How do I test my vacuum gauge?

We get a lot of calls inquiring about how to test vacuum gauges. “How do I test my core tools and how do I test my hoses to see if things are leaking?”

It’s actually a pretty tricky science because it’s not about leakage. It’s about a process called adsorption (Yes, with a “d”).

Adsorption is a surface’s ability to hold moisture at a molecular level. On a molecular level moisture is bonded to a surface. So it’s not anything like liquid water that you could see. It’s almost like a sponge where the metal will absorb moisture to bond at a molecular level and then those moisture bonds have to break through a process called desorption. Desorption takes time.

About the only reference we have in the field to see if things are working correctly, first of all, is with our vacuum pump. What we show in the video below is a NAVAC 12 cfm pump. Before we run our test, there are a couple things to be aware of.

Be aware of how your micron gauge is rated. For instance, the AccuTools micron gauge is rated at +/- 5 microns. So if your gauge is reading 25 microns, it could be as low as 20 or has high as 30.

The pump is rated at 5 microns, so if you see your gauge at 25 microns then you might think that your pump is defective, but it’s not. The pump is rated for 5 microns, with fresh dry oil in it, with the right type of oil. Using a higher quality oil will give you better performance than a lower quality. So as a general rule, any pump that can pull below 30 microns is performing well.

As we test the core tools in the video there are things to consider. Once you hook up a core tool, you will lose some vacuum because you’re including more components for the pump to pull down. We have increased the surface area for desorption to take place, which means those things are wet. For instance, if your core tool is rated to 20 microns, it may not achieve 20 microns right away because it’s the leak rate that it has to overcome. So when they say 20 microns, I have to have something that can pull down below 20 microns. In other words, if my pump can pull down to zero, then the core tool would show 20. So if my pump can only pull down to 20 then I should expect my core tool to read around 40 microns.

So if you have your micron gauge hooked up to a vacuum pump with a core tool and you close off the ball valve, you will see your micron gauge climb. This is not a leak. This is trapped gases and moisture that’s on the side of the ball valve, that’s increasing in pressure. So when it’s completely isolated, you will still see the gauge climb and you might think “I have a defective core tool.” But you don’t. There is a small bit of volume (between the closed off ball valve and gauge) and a lot of surface area, so it’s just moisture desorption taking place and building up pressure against the gauge. Take a look at the video to see the difference in performance between the AccuTools core tool and the Appion core tool.