Temperature Measurement

Summary

Temperature is a measure of energy. Objects are in thermal equilibrium when they have the same temperature. Accurate temperature measurements are essential to determine the rate of chemical reactions, the weather, and the health of a living creature, to cite a few examples. Measurements are said to be made in absolute units when referred to as absolute zero or relative units when referred to some other standard state. Measuring temperature is done with a thermometer, including those using mechanical probes, chemical paints, and nonintrusive optical and sonic methods. The temperature range of interest depends on the application. Temperatures in the Earth's atmosphere range from around 200 to 320 Kelvin (K), while Mercury's varies from around 70 K on the dark side to more than 670 K on the side facing the Sun. The Sun's surface temperature is nearly 6,000 K.

Definition and Basic Principles

Temperature measurement, or thermometry, determines the quantitative value of the degree of heat, or the amount of sensible thermal energy contained in matter. The temperature of matter is a manifestation of the motion of atoms and molecules, either relative to each other or because of motions within themselves. Absolute zero is a temperature that creates a state in which molecules come to complete rest.

Temperature can be measured from heat transfer by conduction, convection, or radiation. Household thermometers use either the expansion of metals or other substances or the increase in resistance with temperature. Thermocouples measure the electromotive force generated by temperature difference. Pyrometers measure infrared radiation from a heat source. Spectroscopic thermometry compares the spectrum of radiation against a blackbody spectrum. Temperature-sensitive paints and liquid crystals change the intensity of radiation in certain wavelengths with temperature.

Temperature is measured in degrees on either an absolute or a relative scale, with the value of a degree differing from one system of units to another. Two major systems of units are degrees Fahrenheit (F) and degrees Celsius (C). The absolute scales of temperature using these units are called the Rankine (R) and the Kelvin scales, respectively, and both refer to the absolute zero temperature. A change of 5 Kelvin or 5 degrees Celsius corresponds to a change of 9 degrees Rankine or 9 degrees Fahrenheit. Since the freezing point of water at normal pressures is set at 32 degrees Fahrenheit and 0 degrees Celsius, temperatures in degrees Celsius are converted to degrees Fahrenheit by multiplying by 1.8 and adding 32.

Background and History

Researchers long sought ways to objectively quantify heat, but efforts were largely unsuccessful until the seventeenth century. For example, Galileo developed an effective thermoscope, but the effects of barometric pressure, which were not understood, made the device inaccurate as a thermometer. The invention of the clinical thermometer has been credited to Italian physician Sanctorius, who, in 1612, published a description of a calibrated heat-measuring device. Danish astronomer Ole Christensen Rømer used the expansion of red wine as the temperature indicator in a thermometer he created later in the seventeenth century. He set zero as the temperature of a salt-ice mixture, 7.5 as the freezing point of water, and 60 as its boiling point.

German physicist Daniel Gabriel Fahrenheit invented a mercury thermometer and the Fahrenheit scale, where 0 and 100 were roughly the coldest and hottest temperatures encountered in the European winter and summer. Swedish astronomer Anders Celsiusinvented the inverted centigrade scale, which was converted to the current Celsius scale by Swedish botanist Carl Linnaeus in the nineteenth century, so 0 represents water's freezing point and 100 its boiling point. British physicist William Thomson, also known as Lord Kelvin, devised the absolute scale in which absolute zero (0 K) corresponds to −273.15 C. Thus, 273.16 K is the triple point of water, defined as the temperature where all three states—ice, liquid water, and water vapor—can coexist. Scottish engineer William John Macquorn Rankine devised an absolute scale measured in units of degrees F, with absolute zero being -459.67 F.

How It Works

Probe Thermometers. Volume expansion thermometers use the expansion of liquids with rising temperature through a narrow tube. The expansion coefficient, defined as the increase in volume per unit volume per unit rise in temperature, is 0.00018 per Kelvin for mercury and 0.00109 per Kelvin for ethyl alcohol colored with dye. Calculating temperature from the actual random thermal motion velocity of every molecule, or the energy contained in a vibrational excitation of every molecule, is impractical. So, temperature is measured indirectly in most applications. Different metals expand to different extents when their temperature rises. This difference is used to measure the bending of two strips of metal attached to one another in outdoor thermometers.

Thermocouples use the Seebeck or thermoelectric effect discovered by German physicist Thomas Johann Seebeck, in which a voltage difference is produced between two junctions between wires of different metals when the two junctions are at different temperatures. Some metal combinations produce a linear voltage with temperature, which produces thermocouple sensors. Resistance temperature detectors (RTDs) use the temperature sensitivity of the resistance of specific materials for measurement by including them in an electrical "bridge" circuit of interconnected resistances, such as a Wheatstone bridge, and measuring the voltage drop in a part of the circuit when the resistances are equalized. Probes using carbon resistors are the most stable at low temperatures.

