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29 September 2014

Windmill Software Ltd
Data Acquisition Intelligence
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Glossary of Data Acquisition and Control

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

AHave we missed your DAQ term?

Absolute Accuracy
How closely the measured value approaches the real value. For more on accuracy in temperature measurements, see Issue 50 of our Monitor Newsletter.
Acquisition Time
In analogue to digital conversion, the time taken for the sample circuit to settle to the a input voltage. Also known as Sample and Hold Acquisition Time. For more information see Issue 3 of our Monitor newsletter.
A/D, A-D, ADC, Analogue-to-Digital Converter
Converts an analogue signal (such as a voltage signal from a temperature sensor) into a digital signal suitable for input to a computer. See Issues 3 and 4 of our newsletter for more information on A-D converters.
Alternating Current (ac)
Electric current whose flow alternates in direction. The number of times the current changes direction in one second is called the frequency. The usual waveform of ac is sinusoidal.
Alias
A false picture of a high frequency waveform that has been sampled at too low a rate. See also anti-alias filter.
Alumel
Trade name for an alloy of nickel with up to 5% aluminium, manganese and silicon, used with chromel in K-type thermocouples.
Ampere (A)
SI unit of electric current.
Amplifier
A circuit that produces a larger output power, voltage or current than was applied at its input.
Amplitude
The size or magnitude of a signal.
Analogue-to-Digital (A-D) Converter
Converts an analogue signal (such as a voltage signal from a temperature sensor) into a digital signal suitable for input to a computer. See Issues 3 and 4 of our newsletter for more information on A-D converters.
Analogue Input
An infinitely variable signal going into a data acquisition device. This is generally a voltage signal. Thermocouples, strain gauge bridge circuits and gas concentration probes, for example, all produce an analogue voltage. Alternatively the signal may be a milliamp current. In this case the data acquisition hardware will convert the current to a voltage before accepting it.
Analogue Output
A waveform or control signal generated by the data acquisition and control equipment. See Issue 36 of our newsletter for more on analogue output.
Anti-Alias Filter
An anti-alias (or anti-aliasing) filter allows through the lower frequency components of a signal but stops higher frequencies, in either the signal or noise, from introducing distortion. Anti-alias filters are specified according to the sampling rate of the system and there must be one filter per input signal. See Issue 8 of our newsletter for more on anti-alias filter.
Argument
Input parameter to a program.
ASCII
American Standard Code for Information Interchange. Coding for text files.
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Backbone
The major multi-channel link in a network, from which smaller links branch.
Background noise
Extraneous signals that might be confused with the required measurement.
Batch process
Any process on which operations are carried out on a limited number of articles, as opposed to continuous process.
Bathymetry
Measurement of the depths of features at the bottom of the sea, especially by echo-sounding.
Bipolar
A signal that ranges between a negative and a positive value, -10 V to +10 V for example.
Bluetooth
Short-range wireless communication. See Issue 110 of our Monitor newsletter for more on Bluetooth systems.
B-Type Thermocouple
Platinum-rhodium thermocouple with a temperature range of 600 to >1700 oC.
Bus
Transfers data from the data acquisition device to the computer. Examples include Universal Serial Bus, Modbus and General Purpose Interface Bus. Network communications like Ethernet are not generally regarded as buses.
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<CR>
A Carriage Return. A "non-printing" character which often terminates a message from an instrument plugged into the computer's COM port.
Cable Gland
Secures an electric cable entering equipment and provides a seal between the external and internal surfaces of the equipment.
Calibration
Calibration compares a data acquisition device's performance to an accuracy standard, and adjusts the performance as necessary. See Issue 96 of Monitor for more on drift and calibration.
Capacitance
The ability to store an electrical charge, or, more precisely, the ratio of the total charge on a capacitor to its potential. The unit is the Farad.
CE
Conformite Europeene. A mark that is affixed to a product to designate that it is in full compliance with all applicable European Union legal requirements.
Celsius
A temperature scale where the freezing point of water is 0 degrees and the boiling point 100 degrees. Symbol is C.
Channel
During data acquisition different parameters are collected, for example latitude, longitude, humidity, wind speed, strain, force and so on. Each parameter is said to be collected through a "channel". So you might have one storing temperature readings and another storing pressure readings. If, say, you were measuring temperatures at four different points on a cooker, you would have four channels of temperature data.
Chromel
An alloy of nickel with about 10% chromium, used with Alumel in K-type thermocouples.
