(1) (Techn) An instrument that indicates pressure. Note: Vacuum gages indicate pressure below atmospheric (negative pressure), straight pressure gages indicate pressures above atmospheric (positive pressure), and compound gages indicate both positive and negative pressures. Zero on pressure gages usually represents atmospheric pressure. Oil-pressure, fuel, and temperature gages are often electrical. Mechanical pressure gages generally operate on the Bourdon principle. See: Bcurdon-Gage. (2) (Techn) An instrument for determining the size or shape of an object and whether it falls within an allowable tolerance. Note: The spelling "gage" is often used for indicating instruments while "gauge" is used for nonadjustable instruments, including feelers, depth, go-no-go, wire, and thread gages that measure thickness, length, or tolerance. This distinction seems to be disappearing in favor of the single form "gage. (3) (Build) a. The proportion of plaster of paris added to mortar to hasten setting. b. To mix plaster in definite proportions