Watlow Series 998 - Manual (Page 118)

A.4 WATLOW Series 998 User’s Manual Appendix
Glossary, R-Z
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.