Entonox, a mixture of 50% oxygen and 50% nitrous oxide, exists as a gas. The pseudo critical temperature of Entonox in pipelines at 4.1 bar is below −30°C. Nitrous oxide in an Entonox cylinder, however, begins to separate out from Entonox if the temperature falls below −6°C. A homogenous mixture is again obtained when the temperature is raised above 10°C and the cylinder is agitated.

Table 1.5 Relative sizes and specifications of commonly used carbon dioxide cylinders

Table 1.6 Relative sizes and specifications of commonly used Heliox21 cylinders

CYLINDER SIZE	HL	HX
Contents (L)	8200	1780
Nominal cylinder pressure at 15°C (bar)	200	200
Valve type	Side outlet	Integral
Dimensions (mm)	1540 × 230	940 × 140
Empty weight (kg)	50	15.5

Table 1.7 Relative sizes and specifications of commonly used xenon, nitric oxide and carbon monoxide cylinders

Cylinder filling and maintenance

Most gasses such as oxygen, medical air, helium and Heliox21 are stored in cylinders in a compressed gaseous state and are normally filled by pressure. Nitrous oxide and carbon dioxide, however, are liquefied gasses under pressure. The liquid is in equilibrium with the gas, the pressure being dependent on the temperature. These gasses are charged by weight and not by pressure. The maximum weight of gas filled into the cylinder divided by the weight of water that would completely fill the cylinder is called the maximum filling ratio and is governed by legislation. This ‘fill ratio’, termed ‘fill density’ elsewhere, is critical. In the UK this value is 0.75 at 15.5°C for nitrous oxide and carbon dioxide. Note that due to the difference in the densities of liquefied gasses and water this ratio is not the same as the proportion of the volume of the cylinder filled by the liquid phase, which is nearer 90 to 95% for nitrous oxide. If there is insufficient gas space left in the cylinder after filling, a comparatively small increase in temperature will cause a significant increase in pressure and could, in extreme circumstances, cause the cylinder to rupture.

Cylinders of nitrous oxide and carbon dioxide should always be used in the vertical position with the outlet uppermost; this prevents liquefied gas from being vented and causing cold thermal burns or equipment damage. Also when a high flow rate is drawn, the temperature of the liquefied gas will fall – this can result in a significant pressure drop which may cause poor performance when used with equipment such as a cryogenic probe. A good indicator of this phenomenon is the appearance of water vapour condensing/freezing on the outer surface of the cylinder. The effect is most common on smaller liquefied gas cylinders up to size F (9.43 litres).

Some applications require carbon dioxide in liquid form. This is supplied using cylinders fitted with a ‘dip tube’ connected to the cylinder valve, which allows liquid from the bottom of the cylinder to be drawn up through the valve (Fig. 1.2). These cylinders, supplied in F size and known as LF, have a white stripe down the length of the cylinder body. For other CO₂ applications where vapour is required, VF cylinders should be used. It is important to ensure that the correct cylinder is used to prevent damage to equipment.

Figure 1.2 LF type CO₂ cylinder with dip tube for delivering liquid CO₂.

Checking for cylinder contents is done in one of two ways: compressed gasses such as oxygen, medical air, helium, Heliox and Entonox (which, although 50% nitrous oxide, remains as a gas in the cylinder under normal ambient temperatures) are assessed using a pressure gauge, as the pressure in the cylinder is directly proportional to the volume. By contrast, the contents of cylinders containing liquefied gasses (pure nitrous oxide and carbon dioxide) can only be determined by weight, as the pressure only begins to fall once all the liquid is exhausted. Subtracting the tare weight (stamped on the neck of the cylinder) from the total weight will give an estimate of contents.

Cylinder identification

The correct method of identifying the contents of a cylinder is to read the collar identification label (rather than assuming that the colour of the cylinder or the type of valve fitted reliably indicates gas inside it). The label (Fig. 1.3) is a legal requirement and contains all the key information for the user:

Figure 1.3 Cylinder label.

• product name, chemical symbol and pharmaceutical form of the product

• product specification

• hazard warning diamond(s)

• product licence number

• cylinder contents in litres

• maximum cylinder pressure

• cylinder size code

• directions for use and information for storage and handling.

The cylinder label also has a unique batch label (Fig. 1.4), which contains: the batch number; fill and expiry date; and the size and type of gas. The label is changed every time the cylinder is filled and serves two important functions: (a) it provides vital information for a batch recall should the cylinder be involved in an incident; and (b) it provides information for proper cylinder rotation.

Figure 1.4 Batch label.

