Temperature is a variable, unlike others, should be measured in terms of indirect effects it has on the physical properties of materials or changes in electrical circuits (voltage or resistance). Changes like these must be related to reproducible laboratory phenomena such as boiling and freezing points of water. The laboratory calibration points are often based on the temperatures at which there is an equilibrium liquid - vapor of pure substances such as oxygen, water, sulfur, silver and gold.
Local Meters: Thermometers
Over a period of years, at least five different temperature scales have been used in measuring this variable. The two most commonly used, Fahrenreit and Centigrade, using scopes (span) arbitrary 180 ° F and 100 ° C respectively for the boiling and freezing points of water. Two other scales (RankSne and Keiwin) that are referenced to absolute zero.
From a historical standpoint, the first practical device for measuring temperature was the glass thermometer, which is why he will begin to examine the various ways that exist to measure temperature.
Glass Thermometers mineral substances constrict or expand a certain amount for every degree of temperature change. This is the principle of thermal expansion. When heat is applied to a glass thermometer containing mercury such, it expands more than glass bulb that contains it. The difference in expansion, forcing the mercury up a capillary tube uniformly with respect to temperature change, so that calibration of the tube with a certain scale, you will have a direct reading of temperature. Mercury thermometers can be used from 33 ° F to +800 ° C. However, for very low temperatures using thermometers that contain alcohol (-300 ° C to +600 ° C).
Bimetallic Thermometers: The operation of these devices is based on the principle that different metals have different coefficients of thermal expansion. If two different metal alloys are welded to form a coiled bimetal element has. When this joint is heated, it tends to develop due to different thermal expansion of each alloy. If you connect a pointer to the spiral by means of a shaft, the pointer will move and indicate the temperature on a circular scale calibrated.
Bimetal Thermometers
Filled thermal systems
One of you older systems for measuring temperature based on the use of pressure-actuated thermometers, using systems "filling" (systems with liquids, gas or steam) that respond to temperature variations. All fluids are liquids, vapors or gases expand when heated and contract when cooled.
This phenomenon is used to expand an element of pressure, usually a Bourdon tube which transmits the movement of an indicator or with other elements to transmit or register. They are basically simple and robust systems that have been disappearing but giving way to other forms of measurement can be seen below.
Electrical methods for measuring temperature
There are several ways to obtain an electrical signal representing the measured temperature. Among these we can point to the measurement systems using thermocouples, RTD and others.
- The most famous: Thermocouples
The termocupia is one of the simplest and most common methods used ^ to determine the process temperature. When you require remote indication or when you need displayer temperature of several points, this method is most appropriate. In 1821 TJ. Seebeck discovered that when heat is applied to the junction of two dissimilar metals, an electromotive force is generated, which can be measured at another juncture (cold) of these two metals (conductors); these drivers form an electrical circuit and current flows as a result of the emf generated. This is valid as long as the temperatures in the two junctions are different.
Figure
For a given combination of materials, the output voltage (in millivolts) varies in direct proportion to the temperature difference between these joints or seams. To the extent appropriate to the actual temperature, the cold junction (physically located at the entrance of the instrument receiver) should remain constant, commonly referred to zero degrees Celsius. To achieve time have appeared in various methods currently being used electronics for this purpose. The measuring junction (hot junction) of course, be located at the place where temperature measurement is required.
Fig 3 - Seebeck Effect
For moderate temperatures (up to about 260 ° C), combinations of iron and copper, iron and constantan (an alloy of copper and nickel) are used frequently. At high temperatures (up to about 1640 ° C), the threads are made of platinum or platinum and rhodium alloy.
Different types of thermocouples
The thermocouples are commonly designated with a letter. For example, a thermocouple type J Iron / Constantan (the spreader bar is to indicate the materials in each thread) and is K-type Chromel / alumel (the Chromel is an alloy of chromium and nickel and alumel is aluminum and nickel).
Figure 5 thermowell (courtesy of Omega)
There are several combinations used in the manufacture of thermocouples and the proper selection of these sensors depends on its range of use, output in mV / ° C and the maximum errors in measurement, in addition to the desired mechanical characteristics. Thermocouples are not always in direct contact with the process. A protective elements are often used at the same time allow a thermocouple to remove without interrupting the process. Such is the case of the thermowell.
The no less famous, the RTD's
These devices whose acronym in English mean temperature resistive detectors have been used for years and are still popular today. Is predicated on increasing resistance of a wire with increasing temperature. The magnitude of this change with respect to temperature change in it, is called "thermal coefficient of resistance" of conductive material.
For most pure metals, this is constant over a certain temperature range. For example, the ratio of platinum (a) is 0.00392 ohm / (ohm) (° C) over a range of 0 ° C to 100 ° C, having a resistance of 100 ohms to a temperature of 0 ° C, so Pt called -100. For most drivers, the coefficient referred to (a) is positive. Commonly used materials include platinum, nickel, copper, nickel - iron and tungsten. Among them, platinum is the most widely used due to its linear characteristic over most of its range, but also nickel, for its high rate of resistance, although it has a linear characteristic. For the Pt -100, you can use the following formula for the approximate response for a given temperature sensor:
R = 100 (1 + T)
Because such a small diameter wire used in these RTD (0.05 mm), its construction includes shielding against mechanical shock. Often resistance probes are manufactured with three or four output wires in order to eliminate the effects of change of resistance in extension wires for temperature changes. Common measurement circuits used Wheatstone bridge.
Still others, thermistors
Are semiconductors made of carbon, germanium, silicon and mixtures of certain metal oxides, which exhibit high temperature coefficients, usually negative (NTC). Its characteristic is nonlinear and exhibit the greatest changes in cryogenic temperature range below 100 ° K. Its strength is a function of absolute temperature. The details of these devices vary with temperature range. For example, a germanium sensor can have a variation of + 0.005 ° K on a cryogenic range of 1.5 ° to 5 ° K and ± 0.1 ° K over a range of 40 ° to 100 ° K. This can even vary with time of use of the sensor.
Additionally, the use of thermistors and temperature devices are used in voltage regulation, power level control, compensation of other temperature sensors, temperature control and as detectors in analyzers.
Solid State Sensors
They are small transducers that convert a temperature input into a current output, proportional to it. They are especially used in applications within the range of -55 ° C to 150 ° C where high reliability is required, linearity and accuracy. One of the most important applications is in the cold junction compensation for thermocouple measurement systems.
Pyrometers, without contact with the process
These are devices that measure temperature over the range applicable to thermocouples, though, that certain alloys, allowing the latter to reach 3000 ° C even for short periods. Some pyrometers can be used to measure temperatures as low as 0 ° C and as high as 5000 ° C with high accuracy.
The pyrometers are classified into two groups: the so-called total radiation pyrometers and so-called partial radiation pyrometers. Radiation pyrometry uses the property of thermal radiation that is emitted by all materials (except inert gases) at a temperature of absolute zero. It is particularly interesting because of no need for direct contact with the material whose temperature is measured.
Of radiation pyrometers are currently employed by infrared digital technology have become increasingly the more versatile than their predecessors, allowing for example automatically make amends, by variations in temperature, emissivity setting etc..
Figure 6 Radiation Pirómeíro
