Human body temperature to give the most accurate assessment of core body temperature, particularly in hypothermia. In the early 2000s, ingestible thermistors in capsule form were produced, allowing the temperature inside the digestive tract to be transmitted to an external receiver; one study found that these were comparable in accuracy to rectal temperature measurement. In the 19th century, most books quoted "blood heat" as 98 °F, until a study published the mean (but not the variance) of a large sample as . Subsequently that mean was widely quoted as "37 °C or 98.4 °F" until editors realised 37 °C is closer to 98.6
Operating temperature Similarly, biological systems have a viable temperature range, which might be referred to as an "operating temperature". Most devices are manufactured in several temperature grades. Broadly accepted (?) grades are: Nevertheless, each manufacturer defines its own temperature grades so designers must pay close attention to actual datasheet specifications. For example, Altera uses five temperature grades for its products: 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.