It is this _difference_ that is meant when the
_pressure_ of a gas in a pipe or piece of apparatus is spoken of,
and it must of necessity in the case of a gas-supply have a positive
value. That is to say, the "pressure" of gas in a service-pipe expresses
really by how much the pressure in the pipe _exceeds_ the
atmospheric pressure. (Pressures less than the atmospheric pressure will
not occur in connexion with an acetylene installation, unless the
gasholder is intentionally manipulated to that end.) Gas pressures are
expressed in terms of inches head or pressure of water, fractions of an
inch being given in decimals or "tenths" of an inch. The expression
"tenths" is often used alone, thus a pressure of "six-tenths" means a
pressure equivalent to 0.6 inch head of water.
The pressure gauge is for convenience provided with an attached scale on
which the pressures may be directly read, and with a connexion by which
the one limb is attached to the service-pipe or cock where the pressure
is to be observed. A portable gauge of this description is very useful,
as it can be attached by means of a short piece of flexible tubing to any
tap or burner. Several authorities, including the British Acetylene
Association, have recommended that pressure gauges should not be directly
attached to generators, because of the danger that the glass might be
fractured by a blow or by a sudden access of heat.
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