The Fleisch pneumotachograph is an established technology, developed by Professor Alfred Fleisch in Lausanne in 1953. It makes an accurate, linear and reliable type of flow measurement spirometer as the linear signal is simple to process and the device is extremely robust. In addition, the large diameter and effective cross sectional area gives extremely low flow impedance, so that even with a Bacterial/Viral Filter (BVF) essential for effective hygiene between the patient and the measuring system, the system flow impedance is low even at high flow rates and well below the requirements of the current spirometer standard ISO26782:2009. Both Fleisch and Lilly flowmeters may be heated or unheated. Laboratories conducting extended test methodologies tend to use heaters to make measurements when conditions reach equilibrium. But heating adds variables and does not improve accuracy; its purpose is to reduce surface condensation from the breath. In office spirometry flowmeters are unheated for safety, speed of use and accuracy. Vitalograph spirometers feature a Fleisch Pneumotachograph.
A Fleisch Pneumotachograph consists of a bundle of small capillary tubes which create pseudo-laminar flow. The consequence of this is a linear output pressure signal created by increasing air flow. Two annular rings of pressure measuring points in the Fleisch are ported out to an extremely low range differential pressure transducer, operating in the linear range of the transducer. The differential pressure is sampled by the associated electronics at a frequency of at least 100Hz, effectively continuously measuring the pressure to allow the instantaneous and accurate calculation of flow rates and, by differentiation over time, the accumulated volume. The turndown ratio of such a device is a staggering 20:1, which allows accurate measurement of very low flows as well as being capable of measuring very high flows.The advantages of the Fleisch diagnostic flow measuring system are that it is: