A
fluid supply apparatus with a plurality of flow lines branching out from one
regulator for adjustment of pressure, the flow lines being arranged in parallel, wherein a measure is taken that the operation, that is, opening or closing of one flow passage will have no transient effect on the steady flow of the other flow passages. For this purpose, each flow passage is provided with a time
delay-type
mass flow controller MFC so that when one closed fluid passage is opened, the
mass flow controller on that flow passage reaches a set flow rate Qs in a specific
delay time DELTAt from the starting point. Also provided are a method and an apparatus for the above in which a plurality of gas types can be controlled in flow rate with high precision by one pressure-type flow
control system. To that end, a formula for calculating the flow rate of a gas is theoretically derived that flows with a pressure ratio not higher than the critical pressure ratio. From that formula, the flow factor is defined, so that the formula may be applied to a number of gas types using flow factors. The method includes calculating the flow rate Qc of a
gas passing through an orifice according to formula Qc=KP1 (K=constant) with a pressure P1 on an upstream side of the orifice set at twice or more higher than pressure P2 on a downstream side, wherein the flow factor FF for each kind of gas is calculated as follows: <
paragraph lvl="0"><in-line-formula>FF=(k / gammas){2 / (kappa+1)}1 / (kappa-1)[kappa / {(kappa+1)R}]½< / in-line-formula>and wherein, if the calculated flow rate of gas type A is QA, and, when gas type B is allowed to flow through the same orifice under the same pressure on the upstream side and at the same temperature on the upstream side, the flow rate QB is calculated as follows: <
paragraph lvl="0"><in-line-formula>QB=(FFB / FFA)QA < / in-line-formula>where gammas=concentration of gas in
standard state; kappa=ratio of
specific heat of gas; R=constant of gas; K=proportional constant not depending on the type of gas; FFA=flow factor of gas type A; and FFB=flow factor of gas type B.