The usual configuration is a back mounted single cylinder supported by the diver's safety harness, with first stage regulator connected by a low-pressure hose to a bailout block, which may be mounted on the side of the helmet or band-mask or on the harness to supply a lightweight full-face mask. Where the capacity of a single cylinder in insufficient, plain manifolded twins or a rebreather may be used. For closed bell bounce and saturation dives the bailout set must be compact enough to allow the diver to pass through the bottom hatch of the bell. This sets a limit on the size of cylinders that can be used. Diving bells are required to carry an onboard supply of breathing gas for use in emergencies. The cylinders are mounted externally as there is insufficient space inside. They are fully immersed in the water during bell operations, and may be considered diving cylinders. Suit inflation gas may be carried in a small independent cylinder. Sometimes argon is used for superior insulation properties. This must be clearly labelled and may also need to be colour coded to avoid inadvertent use as a breathing gas, which could be fatal as argon is an asphyxiant. Divers also use gas cylinders above water for storage of oxygen for first aid treatment of diving disorders and as part of storage "banks" for diving air compressor stations, gas blending, surface supplied breathing gas and gas supplies for decompression chambers and saturation systems. Similar cylinders are also used for many purposes not connected to diving. For these applications they are not diving cylinders and may not be subject to the same regulatory requirements as cylinders used underwater. It is necessary to know the approximate length of time that a diver can breathe from a given cylinder so that a safe dive profile can be planned. There are two parts to this problem: The capacity of the cylinder and the consumption by the diver. Two features of the cylinder determine its gas carrying capacity:   At the pressures which apply to most diving cylinders, the ideal gas equation is sufficiently accurate in almost all cases, as the variables that apply to gas consumption generally overwhelm the error in the ideal gas assumption. To calculate the quantity of gas:  In those parts of the world using the metric system the calculation is relatively simple as atmospheric pressure may be approximated as 1 bar, So a 12-litre cylinder at 232 bar would hold almost 12 × 232 / 1 =  of air at atmospheric pressure (also known as free air). In the US the capacity of a diving cylinder is specified directly in cubic feet of free air at the nominal working pressure, as the calculation from internal volume and working pressure is relatively tedious in imperial units.