Carrier: An electrical
transmission that does not represent data but is used as a messenger on
which the data is transported.
Calorie: The
quantity of thermal energy required to raise one gram of water 1°C at
150°C.
Celsius (Centigrade): A temperature scale defined by 0°C at
the ice point and 100°C at the boiling point of water.
Ceramic Insulation:
High-temperature compositions of metal oxides used to insulate a pair
of thermocouple wires. The most common are Alumina (Al O) Beryllia (BeO)
and Magnesia (MgO) Their application depends upon temperature and type of
thermocouple. High-purity Alumina is required for platinum alloy
thermocouples. Ceramic insulators are available as single and multihole
tubes or as beads.
CHROMELA chromium-nickel alloy which makes up the positive
leg of Type K and Type E thermocouples.
Clear-To-Send
(CTS): A signal that tells the transmitting device to start
transmitting data.
Closeness of Control: Total temperature variation from a
desired set point of system expressed as “closeness of control” is ±2°C or
a system bandwidth with 4°C, also referred to as amplitude of
deviation.
CMR (Common-Mode
Rejection Ratio): The ability of a panel meter to eliminate the effect of AC or DC
noise between signal and ground. Normally expressed in dB at dc to 60 Hz.
one type of CMR is specified between SIG LO and PWR GND. In differential
meters a second type of CMR is specified between SIG LO and ANA GND (METER
GND).
CMV (Common-Mode Voltage): The AC or DC voltage which is
tolerable between signal and ground. One type of CMV is specified between
SIG LO and PWR GND. In differential meters a second type of CMV is
specified between SIG HI or LO and ANA GND (METER
GND).
Color Code: The ANSI established color code for
thermocouple wires in the negative lead is always red. Color code for base
metal thermocouples is yellow for Type K, black for Type J, purple for
Type E, and blue for Type T.
Compensating Alloys:
Alloys used to connect thermocouples to instrumentation. These alloys
are selected to have similar thermal electric properties as the
thermocouple alloys (however, only over a very limited temperature
range).
Compensated Connector: A connector made of thermocouple
alloys used to connect thermocouple probes and wires.
Conductance: The
measure of the ability of a solution to carry an electrical
current.
Conformity Error: For thermocouples and RTDs, the
difference between the actual reading and the temperature shown in
published tables for a specific voltage input.
Counts: The number of
time intervals counted by the dual-slope AD converter and displayed as the
reading of the panel meter, before addition of the decimal
point.
Constantan: A copper-nickel alloy used as the negative lead
in Type E, Type J. and Type T thermocouples.
Control Mode:
The output form or type of control action used by a temperature controller
to control temperature, i.e., on/off, time proportioning,
PID.
Control Point: The temperature at which a system is to be
maintained.
Current Loop:
A two wire communications link in which the presence of a 20 milliamp
current level indicates a binary "1" (mark), and its absence indicates no
data, a binary "0" (space).
Current
Proportioning: An output form of a temperature controller which
provides a current proportional to the amount of control required. It is
normally a 4 to 20 milliamp current proportioning band.
Current Sink:
A device that receives current.
Current Source:
A device that supplies current.
Current
Transformer (CT): A transformer designed for AC measurement circuits.
It provides isolation and a step-down current output.
Cyclic Redundancy
Check (CRC): An error detection scheme in which all the bits in a
block of data are divided by a predetermined binary number. A check
character is determined by the remainder.
Cycle Time: The time
usually expressed in seconds for a controller to complete one on/off
cycle.
Data: A
general term for any type of information.
Data Carrier
Detect (DCD): A signal that indicates the carrier is being
received.
Data Communication
Equipment (DCE): Equipment that provides the functions required to
establish, maintain, or terminate a connection.
Data Set Ready
(DSR): A signal that indicates the modem is connected, powered
up, and ready for data transmission.
Data Terminal
Equipment (DTE): Equipment which is attached to a network to send
and/or receive data.
Debouncing:
The act of removing intermediate noise states from a discrete
switch.
dB (Decibel):
20 times the
log to the base 10 of the ratio of two voltages. Every 20 dB’s correspond
to a voltage ratio of 10:1, every 10 dB’s to a voltage ratio of 3.162:1.
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.
Decimal Number
System: A number system that uses ten numeral digits (decimal digits),
0,1,2,3,4,5,6,7,8,9. Each digit position has a place value of
1,10,100,1000, and so on, beginning with the least significant (right
most) digit. Also known as Base 10.
