What happens to the meter reading when a thermistor is connected to a system being evacuated?

When a thermistor is connected to a system being evacuated, it measures the changes in pressure as the vacuum is created. The behavior of the thermistor during this process is critical to understanding its functioning.

As the system is evacuated, the gas molecules are gradually removed, leading to a reduction in pressure. However, thermistors can experience a phenomenon known as "thermistor heating" where the electrical current passing through the thermistor causes it to generate heat. This heating can influence the readings by changing the resistance of the thermistor, which in turn affects the output that the meter displays.

When significant vacuum levels are achieved, the small residual gas present can lead to what is termed a "micron reading" because the thermistor is essentially still detecting the minuscule amount of gas in the system. Therefore, as the vacuum level increases, the meter will register these changes in pressure, evidenced by readings measured in microns.

This understanding of how thermistors function in an evacuated system helps to clarify why the correct answer highlights that the meter indicates a micron reading due to thermistor heating. The unique attributes of the thermistor in relation to pressure measurement during the evacuation process are crucial for effective monitoring and troubleshooting in refrigeration and air conditioning systems.