Operating temperature

Anoperating temperatureis the allowabletemperaturerange of the local ambient environment 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 theminimum operating temperatureto themaximum operating temperature(orpeak operating temperature). Outside this range ofsafe operating temperaturesthe device may fail.

It is one component ofreliability engineering.

Similarly, biological systems have a viable temperature range, which might be referred to as an "operating temperature".

Ranges

Most semiconductor devices are manufactured in several temperature grades. Broadly accepted gradesare:

• Commercial: 0 °C to 70 °C (32 to 158 °F)
• Industrial: −40 °C to 85 °C (−40 to 185 °F)
• Military: −55 °C to 125 °C (−67 to 257 °F)

Nevertheless, each manufacturer defines its own temperature grades so designers must pay attention todatasheetspecifications. For example,Maxim Integrateduses five temperature grades for its products:

• Full Military: −55 °C to 125 °C (−67 to 257 °F)
• Automotive: −25 °C to 125 °C (−13 to 257 °F)
• AEC-Q100 Level 2: −40 °C to 105 °C (−40 to 221 °F)
• Extended Industrial: −40 °C to 85 °C (−40 to 185 °F)
• Industrial: −20 °C to 85 °C (−4 to 185 °F)

The use of such grades ensures that a device is suitable for its application, and will withstand the environmental conditions in which it is used. Normal operating temperature ranges are affected by several factors, such as the power dissipation of the device.These factors are used to define a "threshold temperature" of a device, i.e. its maximum normal operating temperature, and a maximum operating temperature beyond which the device will no longer function. Between these two temperatures, the device will operate at a non-peak level.For instance, aresistormay have a threshold temperature of 70 °C (158 °F) and a maximum temperature of 155 °C (311 °F), between which it exhibits a thermalderating.

For electrical devices, the operating temperature may be thejunction temperature(T_J) of thesemiconductorin the device. The junction temperature is affected by the ambient temperature, and forintegrated circuits,is given by the equation:

$T_{J}=T_{a}+P_{D}\times R_{ja}$

in which T_Jis the junction temperature in °C, T_ais the ambient temperature in °C, P_Dis the power dissipation of the integrated circuit inW,and R_jais the junction to ambientthermal resistancein °C/W.

Aerospace and military

Electrical and mechanical devices used in military and aerospace applications may need to endure greater environmental variability, including temperature range.

In theUnited States Department of Defensehas defined theUnited States Military Standardfor all products used by the United States Armed Forces. A product's environmental design and test limits to the conditions that it will undergo throughout its service life are specified inMIL-STD-810,theDepartment 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 extremetemperature loads.

Military engine turbine blades experience two significant deformation stresses during normal service,creepandthermal 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,microprocessorsproduced byIntel Corporationare manufactured to three grades: commercial, industrial and extended.

Because some devices generate heat during operation, they may requirethermal managementto ensure they are within their specified operating temperature range; specifically, that they are operating at or below the maximum operating temperature of the device.Coolinga 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 mitigationstrategies 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 (68 °F) versus temperature.Ohmic loadand operating temperature often jointly determine a battery's discharge rate.Moreover, if the expected operating temperature for aprimary batterydeviates from the typical 10 °C to 25 °C (50 to 77 °F) range, then operating temperature "will often have an influence on the type of battery selected for the application".Energy reclamation from partially depletedlithium sulfur dioxide batteryhas been shown to improve when "appropriately increasing the battery operating temperature".

Biology

Mammals attempt to maintain a comfortable body temperature under various conditions bythermoregulation,part of mammalianhomeostasis.The lowest normal temperature of a mammal, thebasal body temperature,is achieved during sleep. In women, it is affected by ovulation, causing a biphasic pattern which may be used as a component offertility awareness.

In humans, thehypothalamusregulatesmetabolism,and hence thebasal 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'sCircadian rhythm.

Changes to the normalhuman body temperaturemay result in discomfort. The most common such change is afever,a temporary elevation of the body's thermoregulatory set-point, typically by about 1–2 °C (1.8–3.6 °F).Hyperthermiais an acute condition caused by the body absorbing more heat than it can dissipate, whereashypothermiais a condition in which the body's core temperature drops below that required for normal metabolism, and which is caused by the body's inability to replenish the heat that is being lost to the environment.

References

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