Mass flow controllers and methods for auto-zeroing flow sensor without shutting off a mass flow controller

A mass flow controller, comprising:

• a proportional valve having a first port connected to a first external line;
• a mass flow sensor;
• a first flow line connecting from a second port of the proportional valve through the mass flow sensor to a second external line, wherein one of the first external line and the second external line is an entry line for connection to a fluid source and the other of the first external line and the second external line is an exit line;
• a second flow line joining the first flow line at a first junction located upstream of the mass flow sensor and at a second junction located downstream of the mass flow sensor;
• a switching valve placed at the first junction or the second junction to join the first flow line and the second flow line such that the switching valve can regulate a flow of a fluid through the first flow line or the second flow line, wherein the switching valve is switchable between a first state at which the switching valve allows the fluid to flow through the first flow line, and a second state at which the switching valve allows the fluid to flow through the second flow line while producing a zero-flow condition at the mass flow sensor;
• and
• a feedback control comprising control electronics and a feedback control loop connecting the control electronics with the proportional valve and the mass flow sensor, said feedback control configured to regulate the proportional valve based on signals measured by the mass flow sensor, wherein the feedback control is configured to send a control signal to the proportional valve effective for keeping a flow rate of fluid exiting the exit line substantially constant while the switching valve is in the second state and the fluid is flowing through the second flow line.