I've had better luck using a knife valve as opposed to a flapper valve as shown... My flapper always kicked sand everywhere when it snaps shut... The problem with a knife valve is that you need to position the actual "knife" far enough up the tube so there is never a chance of finger amputation! We have ours attached to a spring, and when the structure breaks, the supervisor can pull a string which immediately disconnects the knife valve from the student lever and allows the valve to snap shut. This eliminates any possibility of excess sand being run into the bucket after failure (in theory). Also, we use a "Black Diamond" sand blasting sand. no dust and a little larger grains, so there is minimal leakage around the valve blade. We bought ours a TSC.
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