Pressure – The Inner Working of the Basic Pressure Gauge

Everyday here at EXAIR we talk about pressure, specifically compressed air pressure. The other day I was looking up our model 9011, 1/4″ NPT Pressure Gauge , and it got me to wondering just how does this small piece of industrial equipment work. The best way to find out is to tear it apart.


Most mechanical gauges utilize a Bourdon-tube. The Bourdon-tube was invented in 1849 by a French watchmaker, Eugéne Bourdon.  The movable end of the Bourdon-tube is connected via a pivot pin/link to the lever.  The lever is an extension of the sector gear, and movement of the lever results in rotation of the sector gear. The sector gear meshes with a spur gear (not visible) on the indicator needle axle which passes through the gauge face and holds the indicator needle.  Lastly, there is a small hair spring in place to put tension on the gear system to eliminate gear lash and hysteresis.

When the pressure inside the Bourdon-tube increases, the Bourdon-tube will straighten. The amount of straightening that occurs is proportional to the pressure inside the tube. As the tube straightens, the movement engages the link, lever and gear system that results in the indicator needle sweeping across the gauge.

Pressure Gauge Top

The video below shows the application of air pressure to the Bourdon-tube and how it straightens, resulting in movement of the link/lever system, and rotation of the sector gear –  resulting in the needle movement.

If you need a pressure gauge or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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Why 5 PSIG Matters

Last week I pointed out the important locations for measuring your compressed air system pressure throughout your compressed air system.   One of the critical points to measure system pressure was before and after each filter.  This leads into another question that I receive every once in a while, “How do I tell when the filter needs to be changed?”  The answer to this is easy, when you see more than a 5 PSIG pressure drop across the filter.  This means that the element within the filter has become clogged with sediment or debris and is restricting the volume available to your downstream products.

EXAIR 5 micron Auto Drain Filter Separator


This can lead to decreased performance, downtime, and even the possibility of passing contaminants through the filter to downstream point of use components.  In order to maintain an optimal performance when using EXAIR filter separators and oil removal filters, monitoring the compressed air pressure before and after the unit is ideal.

Replacement filter elements are readily available from stock, as well as complete rebuild kits for the filter units. Changing the filters out can be done fairly easily and we even offer a video of how to do it.

The life expectancy of a filter element on the compressed air is directly related to the quality of air and the frequency of use, meaning it can vary greatly.  If you tie a new filter onto the end of a compressed air drop that has not been used in years, you may get a surprise by the filter clogging rather quickly.   However, if you maintain your compressor and your piping system properly then the filters should last a long time. Generally we recommend checking your filters every 6 months.

If you have questions about where and why to filter your compressed air contact us.

Brian Farno
Application Engineer


Line Vac Troubleshooting / Alumina Ball Conveying

Recently, I had a good interaction with a customer on a Line Vac Application. You can read through the e-mail below to get a better understanding of the issues at work in a typical conveying application.

Dear Sir,

We have Line Vac model 6084 operating at between 80 & 90 psig. We are trying to convey alumina balls to a height of about 12 ft. The balls are spherical with diam. between 4 & 7mm (about 1/4 inch). Packed density =50lbs/ ft3 actual density must be in region of 60lbs/ft3. We are only able to convey a very small amount of alumina. Feeding more than a nominal amount blocks the feed tube. Can you please help?


Hello Robert,

Yes, I think we have a few ideas you can consider to make the application work a little better.

First thing you must do is to verify with zero doubt about the actual, net operating pressure at the Line Vac inlet. The way you do this is to install a pressure gauge onto a pipe tee and install the pipe tee into the inlet of the Line Vac. Then re-connect your compressed air supply to the 3rd leg of the pipe tee. Operate the Line Vac and note your working pressure on the gauge. If it is less than 80 PSIG, you could do better on the pressure by up-sizing your feed lines and all fittings that are included within. Re-test the Line vac as outlined above and note any improvement in net working pressure.

OK, let us assume you were able to fix the supply problems that contributed to the excessive pressure drop. You can then make a modification to the Line Vac itself by taking it apart and removing the internal part called a generator. This is the part with the air jet holes drilled into it. You can proceed by enlarging these holes to a larger diameter. This will give you higher vacuum performance similar to our Heavy Duty Line Vac series of Line Vac. Do note that if you do not check your plumbing supply lines as indicated in the first paragraph, making this modification could make things worse. So, you do need to be confident that your compressed air system is up to the task.

Another direction you can take will be to go to a smaller size Line Vac to improve your performance. This may seem counter-intuitive at first, but have an open mind.  Going smaller size on Line Vac reduces your air consumption requirement which makes the demand on your compressed air system less which reduces the chances of you have an extreme pressure drop at the Line Vac. The other idea at work here is that you are essentially trying to pull a vacuum over a smaller cross-section area with a smaller Line Vac which enables higher air velocity within the conveying tube with less effort. That higher conveying air velocity is what you need to pull these relatively dense alumina balls without having them stall inside the conveying hose.

If I were to suggest a size to move down to, I’d go with a 1-1/4″ unit from the 2″ model you have now. That would take your air requirement at 80 PSIG down from 45 SCFM to 26 SCFM. In other words, cutting the air required by almost half.  Hopefully, now you are beginning to have the understanding. And then, you can make the same hole enlarging modification to the 1-1/4″ unit that I described above if you wanted to in order to improve through put by about 20 – 30%.  Do note that you have to be able to maintain input pressure right at the Line Vac in all cases.

Best regards,
Neal Raker, Application Engineer