Radiation Thermometers. Infrared thermometers use the different emission rates 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. The dependence on emissivity is eliminated using the two-color ratio infrared thermometer, which compares the emission at two different parts of the spectrum against the blackbody spectrum.

Ultrasonic Thermometer. Ultrasonic temperature measurement considers the change in the frequency of sound of a given wavelength traveling through a medium. The frequency increases as the speed of sound increases proportional to the square root of temperature. This technique is difficult to implement with good spatial resolution since the changes in temperature over the entire path of the beam affect the frequency. However, it is suitable for techniques based on mapping multidimensional temperature fields.

Relation Between Temperature and Internal Energy. Nonintrusive laser-based methods generally use the fundamental properties of atoms and molecules. Temperature is a measure of the kinetic energy of the random motion of molecules and is hence proportional to the square of the speed of their random motion. Some energy also goes into the rotation of a molecule about axes passing through its center of mass and into the potential and kinetic energy of vibration about its center of mass. Other energy goes into the excitation of electrons to different energy levels. Quantum theory holds that each of these modes of energy storage occurs only in discrete steps called energy levels. Absorption and emission of energy, which may occur during or after collision with another atom, molecule, subatomic particle, or photon of energy, occurs with a transition by the molecule or atom from one energy level to another. The quantum of energy released or absorbed in such a transition is equal to the difference between the values of energy of the two levels involved in the transition. If the energy is released as radiation, the energy of the transition can be gauged from the frequency of the photon using Planck's constant.

Applications and Products

Temperature measurement is used in many applications. Volume expansion thermometers work in the range from about 250 to 475 K, but each thermometer is usually designed for a much narrower range for specific purposes. Examples include measuring human body temperature, atmospheric temperature, and the temperature in ovens used for cooking.

Mercury thermometers were used widely to measure room temperature as well as the temperature of the human body. Human body temperature is usually 98.6 to 99 degrees Fahrenheit, and variations of a few degrees may indicate illness or hypothermia. Alcohol thermometers are used in weather sensing and homes. The atmospheric temperature varies between roughly 200 K in the cold air above the polar regions and more than 330 K above the deserts. Concerning food preparation, temperatures vary from the 256 K of a refrigerator freezer to the 515 K of an oven.

Since the 1990s, expansion thermometers for the lower temperature ranges have mostly been replaced with solid-state devices, generally using RTDs with liquid crystal digital (LCD) displays. Platinum film RTDs offer fast and stable responses with a constant temperature coefficient of resistance (0.024 ohms per K), whereas coil sensors offer higher sensitivity. Pt100 (platinum-coiled) RTDs have a standard 100 ohms resistance at 273 K and a 0.385 ohms per degree Celsius sensitivity. RTDs are generally limited to temperatures well below 600 K. Thermocouples are made of various metal alloy pairs. Type K (chromel-alumel) thermocouples provide a sensitivity of 41 microvolts per Kelvin in the range −200 C to 1350 C. Type E (chromel-constantan) generates 68 microvolts per Kelvin and is suited to cryogenic temperatures. Types B, R, and S are platinum or platinum-rhodium thermocouples offering around 10 microvolts per Kelvin with high stability and temperature ranges up to 2,030 Kelvin. The thermocouples' chemical reaction or catalytic effect on the environment is important in using thermocouples. Some metals, such as tungsten, may get oxidized, while others, such as platinum, catalyze reactions containing hydrogen.

Light waves scattered from different parts of an object interfere with each other. In the case of Rayleigh scattering, where the object, such as a molecule, is much smaller than the wavelength, the scattering intensity is independent of the direction in which it is scattered. It is much lower in intensity than the Mie scattering that occurs when the particle size is comparable to wavelength. Rayleigh scattering measurement can work only where the Mie scattering from relatively huge dust particles does not drown it out. Rayleigh scattering thermometry uses the fact that in a flow where the static pressure is mostly constant, such as a subsonic jet flame, the amount of light scattered by molecules from an incident laser beam is proportional to the density and inversely proportional to temperature if the composition of the gas does not vary. This simple technique is used to obtain high-frequency response. However, there is often a substantial error in assuming the gas composition is constant. Rayleigh scattering occurs at the same wavelength as the incident radiation, but the Doppler shift of molecules broadens the frequency band of the scattered spectrum because of their speed relative to the observer. Where the Doppler shift is due to random motion, such as in the stagnant or slow-moving air of the atmosphere, the Doppler broadening gives the temperature. This fact is used in high spectral resolution lidar (HSRL) measurements of atmospheric conditions used from ground-based weather stations and satellites.

