Accuracy: The difference between the reading of an instrument and the true value of what is being measured, expressed as a percent of full instrument scale.

Action: The function of a controller, specifically, what is done to regulate the final control element to effect control; types of action include: On-Off, proportional, rate, and reset.

Active Device: A device capable of producing gain or control; for example, transistors, diodes, and ICs.

Ambient Compensation: The ability of an instrument to compensate for changes in the ambient temperature so that the changes do not affect control accuracy.

Ambient Temperature: The temperature of the immediate surroundings in which equipment is to operate.

Analog Set Point: A potentiometer adjustment of the control setting.

Analog Indication: A meter with a graduated scale and a pointer that moves to indicate process condition.

Anti-Reset Windup: A feature in PID controls which prevents the integral (automatic reset) circuit from functioning when the temperature is outside the proportional band.

Automatic Reset: The integral function of a control that automatically compensates for the difference between the set point and the actual process temperature.

Automatic Tuning: A control function that calculates the optimum PID parameters with a built-in software algorithm to eliminate manual tuning effects.

Closed Loop: A combination of control units in which the process variable is measured and compared with the desired value (or set point). If the measured value differs from the desired value, a corrective signal is sent to the final control element to bring the controlled variable to the proper value.

Common Mode: The noise signal that is common to all sensor wires.

Dead Band: The temperature band where no heating or cooling takes place, expressed in degrees.

Derivative Action: (See Rate Action)

Differential Gap: Applied to On-Off controller action. It is the smallest range of values that the controlled variable must pass in order to move the controller output from its On to its Off position or vice versa.

DIN: Deutsche Industrial Norms, a German agency that sets engineering standards.

Drift: Gradual departure of instrument output from the correct value.

Droop: Same as offset.

Dual Output: The primary output will regulate the process temperature. A second output will be used for process cooling or as an alarm.

Feedback Controller: A mechanism that measures the value of the controlled variable, accepts the value of the command, and as a result of a comparison, manipulates the controlled system to maintain an established relationship between the controlled variable and the command.

Frequency Response: The response of a component, instrument, or control system to input signals at varying frequencies.

Gain: Amount of increase in a signal as it passes through any part of a control system. If a signal gets smaller, it is attenuated, If it gets larger, it is amplified.

Integral Action: (See Automatic Reset)

Mechanical Relay (Contactor): An electromechanical device that makes or breaks a circuit by opening or closing electrical contacts.

Normal Mode Rejection Ratio: The ability of an instrument to reject interference of line frequency across the input terminals.

Offset: A sustained deviation of the controlled variable from set point. Offset is caused by load changes. Also referred to as droop.

PID: Three mode temperature control: proportional, integral (automatic reset), derivative (rate).

Proportional Action: Produces an output signal proportional to the magnitude of the input signal. In a control system, proportional action produces a value correction proportional to the deviation of the controlled variable from the set point.

Proportional Band: The amount of deviation of the controlled variable required to move the final control element through the full range (expressed in % or ° of temperature).

Proportional Control Mode: When the process temperature approaches the set point and enters the proportioning band, the output is switched on and off at the established cycle time. The change in power to the load provides a throttling action which results in less temperature overshoot.

Rate Action: Control function that produces a corrective signal proportional to the rate at which the controlled variable is changing. Rate action produces a faster corrective action than proportional action alone. Also referred to as derivative action.

Reproducibility: The ability of an instrument to duplicate with exactness the measurements of a given value. Usually expressed as a % of the span of the instrument.

Resolution Sensitivity: The amount of temperature change that must occur before the control will activate.

RTD: A temperature sensing probe of finely wound platinum wire that displays a linear resistance increase for a corresponding temperature rise. A base resistance of 100 Ohms at 32°F is the industry (DIN) standard.

SCR: Silicon controlled rectifier. A PNPN silicon junction rectifier whose forward characteristic can be triggered from the blocking state to the conducting state by a pulse applied to a gate electrode.

Sensor Break Protection: Circuitry that ensures safe process shutdown in the event of sensor failure.

Series Mode: A condition in which a noise signal appears in series with a sensor signal.

Set Point: The position to which the control point setting mechanism is set.

Soft Start: Reduces voltage on initial start-up, which reduces power to the heaters. If a heater has accumulated moisture internally during a shutdown, soft start will allow moisture to escape before full voltage is applied.

Solid State Relay: A solid state switching device that completes or breaks a circuit electrically with no moving parts.

Span: The difference between the top and bottom scale values of an instrument.

Speed of Response: The time needed for a temperature change occurring at the sensor to be translated into a control action.

Thermistor: A resistor whose resistance changes with temperatures, typically several % of resistance per °F.

Thermocouple: A device for measuring temperature where two electrical conductors of dissimilar metals are joined at the point of heat application and a resulting voltage difference, directly proportional to the temperature, is developed across the free ends. Reference to standard tables is made to determine the temperature.

Thyristor: Any of a group of solid-state controlling devices; these devices are referred to as TRIACs, SCRs, and DIACs.

TRIAC: A device, similar to the controlled rectifier, in which both the forward and reverse characteristics can be triggered from blocking to conducting.

Zero Voltage Switching: Completing or breaking a circuit when the voltage wave form crosses zero voltage.