Accuracy
Accuracy is the deviation of a measurement from a known standard. Accuracy is normally specified in percent.
ADCs - Analog-to-Digital Converter
ADC is used to convert DC voltage from transducers into digital words (data). The voltage represents a temperature. pressure, flow, pH, or speed and must converted to a digital word before it can be passed to an intelligent device like a computer.
Alarm Reporting/logging
An alarm reporting/logging can show the steps that led to a production line being shut down and the points on the line to be checked before starting it again.
Amplifier
Amplifiers are used to boost the analog level (voltage) of the signal.
ANSI C - American Electronics Standards Institute C language
The ANSI sets standards for data and communication codes, and for programming and data management languages.
Annunciator Panel/Screen
An annunciator panel/screen shows both essential information and alarms to provide danger level coding and display multiple alarm messages.
AO - Analog Output
The D/A converter performs the opposite function of an A/D converter. It interprets commands from the computer and outputs the proper DC voltage or current. The output stays at this output level until the computer tells the D/A converter to output a new value.
API - Application Programming Interface
API allows access to two layers of the open systems interconnect (OSI) model and provides high-level calls to the program for general purpose, name, data, and session support.
Attenuator
Attenuators are used to lower the analog level (voltage) of a signal.
AUI - Attachment Unit Interface
BASIC - Beginners All Purpose Symbolic Instruction Code
BCD - Binary Coded Decimal
Bit-mapped graphics
Bit-mapped graphics require you to use graphics-oriented software, or drawing tools and libraries of symbols which you may be able to modify.
Bridge Measurement
Strain gages change resistance so slightly (0.00024 ohms per microstrain) that a bridge circuit is needed to see it. The bridge in effect highlights this small resistance change in the form of a voltage. Strain gages multiplexers normally contain this bridge circuitry including the DC voltage power supply. Quarter-, Half-, and Full-bridge configurations can be used for increased sensitivity.
CAN - Control Area Network
The CAN is a serial bus system especially suited for networking intelligent I/O devices as well as sensors and actors within a machine or plane.
Character graphics
Character graphics offer special graphics characters (such as motors, pumps, pipes, and tanks) which you display as special, or “alternate” letters or numbers on the screen.
CJC - Cold- Junction Compensation
The CJC let you make direct temperature measurements from thermocouple transducer.
Close-loop Control
The close-loop control is more sophisticated. The measurement hardware makes a measurement and the results is compared to the desired value (called the setpoint). Using the error value (difference between measured value and desired value), the computer performs one or more calculations and sends out a control value. The control value is used by the control hardware to effect a change.
CMOS - Complementary Metal Oxide Semiconductor
CMRR - Common Mode Rejection Ratio
Common Mode Noise
Electrical interference on both signal leads of an analog measurement which change simultaneously relative to ground. Common mode noise most often results when the ground potential between the measuring instrument and the device being measured are different. The difference in grounds results in a ground loop, a current flowing through ground and the low lead. Once this current appears in the low lead wire it will cause a voltage because the wire has some resistance. The longer the lead, the more lead resistance and greater the voltage error.
TIP : To reduce common mode noise, use a guarded voltmeter. Tie the guard to the low side of the device being measured. This will shunt any ground loop currents away from the high and low measurement wires.
Crosstalk
The coupling of signals from one conductor to another nearby conductor, by magnetic or capacitive coupling.
C/T - Counter/Timer
The counter card can be used to sense the presence or absence of a voltage, much like digital input card. The counter is used to count the number of electronic pulses (totalize), the duration of the pulse (pulse width), or the rate of pulses (frequency) coming out of an external device.
DAS products Abound
DAS products can be broken down into the following categories : board level products, data loggers and packaged box/level products, integrated computer driven systems, and products from software-only supplier with choices of I/O hardware drivers.
Data Acquisition
Data Acquisition systems make various types of physical measurements such as temperature, pressure, flow, strain, position and speed using electronic equipemnts. The data collected is usually sent to a computer for analysis and display.
