Start SLOWLY. It takes time for your penis to get used to pumping. At least initially, keep the pressure steady at 3.5Hg for your first few sessions so you can see how your penis reacts. Watch for bruising and edema that doesn't go away. After two weeks or so, you can go to 5Hg for brief periods, again watching for anything like blood blisters, bruising, or edema. I believe that a 5min warm-up and warm-down is important to help prevent complications. Some guys will find that 5Hg for extended periods is possible, others will not. Much depends on your particular body. These low pressures may not feel like much but they do the job.