Visibility of the tephra is enhanced if a white sheet or cardboard is used as a backdrop and drop cloth. Gravity will help pull the sand down the tube. If the tube gets clogged tap it with your fingers or a pencil. Keep the tube as dry as possible. Here is an alternative set up.
Air pressure is supplied by a can of compressed air.
The sand is deposited downwind just like tephra blown downwind from lava fountains. If this set up ran long enough a cone would form. Coarse sand particles would be close to the vent and finer sand particles would be farther away. This set up with the air jet worked differently. By placing a thumb over the open nozzle by the T-joint pressure would build up in the tube and then eject much of the sand in the tube all at once.