Data Loggers
Data logging is the process of collecting information about a process while it is running. Data logging can be as simple as counting the number of parts number, or as complex as recording process times, pressures, temperatures, and part measurements.
Data logging not only “keep score”, but also certifies that a process was performed properly, observed and helps predict normal and abnormal process changes and trends, and analyzes what happened in the event of a process failure.
A well designed data logging screen can graphically show trend data, current data values, and statistics for various historical conditions.
DCE - Data Communication Equipment/Data Circuit terminating Equipment
Use to specify the receiving station.
DDC - Direct Digital Control
The control of a process by a digital computer by directly adjusting the correcting conditions of that process. The computer thereby replaces the function of conventional instrument controllers.
DDE - Dynamic Data Exchange
DI - Digital Input
A digital input card is used to determine whether an external device is on or off by sensing the presence or absence of a voltage. The DI can only report ON/OFF status and not the value of the voltage on each channel (sometimes called a bit). The bit is considered to be ON if the voltage exceeds a certain value.
Digital cards are usually 8, 16, or 32 channels. They can monitor a number of devices. For example, a digital card can be attached to a single operator panel to sense the position of switches on that panel.
Digital Noise
Digital circuitry passes information in the form of voltages, switching from high to low at high speeds. This activity creates high frequency noise that can easily be transferred to analog signals that come in close proximity.
TIP : Never allows sensitive signals to pass near digital circuitry like that found in a computer. (Cards plugged into a PC may be susceptible to digital noise present in the computer).
DLL - Dynamic Link Library
DMA - Direct Memory Access
A method of transferring data from or to memory at a high rate without involving the CPU.
DMA is the hardware/software technique that allows the highest speed transfer of data, to or form random memory (RAM) . Given the potentially more expensive hardware. DMA can provide the means to read or write data at precise times without restricting the microprocessor‘ tasks. For example, one system under DMA control can read or write any combination of analog, digital or counter/timer data to or from RAM at rates up to 360KB/second. This is accomplished without taking time from the other tasks of the microprocessor. The amount of time required to respond to a DMA request is very small compared to the time required to service an interrupt. This makes the goal of foreground/background operation, at high speed, possible.
DO - Digital Output
The digital output card interprets a command from the computer and outputs a high or low voltage on each of its channels (bits). It is commonly used to turn on/off small lights or to send digital words to machinery.
DSP - Digital Signal Processors
DTE - Data Terminal Equipment
Use to specify the transmitting station.
DTL - Diode Transistor Logic
DVM - Digital Voltmeter
Electrical instrument that measures electrical voltages, and display with digital number.
Electromagnetic noise
A magnetic field that is changing will cause a current to flow in a conductor running through it. Conversely, a current flowing is a wire has a magnetic field associated with you. Therefore, one wire with a current flowing in it can cause a current to flow in another wire (conductive coupling).
TIP : Whenever possible, keep signal lines away from noise sources (i.e. other wires carrying large currents).
TIP : To eliminate interference currents, twist your wire pairs so that the current induced into one loop cancels out the current induced in an adjacent loop. This is a simple, no-cost way to reduce electromagnetic noise.
Electrostatic noise
A voltage on one wire can be capacitively coupled to an adjacent parallel wire as a result of an electric field. The signals from power lines are the most common noise sources.
TIP : Whenever possible, run signal lines away from noise sources (i.e. other unshielded wires carrying large voltages).
TIP : Use shielded wire to reduce the effects of capacitive coupling. The voltage will couple to the shield and not to the protected wire.
TIP : Ground one end of the shield to keep voltages built-up on the shield from capacitively coupling to the protected wire. NEVER ground both ends of the shield!
TIP : For the best possible temperature measurements, use a shielded thermocouple and a guarded voltmeter. Tie the shield to the thermocouple junction and to guard. Do NOT ground the shield.
TIP : If not using a three-wire (shielded) thermocouple, tie guard to low. Leaving guard unattached will not make use of its ability to reduce noise.
FET - Field Effect Transistor
FET switch - Field Effect Transistor Switch
Field-effect transistor switch - a solid state device that either allows or prohibits a signal from passing through it - commonly used in solid state multiplexers.
