Magnetic Surface thermometers are thermodynamically designed to measure surface temperatures. It has been calibrated for use in an ambient temperature of approximately 70°F (21°C). Higher surrounding air temperatures will cause very slightly higher readings and lower surrounding air temperatures will cause very slightly lower readings. For accurate measurements the thermometer should be used in still air. The surface to be measured should be flat and smooth as more intimate contact will provide more accurate readings. The greater the thermal mass of the source, the more accurate the readings.