I've been in the process of doing experiments requiring 28- 30 Hg vacuum. For the last couple of weeks, everything seemed to be going well with a glass aspirator I acquired as it would pull the necessary vacuum, when hooked up to the tap, in about 45 seconds. Then, as luck would have it, while trying to unhook the tubing, the small wrench I was using fell and smashed the aspirator. Since the same firm I purchased from no longer had any more, I decided to go with a metal unit with the name in the subject. With virtually no change in the setup, just switching out the broken glass unit for the new metal one, I proceeded to try and draw vacuum tonight but despite all efforts, this until will only pull up to 16 Hg, and that's with both faucets turned to full. I'm not sure what could be wrong. There are no leaks and hook up is correct. Is it possible that this unit requires much more pressure than the prior glass unit, or could it possibly be defective? Anything I could try would be welcome. Thanks in advance.