FFT - Fast Fourier Transform
Flash Convertor
The Flash convertor compares the input voltage (Vin) to a set of known (reference) voltages (comparator). A digital value (0 or 1) is assigned based on the results. If any noise is present on the signal at the time it is digitized, the digital word will reflect that error. Flash converters tend to expensive because of the cost of accurate voltage references.
Flow chart
Graphical representation of the major stages performed by a program.
Filter
Filters are electrical circuits that are used to reduce noise on signals. A low pass filter only allows low frequency signals to get to the measurement hardware - high frequency (noise) signals are attenuated. Filters have the disadvantage of slowing down the response the signal.
FINS - Factory Intelligent Network Service
Frequency
Frequency measurements are common measurements made on flow and RPM transducers. These transducers output a series of pulses proportional to the rate of movement. The counter records the number of pulses (counts) per unit of time (gate). Frequency is the count divide by the gate time.
FSR - Full Scale Range
GAL - Generic Array Logic
Gain - Magnitude ratio
For a linear system or element, the ratio of the magnitude (amplitude) of a steady-state sinusoidal output relative to the causal input; the length of a phasor form the origin to a point of the transfer locus in a complex plane.
GPIB - General Purpose Interface/ Instrument Bus
A standard for IEEE-488 communication interface.
Grounding
There are two definitions of ground : First is a signal ground is an equippotential circuit reference point for a circuit or a system; The other is a signal ground is a low impedance path for current to return to the source. The design objectives of a grounding system are : to minimize the noise voltage generated by currents from two or more circuits flowing through a common ground impedance; to avoid creating ground loops which are susceptible to magnetic fields and differences in ground potential.
Improper grounding can be a primary noise source. Grounds can be divided into two parts; safety and signal grounds. Safety grounds are provided so that a breakdown in impedance between the equipment chassis and the high voltage line of the power line will result in a low impedance path to ground.
Ground loops in very noisy environments can be broken by the use of isolation transformers or devices. The high frequency circuits require multiple grounding to reduce noise in the signal path.
GUI - Graphic User Interface
The GUI are the portion of the software that the user deals with directly. It can vary from a language editor to menu-driven displays to an iconic display.
HP-IB - Hewlett-Packard Interface Bus
HPIB - implementation of the IEEE-488 standard - mechanical and electrical protocol (parallel) for sending data and commands between instruments and computers.
LPF - Low Pass Filter
A low pass filter only allows low frequency signals to get to the measurement hardware - high frequency (noise) signals are attenuated. Filters have the disadvantage of slowing down the response the signal.
LSB - Least Significant Bit
HMI - Human Machine Interface
The HMI are the portion of the software that the user deals with directly. It can vary from a language editor to menu-driven displays to an iconic display.
I.C. Sensor
The IC sensor is a transducer for temperature measurement. Its advantages are most linear, highest output, and inexpensive, usually used for CJC circuit. Its disadvantages are power supply required, slow response, self-heating, and limited configurations.
Integrating A/D convertor
The integrating A/D convertor integrates the input signal over a period of time. Over that time period the noise on the input signal is integrated to zero, leaving the actual signal value. Most noise is related to power lines and exists at 60Hz in the U.S. and Canada and at 50 Hz in Europe and other parts of the world.
Interrupts data transfer
Interrupts to provide a means of tightly controlling the timing of events, while allowing the processing of more than one task. Multitasking systems are also known as “foreground/background” systems. One way of putting data acquisition in the background, is to relegate it to an interrupt routine. The clock or external timing signal, rather than being polled continuously, is used to generate an interrupt to the computer. Whenever the interrupt occurs, the computer suspends current activity, and executes an “interrupt service routine”. The interrupt service routine in this case might be a short program which acquires one frame of data, and stores it in memory. The computer can perform other operations in the foreground while collecting data in the background. Whenever a clock tick or external interrupt occurs, the computer will automatically stop the foreground processing, acquire the data, and then resume where it left off.
