GLOSSARY OF TERMS: Temp. Measurement

AMBIENT CONDITIONS: Environment surrounding equipment or system, such as humidity, temperature, pressure, vibration, shock, magnetic fields, etc.

  BIMETALLIC STRIP: Temperature sensing instrument in which two dissimilar metals are bonded together so that differential expansion of the dissimilar metals actuates pointer, indicating temperature. 

 BLACKBODY: Ideal surface that absorbs all incident radiation, regardless of wavelength, direction of incidence, and polarization; radiates the maximum energy possible for given spectral and temperature conditions and has emissivity of 1.00.

  COLD JUNCTION: (C/J) Reference junction of thermocouple, which is held at stable, known temperature, allowing a comparison with measured junction to determine that unknown temperature.  

COMPENSATING LOOP: In thermal measurements using Resistance Temperature Detector (RTD), the lead wire resistance compensation for this type of element where an extra length of wire is run from instrument to RTD and back to instrument, with no connection through the RTD.

  CONSTANTAN: Alloy of copper (50% to 60%) and nickel that has very low temperature coefficient of resistance with comparatively high resistance, making it very suitable for precision wire-wound resistors and to form thermocouples with iron, copper, silver, etc.; used as negative lead in types E, J, and T thermocouples. 

 CONVECTION: Circulatory action that occurs in fluid at nonuniform temperature due to variation of its density and action of gravity; the transfer of energy (heat) by this automatic circulation of medium (gas or liquid fluid).

  DEW POINT: (DP) Temperature referred to a specific pressure, at which water vapors condense; the temperature at which condensation of moisture from vapor phase begins.  

EUTECTIC TEMPERATURE: Lowest possible melting point of a mixture of alloys.

  HALT: (Highly Accelerated Life Test) Typically rapid ramp-soak cycling of electronic equipment between the extremes of its temperature rating while operating over 2-3 days to quickly identify failures (infant mortalities) that would otherwise take years to surface. 

 HOT JUNCTION: Thermocouple junction used to measure an unknown temperature (measuring junction) for comparison to a known.  

ICE POINT: Temperature at which pure water freezes (0 C or 32 F).

  INFRARED: Radiation emission (heat) used in some thermometer types to determine temperature of an object that is generally not suitable for contact measurement.

  IPTS: (International Practical Temperature Scale) Provides fixed points in thermometry as specified by General Conference of Weights and Measures.

  ISOTHERMAL: Process or area that is at constant temperature.

  KELVIN: Absolute thermodynamic temperature scale based upon Celsius scale with 0 C = 273.15K. 

 MEASURING JUNCTION: Thermocouple junction referred to as the hot junction used to measure as unknown temperature.   

NTC: (Negative Temperature Coefficient) NTC resistor (thermistor) that will make large, abrupt change in resistance when undercurrent or low temperature cools it below some specific point, thereby effectively "switching OFF," or in reality, acting like a solid-state fuse.  

PTC: (Positive Temperature Coefficient), such as in PTC resistor (thermistor), that will make large, abrupt change in resistance when an overcurrent or high temperature heats it above a specific point, thereby effectively "switching OFF," or in reality, acting like solid-state fuse.  

RANKINE: An absolute temperature scale based upon the Fahrenheit scale with 0 F = 459.67 R.

  REFERENCE JUNCTION: Cold junction in thermocouple circuit, which is held at stable known temperature.  

RTB: (Resistance Thermometer Bulb) Temperature measuring device in which expansion and contraction of a fill, such as mercury and certain other gasses and liquids, is calibrated to reflect change in temperature.  

RTD: (Resistance Temperature Detector) Temperature measuring device in which the resistance of sensing element is an accurately known function of temperature.

  SEEBECK EFFECT: When two junctions in two dissimilar metals form a circuit, and each junction is held at different temperatures, a current flows in that circuit which is caused by difference in temperature between those two junctions.

  SPECIFIC HEAT: Ratio of thermal energy required to raise temperature of a body one degree to thermal energy required to raise an equal mass of water one degree.

  TEMPERATURE ERROR: Maximum change in output, at any input signal within some specified range, resulting from a change in ambient temperature.

  THERMAL CONDUCTIVITY: Property of a material to conduct heat in the form of thermal energy.

  THERMAL DRIFT: Operating variance due to changes in temperature.

  TRIPLE POINT: Temperature and pressure at which solid, liquid, and gas phases of a given substance are present simultaneously in varying amounts.

  THERMAL EXPANSION: Increase in size due to an increase in temperature expressed in units of an increase dimension per degree. 

 THERMAL GRADIENT: Distribution of a differential temperature through a body or across a surface.

  THERMISTOR: (Thermal Resistor) Resistive circuit component having high negative temperature coefficient of resistance so that its resistance decreases as temperature increases.  

THERMOCOUPLE: Device consisting of two dissimilar metals joined together at both ends. The voltage developed between these two junctions is proportional to temperature difference between those junctions; used to measure temperature at one end while other is held fixed as the reference (cold) junction.

  THERMOPILE: Arrangement of thermocouples in series such that alternate junctions are at the measuring temperature and the reference temperature to amplify the thermoelectric voltages; often used in radiation pyrometry.  

This glossary of terms and definitions was contributed by Sam Herb of JAOMAD Consultancy, a marketing, engineering, & training services firm for the automation industries. For more than 5,500 glossary terms & abbreviations, see www.JAOMAD.com.

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