Chapter 3 Temperature

Filled thermal system

A temperature- sensing bulb filled with a liquid, vapor, or gas connected by means of a capillary tube to a pressure-measuring element.

Glass-stem thermometer

A temperature-measuring device constructed of a glass bulb and a capillary (small) tube made of glass with a numbered scale; the bulb and the capillary tube contain a liquid such as mercury or colored alcohol that expands whereby a reading can be made from the etched scale on the length of the tube.

Thermowell

A thick-walled, typically stainless steel device shaped like a tube that is inserted into a hole in piping or equipment specifically prepared to house a temperature sensing and measuring element.

Thermistor

A type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature.

Temperature differential (delta)

Difference between two temperatures, normally across two points in a control loop.

Kelvin

Scale of measurement that is sometimes called an absolute scale because 0 K is the point at which no heat exists; freezing point of water is 273 K and boiling point of water is 373 K.

Fahrenheit

Scale of measurement to determine temperature; freezing point of water is defined as 32 degrees F and the boiling point of water is at 212 degrees F.

Celsius/centigrade

Scale of measurement to determine temperature; freezing point of water is zero degrees C and boiling point of water is at 100 degrees C.

Heat

The energy that flows between bodies of differing temperatures.

Rankine

An absolute scale of measurement to determine temperature; freezing point of water is 492 degrees R and boiling point of water is 672 degrees R.

Temperature gauge

An independent device with a sensing element such as a bimetallic strip, bourdon tube or bellows that is linked to a pointer displaying the temperature on a calibrated face.

Thermocouple

Primary element consisting of two wires of dissimilar metals connected at one end.

Resistance temperature device (RTD)

Primary element that measures temperature changes in terms of electrical resistance.

British thermal unit (BTU)

The amount of heat required to raise the temperature of a pound of water 1 degree Fahrenheit.

Radiation

The transfer of heat through emitting radiant energy in the form of waves or particles.

Conduction

The transfer of heat through solid matter by moving from one molecule to the next.

Convection

The transfer of heat through the circulatory motion occurring in a fluid when there is a difference in temperature from one region to another; density differences between the hot and cold molecules causes convection motion in fluids.

Bimetallic Strip

Two dissimilar strips of metal bonded together that expand and contract at different rates when exposed to temperature change, causing a blending or rotating effect; used as the primary element in a temperature gauge or bimetallic thermometer.