The reaction speed of the interrupt system is much higher than that of a polling loop. Speed, for a PC, is about 10K ~ 30K Hz in the interrupt mode.
Isolation
Isolation circuits are used to protect sensitive measurement circuitry from interference currents or voltages. These circuits are useful when the external voltage (from a transducer) is different than expected by the measurement hardware.
Isothermal Block
Terminal block specially designed to be at a constant temperature from one end to the other - used with thermocouples.
ISR - Interrupt Service Routine
Memory models for the assembler - Compact type
All code fits into a single 64K byte code segment, and data fits into more than one data segment.
Memory models for the assembler - Flat type
Not available in MASM version 5.10. The flat memory model uses one segment with a maximum length of 512M bytes to store data and code.
Memory models for the assembler - Huge type
Same as large, but allows data segments that are larger than 64K bytes.
Memory models for the assembler - Large type
Both code and data fit into multiple code and data segments.
Memory models for the assembler - Medium type
All code fits into a single 64K byte code segment, and all code fits into more than one 64K byte code segment. This allows code to exist in multiple segments.
Memory models for the assembler - Small type
All code fits into a single 64K byte code segment and all code fits into another single 64K byte code segment. This allows all code to be accessed with near jumps and calls.
Memory models for the assembler - Tiny type
All data and code fit into one segment, the code segment. Tiny model programs are written in the .COM file format, which means that the program must be originated at memory location 0100H. This model is most often used with small programs.
MSB - Most Significant Bit
Multidrop communication
A multidrop communication line connects multiple devices together on a common transmission line for communication with each other.
Multiplexer
Multiplexers are electronic devices that are used to switch multiple channels to a single measurement instrument, one at a time. They operate much the same way as television set; although several channels are available, only one channel is viewed at a time.
Network
A communication link between PLCs, workstations, industrial equipment, and other devices.
Normal Mode Noise
Voltage differentially between two wire that appears to be part of the input signal. The noise is the most difficult to eliminate because it appears to be part of the signal. Normal mode noise can occur as a result of common mode, electrostatic, or electromagnetic noise finding its way to the signal. In most cases, the noise is related to AC power lines. If so, it appears as a 50 or 60 Hz sine wave added to the DC voltage that we want to measure. A filter can be used to reduce 50 or 60 Hz but the slow response of the filter will slow down the measurements speed. Filters also inject some errors of their own.
TIP : The best way to eliminate power line noise is with an integrating voltmeter. The noise signal is integrated out with only minimal loss of speed.
NTC - Negative Temperature Coefficient
ON/OFF control
ON/OFF control is the simplest of control. A switch card is used as the control hardware to turn something on or off.
Open-loop Control
Open-loop control is generally used in the characterization of a product or process. The product, for instance, is run through a series of tests and the results are collected.
Optical isolators
Optical isolators are used with digital circuitry to shield high voltage signals from affecting digital circuitry.
PAL - Programmable Array Logic
PCLD-RMK-3U
19 inch Rackmount Adapter for PCLD-series Daughterboards(3 units wide).
PID - Proportional, Integral, Derivative
A PID controller or PID loop, constantly reads and manipulates an output condition so that a process variable is maintained at a set-point.
The PID algorithm combines P-value proportional to the error value, I-value a time integral (accumulation of past errors), and D-value a time derivative (rate of change) of the error value. The P, I, and D factors may vary depending on the process being controlled.
PID control has the advantage of being able to respond to rapidly changing errors before they get too large and yet send out control values that will not cause the process to overreact.
PLC - Programmable Logic Controller
A standard hardware device to control plant startup, running and shutdown sequencing, which may be readily altered to suit changed operating conditions.
PLC are electronic replacements for traditional industrial hard-wired relay panels. Available from a wide variety of manufacturers for all purposes, PLCs offer more control and are much easier to install and use than relay panels.
PO - Pulse Output
The pulse output card is a combination of a digital output card and an arbitrary waveform generator. Like the waveform generator it contains a clock. It outputs a series of pulses at varying rates. The most common use of a pulse output card is to control stepper motors.
