From Measurement and Control Basics Fourth Edition

Many physical properties change with temperature, such as the volume of
a liquid, the length of a metal rod, the electrical resistance of a wire, the
pressure of a gas kept at constant volume, and the volume of a gas kept at
constant pressure. Filled-system thermometers use the phenomenon of
thermal expansion of matter to measure temperature change.

The filled thermal device consists of a primary element that takes the form
of a reservoir or bulb, a flexible capillary tube, and a hollow Bourdon tube
that actuates a signal-transmitting device and/or a local indicating temperature
dial. A typical filled-system thermometer is shown in Figure 7-1.
In this system, the filling fluid, either liquid or gas, expands as temperature
increases. This causes the Bourdon tube to uncoil and indicate the
temperature on a calibrated dial.

Figure 7-1. Filled bulb thermometer

The filling or transmitting medium is a vapor, a gas, mercury, or another
liquid. The liquid-filled system is the most common because it requires a
bulb with the smallest volume or permits a smaller instrument to be used.

The gas-filled system uses the perfect gas law, which states the following
for an ideal gas:

 T = kPV(7-5)


If the volume of gas in the measuring instrument is kept constant, then the
ratio of the gas pressure and temperature is constant, so that


The only restrictions on Equation 7-6 are that the temperature must be
expressed in degrees Kelvin and the pressure must be in absolute units.

Example 7-3 shows how to calculate the temperature for a change in pressure
of a fixed volume temperature detector.




Products & Services
Pressure Sensors
Pressure sensors include all sensors, transducers and elements that produce an electrical signal proportional to pressure or changes in pressure.
Resistive Temperature Devices (RTD) Elements
Resistive temperature device (RTD) elements are wire windings or other thin-film serpentines that exhibit changes in resistance with changes in temperature. They are usually made of metallic elements or alloys such as copper, nickel, or nickel-iron.
Level Sensors
Level sensors are used to detect liquid or powder levels, or interfaces between liquids. There are two basic level-measurement types: continuous and point or multi-point.
Noncontact Infrared Temperature Sensors
Noncontact infrared temperature sensors absorb ambient infrared (IR) radiation given off by a heated surface. They are used in applications where direct temperature measurement is not possible.
Dial Thermometers
Dial thermometers are bimetal or bi-metallic, liquid or gas-filled, and vapor-tension-based. These devices display temperatures in Fahrenheit, Celsius, or both.

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