Following is a definition of terms frequently used in the instrumentation, industrial automation and test & measurement communities.
Calender-van Dusen Equation: An equation that defines the resistance-temperature value of any pure metal that takes the form of RT = RO(1 + AT + BT2) for values between the ice point (0°C) and the freezing point of antimony (630.7°C) and the form RT = RO[1 + AT + BT2 + C(T-100)T2] between the oxygen point (-183.0°C) and the ice point (0°C).
Calibration: The process of adjusting an instrument or compiling a deviation chart so that its reading can be correlated to the actual value being measured.
Calorie: The quantity of thermal energy required to raise one gram of water 1 °C at 15 °C.
Cation: A positively charged ion (Na+, H+).
Cavitation: The boiling of a liquid caused by a decrease in pressure rather than an increase in temperature.
Celsius (centrigrade): A temperature scale defined by 0 °C at the ice point and 100 °C at boiling point of water at sea level.
Center of Gravity (Mass Center): The center of gravity of a body is that point in the body through which passes the resultant of weights of its component particles for all orientations of the body with respect to a uniform gravitational field.
Centripetal Force: A force exerted on an object moving in a circular path which is exerted inward toward the center of rotation.
Ceramic Insulation: High-temperature compositions of metal oxides used to insulate a pair of thermocouple wires The most common are Alumina (Al2O3), Beryllia (BeO), and Magnesia (MgO). Their application depends upon temperature and type of thermocouple. High-purity alumina is required for platinum alloy thermocouples. Ceramic insulators are available as single and multihole tubes or as beads.
Ceramic: Polycrystalline ferroelectric materials which are used as the sensing units in piezoelectric accelerometers. There are many different grades, all of which can be made in various configurations to satisfy different design requirements.
CFM: The volumetric flow rate of a liquid or gas in cubic feet per minute.
Character: A letter, digit or other symbol that is used as the representation of data. A connected sequence of characters is called a character string.
Charge Sensitivity: For accelerometers that are rated in terms of charge sensitivity, the output voltage (V)is proportional to the charge (Q) divided by the shunt capacitance (C). This type of accelerometer is characterized by a high output impedance. The sensitivity is given in terms of charge; picocoulombs per unit of acceleration (g).
Chatter: The rapid cycling on and off of a relay in a control process due to insufficient bandwidth in the controller.
Clear: To restore a device to a prescribed initial state, usually the zero state.
Clipping: The term applied to the phenomenon which occurs when an output signal is limited in some way by the full range of an amplifier, ADC or other device. When this occurs, the signal is flattened at the peak values, the signal approaches the shape of a square wave, and high frequency components are introduced. Clipping may be hard, as is the case when the signal is strictly limited at some level; or it may be soft, in which case the clipping signal continues to follow the input at some reduced gain.
Clock: The device that generates periodic signals for synchronization.
Closeness of Control: Total temperature variation from a desired set point of system. Expressed as “closeness of control” is ±2 °C or a system bandwidth with 4 °C, also referred to as amplitude of deviation.
CMR (Common-Mode Rejection): The ability of a panel meter to eliminate the effect of AC or DC noise between signal and ground. Normally expressed in dB at dc to 60 Hz. One type of CMR is specified between SIG LO and PWR GND. In differential meters, a second type of CMR is specified between SIG LO and ANA GND (METER GND).
CMV (Common-Mode Voltage): The AC or DC voltage which is tolerable between signal and ground. One type of CMV is specified between SIG LO and PWR GND. In differential meters, a second type of CMV is specified between SIG HI or LO and ANA GND (METER GND).
Coherence Function.: A frequency domain function computed to show the degree of a linear, noise-free relationship between a system’s input and output. The value of the coherence function ranges between zero and one, where a value of zero indicates there is no causal relationship between the input and the output. A value of one indicates the existence of linear noise-free frequency response between the input and the output.
Color Code: The ANSI established color code for thermocouple wires in the negative lead is always red. Color Code for base metal thermocouples is yellow for Type K, black for Type J, purple for Type E and blue for Type T.
Common Mode Rejection Ratio: The ability of an instrument to reject interference from a common voltage at its input terminals with relation to ground. Usually expressed in db (decibels).
Common Mode: The output form or type of control action used by a temperature controller to control temperature, i.e. on/off, time proportioning, PID.