Polling
Polling is the simplest method for detecting a unique condition and then taking action. This involves a software loop that contains all of the required measurement, analysis, decision-making algorithms and planned actions. The data acquisition program periodically tests the system‘s clock or external trigger input to sense a transition. Whenever a transition occurs, the program then samples each of the inputs and stores their values in a “frame”. A frame is simply a list that contains the values representing the specified inputs at a given time. The frames can be stored in RAM, disk or other types of memory. Each time the program senses a clock “tick”, the inputs are scanned and converted, and a new frame is added to memory. In this mode, generic PC/AT’s can support an acquisition rate of about 10Khz.
In addition, the PC is continuously busy when the polling loop is operational, and hence no other tasks can be serviced. When an application cannot tolerate these characteristics, interrupt techniques may be needed.
Power Line Noise
Electrical interference caused by radiation from power cables or machinery in the vicinity of the signals to be measured.
PRTD - Platinum Resistance Temperature Detector
A temperature transducer RTD made by platinum metal.
Pseudo Diff. Input - Pseudo Differential Input
The pseudo-differential configuration brings the common lines of all channels back to the systems front end on two lines : AGND and IN LO. Choose pseudo-differential inputs over single-ended inputs whenever your setup can accommodate more than one ground wire.
If your sensor is not grounded, you should install a resistor between IN LO and AGND. You should also install a resistor between each channels input line and AGND. The resistors will protect your system from common mode voltages. Any induced currents in the sensor line will travel down the low impedance AGND line to ground and away from the high impedance IN LO line, ensuring the integrity of your measurements.
PTC - Positive Temperature Coefficient
Pulse Width
Pulse Width measurements are used to determine the length of time a pulse stays at a high (or low) voltage level. One practical application is to use a sensor and measure the amount of time it takes an objects to pass this sensor. This measurement can be used to determine the speed of a known object or if speed is known, the length of an object where size is not known.
RDAC - Remote Data Acquisition and Control
Rectifiers
Rectifiers are used to convert an AC signal into a DC signal. Some application call for an AC measurement. If your A/D converter does not measure AC signals, a rectifier would be needed to convert the signals to DC voltage levels.
Relay Configurations
Simple relay configurations may be used in applications from the power to the device under test (DUT) to forming a complex matrix topology for measurements. Some of the most common configurations consist of from A, form B, and form C configurations which can be linked together to form binary switching networks which guarantee that only one point can be connected to any other point at a time. Similarly, these can be configured to form measurement buses for the connection of many points to one point at a time. For the most part, this configuration is used for simple on-off switching of power rather than signals.
Resolution
Resolution is the smallest change that a measurement instrument can sense. Resolution is normally specified in bits.
RS-232
RS-232 a low-cost and reliable standard for low speed, point to point communications at distances up to 50 feet (15 meters). The biggest drawback to RS-232 communications is that plant floor environments often generate electrical noise and transients which can cause errors in transmission and damage to the interface components.
RS-422
RS-422 communications, with its balanced transmission and receiving modes, improve the reliability of transmitted data to make RS-422 preferred over RS-232 communications. Additionally, RS-422 communications allow you to specify a master workstation for multidrop connections to create a small network. The RS-422 standard permits transmissions up to 4000 feet (125 meters).
RS-485
The RS-485 standard includes all of the improved communications features of RS-422, but also allows you to operate using multiple master workstations with only a smaller and less expensive communication cable. half duplex, differential signal, two wire connection
RTD - Resistive Temperature Detector
A transducer that changes resistance proportional to its temperature. Its advantages are most stable, most accurate, and more linear than thermocouple. Its disadvantages are expensive, slow response time, current source required, small resistance change, 4 wire measurement, and self-heating.
RTD requires a four-wire ohms measurement in order to get a reliable reading, especially if the RTD is located several feet from the DAC system.
RTL - Resistor Transistor Logic
RTS - Request To Send
Handshaking signals, are normally used to control the direction of the data flow for RS-485.
