To obtain the correct superheat reading in a capillary tube system, the measurement must be taken:

In a capillary tube system, the correct superheat reading must be taken after the capillary tube suction line heat exchanger to ensure an accurate measurement. This is because the suction line heat exchanger is designed to increase the temperature of the refrigerant vapor coming from the evaporator before it enters the compressor. When the superheat is measured after passing through this heat exchanger, it reflects the true latent heat absorption that has occurred in the evaporator, allowing for an accurate assessment of the system's efficiency and proper refrigerant flow.

Measuring superheat before this heat exchanger would give a reading influenced by the lower temperature of the refrigerant before it has been heated appropriately. This could misrepresent the actual thermal state of the refrigerant, leading to potential diagnostic errors and improper adjustments. Accurate superheat measurements are critical for ensuring the system operates efficiently, preventing compressor damage, and maximizing overall system performance.