Pneumotachographs (also called pneumotachometers and anemometers) measure airflow against a fixed resistance and may be found in respirometers, mechanical ventilators, incentive breathing devices, diagnostic spirometers, and other pulmonary function testing instruments. The Fleisch design is the oldest, but it requires a costly pressure transducer and analog-to-digital converter and its accuracy is affected by gas-viscosity differences and by airflow turbulence. Besides the Fleisch, ceramic, screen, orifice, hot-wire, turbine, vortex, and ultrasonic pneumotachographs are now available. They solve some of the Fleisch's problems but cannot be used in all applications. All instruments with these devices must be carefully and completely calibrated at the time of installation and routinely calibrated or checked thereafter.