Installation errors may occur when an instrument is used in a manner different from how it was calibrated. For example, suppose that a temperature sensor is calibrated in a stirred, constant temperature bath. During calibration, the sensor is in thermal equilibrium with the circulating fluid, and the fluid and sensor temperatures are the same. Next, let the same sensor be used to measure the temperature of natural gas flowing through a pipe at low velocity. If the pipe wall temperature is different from the flowing gas temperature, convection heat transfer will occur between the gas and the pipe wall, conduction heat transfer will occur between the pipe wall and the sensor, and convection heat transfer will occur between the sensor and the flowing gas. In this example, the sensor would not be in thermal equilibrium with the flowing gas and the sensor temperature would be different from the flowing gas temperature.
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