Which instrument is used to check the pressure drop across an indoor coil?

The correct instrument for checking the pressure drop across an indoor coil is an inclined manometer. This device is specifically designed to measure small pressure differences, making it ideal for applications where precise pressure readings are necessary, such as across coils in heating, ventilation, and air conditioning (HVAC) systems.

An inclined manometer operates on the principle of hydrostatic pressure, using a column of liquid (often water or mercury) to measure pressure differentials. The inclination of the tube allows for more sensitive readings over small pressure changes, which is crucial when assessing the performance of indoor coils that may have subtle pressure drops that impact efficiency.

In contrast, a velometer measures air velocity, which does not provide the necessary information regarding pressure drops. An anemometer is primarily used to measure airflow; while it can provide insights into system performance, it does not directly measure pressure. A psychrometer measures humidity and temperature, making it irrelevant for assessing pressure drop across a coil. Thus, the inclined manometer is the appropriate choice for accurately evaluating pressure differentials in this context.