Degree: An incremental
value in the temperature scale, i.e. there are 100 degrees between the ice
and the boiling point of water in the Celsius scale and 180°F between the
same two points in the Fahrenheit scale.
Delimiter: A
character that, when placed before and/or after a string of data, causes
the data to be interpreted in a predetermined manner.
Deviation: The
difference between the value of the controlled variable and the value at
which it is being controlled.
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.
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.
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.
DIN (Deutsche Industries 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 = 0.00385 ohms per ohms per
degree C.
Double Precision: A method of increasing the range
of numbers by using two words (four bytes) to represent single
numbers. Signed Double Precision ranges from -2,147,483,648 to
+2,147,483,647.
Drift: A change of a reading or a
setpoint 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 setpoint 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.
Duty Cycle: The total
time to complete one on/off cycle usually refers to the on/off cycle time
of a temperature controller.
EEPROM: Electrically
Erasable Programmable Read Only Memory.
EPROM: Erasable Programmable Read Only Memory.
Exclusive OR (XOR): A logical operation that has only two
inputs, and yields a logic "1" output if either of the two inputs is "1,"
and a logic "0" output if both inputs are "1" or "0."
Exposed Junction: A form of construction of a
thermocouple probe where the hot or measuring junction protrudes beyond
the sheath material so as to be fully exposed to the medium being
measured. This form of construction usually gives the fastest response
time.
Fahrenheit: A temperature scale defined
by 32° at the ice point and 212° at the boiling point of water at sea
level.
Filter (ABC): Adaptive Bandwidth Filter–Automatically
adjusts filter time constant to adapt to noise and noise spikes.
Automatically reduces filter time constant in successive 2:1 steps to
respond to step changes in the input signal.
Frequency Shift Keying (FSK): A signal modulation
technique, in which a carrier frequency is shifted high or low to
represent a binary one or zero, respectively. Offers a high degree
of noise immunity.
Full
Duplex: A communication link in which data transmission can take
placed in both directions simultaneously.
Gain: The amount of amplification used in an electrical
circuit.
Ground: 1. The electrical neutral line having the
same potential as the surrounding earth. 2. The negative side of dc power
supply. 3. Reference point for an electrical system.
Grounded
Junction: A form of construction of a thermocouple probe where the hot
or measuring junction is in electrical contact with the sheath material so
that the sheath and thermocouple will have the same electrical
potential.
Half Duplex: A communication link in
which data transmission is limited to one direction at a time.
Hertz
(Hz): Units
in which frequency is expressed. Synonymous with cycles per second
(cps).
Hexadecimal Number System: A number
system that uses the numerals 0,1,2,3,4,5,6,7,8,9, and the letters
A,B,C,D,E,F to represent numbers and codes. Base 16.
Hysteresis: (see Deadband)
Ice Point: The temperature at which pure
water freezes, 0°C, 32°F.
Input Impedance: The resistance of
a panel meter as seen from the source in the case of a voltmeter. This
resistance has to be taken into account when the source impedance is high;
in the case of an ammeter when the source impedance is
low.
Insulated Junction: See ungrounded
junction.
Insulation Resistance: The resistance value of the
cover material of an electrical conductor, expressed in
ohms.
Integral: A form of temperature control. See automatic
reset, #2.
Isothermal: A process or area that is at a
constant temperature.
Junction: The point in a thermocouple
where the two dissimilar metals are joined.
K: 1K=1024. Used to denote size of
memory and can be expressed in bits, bytes, or words.
Kelvin: Symbol K. The unit of absolute or thermodynamic temperature scale
based upon the Celsius scale with 100 units between the ice point and
boiling point of water 0°C = 273.15K (there is no degree (°) symbol used
with the Kelvin scale).
Lag: 1. A time delay between the
output of a signal and the response of the instrument to which the signal
is sent. 2. A time relationship between two waveforms where a fixed
reference point of one wave occurs after the same point of the reference
wave.
Limits of Error: A tolerance band for the thermal
electric response of thermocouple wire expressed in degrees or percentage
defined by ANSI specification MC96.1 (1975).
Load: The
electrical demand of a process expressed as power (watts), current (amps)
or resistance (ohms).
Loop Resistance: The total resistance
of a thermocouple circuit caused by the resistance of the thermocouple
wire. Usually used in reference to analog pyrometers which have typical
loop resistance requirements of 10 ohms.
LSD (Least-Significant
Digit): The rightmost active (non dummy) digit of the
display.
