O.D. (Tubing VS. Pipe)
Outside diameter, the proper way to size T/C or RTD sheath tubing vs. inside diameter used for pipe sizing.

Offset Current
The difference between two bias currents drawn by the inputs of a differential input stage of an electronic meter.

Offset Voltage
The deviation from zero output voltage of an amplifier having zero input voltage.

OFHC
Oxygen free high conductivity copper, one leg in type T thermocouples, the other leg being an alloy of approximately 60/40 copper nickel.

OHMS/Volts
Relates to total terminal resistance of voltmeters. For example, a meter with 1000 ohms/V, and 300 volts full scale has a total resistance of 3000,000 ohms, using ohms law l=E/R or 1V/1000 ohms = 1 MADC

OHMS Per Volt
Indication of the total terminal resistance of an analog voltmeter. A measure of the sensitivity of an analog voltmeter. A 1000-ohms-per volt instrument has resistance of 150,000 ohms on its 150 volt (full scale) range, and 300,000 ohms on it 300 volt range. Its basic movement is a 1-milliampere meter (1 = E/R = 1/1000 = .001A = 1 milliampere).

Opto-Isolator
An isolation device that provides an electrical barrier between related circuits.