Cylinder testing

All cylinders must undergo hydraulic testing and internal inspection at regular intervals, to ensure they remain safe to use. The test is carried out every 10 years for steel cylinders and every 5 years for composite cylinders. A colour-coded plastic ring between the valve and the cylinder neck indicates when the next test date is due. In general, cylinders have a long service life and tend to be withdrawn for reasons of technical obsolescence rather than deterioration per se.

Colour coding

Medical cylinders in the UK conform to colour codes specified in ISO 32:1972 and EN 1089-3. The colour coding relates only to the shoulder of the cylinder. Figure 1.5 shows the colour codes for medical gas cylinders in the UK.

Figure 1.5 Cylinder colour codes.

Cylinder valves

Medical cylinder valves can generally be categorized as follows:

• Non-integral valves (which require manual attachment of an external pressure regulator):

Small pin index and side spindle pin index outlet valves

Bull nose outlet valves

Handwheel actuated valves

• Integral valves (which have a built in regulator and possibly flowmeter).

Pin index system

Small pin index valves (Fig. 1.6) are fitted to small cylinders (less than 5 litre capacity) which are commonly connected directly to medical equipment such as anaesthetic machines. Newer designs using a thumbwheel do away with the need for a spanner to operate the valve.

Side spindle pin index valves (Fig. 1.6) These are fitted to large cylinders of medical oxygen, medical air, Entonox used in pipeline manifolds and F size Entonox cylinders.

Figure 1.6 A, Small pin index valve on E size cylinder. B, Side spindle pin index valve.

Both types of pin index valves conform to BS EN ISO 407:2004 and adopt an indexed outlet system which incorporates a gas-specific combination of holes positioned to correspond to pins located on the receiving equipment, making it impossible to connect the cylinder to an incorrect gas connection. Fig. 1.1 shows the different pin positions. The pin index system also prevents charging with the wrong gas, as the gas suppliers use the same non-interconnectable system for their filling connections.

Pin index cylinders require a washer (seal) between the face of the cylinder valve outlet and the equipment to which it is fitted. This bonded non-combustible seal, known as a ‘Bodok’ washer (Figs 1.7 and 1.8), must be kept clean and should never become contaminated with oil or grease. If a gas tight seal cannot be achieved by moderate tightening of the screw clamp, it is recommended that the seal be renewed. Excessive force should never be used.

Figure 1.7 Bodok washer.

Figure 1.8 Bodok washer fitted to a regulator.

Bull nose outlet valve

This type of valve (Fig. 1.9) is fitted to F and G size cylinders including medical oxygen, medical air, helium, and mixed gasses such as carbogen and Heliox. The valve is spindle key operated and has a 5/8-inch female outlet thread into which a regulator is fitted. The spindle mechanism is assembled in two parts. This permits a gas tight seal to be achieved without the use of excessive force and increases its operational life.

Figure 1.9 Bull nose cylinder valve.

All bull nose valves are fitted with an RP (residual pressure) device, to ensure that a positive pressure of approximately 3 bar is retained in the cylinder (Fig. 1.10). This prevents the ingress of moisture should the valve be left open when the cylinder is empty. When connecting regulator equipment to the valve, the user should always adopt the proper connecting procedure:

Figure 1.10 Schematic showing ‘residual pressure’ device of a bull nose cylinder valve.

Redrawn using material kindly provided by Müller Gas Equipment A/S, Denmark.

• Ensure the ‘O’ ring (Fig. 1.11) fitted to the regulator is in good condition and hand tighten the regulator only.

• Once fitted, open the spindle valve slowly (to prevent a gas surge) at least one full turn and note the position of the regulator gauge needle.

• A simple test for leaks can be carried out by closing the cylinder spindle valve and noting any drop in pressure shown on the gauge. If a leak does exist, spraying the joints with a leak detection spray will identify it. If a leak is evident at the valve outlet, replace the ‘O’ ring and repeat the procedure. If the leak persists, fit a replacement regulator. (Note: It is vitally important that any leak detection spray has been approved by the cylinder supplier as being compatible with their equipment and the gas.)

• When changing an empty cylinder, close the cylinder valve and vent the regulator completely before attempting to disconnect it.

Figure 1.11 ‘O’ ring fitted to regulator.

Handwheel valves

Large nitrous oxide cylinders for use on cylinder manifolds and carbon dioxide cylinders of size F and G are fitted with handwheel valves which are surrounded by a protective guard (Fig. 1.12), and have a gas specific, male thread, side outlet.

Figure 1.12 CO₂ cylinder with handwheel valve.

Integral valves

The introduction of integral valves (Fig. 1.13), which have their own built-in regulator, has revolutionized the industry by greatly improving safety and eliminating the need for regulator maintenance by hospitals. They have a number of advantages: