Thermometers, infrared

An instrument for measuring temperatures, in the rubber industry the term is usually applied to an instrument for determining the surface temperature of mill and calender rolls, moulds, etc. The instrument is usually based on thermocouples or, where higher accuracy is required, platinum resistance thermometers. Infrared (IR) techniques are now used which have the advantage of non contact but require careful calibration for the emissivity of the surface. 

Radiation. The thermal radiation emitted by a body is a function of the temperature of the body hence, measurement of the radiant energy can be used to indicate the temperature. Commonly employed sensors in this category are optical thermometers, infrared scanners, spectroscopic techniques, and total-radiation calorimeters. 

Radiation Thermometers. Infrared thermometers use the different rates of emission in the infrared part of the electromagnetic spectrum from objects, compared to the emissivity of a perfectly absorbing blackbody. The device is calibrated by comparing the radiation coming from the object against that from a reference object of known emissivity at a known... 

Temperature is measured by thermometers. The first temperaturemeasuring device was a special type of liquid thermometer, the thermoscope (discovered by Galilei in the sixteenth century). These days there are gas thermometers, liquid thermometers, infrared thermometers, liquid crystal (LC) thermometers (cholesteric), thermocouples, and resistance thermometers (these latter ones are the most important in thermal analysis). 

Collecting optics, radiation detectors and some form of indicator are the basic elements of an industrial infrared instrument. The optical system collects radiant energy and focuses it upon a detector, which converts it into an electrical signal. The instrument s electronics amplifies the output signal and process it into a form which can be displayed. There are three general types of instruments that can be used for predictive maintenance infrared thermometers or spot radiometers line scanners and imaging systems. 

Infrared thermometers or spot radiometers are designed to provide the actual surface temperature at a single, relatively small point on a machine or surface. Within a predictive maintenance program, the point-of-use infrared thermometer can be used in conjunction with many of the microprocessor-based vibration instruments to monitor the temperature at critical points on plant machinery or equipment. This technique is typically used to monitor bearing cap temperatures, motor winding temperatures, spot checks of process piping temperatures and similar applications. It is limited in that the temperature represents a single point on the machine or structure. However when used in conjunction with vibration data, point-of-use infrared data can be a valuable tool. 

Point-of-use infrared thermometers are commercially available and relatively inexpensive. The typical cost for this type of infrared instrument is less than 1,000. Infrared imaging systems will have a price range between 8,000 for a black and white scanner without storage capability to over 60,000 for a microprocessor-based, color imaging system. 

Reliable micro-scale measurement and control of the temperature are required in developing thermal micro-devices. Available measurement techniques can be largely classified into contact and non-contact groups. While the resistance thermometer, thermocouples, thermodiodes, and thermotransistors measure temperature at specific points in contact with them, infrared thermography, thermochromic liquid crystals (TLC), and temperature-sensitive fluorescent dyes cover the whole temperature field (Yoo 2006). 

Further, we examined the Heck reaction between w-butyl acrylate and 4-bromobenzotrifluoride 5 in the presence of 2 mol% Pd clusters in a singlevessel monomode m/w oven fitted with an infrared thermometer. 100% conversion with quantitative yield to the cinnamate was obtained after 5 min irradiation at 75 W/240 °C. We then repeated the reaction under conventional heating at 240 °C. After 3.5 min a black tarry gel formed. Extraction followed by GC analysis showed only cinnamate, but the tarry material (probably acrylate polymers/oligomers) could not be analysed. These experiments show that when clusters are present different results are obtained depending whether m/w heating or conventional heating is used. In principle, this could be the result of hot spots created on the metal clusters. 

The other limit is the problem of temperature measurements. Classical temperature sensors could be avoided in relation to power level. Hence, temperature measurements will be distorted by strong electric currents induced inside the metallic wires insuring connection of temperature sensor. The technological solution is the optical fiber thermometers [35-39]. However, measurements are limited below 250 °C. For higher values, surface temperature can be estimated by infrared camera or pyrometer [38, 40], However, due to volumic character of microwave heating, surface temperatures are often inferior to core temperatures. 

