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Measuring air temperature in a room means adequate immersion is almost guaranteed.
However, air and gases are not efficient heat transfer media so even a modest heat input to the sensor, such as from lighting or other sources of radiation, can cause it to read high. Sensor should be protected from these, as well as from sunlight, draughts and convection heaters.
Suspend the sensor in a protective perforated cage some distance below the ceiling, away from heat sources, doors and windows. It helps if the air is gently circulated in a controlled manner, as this improves both the temperature uniformity and the heat exchange with the sensor, and so makes the measurement more meaningful. It may also be useful to attach fins to the probe, to increase the area for heat exchange. Make sure that the measurement process does not dissipate too much heat in the sensor.
Similar considerations apply to air temperature measurements in an environmental chamber or oven, whose temperature may be very different from ambient. Good immersion may be more easily achievable than in liquids, because the probe is more compatible with the medium, but the poor heat exchange with the air makes small heat flows more significant.
Follow the advice given above for room air, but more vigorous circulation of the air will be needed to achieve uniform conditions. Take particular care to shield the sensor from heaters (using a shiny probe will help).
MIMS (mineral-insulated metal-sheathed) thermocouples are commonly used in industrial sensing, especially at very high temperatures, and in reactive gases such as in combustion processes (power generation, aeroengines, etc). The sheath provides protection against the environment, and electrical insulation in the cable is provided by high purity compacted magnesia or alumina powder. The cable diameter may be ~1 mm in diameter or up to 8 mm, and in practice there may be a difficult compromise between speed of response, which requires a lightweight sensor, and physical and chemical durability, which calls for more substantial protection.
The sheath material, stainless steel, Inconel or another special alloy, must be chosen to provide an effective barrier to chemical attack from the medium. It should not contain elements, such as manganese, which may diffuse in the cable and contaminate the thermocouple wires. The supplier should advise, and some guidance is given in documentary standards.
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