What The Vacuum Gauge Says

Earth science experts easily understand what a simple vacuum gauge indicates, but it can become confusing for the rest of us. As straightforward as it may seem, it’s complicated in the coolest way. Even after developing vacuum equipment for over a decade, I still find myself speaking very slowly when the topic arises so my mouth doesn’t outpace my brain while I struggle to articulate my thoughts. Too many variables and scientific terms get intimidating and confusing fast.  Let’s fix that by approaching the topic in terms that relate directly to our interactions with a standard vacuum chamber and gauge like the Torr Kitchen equipment. 

The simple explanation is…..simple. Once a pump begins pulling a vacuum on a chamber, the pressure inside becomes lower than the outside environment. The gauge reading shows that difference. This is easy and mostly common knowledge. However, understanding what’s actually happening and what the gauge really says is a bit more involved.

Mechanics: How it Says What it Says

The mechanics of the gauge are old and simple.  It only measures the difference in ambient pressure between the inside and outside of the chamber. The pressure in the vacuum chamber can only be equal to or less than the ambient pressure, resulting in negative pressure readings. Conversely, a tire pump creates positive pressure because it pumps air into the tire rather than removing it, as we do with a vacuum. The internal mechanism employs a Bourdon tube or diaphragm that divides the case into two sealed areas.  One area maintains static ambient pressure, while the other is exposed to the chamber environment, which varies in pressure.

 

 

As the chamber pressure changes, the Bourdon tube or diaphragm is pushed or pulled, which mechanically deflects an arm and gear that moves the gauge needle to indicate the pressure difference between the two areas. 

The liquid inside the case is usually glycerin, which dampens and provides slight resistance to the needle as it moves. This aids the needle in moving smoothly and holding steady, and most importantly, protects the connected mechanism from damage during shipping and handling.

Once a pump pulls a vacuum on a chamber, the chamber's pressure becomes lower than the surrounding environment. The gauge needle indicates only that difference, but what it actually says is more interesting.

INSIDE & OUTSIDE

Simple concepts.  Lower pressure inside.  Higher pressure outside.  The difference between the two is shown on the gauge.

Lower pressure inside.  Higher pressure outside.  The difference is on the gauge.
Inside Pressure – Outside Pressure = Gauge Reading

 

The gauge reads in negative numbers to show the relation of each unit of pressure inside the chamber to be less than the outside.  The only information provided in the image is the pressure differential of -25 inHG.  The actual pressure inside or outside the chamber isn't shown anywhere.  What the gauge shows has little value without additional information.  The ambient pressure must be known and used with the gauge reading to determine the chamber pressure.

Ambient Pressure + Gauge Reading = Chamber Pressure    
(the gauge reading is a negative number)

Determining Ambient Pressure

(Numbers disclaimer:  The goal here is to understand the pressure dynamics without being weighed down by numbers.  Numbers will be rounded to easy-to-use numbers, and curves will be made linear.  For our purposes, the number simplification will have no appreciable effect. For a more in-depth study of the exact numbers, search “atmospheric pressure lapse rate,” for tons of
information.)

Ambient pressure, also called atmospheric pressure, is the force that the atmosphere exerts downward due to its weight. At sea level, it measures 14.7 pounds per square inch, or 29.92 inches of mercury (inHg). Other measures, like torr, bar, and kPa, are also used.  The inHg unit is convenient because it simplifies the challenge of estimating the approximate ambient pressure, and it is typically displayed on gauges for easy reading.

Ambient pressure constantly changes with temperature and air density due to weather systems, and more predictably with altitude. Weather variations shift too much and aren't particularly consequential for our purposes (but keep them in mind), so we will focus on how ambient pressure decreases as altitude increases. For every 1,000 feet of elevation gain, ambient pressure reliably decreases by 1 inHg up to around 10,000 feet. At sea level, ambient pressure is about 30 inHg (actually 29.92), which is why the last number on a vacuum gauge reads -30. If all ambient pressure is removed, that results in a perfect vacuum. A "perfect" or "full" vacuum doesn't truly exist, even in space, but we get so close that for practical purposes, these terms are used with an understanding of the very minute error. Therefore, if all pressure is removed at sea level, the gauge shows -30. What it indicates is that the chamber pressure is 0. 

Ambient Pressure + Gauge Reading = Chamber Pressure
30 + (-30) = 0

At or near sea level is the only time the gauge can reach a maximum of -30. Likewise, it's also the only location where the gauge reading can be used without adjustment. At -25 on the gauge, the chamber pressure would be 5 inHg. I know it might seem like I’m beating a dead horse by restating the same point in different ways, but here’s why.

UP IN ELEVATION, DOWN IN PRESSURE

Bakers and chefs are well aware of the effects of higher elevation and lower pressure. If overlooked, complications can arise and efforts may be wasted. This is precisely why understanding that for every 1,000 feet gained in elevation, ambient pressure decreases by 1 inHg is important. Consequently, for each 1,000-foot increase in elevation, the maximum reading on the gauge drops by 1 inHg (actually "increased" when discussing mathematically with negative numbers) from the maximum of -30. For example, at 3,000 feet in Boise, Idaho, the ambient pressure is 27 inHg, making the maximum possible gauge reading -27 inHg.

 

This pressure lapse rate is why most people won't see the needle on the gauge reach -30 and may think their pump isn't working well. Using the elevation pressure adjustment makes it easy to see what the maximum reading on the gauge should be.

Burp The Gauge

Earlier, it was noted that the gauge is divided into two sections, one of which measures ambient pressure. It operates with minimal maintenance, but one minor adjustment requires periodic attention.  The gauge must be periodically equalized to function correctly and recognize the ambient pressure as 0.

A small black plug at the top of the case serves two purposes: it allows ambient pressure to enter and seals tightly to prevent the glycerin from spilling out. These two functions contradict each other, meaning the seal must be occasionally burped to equalize it with the ambient pressure. This should be done for new units and whenever relocating. Burp the gauge by removing the plug for a few seconds and then replacing it.

Gauge reading indicates the direct differential reference between chamber and ambient pressures, not the actual chamber pressure: Chamber Pressure – Outside Pressure = Gauge Reading.

The negative numbers on the gauge easily show where full vacuum would be at the location:
Boulder, Colorado ambient pressure is 25 inHG, so full vacuum on gauge would be -25.

If the dial indicates anything other than 0 when no vacuum is applied, the gauge says, "Burp Me!"