ABSTRACT
All commonly used measurement techniques are indirect in that they rely on physical interpretation of a measured surrogate quantity. For example, when pressure is measured with a membrane-based transducer, the deflection of the membrane is sensed and the resulting output is interpreted as being due to a change in pressure. Interactions between the flow, the facility, and the measurement instrument affect the physical relation between the quantity that is being measured and the output of the instrument. To examine instrument-flow and instrument-facility interaction, velocity measurements may be considered for illustration purposes. Most commonly used velocity measurement techniques fall into one of two categories: Intrusive instruments that directly measure a flow property that can be related to flow velocity; and, Non-intrusive instruments that track the motion of particles or tracers suspended in a flow. The sensors of all intrusive instruments and some non-intrusive instruments must be immersed in the flow to collect data.
