Following is a definition of terms frequently used in the instrumentation, industrial automation and test & measurement communities.
Damping: The reduction of vibratory movement through dissipation of energy. Types include viscous, coulomb, and solid.
dB (Decibel): 20 times the log to the base 10 of the ratio of two voltages. Every 20 dBs correspond to a voltage ratio of 10, every 10 dBs to a voltage ratio of 3.162. For instance, a CMR of 120 dB provides voltage noise rejection of 1,000,000/1. An NMR of 70 dB provides voltage noise rejection of 3,162/1.
DC: Direct current; an electric current flowing in one direction only and substantially constant in value.
Dead Band: (1). For chart records: the minimum change of input signal required to cause a deflection in the pen position. (2) For temperature controllers: the temperature band where heat is turned off upon rising temperature and turned on upon falling temperature expressed in degrees. The area where no heating (or cooling) takes place.
Dead Volume: The volume of the pressure port of a transducer at room temperature and ambient barometric pressure.
Debye-H¸ckel Equation: Used in relating the activity coefficient (fi) to ion strength (see Activity coefficient):where I is the ionic strength, A and B the temperature-dependent constants (see Table A.5), Zi the valence of the ion (i), and Â the ion-size parameter in angstroms.
Decimal: Refers to a base ten number system using the characters 0 through 9 to represent values.
Default: The value(s) or option(s) that are assumed during operation when not specified.
Degree: An incremental value in the temperature scale, i.e., there are 100 degrees between the ice point and the boiling point of water in the Celsius scale and 180°F between the same two points in the Fahrenheit scale.
Density: Mass per unit of volume of a substance. I.E.: grams/cu.cm. or pounds/cu.ft.
Derivative: The derivative function senses the rate of rise or fall of the system temperature and automatically adjusts the cycle time of the controller to minimize overshoot or undershoot.
Deviation: The difference between the value of the controlled variable and the value at which it is being controlled.
Diaphragm: The sensing element consisting of a membrane which is deformed by the pressure differential applied across it. Also known as a pressure responsive element.
Dielectric Constant: Related to the force of attraction between two opposite charges separated by a distance in a uniform medium.
Differential Input: A signal-input circuit where SIG LO and SIG HI are electrically floating with respect to ANALOG GND (METER GND, which is normally tied to DIG GND). This allows the measurement of the voltage difference between two signals tied to the same ground and provides superior common-mode noise rejection.
Differential Pressure: The difference in static pressure between two identical pressure taps at the same elevation located in two different locations in a primary device.
Differential: For an on/off controller, it refers to the temperature difference between the temperature at which the controller turns heat off and the temperature at which the heat is turned back on. It is expressed in degrees.
Digit: A measure of the display span of a panel meter. By convention, a full digit can assume any value from 0 through 9, a 1/2-digit will display a 1 and overload at 2, a 3/4-digit will display digits up to 3 and overload at 4, etc. For example, a meter with a display span of ±3999 counts is said to be a 3-3/4 digit meter.
Digital Output: An output signal which represents the size of an input in the form of a series of discrete quantities.
Digital-to-Analog Converter (D/A or DAC): A device or circuit to convert a digital value to an analog signal level.
DIN (Deutsche Industrial Norm): A set of German standards recognized throughout the world. The 1/8 DIN standard for panel meters specifies an outer bezel dimension of 96 x 48 mm and a panel cutout of 92 x 45 mm.
DIN 43760: The standard that defines the characteristics of a 100 ohm platinum RTD having a resistance vs. temperature curve specified by a = 0.00385 ohms per degree.
Discharge Time Constant: The time required for the output-voltage from a sensor or system to discharge 37% of its original value in response to a zero rise time step function input. This parameter determines a low frequency response.
Disk Operating System (DOS): Program used to control the transfer of information to and from a disk, such as MS DOS.
Displacement: The measured distance traveled by a point from its position at rest. Peak to peak displacement is the total measured movement of a vibrating point between its positive and negative extremes. Measurement units expressed as inches or millinches.
Dissipation Constant: The ratio for a thermistor which relates a change in internal power dissipation to a resultant change of body temperature.
Dissociation Constant (K): A value which quantitatively expresses the extent to which a substance dissociates in solution. The smaller the value of K, the less dissociation of the species in solution. This value varies with temperature, ionic strength, and the nature of the solvent.
DMA: Acronym direct memory access. A high speed data storage mode of the IBM PC.
Double Precision: The degree of accuracy that requires two computer words to represent a number. Numbers are stored with 17 digits of accuracy and printed with up to 16 digits.
Drift: A change of a reading or a set point value over long periods due to several factors including change in ambient temperature, time, and line voltage.
Droop: A common occurrence in time-proportional controllers. It refers to the difference in temperature between the set point and where the system temperature actually stabilizes due to the time-proportioning action of the controller.
Dual Element Sensor: A sensor assembly with two independent sensing elements.
Dual-slope A/D Converter: An analog-to-digital converter which integrates the signal for a specific time, then counts time intervals for a reference voltage to bring the integrated signal back to zero. Such converters provide high resolution at low cost, excellent normal-mode noise rejection, and minimal dependence on circuit elements.
Duplex Wire: A pair of wires insulated from each other and with an outer jacket of insulation around the inner insulated pair.
Duplex: Pertaining to simultaneous two-way independent data communication transmission in both direction. Same as “full duplex”.
Duty Cycle: The total time to one on/off cycle. Usually refers to the on/off cycle time of a temperature controller.
Dynamic (Two-Plane) Balancing Machine: A dynamic balancing machine is a centrifugal balancing machine that furnishes information for performing two-plane balancing.
Dynamic Calibration: Calibration in which the input varies over a specific length of time and the output is recorded vs. time.
Dynamic Pressure: The difference in pressure levels from static pressure to stagnation pressure caused by an increase in velocity. Dynamic pressure increases by the square of the velocity.
Dynamic Unbalance: Dynamic unbalance is that condition in which the central principal axis is not coincident with the shaft axis.