Final answer:
An orifice plate is the correct metering device for applications with a constant load and when manual adjustments can be made by an operator. Although it's simple and effective for certain conditions, it is not commonly used where load varies or no operator is present, unlike Venturi tubes or Pitot tubes used for different purposes.
Step-by-step explanation:
The metering device described in the question is typically used in systems where a constant load is present, and adjustments can be manually made by an operator. Among the devices listed which include an orifice plate, Venturi tube, flow nozzle, and Pitot tube, the correct answer is the orifice plate. Orifice plates are basic devices used to measure flow rate or reduce pressure in a pipeline; they provide a restriction through which fluid passes, creating a pressure drop that can be measured to infer the flow rate. However, due to their simplicity and the need for manual adjustments to handle varying flow conditions, they are not commonly used in situations where the load is variable or no operator is present.
On the other hand, a Venturi tube, which is also mentioned in the question, uses a constriction to measure flow but is more commonly used due to its ability to handle varying flow rates without the need for constant adjustment. Flow nozzles are similar to Venturi tubes but are used for high-velocity, non-viscous flows. A Pitot tube, as described in the reference material, is primarily employed for velocity measurements in a flow and not for metering purposes.