MSD (Most-Significant Digit): The left-most digit of the
display.
Manual Reset (Adjustment): The adjustment of a
proportioning controller which shifts the proportioning band in
relationship to the setpoint to eliminate droop or offset
errors.
Manual Reset (Switch): The switch in a limit
controller that manually resets the controller after the limit has been
exceeded.
Mask: A logical function used to always set
certain bits in a word to an established state.
Maximum Operating Temperature: The maximum temperature at
which an instrument or sensor can be safely operated.
Measuring
Junction: The thermocouple junction referred to as the hot junction
that is used to measure an unknown temperature.
Microvolt:
One millionth of a volt, 10-6 volts.
Mil: One
thousandth of an inch (0.001),10-3.
Milliamp: One
thousandth of an amp, 10-3 amps, symbol mA.
Microsecond: One millionth of a second. 1 x
10-6 second or 0.000001 seconds.
Millimeter: One
thousandth of a meter,10-3 meters, symbol mm.
Millisecond: One thousandth of a
second. 1 x 10-3 second or 0.0001 seconds.
Millivolt: One thousandth of a volt, 10-3
volts symbol mV.
Mineral-insulated Thermocouple: A type of
thermocouple cable which has an outer metal sheath and mineral (magnesium
oxide) insulation inside separating a pair of thermocouple wires from
themselves and from the outer sheath. This cable is usually drawn down to
compact the mineral insulation and is available in diameters from .375 to
.010 inches. It is ideally suited for high-temperature and severe-duty
applications.
Multiplex: A technique which allows different
input (or output) signals to use the same lines at different times,
controlled by an external signal. Multiplexing is used to save on wiring
and I/O ports.
N/C (No Connection): A connector point for which
there is no internal connection.
NAND: A logical operation
that yields a logic "1" output if any input is "0," and a logic "0" if all
inputs are "1." The negated AND function. Result of negating
the output of an AND gate by following it with a NOT symbol.
Negative Logic: The use of binary logic in such a way that
"0" represents the voltage level normally associated with logic 1 (e.g. 0
= +5V, 1 = 0V). Positive Logic is more conventional
(1 =
+5V, 0 = 0V).
NEMA
1/ UL Type 1: Intended for indoor use, to provide a degree of
protection against contact with the enclosed equipment and against limited
amount of falling dirt.
NEMA 4/ UL Type 4: A standard from
the National Electrical Manufacturers Association which defines enclosures
intended for indoor or outdoor use primarily to provide a degree of
protection against windblown dust and rain, splashing water, and
hose-directed water.
Testing - Hosedown: Enclosure is
subjected to a stream of water from a hose which has a 1 inch diameter
nozzle and which delivers at least 65 gallons per minute from a distance
of 10 to 12 feet for a minimum of 5 minutes.
NEMA 7/ UL Type 7:
A standard from the National Electrical Manufacturers Association,
which defines explosion-proof enclosures for use in locations classified
as Class 1, Groups A, B. C or D, as specified in the National Electrical
Code.
NEMA 12/ UL Type 12: A standard from the National
Electrical Manufacturers Association, which defines enclosures with
protection against falling dirt, circulating dust, and dripping
non-corrosive liquids.
Testing - Drip: Enclosure is subjected to a drip
pan which produces both splashing and dripping.
Testing - Atomized Water: Enclosure is subjected
to a spray of atomized water by using a nozzle that produces a 3" round
diameter spray, 12" from closure. The air pressure is at
30psi.
NMR (Normal-Mode Rejection): The ability of a panel
meter to filter out noise superimposed on the signal and applied across
the SIG HI to SIG LO input terminals. Normally expressed in dB at 50/60
Hz.
Nicrosil/nisil: A nickel chrome/nickel silicone thermal
alloy used to measure high temperatures. Inconsistencies in thermoelectric
voltages exist in these alloys with respect to the wire
gage.
Noise: An unwanted electrical interference on the
signal wires.
NOR: A logical operation that yields a logic "1" output if
all inputs are "0" and a logic "0" output if any input is "1." The
negated OR function. Result of negating the output of an OR gate, by
following it with a NOT symbol.
NOT: A logical operation that yields a logical "1" output
if a logic "0" is entered at the input, and a logic "0" output if a logic
"1" is entered at the input. The NOT, also called the inverter, is
normally used in conjunction with the AND and OR functions.
NPT: National Pipe Thread.
Octal Number System: A number system that uses eight
numeral digits 0,1,2,3,4,5,6,7. Base 8.
