What type of metering device consists of seamless tubing with a small inside diameter?

The capillary tube is a type of metering device characterized by its seamless construction and small inside diameter. This design allows the tube to control the flow of refrigerant in a cooling system effectively. By relying on the pressure difference between the high-pressure side and low-pressure side of the system, the capillary tube facilitates the precise throttling of refrigerant, which is essential for maintaining the desired cooling effect.

In systems utilizing capillary tubes, they do not have any moving parts or additional mechanisms, making them straightforward and reliable for certain applications like small cooling systems, refrigerators, and air conditioning units. Their simplicity in design aids in reducing maintenance needs, which can be advantageous in various setups.

While the other metering devices listed have their specific applications and mechanisms—such as electronic expansion tubes that utilize electronic controls for more nuanced flow adjustments, and thermal and automatic expansion valves that rely on temperature and pressure to regulate refrigerant flow—they differ notably in their construction and operational methodology when compared to the basic functionality of the capillary tube.