A sensor chip (1030) for gas has cells (200) for emitting and receiving ultrasound and is configured for a sufficiently large frequency range and for measuring concentration of at least one of the gas components based on at least two responses within the range. The frequency range can be achieved by varying the size of cell membranes (230), varying bias voltages, and/or varying air pressure for an array (205) of cMUTs or MEMS microphones. The sensor chip can be applied in, for example, capnography. A measurement air chamber (515) is implemented in the respiratory pathway (400), and it and/or the pathway may be designed to reduce turbulence in the exhaled breath (120) subject to ultrasound interrogation. The chip (1030) can be implemented as self-contained in the monitoring of parameters, obviating the need for off-chip sensors.