SCADA - Supervisory Control and Data Acquisition
SCADA is the name applied to the control of pipe lines, production wells, etc., through telemetry to a central computer.
Seebeck
When two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, there is a continuous current which flows in the thermoelectric circuit. Thomas Seebeck made this discovery in 1821. If this circuit is broken at the center, the net open circuit voltage (the Seebeck voltage) is a function of the junction temperature and the composition of the two metals. All dissimilar metals exhibit this effect.
S.E. Input - Single Ended Input
A single-ended configuration is best when you need to make analog measurements with respect to a common external ground. This configuration is also appropriate when there is no practical way to bring both a signal ground and an analog ground (AGND) back to the systems input terminals.
Sensor
Device that converts a physical parameter such as temperature, pressure, flow, strain, or position into an electrical signal - sometimes used synonymously with transducer - a true sensor contains signal conditioning so its output is more easily measured.
S/H - Sample and Hold
Measurement hardware that is used to capture events on multiple channels - when triggered, hardware will hold value of input until a measurement can be made.
Shielding
An extra layer of conductive material surrounding a wire to prevent external electrical signals from interfering with the signal on the wire.
Signal Conditioning
To electrically amplify, reduce, eliminate, or change a signal - used to alter signals (eliminate noise) before they are measured in order to get more accurate measurements.
SPC - Statistical Process Control
SPC is a tool used to improve productivity and reduce costs.
SPEC - System Performance Evaluation Consortium
SPEC benchmarks are to workstations what FFTs are to DSPs - the most popular metric of performance comparison.
SPDT relay - Single-Pole Dual-Throw relay
SPST relay - Single-Pole Single-Throw relay
SQC - Statistical Quality Control
SQC is a tool used to improve productivity and reduce costs.
SQL - Structured Query Language
SSR - Solid State Relay
Stepper Motor
Electro-mechanical device that rotates(or moves) proportional to pulses it receives from a stepper motor controller - DAC system sometimes contain stepper motor controllers - used for positioning equipment.
Strain
Change in length of a material divided by its total length - strain measurements are used in order to calculate the stress on a material.
Successive Approximation A/D
The successive approximation A/D is less expensive than a flash converter. It uses one comparator and generates reference voltages, comparing each one to the input signal. If the input signal is varying a voltage error can occur.
Switching
A switch is a mechanical or electronic device that is used to maker or break the connections in an electrical circuit. Switches can be used to route the output signals from multiple transducers to one measurement instrument (scanning). Switches are also used as control devices to switch power to external fans, heaters, pumps, lights, and motors.
T/C - Thermocouple
A pair of dissimilar conductors that are connected together at a point and produce a voltage proportional to temperature. Its advantages are self-powered, simple, rugged, inexpensive, wide variety of physical forms, and wide temperature range. Its disadvantages are non-linear, low voltage, reference required, least stable, and least sensitive.
When two wires composed of dissimilar metals are jointed as both ends and one of the ends is heated, there is a continuous current which flows in the thermoelectric circuit. Thomas Seebeck made this discovery in 1821.
TIP : Do not attach thermocouple wire directly to your voltmeter. Use an isothermal block.
T/C B type - Platinum -6% Rhodium (+) Versus Platinum -30% Rhodium (-)
Seebeck coefficient = 6 mV/°C, and DVM sensitivity for 0.1°C = 0.6 mV, Range = 0°C to 1820°C, 4.4 ~ 8.6°C standard wire error.
T/C E type - Nickel -10% Chromium (+) Versus Constantan (-)
Seebeck coefficient = 62 mV/°C, and DVM sensitivity for 0.1°C = 6.2 mV, Range = -100°C to 1000°C, 1.7 ~ 4.4°C standard wire error.
T/C J type - Iron (+) Versus Constantan (-)
Seebeck coefficient = 51 mV/°C, and DVM sensitivity for 0.1°C = 5.1 mV, Range = 0°C to 760°C, 1.1 ~ 2.9°C standard wire error.