Offset: The difference in engineering units between the setpoint and the
actual process temperature. Also, referred to as
droop.
Ohmmeter: An instrument used to measure electrical
resistance.
On/Off Controller: A controller whose action is
fully on or fully off.
OR:
A logical operation that yields a logic "1" output if one of any
number of inputs is "1", and a logic "0" if all inputs are
"0."
Overshoot: The number of degrees that a process exceeds
the setpoint temperature when coming up to the setpoint
temperature.
PH(S) (Standard pH Scale): The conventional standard pH
scale established on the basis that an individual ionic activity
coefficient can be calculated from the Debye-Hückel law for primary
buffers.
PPM: Abbreviation for “parts per million”,
sometimes used to express temperature coefficients. For instance, 100 ppm
is identical to 0.01%.
Platinel: A non-standard, high
temperature platinum thermocouple alloy whose thermoelectric voltage
nearly matches a Type K thermocouple (trademark of Englehard
Industries).
Platinum: A noble metal which in its pure form
is the negative wire of Type R and Type S
thermocouples.
Platinum 6% Rhodium: The platinum-rhodium
alloy used as the negative wire in conjunction with platinum 30% rhodium
to form a Type B thermocouple.
Platinum 10% Rhodium: The
platinum-rhodium alloy used as the positive wire in conjunction with pure
platinum to form a Type S thermocouple.
Platinum 13% Rhodium:
The platinum-rhodium alloy used as the positive wire in conjunction
with pure platinum to form a Type R thermocouple.
Platinum 30%
Rhodium: The platinum-rhodium alloy used as the positive wire in
conjunction with platinum 6% rhodium to form a Type B
thermocouple.
Platinum 67: To develop thermal emf tables for
thermocouples the National Bureau of Standards paired each thermocouple
alloy against a pure platinum wire (designated Platinum 27 prior to 1973,
and currently Platinum 67). The thermal emf’s of any alloy combination can
be determined by summing the “vs. Pt-67” emf’s of the alloys, i e. the emf
table for a Type K thermocouple is derived from the Chromel vs. Pt-67 and
the Alumel vs Pt-67 values.
Primary Standard (NBS): The
standard reference units and physical constants maintained by the National
Bureau of Standards upon which all measurement units in the United States
are based.
Probe: A generic term that is used to describe
many types of temperature sensors.
Process Meter: A panel
meter with sizeable zero and span adjustment capabilities, which can be
scaled for readout in engineering units for signals such as 420 mA, 1050
mA and 1-5 V.
Process Variable (PV): The parameter that is
controlled or measured. Typical examples are temperature, relative
humidity, pressure flow, level, etc. The high process variable is the
highest value of the process range, expressed in engineering units. The
low process variable is the lowest value of the process
range.
Proportioning Band: A temperature band expressed in degrees
within which a temperature controller’s time proportioning function is
active.
Proportioning Control: A time-proportioning
controller where the amount of time that the relay is energized is
dependent upon the system’s temperature.
Proportioning Control
Plus Derivative Function: A time proportioning controller with a
derivative function The derivative function senses the rate at which a
system’s temperature is either increasing or decreasing and adjusts the
cycle time of the controller to minimize overshoot or
undershoot.
Proportioning Control Plus Integral: A two-mode
controller with time proportioning and integral (auto reset) action. The
integral function automatically adjusts the temperature at which a system
has stabilized back to the setpoint temperature, thereby eliminating droop
in the system.
Proportioning Control with Integral and
Derivative Functions (PID): Three-mode PID controller. A
time-proportioning controller with integral and derivative functions.
Proportional control causes an output to change as a direct ratio of the
error signal variation. Integral control causes an output to change
as a function of the error signal and time duration. The derivative
function senses the rate of change of the system temperature and
automatically adjusts the cycle time of the controller to minimize
overshoot or undershoot.
Protection Head: An enclosure
usually made out of metal at the end of a probe where connections to the
probe are made.
Protection Tube: A metal or ceramic tube
closed at one end into which a temperature sensor is inserted. The tube
protects the sensor from the medium into which it is
inserted.
Range: An area between two limits
within which a quantity is measured. It is usually expressed by stating
the lower limit first and then the upper limit.
Rankine
(°R): An absolute temperature scale based upon the Fahrenheit scale
with 180° between the ice point and boiling point of water. 459.67°R =
0°F.
Rate Action: The derivative function of a temperature
controller.
