Abstract: A hand-held electronic instrument including a unitary, sleeve-shaped housing having a battery compartment and an electronics compartment housing a microcomputer. A sensing probe and a probe mounting member are removably mounted and fluidically sealed to the front end portion of the housing. A battery door assembly fluidically seals the rear end portion of the housing and is removable to provide access to the battery compartment for replacing spent batteries. A latch pin extending laterally from the latch operator of the battery door assembly engages a partition on the housing to retain the assembly to the housing. A battery polarizing plate disposed intermediate the battery and the electrical contacts for the battery ensure that the proper polarity is applied. Software stored in the microcomputer includes a calibration routine for facilitating calibration of the instrument and an energy savings routine for extending the life of the battery.