A
system and method for measuring fluid flow rate in a
system where fluid is pumped, such as a
gas monitoring instrument where gas is pumped from a space such as a room or
enclosure through a conduit to a sensor. The flow of gas through a pump is determined by measuring motor back-e.m.f. which is proportional to
motor speed. In a
system where
motor speed is regulated by
pulse width modulation of the
motor drive voltage, the back-e.m.f. is sampled during intervals between the drive pulses applied to the motor, and in a further aspect the sampling is done at selected, spaced-apart or infrequent intervals such as once for every ten or once for every hundred
motor drive pulses. Advantageously, in an instrument that uses a
microprocessor and analog-to-
digital converter to measure gas-concentration, the same
microprocessor and converter can provide the PWM control of the pump, in response to the back-e.m.f. generated by the pump motor between the drive pulses. The
microprocessor compares the output of the analog-to-
digital converter, corresponding to the back-e.m.f., which in turn corresponds to the pump
motor speed, to a set-point value, representative of the desired pump motor speed. The processor then adjusts the PWM to control the pump motor to achieve and maintain the desired speed. As a result, in a gas monitor, the gas-flow rate may be maintained close to a desired gas flow rate regardless of the
voltage supplied by the battery or other
voltage source, regardless of the degree to which the gas is filtered, and under a wide range of operating conditions.