What measurement tool would be most appropriate for determining pump efficiency?

The flow meter is the most appropriate tool for determining pump efficiency because it measures the volume of fluid that flows through the pump over a specific period of time. To assess pump efficiency, it's essential to understand how much water the pump is capable of moving compared to the energy input it requires to operate.

The efficiency can be calculated by determining the flow rate (using the flow meter) and then comparing it to the power consumed by the pump as it moves that water, which takes into account factors such as head and pressure. The output flow rate and the hydraulic energy required to achieve that flow are key indicators of how well the pump is performing.

Other tools listed, such as the pressure gauge, measure the pressure within the system, which is useful for checking system performance but does not directly provide information on the volume of fluid flow. A thermometer measures temperature, which is not relevant to pump efficiency calculations. A viscometer measures the viscosity of the fluid, which can influence pump operation but does not directly assess the efficiency of the pump itself. Hence, the flow meter stands out as the critical instrument for evaluating how well a pump is functioning in relation to its power input.