortable!

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