What is Negative Pressure?

If you have ever been confused about the term negative pressure, you are not alone. One reason the concept is so confusing is that negative pressure is not really negative at all. It is actually positive pressure, but it is negative in relation to atmospheric pressure.    

Let me explain. Early in my career (while working for an Ashcroft distributor) I had a difficult time with the term ‘negative pressure’.  The only thing that I knew about pressure was filling up a balloon, a ball or a tire. How could pressure be negative? It didn’t make sense! I asked anyone and everyone to explain but never received a satisfactory answer.

Today, as a Territory Sales Manager for Ashcroft, a recognized leader in pressure and temperature instrumentation - and 16 years of experience under my belt - I now have a clear understanding of negative pressure, and I am in a position to share my insight. In this article, you will not only learn how to answer the question, "What is Negative Pressure?", you will also understand how it relates to other types of pressure.

At the end of the article, you will also see links to additional resources that may be beneficial to you.  

What does the term negative pressure mean?

To understand negative pressure, you first need to be familiar with the concept of vacuum (a space devoid of matter). In the beginning, I generally understood that, in refrigeration and air conditioning systems, a technician needs to pull a vacuum (purge a system of all matter, air, contaminants, etc.).  But it wasn’t clear how the terms vacuum, negative pressure, positive pressure and other engineering units all tied together.  So I went back to the basics and referred to some core definitions:

Atmospheric pressure. This is the pressure within the atmosphere of Earth.  Air has weight and exerts a force upon the earth.  This pressure varies slightly but can be approximated at 14.7 psia, 760 mmHg, or 30” Hg (mercury).  We all live our lives under this constant (and somewhat variable) pressure. 
Psi (pounds per square inch).  PSI is generally a gauge pressure where zero corresponds to atmospheric pressure (14.7 psia).  In other words, when a gauge pointer is reading zero, the pressure is already at 14.7psia.  Note: PSI is sometimes referred to as PSIG for pounds per square inch gauge. 

Psia (pounds per square inch absolute). With PSIA, the zero point on the gauge scale begins at absolute zero pressure, also known as full vacuum. It is rare to see a mechanical pressure gauge reading in PSIA.  It is much more common to see a compound gauge (definition below).  A compound gauge and a PSIA gauge are essentially the same gauge but with different markings/ engineering units. 

Vacuum gauge. This is a pressure gauge where zero is positioned on the right side of the dial and the pointer moves left towards a full vacuum of -30” Hg (absolute zero).  Remember, the zero on the right represents gauge zero or 14.7 psia while the -30” Hg represents a full vacuum or absolute zero pressure.  

Figure 1. Vacuum gauge example.

Compound gauge. This pressure gauge starts with a full vacuum (-30” Hg) beginning on the left side of the dial, includes gauge pressure zero (atmospheric pressure) and extends to the positive gauge pressure range as the needle moves to the right.  See illustration.  

Figure 2. Compound gauge example. 

Negative pressure explained.

Understanding the definitions above sets the context for how to explain negative pressure. Basically, negative pressure refers to pressures below atmospheric pressure, but above absolute zero pressure. See the attached illustration.  The term negative pressure is somewhat of a misnomer Because, in reality, pressure cannot be negative—it is always zero or positive. 

However, since vacuum pressures fall below zero on a gauge pressure scale  (psig), we use the term negative pressure because values below zero are considered negative.  Similarly, we refer to everything above gauge pressure zero as being in the positive pressure range when referring to a compound gauge.

Key negative pressure takeaways

If you are new to instrumentation these terms can be confusing.  Hopefully, the information and illustrations presented here help simplify the terms negative and positive pressure and how they are commonly used.  Here are a few points to remember:

• -30” of Mercury (Hg) represents a full vacuum AND absolute zero pressure.
• Zero psig represents atmospheric pressure.  
• A mechanical pressure gauge’s zero is already under 14.7 psia because the gauge itself is under the same atmospheric pressure as everything else.  
• With mechanical devices, there is no way to subtract out the constant atmospheric pressure.  
• Remember to concentrate on understanding why the zero would be at the two different starting points!  
  

Ready to learn more?

Now that you know how to define negative pressure, you may want to deepen your knowledge about the different types of pressure gauges. Here are a few helpful resources to help you take the next step in building your knowledge on the subject: 

•    What is a Pressure Gauge?
•    What is a Differential Pressure Gauge?
•    What are the Advantages of Ashcroft Pressure Gauges?
•    How do I Select a Pressure Gauge Range?

You can also contact us directly. Our industry experts can help answer your questions. In the meantime, feel free to download our guide.