Watlow Series 982 User Manual download pdf (Page 134)

A.4 WATLOW Series 982 User’s Manual

Appendix

Glossary, R-Z

resistance temperature detector (RTD) — A sensor

that uses the resistance temperature characteristic to

measure temperature. There are two basic types of

RTDs: the wire RTD, which is usually made of platinum,

and the thermistor, which is made of a semiconductor

material. The wire RTD is a positive temperature coeffi-

cient sensor only, while the thermistor can have either a

negative or positive temperature coefficient.

RTD — See resistance temperature detector.

thermal system — A regulated environment that con-

sists of a heat source, heat transfer medium or load,

sensing device and a control instrument.

thermocouple (t/c) — A temperature sensing device

made by joining two dissimilar metals. This junction pro-

duces an electrical voltage in proportion to the differ-

ence in temperature between the hot junction (sensing

junction) and the lead wire connection to the instrument

(cold junction).

thermocouple break protection — The ability of a

control to detect a break in the thermocouple circuit and

take a predetermined action.

three-mode control — Proportioning control with inte-

gral (reset) and derivative (rate). Also see PID.

time proportioning control — A method of controlling

power by varying the on/off duty cycle of an output. This

variance is proportional to the difference between the

set point and the actual process temperature.

transmitter — A device that transmits temperature data

from either a thermocouple or a resistance temperature

detector (RTD) by way of a two-wire loop. The loop has

an external power supply. The transmitter acts as a vari-

able resistor with respect to its input signal. Transmitters

are desirable when long lead or extension wires pro-

duce unacceptable signal degradation.

zero cross — Action that provides output switching only

at or near the zero-voltage crossing points of the ac sine

wave. See burst fire.

zero switching — See zero cross.

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Watlow Series 982 User Manual Page 134