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Intrinsically safe: An instrument which will not produce any spark or
thermal effects under normal or abnormal conditions that will ignite a
specified gas mixture.
IPTS-68: International Practical Temperature Scale of 1968. Fixed
points in thermometry set by the 1968 General Conference of Weights
and Measures.
ISA: Instrument Society of America.
ISE: Integrated squared error
Isolation: Electrical separation
Isothermal: A process or area that maintains a constant temperature.
ITS-90: International Temperature Scale of 1990
Joule: The basic unit of thermal energy. 1 Joule equals 1 ampere passed
through a resistance of 1ohm for 1 second.
Junction: A thermocouple junction is the point at which two alloys are
joined. A typical thermocouple circuit would have a measuring and a ref-
erence junction.
Kelvin: The unit of absolute or thermodynamic temperature scale. Zero
Kelvin is absolute zero, where all molecular activity stops. No “°” sym-
bol is used. 0°C=273.15K; 100°C=373.15K.
Kilo: The prefix for one thousand (K).
Kilowatt (kw): 1000 watts or 3412Btu per hour.
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 on one wave occurs after the same
point of the reference wave.
Least Significant Digit (LSD): The digit farthest to the right in a dis-
play.
Linearity: The deviation of an instrument’s response from a straight
line.
Load: The electrical demand of a process expressed as power (watts),
current (amps) or resistance (ohms).
Manual reset: The adjustment on a proportional control which shifts
the proportional band in relation to the set point to eliminate droop of
offset errors.
Maximum operating temperature: The maximum temperature at
which an instrument or sensor can be safely operated.
Maximum power rating: The maximum power in watts that a device
can safely handle.
Reference junction: The cold junction in a thermocouple circuit
which is held at a stable known temperature. The standard reference tem-
perature 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.
Relay (solid state): A solid state switching device which completes or
interrupts a circuit electrically with no moving parts.
Repeatability: The ability of a probe or instrument to give the same
output or reading under repeated identical conditions.
Resistance: The resistance to the flow of electric current measured in
ohms (Ω). For a conductor, resistance is a function of diameter, resistiv-
ity (an intrinsic property of the material), and length.
Resolution sensitivity: The amount of temperature change that must
occur before the control actuates. It may be expressed in temperature or
as a percentage of the control’s scale.
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 con-
ditions. Five time constants are required for the sensor to stabilize at
99.3% of the step change value.
Retransmit output: Analog output scaled to the process or the set
point value.
RFI: Radio frequency interference.
RS232 or RS485 output signal: A serial interface suitable for con-
nection between a digital control and a personal computer, a host com-
puter, or a printer.
RTD: A temperature sensing probe of finely wound platinum wire that
displays a linear resistance change for a corresponding temperature
change. The resistance increases as the temperature rises. A base resist-
ance of 100ohms at 32°F is the industry (DIN) standard.
Saturation temperature: The boiling temperature of a liquid at the
existing pressure.
SCFM: Volumetric flow rate in cubic feet per minute at 60°F (15°C) and
standard atmospheric pressure.
SCR: Silicone Controlled Rectifier
Secondary standard: A standard of unit measurement derived from a
primary standard.
Sensitivity: The minimum change in input signal to which an instru-
ment can respond.
Set point: Control setting to achieve or maintain temperature.
Sheath thermocouple: A thermocouple made out of mineral-insulated
thermocouple cable which has another metal sheath.
Shield: Material surrounding a conductor(s) to prevent electrostatic or
EMI from external sources.
Slide wire feedback: A potentiometer that varies the resistance to
control a valve position.
Soak: To raise the temperature of a metal object in a heated environment
to produce a metallurgical change.
Soft start: Reduces voltage on initial start-up which reduces power to
the heaters. If heater has accumulated moisture internally during a shut
down, soft start will allow heater to dry before full voltage is applied so
as to extend heater life.
Solid State Relay (SSR): A solid state switching device which com-
pletes or breaks a circuit electrically with no moving parts.
Measuring junction: The thermocouple junction referred to as the hot
junction that is used to measure an unknown temperature.
Mechanical relay: An electromechanical device that completes or
breaks a circuit by opening or closing electrical contacts.
Mega: The prefix for one million (M).
Melting point: The temperature at which a substance transforms from
a solid phase to a liquid phase.
Micro: The prefix for one millionth.
Microamp: 10
-6
amps (one millionth of an amp).
Micron: 10
-6
meters (one millionth of a meter).
Microprocessor: The central processing unit (CPU) that performs the
logic operations in a micro-computer system. The microprocessor in a
process or instrument control decodes instructions from the stored pro-
gram, performs algorithmic and logic functions, and produces signals
and commands.
Milli: The prefix for one thousandth.
Milliamp: 10
-3
amps (one thousandth of an amp).
Millivolt: 10
-3
volts (one thousandth of a volt).
NEC: National Electrical Code
NEMA: National Electrical Manufacturer’s Association
NEMA 4X: A front panel rating designating the control as washdown
capable and corrosion resistant.
NIST: National Institute of Standards and Technology, United States
Department of Commerce.
Noise: Undesirable electrical interference on the signal wires.
Noise suppression: A device used to reduce electrical interference.
Normal-mode rejection ratio: The ability of an instrument to reject
interference, usually of line frequency (50–60Hz), across its input termi-
nals.
NPT: National Pipe Thread.
Offset: The difference in temperature between the set point and the
actual process temperature. Also referred to as droop.
ohm: The unit of electric resistance.
On/off controller: A controller whose action is fully on or fully off.
Open Loop Control: A control system with no sensing feedback.
Overshoot: Excursion of temperature above the set point.
Phase: A time-based relationship between a periodic function and a ref-
erence. In electricity, it is expressed in angular degrees to describe the
voltage or current relationship of two alternating waveforms.
Phase proportioning: A form of temperature control where the power
supplied to the process is controlled by limiting the phase angle of the
line voltage.
PID: Three mode temperature control — proportional, integral, (auto-
matic reset), derivative(rate).
Polarity: In electricity, the quality of having two oppositely charged
poles, one positive and one negative.
Potentiometer: 1. A variable resistor often used to control a circuit. 2.
A balancing bridge used to measure voltage.
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.
Process alarm: A fixed alarm or secondary set point value independ-
ent of the primary set point. Should a process value exceed this value, an
alarm condition would register.
Process variable: The parameter being controlled or measured such as
temperature, relative humidity, flow, level, pressure, etc.
Proportional band: A temperature band in degrees within which a
control’s proportioning function is active.
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