Which of the following devices is bidirectional?

a. Turbine flowmeter c. Vortex ultrasonic flowmeter
b. Screen pneumo-tachograph d. Variable-orifice pneumotachograph

ANS: D
The variable-orifice pneumotachograph is a disposable, bidirectional, flow-measuring device that uses a flexible obstruction with a variable area for measuring flow as a function of the pressure differential generated by the obstruction. Turbine flowmeters, thermal anemometers, and vortex ultrasonic flowmeters are all unidirectional.