An extensive series of experiments on pentadecanoic acid monolayers has been made at 25$^\circ$C with several independent methods of surface manometry. The purpose of the study was to establish a set of surface pressures for the three monolayer phase transitions to act as surface manometric standards. It was coincidentally demonstrated in high purity systems that the so-called liquid-expanded-liquid-condensed transition is simply first order. The experimental requirements for reproducible surface manometry are described. Less extensive data on hexadecanoic acid are also given to invite further critical experiments.