The LEO51 is a unit manufactured by Microplan company. It was composed of a stabilized cast iron structure with two right-angled sides, useful for testing the perpendicularity. It has a reaction time of 3 sec typical, a zero setting of ± 1500 µm/m, and a charge power of 220 V a.c. 20 mA. A commutator permits the selection of one of the five possible resolutions (from 250 µm/m up to 1 µm/m), while a potentiometer permits the "zero" setting. Rechargeable battery-powered, with battery status.