More sophisticated nonlinear laser-based diagnostic techniques are used to measure temperature in several research applications. Raman spectroscopy uses the phenomenon of Raman scattering, which occurs when molecules change from one state of vibration to another after being excited into a higher vibrational level by laser energy. A variant of this technique is called coherent anti-Stokes Raman spectroscopy (CARS), where the signature of the emission from molecules in given narrow spectral bands is compared against databases of the emission signatures of various gases to determine the composition and temperature of a given gas mixture at high temperatures. This technique is used in developing burners for jet engines and rocket engines. In laser-induced fluorescence, a strong laser pulse excites the molecules in a given small "interrogation volume." Within a few microseconds, the excited molecules release energy as photons at a different frequency than the laser pulse energy. A highly sensitive camera relates the spectral distribution to the temperature.

Infrared thermometers are used where nonintrusive sensors are needed or other electromagnetic fields might interfere with thermocouples. Infrared thermometry was used to capture the change in temperature due to the change in skin friction between laminar and turbulent regions of the flow in the boundary layer over the skin of the space shuttle to determine if there were regions where the flow separated because of missing tiles or protuberances. After the space shuttle returned to flight following the Columbia disaster in 2003, such diagnostics were performed using ground-based telescope cameras or from chase planes during the upper portions of the liftoff and orbital passes to alert mission controllers if repairs were needed in orbit.

Thermal equilibrium occurs very rapidly when molecules are allowed to collide with each other for some time. At equilibrium, the energy tied up in each of the various modes of energy storage has a specific value. In this case, the temperature measured from the instantaneous value of vibrational energy in the gas will give the same answer as that measured from the rotational or translational energy. However, in some situations, where the pressure and temperature change extremely rapidly, or energy is added to a specific storage mode, the temperature measured from the value of one type of energy storage may be very different from that measured from the translational energy. Examples are strong shocks in supersonic flows and rapid expansion through the nozzle of a gas dynamic laser. A common example is a fluorescent light bulb, where energy is added to the electronic excitation of molecules without much exciting of the translational, vibrational, or rotational levels. The gas glows as if it were at a temperature of 10,000 K (the electronic temperature is 10,000 K), whereas the translational temperature may be only 400 K.

Temperature-sensitive paints of various kinds are used to capture the temperature distribution over a surface using either the changes in color (wavelength of emitted radiation) or the intensity at a particular wavelength. Typically, these use fluorescent materials. Devices range from simple stick-on tapes to expensive paints used on models in high-speed wind tunnel tests.

Temperature detectors are broadly used in various sectors, such as medical, automobile, sports, agriculture, and space technology. Newer technologies like negative temperature coefficient (NTC) thermistors (temperature-dependent resistors) are stable and accurate and used in transportation, industrial processing, missiles, spacecraft, and electronic devices like fire detectors, air conditioners, and microwaves. Fiber optic thermometers became available in the early decades of the twenty-first century and are used in areas like metallurgy, glass manufacturing, and power generation.

Careers and Course Work

A wide variety of physical phenomena can be used to measure temperature. The basic principles come from college-level physics, including optics, and chemistry. Heat transfer is important, as are material science and electronics, with digital signal processing and image processing used to analyze data and control theory used to turn the measurements into feedback systems to control the temperature of a process or to compensate for the imperfections of an inexpensive measuring system and obtain results as good as those from a much more complicated system. Although few careers can be imagined where temperature measurement is the primary job, many careers involve skills in temperature measurement. Medical doctors, nurses, mothers, and cooks regularly use temperature measurement, as do heating, ventilation, and air-conditioning technicians, nuclear and chemical plant technicians, weather forecasters, astronomers, combustion engineers, and researchers. Preparation for temperature measurement includes courses in physics, chemistry, electrical engineering, and heat transfer.

Social Context and Future Prospects

Temperature measurement will continue to be an area that demands curiosity and scientific thinking across many disciplines. Probes, gauges, and nonintrusive instruments, as well as paints, are all still in heavy use, and each technique appears to be suitable to some particular problem. This diversity appears to be increasing rather than decreasing, so that the field of temperature measurement may be expected to expand and broaden. As energy technologies move away from fossil fuel combustion, where temperatures rarely exceed 2,200 K, into systems based on hydrogen combustion and nuclear fission or even nuclear fusion, techniques that can measure much higher temperatures may be expected to become much more common. In the twenty-first century, most temperature measurement methods rely on optical measurement methods. Thus, research regarding noncontact temperature measuring devices continues. As the twenty-first century progressed, increasing advances were made in temperature measurement. Scientists explored quantum, fiber optic, biomolecular, and infrared temperature measurements. Smart homes began using sensors enabled by the Internet of Things to provide real-time temperature measurements and store temperature data. Artificial intelligence and machine learning also created algorithms that allowed scientists to evaluate temperature data sets more accurately. 

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