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Glossary of Terms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Tempco

Abbreviation for “temperature coefficient”; the error introduced by a change in temperature. Normally expressed in %/°C or ppm/°C.

Temperature Limit

The full capability of the system from the lowest point to the highest point; limited by the sensor. Example: Z-44 Temperature limit: -100° to +600°F

Temperature Span

Those two points anywhere within the temperature limit that the recorder can be calibrated to. FORMULA: Maximum temperature – minimum temperature = span.

Temperature Stability-Instability

The unwanted change (error) of an instrument, sensor, amplifier, etc. caused by changes in the temperature surrounding it. Usually expressed as total change between two temperature limits, or % error/°F or C.

Thermistor

A temperature sensitive passive semiconductor which exhibits a large change in electrical resistance when subjected to a small change in temperature. Usually with negative temperature coefficient.

THD (% THD, Total Harmonic Distortion)

The contribution of all harmonic frequency currents or voltages to the fundamental current or voltage, expressed as a percentage of the fundamental

THDF (Transformer Harmonic Derating Factor)

Method of calculating transformer derating established by DBEMA for phase–to–neutral loads.

Thermocouple

Two dissimilar metals with a voltage output proportional to temperature. ANSI types: J Iron Constantan (copper nickel) 2192°F (1200°C) K Chromel Alumel (Ni-Ch vs. Ni-Al) 2501°F (1372°C) T Copper Constantan (copper nickel) 752°F (400°C) E Chromel Constantan (Ni-Ch vs. Cu-Ni) 1832°F (1000°C) R Platinum vs. Plat (13% Rhodium 3214°F (1768°C) S Platinum vs. Plat (10% Rhodium 3214°F (1768°C) B Plat. 6% Rhodium vs. Plat. 30% Rhodium 3308°F (1820°C) C Tu 5% Rhenium vs. 26% Rhenium 5000°F (2760°C)

Thermocouple Break Protection

A Safety feature to indicate when a thermocouple has failed in an open circuit condition. Its purpose is to eliminate the possibility of an ambiguous reading. In the case of a temperature controller, it eliminates the dangerous condition of thermal runaway.

Thermocouple Loop Resistance

The total resistance of the thermocouple and its extension wire.

Thermopile

A number of thermocouple connected in series, arranged so that alternate junctions are at the reference temperature and at the measured temperature, to increase the output for a given temperature difference between the measuring and reference junctions.

Thermowell

The pressure vessel into which an RTD or thermocouple is inserted for easy removal and/or replacement purposes.

Time Constant

The tem required for a sensor to respond to 63% of its total resistance change resulting from a newly impressed temperature. Five time constants are required to attain 99% of the total change. Values given are in a well stirred oil bath. Time constant in still air is approximately ten times longer.

Tracking

The ability of an instrument to indicate at the scale mark being check when energized by the proportional value of actual end-scale excitation.

Tracking Error

The error in indication at a scale mark, expressed in percentage of fiducial value, when the instrument is energized by the proportional value of the actual end-scale excitation.

Triple Point

The Temperature at which the solid, liquid and vapor phases of a pure substance co-exist in equilibrium.

True RMS

The true root-mean-square value of an AC or AC-plus-DC signal, often used to determine power of a signal. For a perfect sin wave, the RMS value is 1.11071 times the rectified average value, which is utilized for low-cost metering. For significantly non-sinusoidal signals, a true RMS converter is required.

Two Wire Transmitter

A specially constructed R vs. 1 signal conditioner in which the output loop contains the power source; minimizes wires to control room by use of only two (2) wires.