Cold Junction
The reference junction of a thermocouple which is kept at a constant temperature.
COM port
A connection on a computer into which a serial device may be plugged. For more details see Making measurements through a COM port.
Common-Mode Signal
A signal applied simultaneously to both inputs of a differential amplifier.
Common-Mode Rejection Ratio (cmrr)
The ability of the differential amplifier to obtain the difference between the + and - inputs whilst rejecting the signal common to both. For more information see Issue 11 of Monitor.
Contact emf
Electromotive force which arises at the contact of dissimilar metals at the same temperature, or the same metal at different temperatures.
Contact Rating
Refers to the power that can be safely switched with a relay. Quoted for non-reactive load, that is without capacitance or inductance.
Constantan
An alloy of 40% nickel and 60% copper, with a high volume resistivity and almost negligible temperature coefficient. Used with copper in T-type thermocouples.
Continuous Process
Method of producing an article continuously.
Conversion Time
The time taken to convert and analogue signal to a digital signal. The sample and hold circuit of an analogue-to-digital converter freezes an otherwise varying analogue voltage at the moment the sample is required. This voltage is held constant whilst the A-D converter digitises it. (Sample and hold circuits are not used with integrating converters.)
CMOS
Complimentary metal-oxide semiconductor.
Crosstalk
When one channel's signal causes an undesired effect on another.
Current
Current is often used to transmit signals in noisy environments because it is much less affected by environmental noise pick-up. Before A-D conversion the current signals are usually turned into voltage signals by a current-sensing resistor. The unit is the Ampere (A). For more on current signals see Issue 69 of our Monitor newsletter.
Current Sink
The maximum current that data acquisition output channels can dissipate.
Current Source
The maximum current that can be supplied by the data acquisition device for output signals.
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DAC, D/A, D-A, Digital-to-Analogue Converter
Used to produce analogue output signals. These may be control signals or synthesised waveforms.
DAC - Data Aquisition and Control
Acronym for data acquisition and control.
DAQ
Another acronym for data acquisition and control.
Data Acquisition
The automatic collection of data from sensors, instruments and devices: in a factory, laboratory or in the field.
Data Logger
An electronic device that collects and stores data over a period of time. May be stand alone devices or interface with a personal computer. An example of the latter is the Microlink 840 data logger.
Data Logging
Making measurements and recording readings against time.
Data Persistence
The time that data is considered valid. Once data has been read from an instrument it is remembered and returned as a valid reading until the persistence time has expired. Thereafter an attempt to read the instrument will cause a new reading to be taken. This is important where messages extract several channels of data. If you set the persistence time to 0, every channel reading will cause a complete message to be executed. With a longer persistence time the message need only be executed once and all channels can be obtained from the stored data.
DCE
DCE stands for Data Circuit-Terminating Equipment or Data Communications Equipment. It is part of the RS232 standard and represents, for example, an instrument or modem attached to your PC.
Decibel
A logrithmic measure of the ratio between two quantities. Symbol dB.
Device
A peripheral which connects to the computer. Mice, keyboards, printers, data acquisition instruments, modules and cards are all devices.
Differential Amplifier
One whose output is proportional to the difference between two inputs.
Differential Inputs
Using differential inputs can reduce noise picked up by the signal leads. For each input signal there are two signal wires. A third connector allows the signals to be referenced to ground. The measurement is the difference in voltage between the two wires: any voltage common to both wires is removed. For more information see Issue 11 of our Monitor Newsletter.
Digital Input
A digital signal going into a data acquisition device.
Digital Output
A digital signal generated by the data acquisition and control equipment. A digital signal has only 2 states. Software controls each digital output by just one bit - setting the digital line high or low. For more information see Issue 71 of our Monitor Newsletter.
Digital-to-Analogue (D-A) Converter
Used to produce analogue output signals. These may be control signals or synthesised waveforms.
Direct Current (dc)
Current which flows in one direction.
Drift
Slow variation of a performance characteristic such as gain, frequency or power output. May be due to, for instance, temperature or ageing. Usually only significant when measuring low-level signals (a few millivolts) over long periods of time, or in difficult environmental conditions. See Issue 96 of Monitor for more on drift and calibration.
Driver
A program that controls a device. Each device has its own set of commands that its driver understands, and can translate for other software like Windmill's Logger and Chart.
DPM
Diesel particulate matter, or defects per million.
DTE
DTE stands for Data Terminal Equipment. It is part of the RS232 standard and represents, for example, the PC.
