The effects of pressure may seem mundane and routine, but they’re incredibly important to our every day lives. Pressure runs much of our industrial processes, and will forever remain one of the most important process measurements of all time.
Absolute pressure is especially cool because it uses a full vacuum as its reference point.
Every pressure measurement we take is a comparative, or relative, measurement – using a reference pressure as a baseline. Gage pressure uses current atmospheric pressure (what we think of zero pressure, which is actually 14.7 psi at sea level). Sealed pressure uses atmospheric pressure at the moment and location it was sealed. But absolute pressure is sealed under a full vacuum, thus using absolute zero as its baseline.
There are a lot of advantages to using absolute pressure transducers, particularly when you want something sealed to eliminate any chance of fouling due to particulate matter like dust and moisture.
A regular sealed pressure sensor uses a moving target as its reference point. That pocket of air will expand and contract based on what is happening in the atmosphere, affected by simple things like weather and elevation. Absolute pressure, however, has a fixed baseline and won’t need the constant attention that a sealed pressure sensor needs in applications at low pressures.
The disadvantage, however, is that you’re never quite sure how much pressure the atmosphere is adding to the equation unless you’re measuring that separately. So in applications where you want to automatically adjust for atmospheric pressure, gage pressure is the way to go.
That said; let’s take a look at the fascinating world of vacuum pressure and how we achieve a full vacuum during manufacturing:
In order to use vacuum as a reference point, the area behind the pressure sensor’s diaphragm must be sealed under a full vacuum. So the first order of business is creating the vacuum. This is done in a vacuum chamber.
A vacuum chamber is a relatively simple gadget. It works by pumping all the air molecules out of a container. The structural integrity of the container and the capabilities of the pump are the two most difficult pieces of the puzzle. The chamber may not be able to create a perfect vacuum, but it gets very close, pumping out all but a negligible amount of air.
Once all the air is pumped out of the chamber, you can then conduct your tests, demonstrate some experiments, or seal a pressure sensor’s reference point.
While vacuum chambers themselves are pretty fascinating, the process of the sealing a pressure sensor inside a vacuum chamber is not. Once the vacuum is created, the reference chamber behind the diaphragm of the sensor is simply welded shut.
This has to be done with either a little robot welder, or through glass with a laser welder. Once welded, the reference chamber in the sensor allows a measurement of any and all pressure, including atmospheric.
Now for some fun. Vacuum chambers give a unique opportunity to see how well adjusted everything on earth is to our atmospheric pressure of approximately 15 psi. Here are three videos that are fascinating in their own right:
Our first one is Professor Brian Cox visiting the world’s largest vacuum chamber. This aired on his BBC show Human Universe. Notice how these seasoned scientists react to an experiment we all learned in elementary school:
Next, a local news channel hosts scientist Carl Nelson demonstrates how peeps (as in the marshmallow treats) react to a vacuum chamber:
Finally, MythBuster’s Adam Savage demonstrates a vacuum chamber of his own construction that's supposed to simulate how difficult the vacuum of space makes simple movements and dexterity:
