I need a vacuum pump to provide at least 21" Hg of vacuum. Looking at the cheap rotary pumps on ebay, this one seems to be way more than fit for the job.
Is there a way though I can regulate it though to provide lets say around 15-20" Hg? This isn't a vacuum chamber or something and I don't want to have a crazy amount of vacuum.
The device you're looking for is called a vacuum regulator and sits between the vacuum source and the process. This page from Equilibar shows some examples and provides links that explain how they work.
You can also use a vacuum breaker, which operates more or less like a pressure relief valve, but a vacuum regulator will offer more precise control.
There are a variety of low cost vacuum regulator and vacuum breaker options on eBay. I have never used any of those, but I have used a few high end, electrically controlled versions on automated systems.
So you just need to add a sensor that will turn off the pump when the pressure is in your acceptable range and turns it on again when the pressure rises too much.
The more volume is in the chamber the more time it will take for the pump or a leak to change the pressure. You can use that to increase the time between duty cycles.