Dynamic Crosstalk
When one channel's signal causes an undesired effect on another.
Dynamic Data Exchange (DDE)
A standard Microsoft Windows protocol that defines a way for Windows applications to share information with one another.
Dynamic IP Address
An Instrument can get its IP Address allocated when it powers up. This uses a process called Dynamic Host Configuration Protocol (DHCP). In this system the unit is identified by a name eg "My Instrument". When it powers up it asks a DHCP server to allocate an IP Address to "My Instrument". If you wish to send a message to "My Instrument" then you need to know its IP Address. This can be obtained from a Name Server. This may sound complicated but you, as the user, need only decide to Address the instrument by Its IP Address or by its Name the rest is done for you by the operating system.
Dynamic Range
The ratio of the largest to the smallest signal that can be measured at one time. Normally expressed in Decibels (dB). The maximum signal is generally the analogue-to-digital converter's full scale signal. The dynamic range of a DAQ device is important when both large and small signals are to be measured.
Dynamic Range (dB) = 20 x log (Max Voltage / Min Voltage).
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Electromotive Force (emf)
Difference of potential produced by sources of electrical energy which can be used to drive currents through external circuits. Unit is the volt.
Endurance limit
In fatigue testing, the number of cycles which may be withstood without failure at a particular level of stress.
EIA
Electronic Industries Association.
Ethernet
A local area network to which you can connect data acquisition devices.
E-Type Thermocouple
Chromel-constantan thermocouple with a temperature range of 0 to 800 oC.
Excitation
The voltage or current applied to a transducer.
External Trigger
A trigger is something that causes data acquisition to start. External triggers let you synchronise data acquisition with external events, for example when you switch on a machine. For more information and examples see Issue 45 of the Monitor newsletter.
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Fall Time
The time a signal takes to change from a specified high value to a specified low value. Usually measured as the time to fall from 90% to 10% of the step height or maximum amplitude.
Farad
Unit of capacitance.
Farenheit
A temperature scale where the freezing point of water is 32 degrees and the boiling point 212 degrees. Superceded for scientific purposes by Celsius. Symbol is F.
Fast Fourier Transfer (FFT)
An analysis algorithm - given a finite set of data points, the FFT expresses the data in terms of its component frequencies.
FIFO buffer
A first in, first out, store. The first value placed in the buffer (queue) is the first value subsequently read.
Filtering
Attenuates components of a signal that are undesired: reduces noise errors in a signal. More details on filters are in Issue 8 of Monitor.
Flow Control
See Handshaking.
Frequency
Measured in hertz (cycles per second), rate of repetition of changes.
Frequency Counter
Counts digital pulses over a defined gate time. A typical gate time is between 0.1 and 10 seconds.
Front panel
The front surface of a unit, generally containing switches and indicator lights.
Full Scale Output
The difference between the minimum output (normally zero) of a data acquisition device and the rated capacity.
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Gain
Amplification of a circuit.
Gain Range
The maximum and minimum voltage that will be digitised by the A-D converter is sometimes called the gain range.
Gateway
When your computer wishes to send a message to a device on another subnet it sends the message via a Gateway. This is another computer which relays the message to the destination address. Your computer needs to know the IP Address of the Gateway. You can view this in the TCP/IP properties of your computer.
GIS
Geographic information system. Where data is assembled, stored, displayed and identified according to its location.
GPIB
General Purpose Interface Bus. Also known as IEEE-488 bus. The GPIB standard was designed to connect several instruments to computers for data acquisition and control. Data can be transferred over GPIB at 200 000 bytes per second, over distances of 2 metres.
GPRS
General Packet Radio Service.
Ground
See earth.
Ground-Truthed
When data is collected by remote sensing techniques, ground-truthing confirms that the information is accurate. Ground-truthing is collecting data by non-remote sensing means.
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Handshaking
The RS232 protocol includes handshaking (also known as flow control). Although this is often not necessary, it has two functions: tt allows the computer to stop your instrument from sending data when the PC is not ready for it; it allows your instrument to prevent the PC from sending data when the instrument not ready for it. More on handshaking.
Hardware Trigger
See External Trigger.
HART
Highway Addressable Remote Terminal. Provides digital communication to microprocessor-based (smart) analogue process control instruments.
Hertz (Hz)
Cycles per second unit of frequency.
Hexadecimal
Counting system based on 16.
