Watlow Series 93 User Manual download pdf (Page 42)

A.10 ■ Appendix Watlow Series 93

F - G

Form A A single-pole, single-throw relay that uses only

the normally open (NO) and common contacts. These

contacts close when the relay coil is energized. They

open when power is removed from the coil.

Form C A single-pole, double-throw relay that uses the

normally open (NO), normally closed (NC) and common

contacts.

hysteresis A change in the process variable required to

re-energize the control or alarm output. Sometimes

called switching differential.

integral Control action that automatically eliminates off-

set, or droop, between set point and actual process tem-

perature. See reset, automatic.

integral control (I) A form of temperature control. The I

of PID. See integral.

isolation Electrical separation of sensor from high volt-

age circuitry. Allows use of grounded or ungrounded

sensing element.

J - K

JIS See Joint Industrial Standards.

Joint Industrial Standards (JIS) A Japanese agency

that establishes and maintains standards for equipment

and components. Also known as JISC (Japanese

Industrial Standards Committee), its function is similar to

Germany’s Deutsche Industrial Norm (DIN).

junction The point where two dissimilar metal conduc-

tors join to form a thermocouple.

junction, cold Connection point between thermocouple

metals and the electronic instrument. See reference

junction.

junction, reference The junction in a thermocouple cir-

cuit held at a stable, known temperature (cold junction).

Standard reference temperature is 32°F (0°C).

limit or limit control A highly reliable, discrete safety

device (redundant to the primary controller) that monitors

and limits the temperature of the process, or a point in

the process. When temperature exceeds or falls below

the limit set point, the limit controller interrupts power

through the load circuit. A limit control can protect equip-

ment and people when it is correctly installed with its

own power supply, power lines, switch and sensor.

manual mode A selectable mode that has no automatic

control aspects. The operator sets output levels.

NEMA 4X A NEMA specification for determining resis-

tance to moisture infiltration and corrosion resistance.

This rating certifies the controller as washable and corro-

sion resistant.

offset (process) The difference in temperature

between the set point and the actual process tempera-

ture. Offset is the error in the process variable that is typ-

ical of proportional-only control. See also droop.

on/off A method of control that turns the output full on

until set point is reached, and then off until the process

error exceeds the hysteresis.

open loop A control system with no sensory feedback.

output Control signal action in response to the differ-

ence between set point and process variable.

overshoot The amount by which a process variable

exceeds the set point before it stabilizes.

P - Q

P control Proportioning control.

parallel circuit A circuit configuration in which the same

voltage is applied to all components, with current divided

among the components according to their respective

resistances or impedances.

parameter A variable that is given a constant value for

a specific application or process.

PD control Proportioning control with derivative (rate)

action.

percent power control Open loop control with output

power set at a particular level.

percent power limit Restriction of output power to a

predetermined level.

PI control Proportioning control with integral (automatic

reset) action.

Appendix

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