Is there a low velocity threshold for the LISST-SL? Does it need a minimum flow speed in order to sample? What is that?

Absolutely! The internal pump in the SL will adjust the intake velocity to match the ambient flow speed in order to make the sample intake isokinetic. However, it is necessary to maintain a minimum pump velocity in order to avoid settling of particles inside the instrument. This cut-off velocity where the pump velocity will start to exceed the ambient stream flow speed is ~ 0.5 m/s. The pitot-static tube will still function at speeds lower than 0.5 m/s, but the sample becomes less and less isokinetic at flow speeds lower than 0.5 m/s.