High Pass Filter
When a low-level transducer signal is superimposed on a large dc output voltage, a high-pass filter might be useful. This attenuates (removes) low frequencies. Using a cut-off frequency of, say, 4 Hz, will eliminate the dc voltage which has a frequency of zero. A high-pass filter will remove "drift". This can be a particular problem with biological and chemical signals, but not usually with modern electronic signals. See Issue 8 of our newsletter for more information.
Human machine interface (hmi)
Also known as man machine interface. The communication between the computer system and the people who use it.
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I
Symbol for electric current.
I2C
A 2-wire serial bus developed by Philips.
I/O
Input/Output. A data acquisition system monitors signals through its inputs, and sends control signals through its outputs.
IC
Integrated Circuit (electronic components fabricated on a semiconductor substrate which cannot be divided without losing its function).
IEEE
Institute of Electrical and Electronics Engineers. An American Society that, amongst other things, establishes international standards in the computing, electronic and telecommunications fields.
IEEE-488 Bus
See GPIB.
IML
Interface Management Language. A programming language used to communicate with measurement instruments.
Inductance
The magnitude of a magnetic field created by a circuit carrying a current. This can cause higher voltages in the circuit.
Input
Data entering a device from the environment. A signal being monitored by a data acquisition system.
Instrument
Any item of electrical or electronic equipment which is designed to carry out a specific function or set of functions. For example an electronic balance, a gas analyser or a chromatograph.
Integrated Circuit
Electronic components fabricated on a semiconductor substrate. An integrated circuit cannot be divided without losing its function.
Integer
A positive or negative whole number, or 0.
Interface
A shared boundary. It might be a piece of hardware used between two pieces of equipment, or a software display communicating between the computer system and the people who use it.
Interpreter
Software enabling a computer to run programs statement by statement.
Interrupt
An external signal causing the execution of a program to be suspended.
Integrating A-D Converter
An integrating A-D converter averages the input signal over a length of time. This is useful when the signal to be measured fluctuates slowly. By averaging the signal the converter helps reduce unwanted signal contamination (noise).
Integration Time
The time over which an integrating A-D converter averages the input signal. If chosen appropriately will average over a complete mains cycle thereby helping to reduce mains frequency interference.
Inverter
A dc-ac converter.
ISA
Industry Standard Architecture. An ISA expansion slot lets you plug data acquisition boards into PCs.
IrDA
Infrared Data Association. A Nonprofit organisation whose goal is to develop globally adopted specifications for infrared wireless communication.
ISO
International Organization for Standardization, which is made up of national members. A member is the "most representative of standardisation in its country". For example BSI (British Standards Institute), DIN (Deutsches Institut für Normung) and ANSI (American National Standards Institute).
IP Address
Each computer on the network has an IP Address. This is in reality a 32 bit binary number. Since such numbers are not user friendly they are usually presented in Dotted Decimal Notation. This splits the 32 bits into 4 eight bit chunks. Each chunk is then converted to a decimal number in the range 0 to 255. For example -
01011001 00011101 11001100 00011000 becomes 89.29.204.24
You can think of this address as being roughly like a postal address arranged as Country / City / Street / House Number. See also Static and Dynamic IP Address.
Isolation
Two circuits are isolated when there is no direct electrical connection between them.
Isolation to Earth or System
A high transient voltage at one input may damage not only the input circuit, but the rest of the data acquisition hardware, and, by propagating through the signal conditioning and A-D circuits, eventually damage the computer system as well. You can prevent this type of damage by isolating the input from the earth of the data acquisition and computer hardware.
Isolation Between Inputs
A transient at an input can also propagate to other equipment connected to that input. This is prevented by providing isolation between inputs.
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J-Type Thermocouple
Iron-constantan thermocouple with a temperature range of 0 to 750 oC.
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k
A symbol for a thousand, from kilo.
K
A unit of stored data. 1K = 210 = 1024. Also stands for a degree on the Kelvin temperature scale.
Kelvin
A temperature scale which is one of the seven base units in the International System of Units (SI). Symbol is K.
K-Type Thermocouple
Chromel-Alumel thermocouple with a temperature range of -200 to 1200 oC.
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<LF>
A Line Feed. A "non-printing" character which often terminates a message from an instrument plugged into the computer's COM port.
LAN
Local area network. A data communication system connecting devices in the same vicinity. Data is transferred without the use of public communications. Examples of LANs are Ethernet, token ring and Modbus.
Least Significant Bit, LSB
In a binary number, the 1 or 0 furthest to the right.
