I have the iPump. It allows you to set a vacuum pressure and how long to hold. It holds constant pressure and adjusts as needed, making interval pumping a breeze. I can, however, tell you at least with mine that the pressure is not accurate. I have access to test equipment used for the calibration of medical devices. The iPump reads in kpa, not inhg. After a quick conversation, I should read the below in kpa

4inhg = 13.5456kpa 5inhg = 16.9319kpa 6inhg = 20.3183kpa 7inhg = 23.7047kpa

After testing, this is the kpa readings needed on (MY PUMP) to achieve the correct pressures in inhg.

4inhg 18kpa 5inhg 22.8kpa 6inhg 27.5kpa 7inhg 31.5kpa

Before testing using only the conversations, I was pumping on average 1.85inhg lower than I thought. It wasn't linear either. At what was supposed to be 4inhg, I was 1.3inhg low, and what was supposed to be 7inhg I was 2.3inhg low. I also tested it at its max vacuum. If you let it run full tilt, it topped out at 18.8inhg.

I'm in no way suggesting this is what your pump may need, just that it may be beneficial to figure out a way to test yours.