Patent ID: 7474968

Claim:
A flow verifier for verifying measurement by a fluid delivery device, the flow verifier comprising: a chamber configured to receive a flow of the fluid from the device; a pressure sensor configured to measure pressure of the fluid within the chamber; a temperature sensor configured to measure temperature of the fluid within the chamber; and a critical flow nozzle located upstream of the chamber along a flow path of the fluid from the device to the chamber; wherein the critical flow nozzle is configured to maintain, during a critical flow time period t cf , flow rate of the fluid through the nozzle substantially constant and substantially insensitive to variation in pressure within the chamber; wherein the critical flow nozzle is configured to allow the fluid flowing through the nozzle to satisfy a critical flow condition during the critical flow time period t cf , and wherein the critical flow condition is given mathematically by: P d P u ≤ a pc = ( 2 γ + 1 ) γ γ - 1 , where P d is pressure of the fluid within the chamber and downstream of the critical flow nozzle, P u is pressure of the fluid upstream of the critical flow nozzle, γ is given by γ=C p /C v and is a ratio of specific heats C p and C v of the fluid, where C p is heat capacity of the fluid at constant pressure, and C v is heat capacity of the fluid at constant volume, and α pc is critical pressure ratio representing the maximum allowable ratio between P d and P u for which flow of the fluid across the nozzle will remain substantially constant and substantially insensitive to any variation in pressure within the chamber.