Category: Mechanical Measurement

The Bourdon tube pressure gauge, named after Eugène Bourdon, is a very popular pressure sensor. Basic Bourdon tubes are made from metal alloys such as stainless steel or brass. They consist of a tube of elliptical or oval cross-section, sealed at one end. There are various shapes of Bourdon tube, including helical, spiral, and twisted.…

Manometers are differential pressure sensors. A differential pressure sensor measures the difference between a pressure being applied to it and a reference pressure (often atmospheric pressure). The U-tube manometer consists of a clear glass or plastic tube shaped into the form of a ‘U’. The tube is partially filled with a liquid, such as water,…

Pressure is force per unit area on a surface. There are some relative terms such as gauge pressure, atmospheric pressure, and absolute pressure those can be understood easily with Figure 18.2. Figure 18.2 Relative Terms of Pressure Gauge Pressure: It is the pressure relative to atmospheric pressure. Gauge pressure is positive for pressures above atmospheric pressure and negative…

Change-of-state temperature sensors consist of labels, pellets, crayons, lacquers, or liquid crystals whose appearance changes once a certain temperature is reached. They are used, for instance, with steam traps—when a trap exceeds a certain temperature, a white dot on a sensor label attached to the trap will turn black. Response time typically takes minutes, so…

Fluid-expansion devices can be divided into two main classes: the mercury type and the organic-liquid type. Versions employing gas instead of liquid are also available. Mercury is considered an environmental hazard, so there are regulations governing the shipment of devices. Fluid-expansion sensors do not require electric power, do not pose explosion hazards, and are stable even…

Bimetallic devices work on the principle that different materials have different rates of thermal expansion. Strips of two metals are bonded together. When heated, one side will expand more than the other, and the resulting bending is translated into a temperature reading by mechanical linkage to a pointer. These devices are portable and they do…

Infrared sensors are non-contacting devices. They infer temperature by measuring the thermal radiation emitted by a material.

Resistive temperature devices work on the principle that the electrical resistance of a material changes with its temperature. There are two key types of the devices: RTD and thermistors. It is well known that resistance of metallic conductors increases with temperature, while that of semiconductors generally decreases with temperature. Resistance thermometers employing metallic conductors for…

When two conductors made from dissimilar metals are connected forming two common junctions and the two junctions are exposed to two different temperatures, a net thermal emf (electromagnetic force) is produced, the actual value being dependent on the materials used and the temperature difference between hot and cold junctions. The thermoelectric emf generated, in fact…

Temperature of a body shows degree of hotness with respect to reference body. There are a number of temperature measurement systems, some of them are thermocouples, resistive temperature devices (RTDs and thermistors), infrared radiators, bimetallic devices, liquid expansion devices, change-of-state devices, etc.