Definition of Terminology

Below is a list of terms and their definitions that are used throughout this manual.

Accuracy

How precise or correct the measured value is to the actual value. Accuracy is an indication of the error in the measurement.

Ambient

The surrounds or environment in reference to a particular point or object.

Attenuation

A decrease in signal magnitude over a period of time.

Calibrate

To configure a device so that the required output represents (to a defined degree of accuracy) the respective input.

Closed loop

Relates to a control loop where the process variable is used to calculate the controller output.

Coefficient, temperature

A coefficient is typically a multiplying factor. The temperature coefficient defines how much change in temperature there is for a given change in resistance (for a temperature dependent resistor).

Cold junction

The thermocouple junction which is at a known reference temperature.

Compensation

A supplementary device used to correct errors due to variations in operating conditions.

Controller

A device which operates automatically to regulate the control of a process with a control variable.

Elastic

The ability of an object to regain its original shape when an applied force is removed. When a force is applied that exceeds the elastic limit, then permanent deformation will occur.

Excitation

The energy supply required to power a device for its intended operation.

Gain

This is the ratio of the change of the output to the change in the applied input. Gain is a special case of sensitivity, where the units for the input and output are identical and the gain is unitless.

Hunting

Generally an undesirable oscillation at or near the required setpoint. Hunting typically occurs when the demands on the system performance are high and possibly exceed the system capabilities. The output of the controller can be overcontrollerd due to the resolution of accuracy limitations.

Hysteresis

The accuracy of the device is dependent on the previous value and the direction of variation. Hysteresis causes a device to show an inaccuracy from the correct value, as it is affected by the previous measurement.

Ramp

Defines the delayed and accumulated response of the output for a sudden change in the input.

Range

The region between the specified upper and lower limits where a value or device is defined and operated.

Reliability

The probability that a device will perform within its specifications for the number of operations or time period specified.

Repeatability

The closeness of repeated samples under exact operating conditions.

Reproducibility

The similarity of one measurement to another over time, where the operating conditions have varied within the time span, but the input is restored.

Resolution

The smallest interval that can be identified as a measurement varies.

Resonance

The frequency of oscillation that is maintained due to the natural dynamics of the system.

Response

Defines the behaviour over time of the output as a function of the input. The output is the response or effect, with the input usually noted as the cause.

Self Heating

The internal heating caused within a device due to the electrical excitation. Selfheating is primarily due to the current draw and not the voltage applied, and is typically shown by the voltage drop as a result of power (I2 R) losses.

Sensitivity

This defines how much the output changes, for a specified change in the input to the device. Setpoint Used in closed loop control, the setpoint is the ideal process variable. It is represented in the units of the process variable and is used by the controller to determine the output to the process.

Span Adjustment

The difference between the maximum and minimum range values. When provided in an instrument, this changes the slope of the input-output curve.

Steady state

Used in closed loop control where the process no longer oscillates or changes and settles at some defined value.

Stiction

Shortened form of static friction, and defined as resistance to motion. More important is the force required (electrical or mechanical) to overcome such a resistance.

Stiffness

This is a measure of the force required to cause a deflection of an elastic object.

Thermal shock

An abrupt temperature change applied to an object or device.

Time constant

Typically a unit of measure which defines the response of a device or system. The time constant of a first order system is defined as the time taken for the output to reach 63.2% of the total change, when subjected to a step input change.

Transducer

An element or device that converts information from one form (usually physical, such as temperature or pressure) and converts it to another ( usually electrical, such as volts or millivolts or resistance change). A transducer can be considered to comprise a sensor at the front end (at the process) and a transmitter.

Transient

A sudden change in a variable which is neither a controlled response nor long lasting. Transmitter A device that converts from one form of energy to another. Usually from electrical to electrical for the purpose of signal integrity for transmission over longer distances and for suitability with control equipment.

Variable

Generally, this is some quantity of the system or process. The two main types of variables that exist in the system are the measured variable and the controlled variable. The measured variable is the measured quantity and is also referred to as the process variable as it measures process information. The controlled variable is the controller output which controls the process.

Vibration

This is the periodic motion (mechanical) or oscillation of an object.

Zero adjustment

The zero in an instrument is the output provided when no, or zero input is applied. The zero adjustment produces a parallel shift in the input-output curve.