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O.D.: Outside diameter.

Octal: Pertaining to a base 8 number system.

Offset: The difference in temperature between the set point and the actual process temperature. Also, referred to as droop.

ofhc: Oxygen-free high-conductivity copper. The industrial designation of the pure copper used in a Type T thermocouple.

Ohmeter: An instrument used to measure electrical resistance.

On/off Controller: A controller whose action is fully on or fully off.

Open Circuit: The lack of electrical contact in any part of the measuring circuit. An open circuit is usually characterized by rapid large jumps in displayed potential, followed by an off-scale reading.

Operating System: A collection of programs that controls the overall operation of a computer and performs such tasks as assigning places in memory to programs and data, processing interrupts, scheduling jobs and controlling the overall input/output of the system.

Operational pH: The determination of sample pH by relating to pH measurements in a primary standard solution. This relationship assumes that electrode errors such as sensitivity and changes in asymmetry potential can be disregarded or compensated for, provided the liquid junction potential remains constant between standard and sample.

Optical Isolation: Two networks which are connected only through an LED transmitter and photoelectric receiver with no electrical continuity between the two networks.

Outboard Rotor: A two-journal rotor which has its center of gravity between the journals.

Output Impedance: The resistance as measured on the output terminals of a pressure transducer.

Output Noise: The RMS, peak-to-peak (as specified) ac component of a transducer's dc output in the absence of a measurand variation.

Output: The electrical signal which is produced by an applied input to the transducer.

Overshoot: The number of degrees that a process exceeds the set point temperature when coming up to the set point temperature.

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