Pin Index Safety System
The Pin Index Safety System, or PISS, is a safety system that uses geometric features on the yoke to ensure that pneumatic connections between a gas cylinder and a machine that uses pressurized gases are not connected to the wrong gas yoke. This system can be seen on an anesthesia machines and portable oxygen administration sets. It has no purpose other than a physical barrier to connecting the wrong cylinder.
The pin index safety system uses a face seal between the cylinder valve and the associated yoke clamp. There are two holes in specific positions on the cylinder valve body below the outlet port , in positions associated with the gas mixture, which prevent connection of the cylinder to a yoke or pressure regulator with a mis-matched set of pins. The holes accept pins 4 mm diameter by 6 mm long which are correctly aligned with the holes.
Pin index configurations
Each gas cylinder has a pin configuration to fit its respective gas yoke. Refer to the diagram for pin numbers; dimensions are in millimeters.
- O2: 2,5
- N2O: 3,5
- Mixture of O2 and N2O (50% / 50%), commonly called Entonox: 7 (a single pin, located in the centre)
- Air: 1-5
- Mixtures of CO2 and O2 with more than 7% CO2: 1,6
- Mixtures of CO2 and O2 with less than 7% CO2: 2,6
- He: No pin
- Mixtures of O2 and He with less than 80% He, commonly called Heliox: 2,4
- Mixtures of O2 and He with more than 80% He: 4-6
- Cyclopropane: 3,6
- Ethylene: 1,3
- Nitrogen: 1-4
- EN ISO 407 : Small medical gas cylinders - Pin-index yoke-type valve connections
The pin index system is a safety system (PISS) designed to ensure the correct gas is filled into the correct cylinder, and that the cylinder will only connect to the correct equipment. The positions of the holes on the cylinder valve correspond with the pins fitted to the yoke attached to the equipment. The pin positions for each medical gas are unique. If an attempt is made to fit the wrong gas cylinder to the yoke a tight seal will not be made, as the pins cannot locate.
It is possible to bypass the pin-index system if the pins are removed, damaged or corroded, if extra washers are used, or on some valves with a short face above the orifice, by inverting the gas cylinder. There is one report of the cylinder being painted the wrong colour leading to error.
Large cylinders are more likely to be fitted with threaded connectors. One such system are the American CGA fittings. In Europe there are British Standard (BS), German Standard (DIN) and French (AFNOR) connections, and Japan has the Japanese Standard (JIS).
Blanking plugs (dummy cylinder heads) can be inserted into empty yokes to ensure that there is no leak out of the yoke when not in use.
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