Because of their high heat capacity, only few of the thermometers described in Chapter 9 can be used as sensors for detectors. Resistance (carbon) sensors were used for the first time in a cryogenic detector by Boyle and Rogers [12] in 1959. The carbon bolometer had a lot of advantages over the existing infrared detectors [13]. It was easy to build, inexpensive and of moderate heat capacity due to the low operating temperature. 

Infrared radiation was discovered by Herschel [58] in 1800, using a mercury thermometer to detect sunlight dispersed by a prism. However, the Latin poet Lucretius in his De rerum Natura (On the Nature of Things, about 50 BC) clearly showed a clear feeling of the infrared radiation. Of course Lucretius s terminology was far from the modern one, and he had no thermometer at his disposal ... 

Early bolometers used, as thermometers, thermopiles, based on the thermoelectric effect (see Section 9.4) or Golay cells in which the heat absorbed in a thin metal film is transferred to a small volume of gas the resulting pressure increase moves a mirror in an optical amplifier. A historical review of the development of radiation detectors until 1994 can be found in ref. [59,60], The modern history of infrared bolometers starts with the introduction of the carbon resistor, as both bolometer sensor and absorber, by Boyle and Rogers [12], The device had a number of advantages over the Golay cell such as low cost, simplicity and relatively low heat capacity at low temperatures. 

Nevertheless the heat capacity of a carbon resistor was not so low as that of crystalline materials used later. More important, carbon resistors had an excess noise which limited the bolometer performance. In 1961, Low [61] proposed a bolometer which used a heavily doped Ge thermometer with much improved characteristics. This type of bolometer was rapidly applied to infrared astronomy as well also to laboratory spectroscopy. A further step in the development of bolometers came with improvements in the absorber. In the early superconducting bolometer built by Andrews et al. (1942) [62], the absorber was a blackened metal foil glued to the 7A thermometer. Low s original bolometer [61] was coated with black paint and Coron et al. [63] used a metal foil as substrate for the black-painted absorber. A definite improvement is due to J. Clarke, G. I. Hoffer, P. L. Richards [64] who used a thin low heat capacity dielectric substrate for the metal foil and used a bismuth film absorber instead of the black paint. 

Fig. 3.42 Body temperature measurement with an infrared ear thermometer.

Fig. 3.46 Cross section of the sensor unit of an IRT 3000 infrared ear thermometer.

For the calibration of most infrared ear thermometers the sensitivities S0 and R0 and the temperature coefficients Sj and a for both sensors have to be determined. Typically a two-step calibration is performed. In the first step the ambient sensor is calibrated by immersing it into two different temperature controlled baths. In the second step the thermopile sensor is calibrated by measuring the output signal while placing it before two different blackbody radiation sources. 

The infrared ear thermometer is a major step in the development of thermometers for body temperature measurements. Compared to traditional mercury-in-glass or electronic contact thermometers it is more convenient, safer and faster. During its 10 years in the consumer market it has been gradually replacing conventional thermometers, especially for temperature measuring in children. 

Thermopiles are also used in new ear thermometers or in forehead thermometers to measure the infrared radiation emitted from the skin. This allows quick and reliable temperature measurement and is easy and comfortable to use. 

A round-bottomed, standard-taper flask with a Claisen head carrying an ebullition capillary and a thermometer and attached to a two-necked flask with one neck for vacuum takeoff is satisfactory. It is important that the setup allow for heating by either flame or infrared lamp to melt the solid distillate and prevent its clogging the vapor passage. 

When sitting on a beach in the sun, or standing in front of an oven, we often refer to waves of heat hitting us. A thermometer would tell us that our temperature was increasing. In fact, this heat comprises photons of infrared light, which is why we... 

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