Measurement stability
Take into account that instruments require sufficient time to adjust to the ambient temperature. The following rule-of-thumb for a high precision measurement applies:
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Temperature difference in degree Celsius (°C) × 2 = duration in minutes required for the instrument to adjust to the new temperature.
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Fahrenheit: Temperature difference in degrees Fahrenheit (°F) = duration in minutes required for the instrument to adjust to the new temperature.
Avoid sighting across fields with intense heat shimmer by sunlight, e.g. at noon.