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What you should know about basic contact temperature sensing |
| There are three
commonly used sensors for measuring temperature in the industrial arena.
None is considered terribly high tech, since each been around for many
years, albeit some much longer than others. Nevertheless, they have all
endured as viable methods—depending on the application—for measuring
the most often-measured process variable. The three basic sensing
elements—in no particular order—are filled systems, thermocouples,
and resistance thermometers. Here is how each works.
The filled system is the most basic sensor. Filled system devices consist of a capillary tube that connects a bulb containing temperature-sensitive fluid to an element that is sensitive to pressure or volume changes. This element (either pressure- or volume-sensitive) may be a bellows, a helix, a diaphragm, or a Bourdon tube. The motion of one of these elements -coupled mechanically to an indicating, recording, or controlling device—is what makes this type of sensor work. Types of fluids used in these devices can vary greatly depending on intended application. Systems can be liquid filled (Mercury, organic fluids, etc.) or vapor filled using a volatile liquid. A thermocouple is an assembly of two dissimilar metals (often wires) joined at the “hot” end. At the other end, the “cold” junction, the open circuit voltage is measured. This voltage, often called the Seebeck voltage or EMF—for the German physicist J. T. Seebeck, depends on the difference in temperature between the hot and cold junctions and the Seebeck coefficient of the two metals used. Simply put, a voltage measurement indicates the temperature of the hot junction when either the cold junction temperature is known or when measuring circuitry compensates for the cold junction temperature. Various types of thermocouples are available—Types T, J, K, R, S, etc—using a wide variety of dissimilar materials to handle various temperature ranges. Accuracy varies with application. Resistance thermometers use a sensing element that has a predictable and stable relationship between its temperature and measured resistance. The specific resistance of the material used (commonly wire or film) must be relatively high so that measuring its resistance is easy. A Wheatstone bridge circuit is attached to the sensor and measures its resistance, translating it to a temperature reading. Just like with thermocouples, materials of the sensor (resistance bulb) are chosen depending on the temperature range of the intended application. Unlike thermocouples, resistance thermometers or RTDs provide excellent stability, repeatability, and sensitivity. Thermistors are a form of resistance sensor that uses a specially prepared solid-state junction (usually a diode) that will change resistance with temperature. Thermistors are a niche product that produces a large signal over a very narrow temperature range. They are have evolved into stable, accurate sensors. They compete with RTDs in cost. Source: www.controleng.com |