A thermo-hygrograph is a simple, accurate and reliable instrument to measure air temperature and relative air humidity displaying a graphic representation of the simultaneous proceeding of the two parametric measurement units. Monitoring these two quantities assures the optimisation of many production stages, and also allows to achieve a satisfactory environmental comfort in public areas and museums, in industrial warehouses, in those destined to food industry, during weather surveys etc. The variation of the measured parameters is detected by sensors which carry out the tracing of the related graph on a paper diagram through a mechanical drive. Measurement sensors are a hair bundle for humidity and a bimetallic leaf (a Bourdon spring) for temperature; the recording time can be set on a daily/weekly or a monthly reading generated by a clockwork motion (a mechanical or quartz movement) which remains reliable and accurate as time passes.