This was developed in the desire for a simpler method.  The bed is connected to a wide-diameter u-tube containing a liquid such as kerosene.  On pressurizing the space between the u-tube and the bed, the liquid is  forced down.  The level of liquid then acts as a measure of both pressure and volume flow.  The liquid level rises as air leaks out through the bed.  The time taken for the liquid level to pass between two pre-set marks on the tube is measured by stop-watch.  The mean pressure and mean flowrate can be derived from the dimensions of the tube and the density of the liquid.
A later development used mercury in the u-tube: because of mercury's greater density, the apparatus could be more compact, and electrical contacts in the tube touching the conductive mercury could automatically start and stop a timer.