I have just installed a new studer vent that is above counter top ( w stainless cap). The drain is coming form the dish washer and up to the vent and supposedly to the outer drain, but some of the water backs up into the drain and I have to cut the dish washer off . Why is this happening

That sounds more like a dishwasher air gap than a Studor AAV. Are you possibly just confusing terms? That chrome cap on the counter has two hoses coming from it, one to the dishwasher drain pump and the other to your sink drain. One hose is larger than the other, where do you have the larger hose going, to the sink or to the washer?

That sounds more like a dishwasher air gap than a Studor AAV. Are you possibly just confusing terms? That chrome cap on the counter has two hoses coming from it, one to the dishwasher drain pump and the other to your sink drain. One hose is larger than the other, where do you have the larger hose going, to the sink or to the washer?
Bob's correct. You have described a dishwasher air gap, (see image). Dishwashers don't require vents. You say,
some of the water backs up into the drain and I have to cut the dish washer off
Do you mean that some of the water drains back into the dishwasher after the pump cuts off? Please explain. Regards, Tom