Abstract: The pump head liner of a small-size precision reciprocating metering pump, which has its application particularly in High Performance Liquid Chromatography (HPLC), is made of transparent material, preferably of synthetic sapphire or monocrystalline zirconium oxide. A pump head casing is designed such that a pump head liner is visually accessible. Cartridge-type check valve units of transparent components are associated with the pump head casing and pump head liner and mounted to be visually accessible. The opening and closing of a ball in a check valve unit can be monitored by an optical sensing system in order to obtain a feed-back signal for governing a control circuitry, having the purpose of compensating the influence of the specific compressibility of the liquid being pumped, upon the pumping efficiency of the displacement system under conditions of high and highest working pressures.