If you could get the vacuum it would be a great idea. My experience with venturis however have always been that they are hard to get right. I seriously doubt that merely inserting a tube in the air stream will give you the vacuum you need. Perhaps a better choice would be a manifold placed somewhere in the middle of the input side of the fan. I know you could get the vacuum you need there, but you would have to be careful not to pull too much. If you started pulling leaves and straw it would probably end up beneath the sieves. Ouch! dirty sample. Ed in MT