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Liquid crystal thermometers portray temperatures as colors and can be used to follow temperature changes caused by heat flow. They can be used to observe that heat flows by conduction, convection, and radiation. In medical applications, liquid crystal thermometers may be used to read body temperature by placing them against the forehead.
Liquid crystal color transitions are used on many aquarium and pool thermometers as well as on thermometers for infants or baths. [84] Other liquid crystal materials change color when stretched or stressed. Thus, liquid crystal sheets are often used in industry to look for hot spots, map heat flow, measure stress distribution patterns, and so on.
Typically, the high temperature state will reflect blue-violet, while the low-temperature state will reflect red-orange. Since blue is a shorter wavelength than red, this indicates that the distance of layer spacing is reduced by heating through the liquid-crystal state. Some such materials are cholesteryl nonanoate or cyanobiphenyls.
Forehead thermometer is an example of a liquid crystal thermometer. Rectal and oral thermometers have typically been mercury but have since largely been superseded by NTC thermistors with a digital readout. [59] Various thermometric techniques have been used throughout history such as the Galileo thermometer to thermal imaging. [45]
A liquid crystal phase is thermotropic if its order parameter is determined by temperature. At high temperatures, liquid crystals become an isotropic liquid and at low temperatures, they tend to glassify. In a thermotropic crystal, those phase transitions occur only at temperature extremes; the phase is insensitive to concentration.
Liquid crystal polymers (LCPs) are polymers with the property of liquid crystal, usually containing aromatic rings as mesogens. Despite uncrosslinked LCPs, polymeric materials like liquid crystal elastomers (LCEs) [ 1 ] and liquid crystal networks (LCNs) can exhibit liquid crystallinity as well.
Such thermometers are usually calibrated so that one can read the temperature simply by observing the level of the fluid in the thermometer. Another type of thermometer that is not really used much in practice, but is important from a theoretical standpoint, is the gas thermometer. Other important devices for measuring temperature include:
An organic liquid is contained in a glass bulb which is connected to a capillary of the same glass and the end is sealed with an expansion bulb. The space above the liquid is a mixture of nitrogen and the vapor of the liquid. For the working temperature range, the meniscus or interface between the liquid and gas/vapor is within the capillary ...