An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the maximum operating temperature (or peak operating temperature). Outside this range of safe operating temperatures the device may fail. Aerospace and military-grade devices generally operate over a broader temperature range than industrial devices; commercial-grade devices generally have the lowest operating temperature range.
It is one component of reliability engineering.
Although biological systems do not have a defined operating temperature, individuals are most comfortable when body temperature fluctuations as a result of environmental factors are minimised.
Aerospace and military 
Electrical and mechanical devices used in military and aerospace applications must endure greater environmental variability, including temperature range.
- Commercial grade: 0 °C to 70 °C (sometimes −10 °C to 70 °C)
- Industrial grade: −40 °C to 85 °C (sometimes −25 °C to 85 °C)
- Military grade: −55 °C to 125 °C (sometimes -65 °C to 175 °C)
These grades ensure that a device is suitable for its application, and may withstand the environmental conditions in which it is used. In the United States, the Department of Defense has defined the United States Military Standard for all products used by the United States armed forces. A product's environmental design and test limits to the conditions that it will experience throughout its service life are specified in MIL-STD-810, the Department of Defense Test Method Standard for Environmental Engineering Considerations and Laboratory Tests.
The MIL-STD-810G standard specifies that the "operating temperature stabilization is attained when the temperature of the functioning part(s) of the test item considered to have the longest thermal lag is changing at a rate of no more than 2.0°C (3.6°F) per hour." It also specifies procedures to assess the performance of materials to extreme temperature loads.
Military engine turbine blades experience two significant deformation stresses during normal service, creep and thermal fatigue. Creep life of a material is "highly dependent on operating temperature", and creep analysis is thus an important part of design validation. Some of the effects of creep and thermal fatigue may be mitigated by integrating cooling systems into the device's design, reducing the peak temperature experienced by the metal.
Commercial and retail 
Commercial and retail products are manufactured to less stringent requirements than those for military and aerospace applications. For example, microprocessors produced by Intel Corporation are manufactured to three grades: commercial, industrial and extended.
Because some devices generate heat during operation, they may require thermal management to ensure they are within their specified operating temperature range; specifically, that they are operating at or below the maximum operating temperature of the device. Cooling a microprocessor mounted in a typical commercial or retail configuration requires "a heatsink properly mounted to the processor, and effective airflow through the system chassis". Systems are designed to protect the processor from unusual operating conditions, such as "higher than normal ambient air temperatures or failure of a system thermal management component (such as a system fan)", though in "a properly designed system, this feature should never become active". Cooling and other thermal management techniques may affect performance and noise level. Noise mitigation strategies may be required in residential applications to ensure that the noise level does not become uncomfortable.
Battery service life and efficacy is affected by operating temperature. Efficacy is determined by comparing the service life achieved by the battery as a percentage of its service life achieved at 20°C versus temperature. Ohmic load and operating temperature often jointly determine a battery's discharge rate. Moreover, if the expected operating temperature for a primary battery deviates from the typical 10°C to 25°C range, then operating temperature "will often have an influence on the type of battery selected for the application". Energy reclamation from partially depleted lithium sulfur dioxide battery has been shown to improve when "appropriately increasing the battery operating temperature".
Mammals attempt to maintain a comfortable body temperature under various conditions by thermoregulation, part of mammalian homeostasis. The lowest normal temperature of a mammal, the basal body temperature, is achieved during sleep. In women, it is affected by ovulation, causing a biphasic pattern which may be used as a component of fertility awareness.
In humans, the hypothalamus regulates metabolism, and hence the basal metabolic rate. Amongst its functions is the regulation of body temperature. The core body temperature is also one of the classic phase markers for measuring the timing of an individual's Circadian rhythm.
Changes to the normal human body temperature may result in discomfort. The most common form is a fever, a temporary elevation in the body's thermoregulatory set-point by about 1–2 °C (1.8–3.6 °F). Hyperthermia is an acute condition caused by the body absorbing more heat than it can dissipate, whereas hypothermia is a condition in which the core temperature drops below that required for normal metabolism and is caused by the body's inability to replenish the heat that is being lost to the environment.
- "MIL-STD-810G: Test Method Standard for Environmental Engineering Considerations and Laboratory Tests" (PDF). United States Department of Defense. 2008-10-31.
- Branco, Carlos Moura; Ritchie, Robert O.; Sklenička, Václav (1996). Mechanical behaviour of materials at high temperature. Springer. ISBN 978-0-7923-4113-0.
- Turner, Martin J. L. (2009). Rocket and Spacecraft Propulsion: Principles, Practice and New Developments. Springer Praxis Books / Astronautical Engineering. Springer. ISBN 978-3-540-69202-7. OCLC 475771458. "Improved performance of the nuclear engine, in terms of the exhaust velocity, is dependent solely on raising the operating temperature of the fuel elements; there is more than adequate power available, from fission, to generate useful thrust."
- "Intel Pentium Processor Packing Identification Codes". Intel Corporation. 2004-05-12. Retrieved 2010-01-27. Intel's packaging indicates the processors operating temperature range by denoting it with a grade: 'Q' (commercial grade), 'I' (industrial grade), and 'L' or 'T' (extended grade). It also has an automotive grade 'A'.
- "Intel Xeon Processor — Thermal Management". Intel Corporation. Retrieved 2010-01-27.
- Crompton, Thomas Roy (2000). "Effects of operating temperature on service life". Battery reference book. Newnes. ISBN 978-0-7506-4625-3.
- Dougal, Robert A.; Gao, Lijun; Jiang, Zhenhua (2 February 2005). "Effectiveness analysis of energy reclamation from partially depleted batteries". Journal of Power Sources (Elsevier B.V.) 140 (2): 409–415. doi:10.1016/j.jpowsour.2004.08.037.
- Benloucif, S.; Guico, M.J.; Reid, K.J.; Wolfe, L.F.; L'Hermite-Baleriaux, M.; Zee, P.C. (2005). "Stability of Melatonin and Temperature as Circadian Phase Markers and Their Relation to Sleep Times in Humans". Journal of Biological Rhythms (SAGE Publications) 20 (2): 178–188. doi:10.1177/0748730404273983. ISSN 0748-7304. PMID 15834114. Retrieved 2010-01-27.
- Marx, John (2010). Rosen's emergency medicine: concepts and clinical practice (7th ed.). Philadelphia, PA: Mosby/Elsevier. p. 1870. ISBN 978-0-323-05472-0.
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