Which of the following is NOT a metering device?

The correct answer is the pump impeller. In the context of an ice facility operator, metering devices are specifically used to measure and control the flow of refrigerants or other fluids in a system. Low side floats and high side floats are common metering devices used in refrigeration systems that help manage the refrigerant levels and flow based on changes in temperature and pressure.

A capillary tube is another type of metering device that controls the flow of refrigerant through a system by utilizing its internal diameter to create a specific restriction, which allows for proper refrigerant expansion and is crucial in refrigeration cycle operation.

On the other hand, a pump impeller is not a metering device; it is a component of a pump that helps move fluids by converting rotational energy into fluid energy. While an impeller moves fluids and may be essential to the operation of a refrigeration system, it does not function to measure or regulate flow in the way that the other options do. Thus, the pump impeller stands apart from the defined category of metering devices.