LIFO
Last in first out. Describes a stack method of data storage.
LIMS
Laboratory information management system.
Linearity
Ideally an A-D or D-A converter converts the input or output range into equal steps. In practice the steps are not exactly equal. Linearity, or non-linearity, is a measure of how close the steps approach equality.
Load Cell
A transducer which converts a force into an electrical signal. It normally comprises four strain gauges in a wheatstone bridge arrangement.
Low Pass Filter
This lets through the lower frequencies and attenuates the higher frequencies. Choose the cut-off frequency to be compatible with the unwanted frequencies, the frequencies present in the signal you are measuring, and the sampling rate of the analogue-to-digital converter. See Issue 8 of our newsletter for more information.
LVDT
Linear Variable Differential Transformer. Used in measuring devices that convert changes in physical position to an electrical output.
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m
Symbol for metre and milli- (thousandth).
M
Symbol for mega (million).
Mains
Source of electrical power, normally the electricity supply system.
Mains Frequency
Electricity ac supply frequency; 50 Hz in UK, 60 Hz in US.
MES
Manufacturing Execution System.
Metre
The SI fundamental unit of length, equal to 1.093 yards. (Meter in US.)
MIS
Manufacturing Information System.
MMI
Man Machine Interface. Also known as human machine interface. The communication between the computer system and the people who use it.
Modular
Form of construction in which hardware or software units, often with differing functions, are quickly interchangeable.
Multiplexing
Where each signal is switched in turn to a single analogue-to-digital converter. As opposed to where one A-D converter is used for each signal in simultaneous sampling.
Modbus
An industrial networking system that uses peer-to-peer communications. More details on Modbus are in Issue 32 of Monitor.
Most Significant Bit, MSB
In a binary number, the 1 or 0 furthest to the left.
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N-Type Thermocouple
Nicrosil-Nisil thermocouple with a temperature range of -200 to 1200 oC.
Name Server
When comunicating with instruments over a TCP-IP network, and your computer wishes to send a message to a named destination, it asks a Name Server to resolve the name to an IP Address. Your computer needs to know the IP Address of the Name Server. You can view this in the TCP/IP properties of the computer.
Negative Edge Trigger
Data acquisition starts when an input signal changes from a high to a low state.
Network
The physical interconnection of devices sharing a communications protocol.
Node
A device with a direct point of access to a network.
Noise
Any unwanted electrical signals contaminating the signal to be measured. This noise may be electronic noise which is an artefact of semiconductor construction techniques and is not reducible.
Alternatively the noise may be caused by environmental factors. This type of noise can be the result of poor positioning or screening of signal wiring. This may result in mains frequency or RF pickup contaminating the required signal.
Non-Conforming
A product outside manufacturing limits but not necessarily defective.
Non-Destructive Testing
Method of inspecting materials and products without affecting their subsequent properties and performance. Abbreviation NDT.
NTSC
The US National Television System Committee and their standard colour television system. Modified or 4.43 NTSC has the colour subscarrier at the European frequency of 4.43 MHz instead of 3.58 MHz.
Nyquist Theorem
The Nyquist theorem demands that a signal be sampled at at least twice its maximum frequency. To get an accurate picture of a waveform however, a sampling rate of 10 - 20 times the highest frequency is better.
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OEM
Original Equipment Manufacturer. A company which makes basic computer hardware for other manufacturers to build into their products.
Offset Error
If you get a reading other than zero for a zero condition then you have an offset error: every reading will be inaccurate by this amount.
Ohm
SI unit of resistance, such that one ampere through it produces a potential difference of one voltage.
On-Off Control
A simple control system which is either on or off.
Output
Information leaving a device.
Output Source Current
See Current Source.
Output Sink Current
See Current Sink.
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PAL
Phase Alternation Line
The colour television coding system generally used for European broadcasting.
Parsing
Phase Alternation Line
The colour television coding system generally used for European broadcasting.
PC
Personal computer. Generally applied to computers conforming to the IBM designed architecture.
PCI
Peripheral Component Interconnect
A local bus standard developed by in 1992. PCI cards plug into your computer and are configured through software. They do not have jumpers or switches.
PCMCIA
Personal Computer Memory Card International Association
Industry group that developed the specification for credit card-sized plug-in cards for laptop computers.
Peer-to-Peer Communication
A communication between networked devices in which any device can initiate data transfer.