T/C K type - Nickel -10% Chromium (+) Versus Nickel - 5% (-) (Aluminum Silicon)
Seebeck coefficient = 40 mV/°C, and DVM sensitivity for 0.1°C = 4.0 mV, Range = 0°C to 1370°C, 1.1 ~ 2.9°C standard wire error.
T/C N (AWG 14) type - Nicrosil (+) Versus Nisil (-)
Seebeck coefficient = 39 mV/°C, and DVM sensitivity for 0.1°C = 3.9 mV, Range = 0°C to 1300°C
T/C N (AWG 28) type - Nicrosil (+) Versus Nisil (-)
Seebeck coefficient = 26.2 mV/°C, and DVM sensitivity for 0.1°C = 2.62 mV, Range = -270°C to 400°C
T/C R type - Platinum -13% Rhodium (+) Versus Platinum (-)
Seebeck coefficient = 7 mV/°C, and DVM sensitivity for 0.1°C = 0.7 mV, Range = 0°C to 1000°C, 1.4 ~ 3.8°C standard wire error.
T/C S type - Platinum -10% Rhodium (+) Versus Platinum (-)
Seebeck coefficient = 7 mV/°C, and DVM sensitivity for 0.1°C = 0.7 mV, Range = 0°C to 1750°C, 1.4 ~ 3.8°C standard wire error.
T/C T type - Copper (+) Versus Constantan (-)
Seebeck coefficient = 40 mV/°C, and DVM sensitivity for 0.1°C = 4.0 mV, Range = -160°C to 400°C, 0.8 ~ 2.9°C standard wire error.
T/C W-Re type - Tungsten - 5% Rhenium (+) Versus Tungsten - 26% Rhenium (-)
Seebeck coefficient = 19.5 mV/°C, and DVM sensitivity for 0.1°C = 1.95 mV, Range = 0°C to 2320°C
Thermal noise
When two dissimilar metals are joined together and the temperature between the two ends are different, it is very easily affected by the nearby device with extreme change in high or low temperature. This temperature changes is called thermal noise.
Transducers like RTDs, thermistors, and strain gages are devices whose output resistance changes. All require power to make them work. When power is applied to a resistive device, it heats up. The rise in temperature causes a change in resistance and an error in the measurement.
TIP : To eliminate self-heating effects, do not apply power to the device any longer than necessary.
TIP : For best results, make two measurements - one without power applied to the transducer and one with power applied. Simple subtraction will eliminate the thermal noise (self-heating).
Thermal Block
A piece of hardware to which wires coming from transducers connect to the data acquisition system.
Thermistor
Temperature transducer that changes resistance proportional to its temperature. Its advantages are high output, fast response time, and two-wire ohms measurement. Its disadvantages are non-linear, limited temperature range, fragile. current source required, and self-heating.
Time Interval
Time interval measurements can be made by the more sophisticated counters. These measurements are the time between one edge of a pulse and another. If the edges are both the same, the time interval is the period. If one edge is rising and the other is falling, the time interval is equal to the pulse width.
Transducer
Devices that convert physical parameters such as temperature, pressure, flow, strain, or position into electrical signals such as voltage, current, or resistance.
Transducer can be used to measure temperature, pressure, flow, level, position, pH, strain, and many other physical parameters. For example, a temperature transducer may output a specific value at 50 degree C. As the temperature changes, the output voltage changes.
Transformers
Transformers are normally used to step-down (reduce) large voltage and eliminate ground loops (noise) in analog measurements.
True Diff. Input - True Differential Input
True differential operation provides a separate return line for each input channel. Using this configuration cuts the channel count in half for a given data acquisition board or system, but because it separates the return currents of each input, it reduces the errors caused by noise.
Although true-differential inputs offer the best immunity from ground noise, voltages common to both a channels HI and LO inputs can provide measurement errors.
TSR - Terminate and Stay Resident
TSR stay resident program is a program that remains in memory until it is activated.
A program (TSR) remains in the memory system even after the program returns to DOS. Access to a TSR is via the interrupt structure of the microprocessor.
TTL - Transistor Transistor Logic
هیچ نظری موجود نیست:
ارسال یک نظر