A new method of studying the linearity of gas flowmeters was tested on different models of Fleisch pneumotachographs. The method applies a steady flow to the test flowmeter, which is increased in a stepwise manner by adding a constant flow-increment. This is achieved using two flow sources in parallel. The method does not require any reference flow channel and may be implemented with standard laboratory equipment. Using this method, the gain of Fleisch pneumotachographs, whatever their size, decreased by about 2-3% from low flows to about 40% of their nominal full scale (FS), and then increased almost linearly with increasing flow. The error was 8-13% at 200% FS. The following equation was devised to correct the data at high flow: Vc = Vt (1-K (Vt-S] where Vc and Vt are the corrected and measured flow respectively, K a gain correction factor and S a flow threshold below which no correction is needed. Applying this correction with suitable coefficients, the maximal error was below 3% from 0-200% FS.