Damping: The reduction of response at the resonant frequency through the use of a damping media such as oil. Usually specified as the ratio of critical damping.

DARPA: Defense Advanced Research Projects Agency. A US Dept of Defense agency that funds high-risk research projects and that funded the development of UNIX 4.2, and the TCP/IP communications protocol. One of the founders of the Internet.

Data Base: A large amount of data stored in a well-organized manner. A data base management system (DBMS) is a program that allows access to the information.

Data Compression: A method of reducing the number of bits that are needed to represent information. Data compression allows gigher communications speeds and allows more information to be stored on a disk.

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 inside the pressure port of a transducer at room temperature and barometric pressure.

Debug: To find and correct mistakes in a program.

Decibel: A unit (dB) for measuring the relative strength of signal power. The number of decibels equals ten times the logarithm (base 10) of the ration of the measured signal power to a reference power. One tenth of a bell.

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.

Deflection: The change in length along the primary axis or distance a diaphragm moves at the center between no-load and rated load conditions.

Degree: An incremental value in the temperature scale, i.e., there are 100° 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.

Desiccant:Any substance which exerts a drying action by chemically absorbing water vapor .

Desorption: Release of adsorbed or absorbed material .

DFT: Discrete Fourier Transform.

Diaphragm: The sensing membrane which is deformed when pressure is applied.

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 Filtering: The process of smoothing, or removing noise from a signal via mathematical functions that are performed on the digital data stream.

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: 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.

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.

Dithering: The technique of adding controlled amounts of noise to a signal to improve overall system loop control, or to smear quantizing error in an A/D convertor application.

DNS: Domain Name System. A mechanism used in the Internet for translating names of host computers into addresses. The DNS also allows host computers not directly on the Internet to have registered names in the same style.

DLL: Dynamic Linked Library.

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.

DRAM: Dynamic Random Access Memory.

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.

Drop: A connection between a terminal and a subscriber.

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.

Dry Bulb Temperature:Measured air temperature, usually paired with a "wet-bulb" temperature to derive a value of relative humidity .

DSP: Digital signal processing or digital signal processor.

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 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 Range: The ratio of the maximum output signal to the smallest output signal that can be processed in a system, usually expressed logarithmically in dB. Dynamic range can be specified in terms of harmonic distortion, signal to noise ration, or other performance criteria.