Connect-Tek Transa PRO 17 Bedienerhandbuch PDF herunterladen (Seite 85)

A–3 Glossary

Absolute zero: The lowest theoretical temperature. At absolute zero, a

body would have no molecular motion of heat energy. Absolute zero is

the zero point on the Rankine and Kelvin scales. (-273.15°C or -

459.67°F)

AC: Alternating Current; an electric current that reverses direction at

regularly occurring intervals.

Accuracy

Calibration accuracy: The potential error of a device compared to

a physical constant or agency standard.

Control accuracy: Maintaining a process at the desired setting. The

errors or combination of errors in the entire system including the sen-

sor, control, power, load and design inefficiencies affect control accu-

racy.

Display accuracy: The amount of potential error between a meas-

ured value and the control’s displayed value.

Set point accuracy: The potential error between a measured value

and the control setting.

Alarm: A control condition or function, indicating that the process is a

predetermined amount above or below the set point.

Alpha (

): The average percent change in resistance per degree of a pure

metal resistance device between 0° and 100°C. Usually designated by the

Greek letter alpha, α with units of ohm/ohm/°C. The common alpha for

a platinum RTD is 0.00385ohm/ohm/°C.

Ambient compensation: The design of an instrument such that

changes in ambient temperature do not affect the readings of the instru-

ment.

Ambient temperature: The average or mean temperature of the sur-

rounding air which comes in contact with the equipment and instruments

under test.

Ampere (amp): A unit used to define the rate of flow of electricity (cur-

rent) in a circuit; units are one coulomb (6.25 x 10

electrons) per sec-

ond.

Analog indication: A meter with graduated scale and a pointer that

moves to indicate process condition.

Analog output: A voltage or current signal that is a continuous func-

tion of the measure parameter.

Analog set point: Potentiometer adjustment of the control setting.

ANSI: American National Standards Institute

Anti-reset windup: This is a feature in a three-mode PID controller

which prevents the integral (auto reset) circuit from functioning when

the temperature is outside the proportional band.

ASME: American Society of Mechanical Engineers.

ASTM: American Society for Testing and Materials.

Automatic reset (Integral): The integral function of a control that

automatically compensates for the difference between the set point and

the actual process temperature. A signal moves the proportional band up

or down to correct for the droop or offset error.

Automatic tuning (of control parameters): A control that calcu-

lates the optimum PID parameters with a built-in software algorithm to

eliminate manual tuning efforts.

AWG: American Wire Gauge.

Bandwidth: A symmetrical region around the set point in which pro-

portional control occurs.

Baud rate: In serial communications, the rate of information transfer in

bits per second.

Blackbody: A theoretical object that radiates the maximum amount of

energy at a given temperature, and absorbs all the energy incident upon

it. A blackbody is not necessarily black. (The name blackbody was cho-

sen because the color black is defined as the total absorption of light

[energy].)

Boiling point: The temperature at which a substance in the liquid phase

transforms to the gaseous phase; commonly refers to the boiling point of

water which is 100°C (212°F) at sea level.

Btu: British Thermal Unit. The quantity of thermal energy required to

raise one pound of water 1°F at or near its maximum density (39.1°F).

Bumpless transfer: The smooth, automatic transition from automatic

control (closed loop) to manual control (open loop). The control output

is maintained during the transfer.

Burst proportioning: A fast-cycling output form on a time proportion-

ing controller (typically adjustable from .2 to 4 seconds) used in conjunc-

tion with a solid state relay to prolong the life of heaters by minimizing

thermal stress.

Calender-van Dusen equation: An equation that defines the resist-

ance-temperature value of any pure metal that takes the form of

(1+AT+BT

) for values between the ice point (0°C) and the freez-

ing point of antimony (630.7°C) and the form R

[1+AT+BT

+C(T-

100)T

] between the oxygen point (-183.0°C) and the ice point (0°C).

Calibration: The process of adjusting an instrument so that the indica-

tion is accurate compared to the actual value.

Calorie: The quantity of thermal energy required to raise one gram of

water 1°C at 15°C.

Cascade: Control in which the output of a secondary or inner control

loop is the set point for a primary or outer loop. The primary loop, in

turn, determines the control action.

CE: A mark that designates compliance with European Union (EU)

requirements for products sold in Europe.

Celsius: (Centigrade) A temperature scale with 0°C defined as the ice

point and 100°C as the boiling point of water at sea level.

cfm: The volumetric flow rate of a liquid or gas in cubic feet per minute.

Chatter: The rapid cycling on and off of a relay in a control process due

to insufficient bandwidth in the controller.

Closed loop control: A control system in which process temperature

changes are detected by a sensor. The feedback from the sensor allows

the control to make adjustments for accurate system regulation.

Cold junction compensation: A temperature sensitive device that

prevents changes in the ambient temperature from affecting the cold

junction of a thermocouple.

Common mode rejection ratio: The ability of an instrument to

reject interference from a common voltage at the input terminals with

relation to ground. Expressed in dB (decibels).

Control loop: The basic control loop of any automatic control system

consists of:

1. variable (process)

2. sensor

3. error detector (of control)

4. control

5. final control element (relay, SSR, SCR)

6. temperature indication

Control mode: The method in which the control restores the system

temperature to set point. On/Off, proportional, and PID are the most

common control modes.

CT: Current Transformer

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