Communication: Transmission and reception of data among data processing equipment and related peripherals.
Compensated Connector: A connector made of thermocouple alloys used to connect thermocouple probes and wires.
Compensating Alloys: Alloys used to connect thermocouples to instrumentation. These alloys are selected to have similar thermal electric properties as the thermocouple alloys (however, only over a very limited temperature range).
Compensating Loop: Lead wire resistance compensation for RTD elements where an extra length of wire is run from the instrument to the RTD and back to the instrument, with no connection to the RTD.
Compensation: An addition of specific materials or devices to counteract a known error.
Compiler: A program that translates a high-level language, such as Basic, into machine language.
Complex Function: Any mathematically defined relationship given by the following expression:
y(x) = a(x) + ib(x)
Where: x = the real variable
a(x) = the real part of y(x)
b(x) = the imaginary part of y(x)
Complex functions are usually expressed in terms of both their amplitude and phase.
Complex Wave: The resultant form of a number of sinusoidal waves that are summed together forming a periodic wave. Such waves may be analyzed in the frequency domain to readily determine their component parts.
Conductance: The measure of the ability of a solution to carry an electrical current. (See Equivalent Conductance)
Conduction: The conveying of electrical energy or heat through or by means of a conductor.
Confidence Level: The range (with a specified value of uncertainty, usually expressed in percent) within which the true value of a measured quantity exists.
Conformity Error: For thermocouples and RTDs, the difference between the actual reading and the temperature shown in published tables for a specific voltage input.
Connection Head: An enclosure attached to the end of a thermocouple which can be cast iron, aluminum or plastic within which the electrical connections are made.
Constantan: A copper-nickel alloy used as the negative lead in Type E, Type J, and Type T thermocouples.
Continuous Spectrum: A frequency spectrum that is characterized by non-periodic data The spectrum is continuous in the frequency domain and is characterized by an infinite number of frequency components.
Control Character: A character whose occurrence in a particular context starts, modifies or stops an operation that effects the recording, processing, transmission or interpretation of data.
Control Mode: The output form or type of control action used by a temperature controller to control temperature, i.e., on/off, time proportioning, PID.
Control Point: The temperature at which a system is to be maintained.
Convection: (1) The circulatory motion that occurs in a fluid at a non-uniform temperature owing to the variation of its density and the action of gravity. (2) The transfer of heat by this automatic circulation of fluid.
Coriolis Force: A result of centripetal force on a mass moving with a velocity radially outward in a rotating plane.
Correction (Balancing) Plane: A plane perpendicular to the shaft axis of a rotor in which correction for unbalance is made.
Coulomb Sensitivity: Charge/unit acceleration, expressed in Pc/g (charge sensitivity).
Coulomb: A measurement of the quantity of electrical charge, usually expressed as pico coulomb (10-12 coulombs).
Counter Weight: A weight added to a body so as to reduce a calculated unbalance at a desired place.
Counts: The number of time intervals counted by the dual-slope A/D converter and displayed as the reading of the panel meter, before addition of the decimal point.
CPS: Cycles per second; the rate or number of periodic events in one second, expressed in Hertz (Hz).
CPU: Central processing unit. The part of the computer that contains the circuits that control and perform the execution of computer instructions.
Critical Damping: Critical damping is the smallest amount of damping at which a given system is able to respond to a step function without overshoot.
Critical Speed: The rotational speed of the rotor or rotating element at which resonance occurs in the system. The shaft speed at which at least one of the “critical” or natural frequencies of a shaft is excited.
Cryogenics: Measurement of temperature at extremely low values, i.e., below -200 °C.
CSA: Canadian Standards Administration.
Cure Point: The temperature at which a normally magnetic material goes through a magnetic transformation and becomes non-magnetic.
Current Proportioning: An output form of a temperature controller which provides a current proportional to the amount of control required. Normally is a 4 to 20 milliamp current proportioning band.
Current: The rate of flow of electricity. The unit of the ampere (A) defined as 1 ampere = 1 coulomb per second.
Curve Fitting: Curve fitting is the process of computing the coefficients of a function to approximate the values of a given data set within that function. The approximation is called a “fit”. A mathematical function, such as a least squares regression, is used to judge the accuracy of the fit.
Cycle Time: The time usually expressed in seconds for a controller to complete one on/off cycle.