Rate Time: The time interval over which the
system temperature is sampled for the derivative
function.
Ratiometric Measurement: A measurement technique
where an external signal is used to provide the voltage reference for the
dual-slope A/D converter. The external signal can be derived from the
voltage excitation applied to a bridge circuit or pick-off supply, thereby
eliminating errors due to power supply fluctuations.
Reference Junction: The cold junction in a thermocouple
circuit which is held at a stable known temperature. The standard
reference temperature is 0°C (32°F); however, other temperatures can be
used.
Relay (Mechanical): An electromechanical device that
completes or interrupts a circuit by physically moving electrical contacts
into contact with each other.
Form “A”–A single-pole,
single-throw (SPST) relay with normally open (NO) and
common
contacts. These contacts close when the relay coil is energized.
They open when power is removed from the coil.
Form “B”–A
single-pole, single throw (SPST) relay with normally closed (NC) and
common contacts. These contacts open when the relay coil is energized.
They close when power is removed from the coil.
Form “C”–A
single-pole, double-throw relay with normally open (NO), normally closed
(NC) and common contacts.
Relay (Solid State): A solid state
switching device which completes or interrupts a circuit electrically with
no moving pans.
Repeatability: The ability of a probe or
instrument to give the same output or reading under repeated identical
conditions.
Request-To-Send (RTS): A request that is sent to
tell another device to prepare to transmit.
Resistance: The resistance to the flow of electric current
measured in ohms. For a conductor resistance is a function of diameter,
resistivity (an intrinsic property of the material) and
length.
Resistance Ratio Characteristic: For thermistors,
the ratio of the resistance of the thermistor at 25°C to the resistance at
125°C.
Resistance Temperature Characteristic: A relationship
between a thermistor’s resistance and the
temperature.
Resolution: The input corresponding to a
one-unit change in the least significant digit. (Good resolution is not
necessarily equal to good accuracy.)
Response Time (Time
Constant): The time required by a sensor to reach 63.2% of a step
change in temperature under a specified set of conditions. Five time
constants are required for the sensor to stabilize at 100% of the step
change value.
RTD: Resistance Temperature
Detector.
SCR: Silicon Controlled
Rectifier.
Seebeck Coefficient: The derivative (rate of
change) of thermal emf with respect to temperature normally expressed as
millivolts per degree.
Seebeck Effect: When a circuit is
formed by a junction of two dissimilar metals and the junctions are held
at different temperatures, a current will flow in the circuit caused by
the difference in temperature between the two junctions.
Seebeck
EMF: The open circuit voltage caused by the difference in temperature
between the hot and cold junctions of a circuit made from two dissimilar
metals.
Secondary Standard: A standard of unit measurement
derived from a primary standard.
Sensitivity: The minimum
change in input signal to which an instrument can
respond.
Setpoint: The temperature at which a controller is
set to control a system.
Settling Time: The time taken for
the display to settle within one digit of the final value when a step is
applied to the meter input.
Sheath Thermocouple: A
thermocouple made out of mineral-insulated thermocouple cable which has an
outer metal sheath.
Signal Conditioner: A circuit module
which offsets attenuates, amplifies linearizes and/or filters the signal
for input to the A/D converter. The typical output signal conditioner is
+2 Vdc.
Single-ended Input: A signal-input circuit where SIG
LO (or sometimes SIG HI is tied to METER GND. Ground loops are normally
not a problem in AC-powered meters, since METER GND is
transformer-isolated from AC GND.
Snubber: A circuit generally used to suppress inductive
loads, it consists of a resistor in series with a capacitor (RC snubber)
and/or a MOV placed across the AC Load.
Span: The difference
between the upper and lower limits of a range expressed in the same units
as the range.
Span Adjustment: The ability to adjust the
gain of a process or strain meter so that a specified display span in
engineering units corresponds to a specified signal span. For instance, a
display span of 200°F may correspond to the 16 mA span of a 420 mA
transmitter signal.
Specific Heat: The ratio of thermal
energy required to raise the temperature of a body f ° to the thermal
energy required to raise an equal mass of water 1°.
SSR:
Solid State Relay: (see Relay, Solid State).
Stability: The
quality of an instrument or sensor to maintain a consistent output when a
constant input is applied.
Strain Gauge: A mechanical
transducer which is used to measure body deformation, or strain, due to
the force applied to the area of a rigid body.
Super
Cooling: The cooling of a liquid below its freezing temperature
without the formation of the solid phase.
Super Heating: 1.
