threshold velocity

1. n. [Production Logging]
In a spinner flowmeter, the theoretical minimum fluid velocity required to initiate spinner rotation, assuming the spinner response is linear. The actual fluid velocity required to start spinner rotation is slightly higher because of additional viscous and mechanical effects. The threshold velocity is determined by extrapolating the spinner response at higher fluid velocities, where it is known to be nearly linear, back to the value that exists when spinner rotation is zero.
Synonyms: bypass velocity