PID
Proportional gain, integral action time and derivative action time. PID software, for example, compares an analogue input value with a set point and if there's a discrepancy outputs an appropriate analogue or digital control value, according the PID calculations.
PI&D
Piping and instrumentation diagram.
Ping Utility
This is a test utility which sends a message to a defined port within your instrument. The instrument replies with a short data message. Virtually every unit on TCP/IP will support this action. If you cannot Ping your instrument, and it is on a TCP/IP network, you will not be able to talk to it.
PLC
Programmable Logic Controller.
Pole
A relay contact.
Port
The external connector on a device.
Postive Edge Trigger
Data acquisition starts when an input signal changes from a low to a high state.
Positive Temperature Coefficient
An increase in resistance due to an increase in temperature.
Protocol
A set of rules used in data communications.
Pulse
A temporary change in voltage of any length.
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QA
Quality assurance.
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Ramp Voltage
A steadily rising voltage.
Range
The maximum and minimum allowable full-scale signal (input or output).
Reed Relay
Consists of two thin magnetic strips (reeds). When a coil close to the reeds is energised, they are magnetised and drawn together making a connection between leads attached to the reeds.
Relay
Electromechanical device that opens or closes contacts when a current is passed through a coil.
Relative Accuracy
How accurately a change in signal is measured. See also Absolute Accuracy.
Repeatability
The ability of an instrument to give the same reading under repeated identical conditions.
Reproducibility
The precision with which a measured value can be repeated.
Resistance Temperature Device (RTD)
Resistance temperature devices (or detectors) rely on the principle that the resistance of a metal increases with temperature. When made of platinum, they may be known as platinum resistance thermometers (PRTs). See Issue 5 of our newsletter for more information on RTDs.
Resolution
A measure of the smallest change that can be detected.
Response Time
The time a system takes to respond to a given input. For example: the time between software sending a message to an instrument and the instrument sending a reply, or the time a sensor takes to indicate a change in conditions.
Rise Time
The time a signal takes to change from a specified low value to a specified high value. Usually measured as the time to rise from 10% to 90% of the step height or maximum amplitude, but sometimes over 5 to 95%.
rms
Root mean square. The square root of the sum of the squares of a set of quantities divided by the total number of quantities. Used when monitoring ac (alternating current) signals. Many power supplies, for example, issue an ac signal. This needs to be converted to a dc (direct current) signal for the PC interface. The solution is a signal conditioning input that produces a dc signal proportional to the rms of the amplitude of the input signal. The rms operation means the reading will always be positive.
RS232
An EIA (Electronic Industries Association) standard that defines a protocol for serial data communications. An RS232 link will run at up 38400 baud (bits per second) over short distances, and at lower speeds as the distance increases. You can plug the RS232 lead directly into the computer's serial (COM) port.
RS422
Again, an EIA protocol for serial communications. Each signal line comprises two twisted wires. The voltage-difference between the two lines shows the signal value, rather than the voltage-level. You can connect 10 devices to a single cable
RS485
Another EIA protocol for serial communications. Allows 32 devices to be connected to a single cable, distributed over a wide area.
RTU
Remote Terminal Unit. A data acquisition device at a remote location which transmits data back to, and accepts commands from, a central PC (or other controller).
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Sample and Hold
A component of a type of analogue-to-digital converter. The analogue signal is frozen in a sample and hold circuit to prevent it changing during digitisation. For more information on A-D converters see 3 of Monitor.
Sample and Hold Aquisition Time
See Aquisition Time.
Sampling Rate
The number of samples, or readings, per second of an analogue signal.
Scan
Normal channel scanning in a data acquisition system involves stepping round and reading each input channel in turn. The scan will return to the first channel once all the channels have been sampled.
SCADA
Supervisory control and data acquisition - a large scale software package usually used to monitor and control a manufacturing process.
Seebeck Effect
The principle that describes how a thermocouple works. In a circuit in which there are junctions between dissimilar metals, an electromotive force (voltage) is set up when the junctions are at different temperatures.
Self-Calibrating
A data acquisition module with a stable on-board reference voltage that software can use for automatic recalibration.
Sensitivity
A measure of the minimum change in an input signal that an instrument can detect.
Sensor
A device that can detect a change in a physical quantity (light or pressure for example) and produce a corresponding electrical signal.
Serial Communication
Where data is transferred one bit at a time.
Settling Time
When a change in signal occurs, the time taken for the input or output channel to settle to its new value.
Set Point
Value of a controlled variable, departure from which causes a controller to operate to reduce the error and restore the intended steady state.
