Conduction. Mode of heat transfer through a substance on a molecular level as a result of a temperature gradient's being present.
Convection. Mode of heat transfer when there is fluid flow. As in conduction, a temperature gradient must be present, but convection is influenced by fluid flow, which alters the temperature gradient.
Emissivity. Ratio of the actual energy emitted by a real body to that emitted by a blackbody. Emissivity can be considered equal to absorptivity for a graybody, i.e., an object whose emissivity is independent of wavelength and that reflects radiation in a diffuse manner. Most objects can be reasonably approximated as graybodies.
Heat. Movement of energy across a thermodynamic barrier; measured in joules (J).
Heat flux. Rate of energy transfer per unit area; expressed in W/m2 or W/cm2. Heat flux can be positive or negative, depending on the direction of heat transfer.
Heat transfer. Rate at which energy moves across the thermodynamic barrier; measured in watts (W), i.e., J/s. Heat transfer occurs in three different modes: conduction, convection, and radiation.
Radiation. Mode of heat transfer that occurs by means of electromagnetic radiation; requires no transport medium or material.
Temperature. Fundamental property that indicates the internal energy of matter. Any temperature scale (Celsius, Fahrenheit, kelvin, etc.) may be used as long as the units are kept consistent.
Thermocouple. Consists of a pair of junctions between two different metals that will produce a voltage proportional to the temperature difference between the junctions of the wires due to the Seebeck effect. Commercially available thermocouples will appear to have only one junction; the second junction is essentially where the two leads are connected to a voltmeter or electronic thermometer. For a thermocouple to give an accurate reading, the second junction must be at a reference temperature, frequently taken to be room temperature. For more accurate measurements, the second junction is lowered to a known temperature, such as the ice point.
Thermopile. Array of thermocouples. Connecting many thermocouples in series increases their temperature sensitivity because the thermocouple voltages add when linked in series. As does a thermocouple, the thermopile reads the temperature difference between two points. In a heat flux sensor these two points are the top and bottom layers of the thermopile.
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