TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Method: The sensor measures a cumulative volume of a fluid flowing into the anode supply manifold to estimate a position of a hydrogen/air front, where the flow rate of the fluid flowing into the anode supply manifold is modified in response to the estimated position of the hydrogen/air front location. The sensor measures a voltage across at least one of the fuel cells, where the flow rate of the fluid flowing into the anode supply manifold is modified in response to the voltage measured. The flow rate of the fluid flowing into the anode supply manifold is modified in response to a measured voltage peak. Preferred Components: The fuel cell system (A) further comprises: an anode inlet conduit and an anode exhaust conduit, where the anode inlet conduit provides fluid communication between the anode supply manifold and a source of hydrogen, and the anode exhaust conduit is in fluid communication with the anode exhaust manifold; at least one of a jet pump in fluid communication with the anode supply manifold, an injector in fluid communication with the anode supply manifold, or a recycle loop in fluid communication with at least one of the anode exhaust manifold or the anode supply manifold; a first valve disposed at a first end of the fuel cell stack, where the first valve is spaced from an anode inlet conduit and is in fluid communication with the anode supply manifold; a second valve in fluid communication with at least one of the anode supply manifold or the anode exhaust manifold, where the second valve includes an inlet for receiving a fluid flow and an outlet for exhausting a fluid; and a resistive load in electrical communication with the fuel cell stack, where the sensor measures a current through the resistive load, and the flow rate of the fluid flowing into the anode supply manifold is modified in response to the current measured. In the fuel cell system (A), the environmental condition is at least one of an air temperature, a shut-down time period, a composition of a fluid flowing through the fuel cell stack, an age of the fuel cell stack, or a pressure level in the fuel cell system. In the fuel cell system (B), the environmental condition is at least one of a temperature, a composition of a fluid flowing through the fuel cell stack, an age of the fuel cell stack, or a pressure level at various points in the fuel cell system. The fuel cell system (B) further comprises an anode inlet conduit and an anode exhaust conduit, where the anode inlet conduit provides fluid communication between the anode supply manifold and a source of hydrogen, and the anode exhaust conduit is in fluid communication with the anode exhaust manifold; at least one of a jet pump in fluid communication with the anode supply manifold, an injector in fluid communication with the anode supply manifold, or a recycle loop in fluid communication with at least one of the anode exhaust manifold and the anode supply manifold; a resistive load in electrical communication with the fuel cell stack, where the sensor measures a current through the resistive load, and the flow rate of the fluid flowing into the anode supply manifold is modified in response to a measured current peak. In the method of controlling a flow rate of a fluid in a fuel cell system: the characteristic of the fuel cell stack measured is at least one of a volume of fluid injected into the anode supply manifold, a voltage across at least one fuel cell, a current supplied from the fuel cell stack, or a shut-down time period. The fuel cell stack further includes at least one of a jet pump in fluid communication with the anode supply manifold, an injector in fluid communication with the anode supply manifold, or a recycle loop in fluid communication with at least one of the anode exhaust manifold or the anode supply manifold.