a process control instrument , in which the resonant frequency of a vibrating string varies in accordance with changes in a process parameter being monitored , employs a pneumatic drive system to induce the string into oscillation . pressurized gas is discharged through a port , such as a jet nozzle , which is disposed to allow the gas to impinge on the outer surface of the string . variations in the alignment between the gas discharge port and the string permits different modes of vibration , one of which is particularly suitable for process measurement purposes . in a particular embodiment of the apparatus , an optical sensor detects the frequency of vibrations , resulting in an intrinsically safe instrument for use in explosive environments .