Sensortechnics Inc., Walpole, MA, offers custom thermal mass flow sensors for air and gases, with measuring ranges up to 500 lpm. For medical ventilators, the company offers custom versions with 2 sensing elements, which offer excellent resolution and accuracy over the measuring range. One sensor chip is placed in the middle of the main flow stream to measure very small flows around the zero flow point. A second chip is arranged in a bypass configuration to measure higher flows up to F.S. A microcontroller provides DSP and can continuously switch between the two sensing elements. Response times are < 10 ms; output is a linear analog signal or digital output via I2C or SPI.