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The Veterinary Anaesthetic Machine Made Simple Part 2: Gas Cylinders

Posted in Operations @ Dec 1st 2016 - By Dr Jen Davis BVMS Masters Vet Anaes. MANZCVS (Anaesthesia and Critical Care) DipECVAA
The Veterinary Anaesthetic Machine Made Simple Part 2 Gas Cylinders

Part 2 in the series The Veterinary Anaesthetic Machine: Made Simple thanks to Dr Gas Vet

Originally published on the Vet Anaesthesia Tips blog 29/7/16: Gas Cylinders

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

In Part 2 of this series of blogs about the anaesthetic machine we will start from the beginning, with gas supply, commencing with gas cylinders.

Purpose ....

Gas supply directly to anaesthetic machine: cylinders may be attached to an anaesthetic machine as a sole method of fresh gas delivery, or as a back-up for oxygen provision in case of pipeline failure.

Cylinder manifold: this is a bank of large cylinders stored externally to the building to provide a gas reservoir which is then piped into the building.

Safety features to avoid inadvertent supply of the wrong gas type

Colour coding

The colour of the shoulders of a cylinder indicates the type of gas it contains (the colour of the body is also standardised). This colour also matches the colour of the hose and connectors used to attach the cylinder to the anaesthetic machine.

  • Oxygen: white (EU, Australia), green (North America)
  • Nitrous oxide: blue
  • Medical air: black and white
  • Entonox (50% oxygen, 50% nitrous oxide): blue and white

Pin-index system

The pin index system further prevents accidental supply of the wrong gas type. The cylinder valve bears two holes (or ‘nipples’) around the gas outlet. The positioning of these holes in relation to each other is specific for each type of gas.

Cylinders will be attached to a yoke (part of the anaesthetic machine that a cylinder is screwed directly into) or to a regulator separate to the machine (some machines do not have a yoke, and instead a regulator is attached to the cylinder, then a short supply pipe runs between the regulator and gas connector on the anaesthetic machine).

The yoke or regulator is specific to a particular gas type, and will have two pins in a position that corresponds to the holes on the cylinder. This means that only the correct type of cylinder can be attached to a particular yoke/regulator.


Gas cylinder sizes are alphabetical with A being the smallest, and J the largest. Cylinders used on anaesthesia machines are usually size E or smaller. Larger cylinders (e.g. J) are used for cylinder manifolds

Safe storage

Gas cylinders are filled to high pressures, and explosion is possible if they are stored incorrectly.

  • Do not subject to extremes of temperature
  • Do not store near flammable materials (e.g. oil, grease) or near a source of heat
  • Prohibit smoking and naked flames from vicinity of storage
  • Store away from corrosive chemicals
  • Store in a designated area of a dry, well ventilated room (avoid areas where they could be knocked, walked into, etc.)
  • Store/rack cylinders in a way that they are secured so they cannot fall over

Cylinder valve

The main purpose of the cylinder valve is to maintain a secure seal to the contents within the cylinder.

Inappropriate handling of these valves may result in explosion.

Cylinders come with plastic wrapping around the valve block, to protect from dust. Leave the wrapping on until the cylinder is to be used.

It is important that lubricants such as grease and oil never come into contact with a cylinder, and especially not with the cylinder valve.

How to open and close a cylinder

  1. Identify the contents of the cylinder (read label, check colour)
  2. Remove the plastic covering around the valve
  3. Prior to opening a cylinder valve, check that there is no dust or particulate matter visible around the valve.
  4. Hold the cylinder so that the valve opening is facing away from you (and anyone else nearby)
  5. “Crack” the valve by opening very slightly prior to connecting to the yoke/regulator in order to blow off any dust sitting on the valve, then close again
  6. Ensure that the yoke/regulator sealing washer (Bodok seal) is present and intact
  7. Ensure that the oxygen flow of the machine the cylinder is to be attached to is turned fully off
  8. Enzure that the holes and pins of the pin index system are aligned so that the cylinder gas outlet sits nicely against the Bodok seal of the yoke/regulator (if it is not fitting consider whether your cylinder contains the correct gas for that yoke/regulator).
  9. When closing the retaining screw of the yoke/regulator ensure that you are tightening it into the conical depression located on the other side of the valve stem to the pin-index system and gas outlet, some cylinders have a pressure relief device below the conical depression that it is important not to accidentally tighten the screw against (this device looks like a screw head).
  10. Open the cylinder valve slowly by turning the spindle anticlockwise. Sudden opening can lead to a rapid increase in pressure which can damage the machine, and even cause explosion or fire.
  11. Open the valve fully while in use.
  12. When the anaesthetic machine is not in use close the valve fully by turning the spindle clockwise(but avoid over-tightening). Do not leave the cylinder standing upright without support. A size E or smaller cylinder may be stored either horizontally or vertically, but must be secured.
  13. When removing a cylinder from the yoke/regulator; close the cylinder by turning the spindle clockwise fully, ensure flowmeters on the machine are closed and that gas within the anaesthetic machine has been purged (by pressing the oxygen flush valve with cylinder closed until no more gas hissing can be heard), then carefully unscrew from the yoke/regulator.

Cylinder contents

Bourdon gauges for measurement of pressure within the cylinder will be found either above the cylinder yoke or regulator, or on the front panel of the anaesthetic machine (newer models). These gauges should be labeled, colour coded and calibrated for the specific type of gas.


When the cylinder valve is switched off the needle of the pressure gauge should drop to zero, however as the valve is opened the needle will move to indicate the pressure within the cylinder.

The method of estimation of how much gas is contained within a cylinder depends on the type of gas. For cylinders containing gases that are present only in their gaseous form within the cylinder (e.g. oxygen, medical air) the pressure of the cylinder decreases as the amount of gas present in the cylinder decreases. This makes it easy to monitor how much gas remains in the cylinder by inspection of the pressure gauge.

For gases present partially in liquid form within the cylinder (e.g nitrous oxide) the pressure gauge will remain full until all of the liquid has evaporated, and will only start to drop when a very small amount of gas remains. The contents of a nitrous oxygen cylinder should therefore be estimated by weighing the cylinder; the weight of these cylinders will decrease proportionately with the cylinder contents.

This is intended as a simple overview of anaesthetic gas cylinders. Feel free to comment with any additional questions you may have on the topic.

Next, I will write about pipeline gas connections, and pressure regulators…

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

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

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



Brooke @ Nov 20th 2019 9:58pm
Haha I stumbled across an oxygen picture on google and noticed it had the same flooring and walls as Murdoch clicked onto this page and seen it was your article!!
Judy @ Nov 26th 2019 6:04pm
Google collects everything Brooke! :)
Lucy mckinna @ Apr 13th 2020 5:54pm
Hi Jen the author I visited your wordpress website last year and then came up with my practice’s new Dom and torb anaesthetic charts based on your calm/ friendly and also nervous/ aggressive dog doses. My boss has asked me to support these charts now so please can you send me the link to your new webpage ... or your charts and text that was on your wordpress website as I cannot find it now!!! As I need to show the info came from a specialist :) Many thanks Lucy BVSc MRCVS
Judy @ Apr 13th 2020 6:22pm
Hi Lucy, Judy here - I manage Vetanswers. Thanks for the reminder, you're right Jen's blog has moved and you can now find it here: I'll need to change the address on all of Jen's blog post :) Thanks for visiting the Vetanswers Blog! Regards Judy

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