Concentration detection method and pressure-type flow rate control device
A method for detecting the concentration of a specified gas contained in a mixed gas includes, in a pressure-type flow rate control device including a restriction portion, an upstream valve provided upstream of the restriction portion, and a pressure sensor for measuring the pressure between the restriction portion and the upstream valve, a step of flowing the mixed gas from the upstream side of the upstream valve in a state in which the pressure on the downstream side of the restriction portion is lower than the pressure on the upstream side of the restriction portion, a step of determining with a pressure sensor pressure drop characteristics occurring after the upstream valve is changed from an open to a closed state, and a step of determining the concentration of the specified gas in the mixed gas based on the pressure drop characteristics.
1. A method for detecting a concentration of a specified gas contained in a mixed gas, in a pressure-type flow rate control device comprising a restriction portion, an upstream valve provided upstream of the restriction portion, and a pressure sensor for measuring a pressure between the restriction portion and the upstream valve, the method comprising:
a step of flowing the mixed gas from the upstream side of the upstream valve in a state such that the pressure downstream of the restriction portion is lower than the pressure on the upstream side of the restriction portion;
a step of detecting by the pressure sensor pressure drop characteristics occurring after changing the upstream valve from an open to a closed state, and
a step of determining the concentration of the specified gas in the mixed gas on the basis of the pressure drop characteristics.
2. The method for detecting a concentration according to claim 1 , wherein the step of detecting the concentration of the predetermined gas comprises:
a step of storing pressure drop characteristics determined when the specified gas has a specified concentration as reference pressure drop characteristics in association with the specified concentration in a storage device, and
a step of determining the concentration of the specified gas by comparing the detected pressure drop characteristics with the reference pressure drop characteristics stored in the storage device.
3. The method for determining a concentration according to claim 1 , wherein the pressure drop characteristics are defined by the time required for a pressure indicated by the pressure sensor to drop to a specified pressure after changing the upstream valve to the closed state.
4. The method for determining a concentration according to claim 1 , wherein the pressure drop characteristics are defined by the pressure reached after elapse of a specified time after changing the upstream valve to the closed state.
5. The method for determining a concentration according to claim 1 , wherein the pressure drop characteristics are determined under conditions satisfying critical expansion conditions.
6. The method for determining a concentration according to claim 1 , wherein the mixed gas contains a diluent gas and a source gas, and the concentration of the source gas is determined as the specified gas.
7. The method for determining a concentration according to claim 1 , wherein the upstream valve is a control valve for adjusting the flow rate of the mixed gas.
8. A pressure-type flow rate control device comprising a restriction portion, an upstream valve provided upstream of the restriction portion, a pressure sensor for measuring a gas pressure between the restriction portion and the upstream valve, and a controller for receiving an output of the pressure sensor, the pressure-type flow rate control device being configured so that a mixed gas flows from the upstream side of the upstream valve,
wherein the controller determines pressure drop characteristics occurring after the upstream valve is changed from an open state to a closed state from output of the pressure sensor and determines a concentration of a specified gas in a mixed gas based on the pressure drop characteristics.