SI
International system of units. Abbreviation for Systeme International (d'Unites).
Signal Conditioning
Makes a signal suitable for input to an analogue-to-digital converter. For example, a signal may be filtered to remove noise, or amplified to meet the range of the A-D converter.
Signal
General term referring to a conveyor of information.
Signal to Noise Ratio
Compares the signal strength to background noise. Abbreviated to SNR or S/N. The ratio is usually measured in decibels (dB). More details on reducing Noise.
Single-Ended Input
An analogue input that is measured with respect to a common earth. Single ended inputs are only suitable for signals that are of good size - 100 mV full scale or above.
Simultaneous Sampling
When all analogue signals are read simultaneously. This is achieved by providing each input with its own A-D converter, and initiating sampling from a single clock. It ensures that there is no reduction in sampling rate when more signals are connected.
Sine Wave
Waveform of a single frequency, indefinitely repeated in time. In practice there must be a transient at the start and finish of such a wave.
Sink Current
See Current Sink.
Slew Rate
The maximum rate of change of an output signal.
Slave
A networked device that is controlled by another, master, device.
SNR
See Signal to Noise Ratio.
Software Trigger
A programmed event, such as a specific key press or mouse click, that triggers data capture.
Solenoid
A coil of wire, usually wrapped around an iron core, that acts as a magnet when carrying a current. Used as an electromagnetic switch or relay.
Source Current
See Current Source.
Spike
Short pulse of voltage or current - usually undesirable.
Square Wave
Wave that alternates between two fixed values. Has very rapid (theoretically zero) rise and fall times.
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Stability
The ability of an instrument or sensor to maintain a constant output when a constant input is applied.
Static IP Address
An Instrument can have a fixed IP Address which must be allocated to it by a Network Administrator. The allocated address will be appropriate to and unique for the network. Your instrument must have a utility supplied by the manufacturer which will allow the address to be set.
Steady State Error
A measure of the accuracy of performance of a control system. In a stable system, the steady state error is the difference between the desired input and the actual input. When the error is large, the resultant output will not match the desired output.
Strain
When a material is distorted by forces acting on it, it is said to be strained. Strain is the ratio of change in dimension to original dimension.
Strain Gauge
A sensor that experiences a change in resistance when it is stretched or strained. It is attached to the body subjected to the strain.
Successive Approximation
A technique used in A-D converters to measure an analogue signal. It compares the signal with progressively smaller values, each step getting nearer the actual voltage. More details are in Monitor Issue 4.
Subnets
Large TCP-IP networks are physically divided into subnets. Two devices which are on the same subnet can send messages directly to each other without the messages being seen by the rest of the network. This reduces traffic in the wider network. The subnet is defined by the Subnet Mask. You can view this via Control Panel and the TCP/IP properties of your computer. This is a dotted decimal arrangement. For two units to be on the same subnet the parts of their IP Addresses which are covered by a 1 in the Mask must be the same. So if the Mask is 255.255.255.0 then the first 3 elements of both IP Addresses must be the same to be on the same subnet.
If a unit whose IP Address puts it onto one subnet is physically plugged into another subnet then you will not be able to talk to it since your messages will be sent to the wrong subnet.
Surge
A large, momentary, increase in the voltage on a power line.
System
Combination of several pieces of equipment to perform in a particular manner.
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Talker
A device on the GPIB (general purpose interface bus) that sends information to a Listener on the bus.
TCP/IP
Transmission Control Protocol/Internet Protocol. Communications protocols used to connect hosts on the internet.
Text Format
Text format refers to information in the ASCII character set. It is unformatted. Each byte in the file contains one character that conforms to the standard ASCII code.
Thermal
Coefficient of Resistance The change in resistance of a semiconductor per unit change in temperature, over a specific range of temperature.
Thermal Conductivity
A measure of the rate of flow of thermal energy through a material in the presence of a temperature gradient. Materials with high electrical conductivities tend to have high thermal conductivities.
Thermocouple
Popular temperature sensor because of its low cost, wide operating range and ruggedness. Consists of two dissimilar metals joined together, making a continuous loop. When one junction has a different temperature from the other an electromotive force (voltage) occurs. There are several types of thermocouples, constructed from different metals and with differing temperature ranges and accuracies. More details of thermocouples are in Issue 5 of our newsletter, Monitor.
