a positive displacement calibration tool useful for simultaneously calibrating a plurality of flow meters , the tool comprising : a frame ; a housing block having a plurality of fluid chambers therein , each fluid chamber being connectible to a respective liquid flow line , each chamber having an axis therethrough , the axis of each fluid chamber being aligned within a predetermined close tolerance with the axis of each of the other fluid chambers formed in the block ; a piston mounted within the block for movement within each fluid chamber ; and an actuator connected to all of the pistons whereby the actuator is operative to displace each piston through its associated chamber to cause a precisely dispensed predetermined flow rate of a liquid to pass simultaneously from each chamber .