The heating of a liquid above its boiling temperature without the
formation of the gaseous phase. 2. The heating of the gaseous phase
considerably above the boiling-point temperature to improve the
thermodynamic efficiency of a system.
TEMPCO: Abbreviation for “temperature coefficient”: the
error introduced by a change in temperature. Normally expressed in %/°C or
ppm/°C.
Thermal Conductivity: The property of a material to
conduct heat in the form of thermal energy.
Thermal EMF: See
Seebeck EMF.
Thermistor: A temperature-sensing element
composed of sintered semiconductor material which exhibits a large change
in resistance proportional to a small change in temperature. Thermistors
usually have negative temperature
coefficients.
Thermocouple: The junction of two dissimilar
metals which has a voltage output proportional to the difference in
temperature between the hot junction and the lead wires (cold junction)
(refer to Seebeck EMF).Standard
thermocouple types are:
|
Thermocouple Type (ANSI Symbol)
|
Material |
|
J |
Iron/Constantan |
|
K
|
Chromel/Alumel |
|
T
|
Copper/Constantan |
|
E
|
Chromel/Constantan |
|
R
|
Platinum/Platinum 13% Rhodium
|
|
S
|
Platinum/Platinum 10% Rhodium
|
|
B |
Platinum 6% Rhodium/Platinum 30%
|
|
Rhodium
|
|
|
G*
|
Tungsten Tungsten 26/ Rhenium
|
|
C*
|
Tungsten 5°/O Rhenium Tungsten 26%
|
|
Rhenium
|
|
|
D*
|
Tungsten 3% Rhenium Tungsten 25%
|
|
Rhenium |
|
|
N |
Nicrosil/Nisil |
| * Not ANSI
symbols |
Thermowell: A closed-end tube designed to protect
temperature sensors from harsh environments high pressure and flows. They
can be installed into a system by pipe thread or welded flange and are
usually made of corrosion-resistant metal or ceramic material depending
upon the application.
Transmitter (Two-wire): A device which
is used to transmit temperature data from either a thermocouple or RTD via
a two-wire current loop. The loop has an external power supply and the
transmitter acts as a variable resistor with respect to its input
signal.
True RMS: The true root-mean-square value of an AC
or AC-plus-DC signal, often used to determine power of a signal. For a
perfect sine wave the RMS value is 1.11072 times the rectified average
value which is utilized for low-cost metering. For significantly
non-sinusoidal signals, a true RMS converter is
required.
TTL-Compatible: For digital input circuits, a
logic 1 is obtained for inputs of 2.0 to 5.5 V which can source 40 µA and
a logic 0 is obtained 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 µA; and a logic 0 is represented
by 0 to 0.6 V with a current sink capability of at least 16
mA.
LS-TTL Compatible: For digital input circuits, a logic 1
is obtained for inputs of 2.0 to 5.5 V which can source 20 pA, and a logic
0 is obtained for inputs of 0 to 0.8 V which can sink 400 uA. 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 uA; and a logic 0 is represented by 0 to
0.8 V with a current sink capability of at least 16 mA. “LS” stands for
low-power Schottky.
TTL Unit Load: A load with TTL voltage
levels, which will draw 40 µA for a logic 1 and -1.6 mA for a logic
0.
LS-TTL Unit Load: A load with LS-TTL voltage levels,
which will draw 20uA for a logic 1 and -400 uA for a logic 0.
UART: Universal Asynchronous
Receiver/Transmitter. Interface device for serial/parallel
conversion, buffering, and adding check bits.
Undershoot:
The
difference in temperature between the temperature a process goes to below
the setpoint, after the cooling cycle is turned off, and the setpoint
temperature.
Ungrounded Junction: A form of construction of
a thermocouple probe where the hot or measuring junction is fully enclosed
by and insulated from the sheath material.
Zero Adjustment:The ability to adjust the display of a
process or strain meter so that zero on the display corresponds to a
non-zero signal, such as 4 mA, 10 mA or 1 Vdc. The adjustment range is
normally expressed in counts.
Zero Offset: 1. The difference
expressed in degrees between true zero and an indication given by a
measuring instrument. 2. See Zero Suppression.
Zero Power
Resistance: The resistance of a thermistor or RTD element with no
power being dissipated.
Zero Suppression: The span of an
indicator or chart recorder may be offset from zero (zero suppressed) such
that neither limit of the span win be zero. (Example: a temperature
recorder which records a 100° span from 400° to 500° is said to have 400%
zero suppression.