Thermistor
A temperature sensor. The name comes from thermal resistor. It is a semiconductor that exhibits a large change in resistance as a function of temperature. Most thermistors exhibit a negative temperature coefficient, where resistance decreases as temperature rises. These are referred to as NTC thermistors.
Throughput
Number of results produced per unit time.
Timeout
The time software should wait for a reply from the instrument before giving up. If no reply is received in this time, an error may be declared. To avoid spurious timeouts set to between five and ten times the normal response time. If you donít know the response time, 5000 milliseconds should be adequate.
Time stamp
Information added to data to indicate the time at which it was collected.
Transducer
A device which converts a physical quantity into an electrical signal. Examples include thermocouples and photocells. Most sensors are also transducers.
Transient
A short surge of current or voltage, often occurring before steady-state conditions have become established.
Trigger
A trigger is something that causes a data acquisition system to start collecting data. It may be as simple as pressing a software button or a set of conditions which when met trigger data capture (internal triggers), or an externally generated, hardware signal (an external trigger). For more information see Issue 45 of the Monitor newsletter.
Truncation
Rejection of the final digits in a number, thus lessening the precision but not necessarily the accuracy.
TTL
Abbreviation for transistor-transistor logic. Referring to logic circuits consisting of two or more directly interconnected transistors, to provide conditional switching capability.
TTL-Compatible
For digital input circuits, a logic 1 is obtained for inputs of 2.0 to 5.5 V which can source 40 microA, and a logic 0 for inputs of 0 to 0.8 V which can sink 1.6 mA. For digital output signals, a logic 1 is represented by 2.4 to 5.5 V with a current source capability of at least 400 microA; and a logic 0 by 0 to 0.6 V with a current sink capability of at least 16 mA.
T-Type Thermocouple
Copper-constantan thermocouple with a temperature range of -200 to 400 oC.
Twisted Pair
Cable that consists of individual wires wrapped around each other for carrying telephone or computer data. Reduces pickup noise levels in signals.
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UART
Universal Asynchronous Receiver/Transmitter. In serial communications, the UART takes bytes of data and transmits the individual bits sequentially. Adds Start, Stop and sometimes Parity bits before and after the transmitted word.
Unipolar
A signal range that is always positive or always negative, for example 0 to +10 V.
UPS
Uninterruptible Power Supply. Used to keep critical equipment, including computers, running in the event of a mains power failure.
USB
Universal Serial Bus A serial bus gradually replacing RS232 on PCs because of its higher speed. Generally fitted as standard in new PCs.
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VAC
AC Voltage
Velocity
The rate of change of displacement; dx/dt.
Volt
SI unit of potential difference such that the potential difference across a conductor is 1 volt when 1 ampere of current in it dissipates 1 watt of power. Named after Count Alessandro Volta (1745-1827). Symbol V.
Voltage
The value of an electromotive force or potential difference expressed in volts.
Voltage-to-Frequency Converter
A device that converts an analogue input voltage into a sequence of digital pulses with a frequency that is proportional to the input voltage.
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WAN
Wide area network. A network of circuits spanning a large region which is used to transmit data.
Wheatstone Bridge
A network of four resistances, an emf (voltage) source, and an indicator connected such that when the four resistances are matched, the indicator will show a zero deflection or "null" reading. Prototype of most other bridge circuits.
Wibree
Wibree is a new interoperable radio technology for small devices like sports sensors. It was designed by Nokia for applications where ultra low power consumption, small size and low cost were the critical requirements. Wibree has recently been brought into the Bluetooth stable and renamed Ultra Low Power (ULP) Bluetooth. More details of Wibree are in issue 110 of Monitor
Wi-Fi
Wireless Fidelity. A wireless communications network conforming to IEEE 802.11 specifications. More details of Wi-Fi are in issue 82 of Monitor
WiMax
Worldwide Interoperability for Microwave Access. Provides wireless data over long distances. More details of Wi-Fi are in issue 110 of Monitor
WSN
Wireless sensor network.
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x-axis
Conventionally, the horizontal axis of any type of graph.
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y-axis
Conventionally, the axis perpendicular to and in the horizontal plane through the x-axis of any type of graph.
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z-axis
Conventionally, the vertical axis in any three-dimensional co-ordinate system.
Zero Balance
With transducers like strain gauges, the output is large compared to the changes caused by the strain. Setting a zero balance subtracts an offset (actually a fraction of the bridge excitation voltage), so the changes caused by the strain can be accurately measured.
ZigBee
Wireless communication method commonly used in home automation.

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