A monitoring technique, previously used in systems with flow control by an orifice plate, has been applied to flows of sand in a downcomer discharging to atmosphere and controlled by an L-valve. A bubble of high-pressure gas is released into the sand flow. This generates a pressure wave, which is sensed at points upstream and downstream of the impulse source. The lag between the arrival of the pressure wave at the downstream and upstream sensors is measured. Time lags increased with increasing sand flux, up to about 120 kg m(-2) s(-1), but at low fluxes in particular, the trend was highly non-linear. (C) 2004 Elsevier B.V. All rights reserved.