Chris Thompson 2003
Yokes, Regulators, Line Connectors etc
YOKES
Mechanical system for fitting cylinders securely to the machine. Components usually include the check valve and gauge all in one piece):
- the male part of the pin indexing system
- bodok seal - neoprene disk with aluminium or brass ring - generates airtight seal
- nipple to enter and open the cylinder outlet valve
- check valve to prevent retrograde loss of gas on cylinder disconnection
- filter - 34 micron - to prevent dust entering and blocking needle valves etc
The junction between cylinder and yoke is rendered airtight by a 'Bodok seal', otherwise known as CGA 870 seal. These consist of a metal ring surrounding a neoprene (or other elastomeric) washer. The metal ring limits compression of the neoprene when the cylinder is screwed onto the yoke, and also stops it expanding outwards under gas pressure. With these type of seals and airtight cylinder -yoke connection requires little force.
typical Bodok seals - cross-section to the right: |
|
CHECK VALVES
Usually placed in-line immediately at the cylinder connection point (ie within the yoke) - may leak, but should not exceed 10ml/min. Unused yokes should be plugged.
PRESSURE GAUGES
Gauges indicating cylinder pressure should be visible from the front of the machine. They are usually Bourdon type (twisted metal tube that straightens as pressure rises, connected by small gears to the needle at front). Require adjustment from time to time. Note that the nitrous cylinder gauge will remain high until about 87% of the cylinder is empty, then fall (relatively) rapidly, because you are not .
REGULATORS
Functions
Reduce high cylinder pressures (up to 20,000kPa) to lower, workable levels (usually 350 - 380kPa) for use in secondary systems in machines (gas mixing, ventilator drive)
maintain constant 'downstream' pressure despite changes in 'upstream' (ie cylinder) pressure, so that gas flows delivered by simple mixers (eg needle valves) won't change as cylinders empty
Design
A diaphragm inside the regulator separates the outlet side from an opposing spring. The location of the diaphragm depends on outlet pressure alone (for a given spring tension). If outlet pressure falls, the spring moves the diaphragm towards the outlet side, and a needle valve attached to the diaphragm opens, increasing gas delivery (and vice versa). The nominal outlet pressure can be fine-tuned by adjusting the spring tension.
INDIRECT ACTING:
- Outlet pressure rises as cylinder empties
- Needle valve with gas flow (valve in high pressure path)
- most common (GIG minireg & O series, Ulco's)
DIRECT ACTING:
- Outlet pressure falls as cylinder empties
- Needle pushes against incoming gas flow (located in low pressure part)
- older machines (GIG M series)
SECOND STAGE / DUAL REGULATORS
- A pair of (usually direct and indirect acting) regulators in series.
- Second regulator regulates line pressure also
- Usually to a lower internal operating pressure.
- Much better pressure regulation
Notes:
Second-stage regulators are required by most 'anti-hypoxia' devices.
Line pressure is nominally 414kPa. Usually this is so much above regulator pressure that the regulator is 'off' all the time; when line pressure drops below regulator opening pressure, the cylinder will automatically supply gas.
Line O2 pressures can drop intermittently to below 350 if the ventilator is driven by O2 (esp Campbell's), if pipes are old and if usage is high. If this happens, an open O2 cylinder can become unexpectedly empty. The other cause is a leaky bodok seal. This is why the general rule is to keep unused cylinders turned off. If a particular machine seems to always be draining O2, the output pressure may be set too close to line pressure.
Old needle-valve machines run the line gas supply directly to the rotameters (at 400kPa). On switching to cylinders, flow falls in proportion to the percentage pressure drop from line to cylinder (up to 10%).
Machines with anti-hypoxia devices run on much lower internal operating pressures, with the first stage regulator working as above and the second stage regulator accurately regulating both line and cylinders. eg Ulco 615 runs internally at 200kPa.
Most new machines with electronic mixers only need a first-stage regulator (for the cylinders), because the mixing system can happily handle a very wide range of input pressures (eg, Drager Primus works down to 270kPa).
These days most 'intermediate pressure' gas lines are stiff, small plastic hoses.
Safety issues:
Overpressure relief valve - usually internal, at 600kPa, to prevent diaphragm rupture.
Non-corrosive, non-flammable construction.
Sudden cylinder opening can cause abrupt pressure rises and compressive temperature increases, which (in the case of O2) could cause a fire if oil etc was present in the regulator.
High gas usage can cause adiabatic (expansion) cooling, potentially leading to freezing and jamming (very rare).
Diaphragm failure in indirect regulators causes spring to fully open needle valve. If this happened on the nitrous reg in an old machine (ie without second stage regulators or electronic mixers), the mixture would become hypoxic. Usually happens when quickly opening full cylinders - open slowly!
LINE INLET CONNECTORS
Most inlet connectors on anaesthetic machines are Sleeve Indexed (SIS) and have integral check valves to prevent loss of gas from the machine when hoses are disconnected.
