# A Brief Introduction to Pressure

## What is Pressure?

Pressure (P) is defined as the amount of force (F) exerted over an area (A), or P = F/A. But, where does the force come from? Imagine a multitude of tiny particles moving in a liquid or gas, bouncing off each other and the container they are in. The force comes from their mass (m) multiplied by the acceleration (a) of these particles as they hit the container wall (F = ma).

We usually speak of the pressure of a gas or a liquid in a container, but pressure also exists in the closed system of our atmosphere and oceans - the highest pressure being at the bottom of the ocean and decreasing to zero pressure as you leave the Earth's atmosphere. As Evangelista Torricelli once said, "We live submerged at the bottom of an ocean of the element air."

## Pressure Types: A Video Explanation

In the pressure calibration business, we get the question "What's the difference between gauge pressure and absolute pressure?" often. The answer is simple and comes down to what the pressure is referenced to. Essentially, all pressure is measured as a difference between a reference pressure and a measured pressure. For gauge pressure, the reference is ambient atmospheric pressure; for absolute pressure, the reference is absolute zero pressure or the absence of pressure.

The following video offers a graphical representation of the differences between gauge and absolute pressure, as well as barometric, bidirectional, and vacuum pressure.

## Absolute Pressure

Absolute pressure is zero-referenced against a perfect vacuum. It is used to measure pressure relative to absolute zero. The ambient barometric pressure on earth is measured as an absolute pressure. It is the difference between the zero pressure of outer space and the pressure created by the atmospheric gases that cover the earth, held there by gravity.

When there is nothing within a space, there is nothing to create a pressure. This is the baseline used for comparison in an absolute pressure measurement. This means the lowest reading an absolute pressure instrument can reach is zero - absolute pressure cannot be negative. However, zero absolute pressure is virtually impossible to achieve. This and other complexities involved with zeroing absolute sensors is explained in the white paper: "Everything You Wanted to Know About Setting Zero on an Absolute Pressure Transducer, but Were Afraid to Ask."

## Gauge Pressure

Gauge pressure is referenced against atmospheric pressure. Gauge pressure is used when it is important to know how much a pressure differs from atmospheric pressure. For example, a vessel containing a pressurized gas is subjected to a force on the interior vessel walls. The exterior walls of the vessel also experience a force from the ambient atmospheric pressure. The difference between these pressures must remain below the burst pressure of the vessel, or the vessel could explode. This difference is called gauge pressure. When a gauge sensor's pressure port and reference port are both open to atmospheric pressure, the reading should be zero.

Low gauge pressure measurement and calibration can be tricky. As a leader in low pressure instruments for many years, Mensor has developed special techniques to characterize and calibrate gauge pressure instruments with full scale ranges below 5 psi. The white paper "Low Pressure Measurements" describes these techniques and limitations.

## Differential Pressure

By convention, what we call differential pressure is similar to gauge pressure in that it also has two sides - the low side and the high side. However, the low side (sometimes called the line pressure) is not always atmospheric pressure. The low side can be atmospheric or any other pressure, limited by the pressure rating of the sensor. The high side can be higher than the low side, resulting in a positive differential, or lower than the low side, resulting in a negative differential. This is complicated by the fact there may be an offset at different line pressures. This type of sensor is sometimes used to measure flow across a restriction in a pipe; the differential pressure across the restriction is proportional to the flow.

Another good example of this type of sensor is the Qc sensor in Mensor's CPA8001 Air Data Test Set, which measures an aircraft's airspeed. Mensor has developed a way to increase the accuracy of this type of sensor to compensate for different line pressures.

## Vacuum Pressure

Vacuum pressure may mean different things to different people. We define vacuum pressure as a sub-atmospheric pressure that is measured relative to the ambient atmospheric pressure. It is expressed as a positive number. The statement "29 inches of Hg vacuum" means the pressure being described is 29 inches of mercury (Hg) below atmospheric pressure. This is equivalent to -29 inches of mercury gauge pressure. The term vacuum in this context is a common way to describe these sub-atmospheric pressures. This is especially true in the HVAC and automotive industries.

Others may refer to vacuum pressure as any pressure - either absolute or gauge - that is less than ambient atmospheric pressure.

## Bidirectional Pressure

Like gauge pressure, bidirectional pressure is zero-referenced against atmospheric pressure. However, bidirectional pressure extends above and below atmospheric pressure. The indication of pressure below atmospheric pressure is denoted with a negative (-) sign and pressure above is indicated with a positive (+) sign, or the sign is omitted. A bidirectional range can be symmetrical (the magnitude of the negative portion is equal to the positive portion) or non-symmetrical (the negative and positive portions are not equal in magnitude). Symmetrical ranges like -15 to +15 are sometimes written as +/-15, and non-symmetrical ranges show the negative and positive portion, such as -15...100. A good example of an application here is in monitoring rooms in hospitals or nuclear power plants, where it is desirable to have the room at either a lower or higher pressure than the exterior.

A bidirectional transducer is really just a gauge transducer that has been calibrated and ranged to include a negative component below atmospheric pressure. As with gauge pressure, the calibration of pressure spans below 5 psig require special consideration.

## Pressure Units

Pressure is measured in any unit of force divided by any unit of area. The SI unit of pressure is Pascal (Pa), which is equivalent to one Newton per meter square (N/m2). This can sometimes also be expressed as kilograms-force per square centimeter (kg/cm2). However, Pascal is often not the most convenient unit to represent pressure. The typical metric and imperial units of pressure used widely are bar and pounds per square inch (psi), respectively.

1 bar = 10,000 Pa = 14.503 psi
1 psi = 6,894.76 Pa = 0.0689 bar

Pressure can also be expressed as a value relative to the atmospheric pressure at sea level, represented by the unit "atm," or in a value relative to zero pressure/vacuum, represented by the unit "Torr." One Torr of pressure is exactly 1/760 of the standard atmospheric pressure.

1 atm = 101325 Pa =  1.01325 bar = 14.6959 psi
1 Torr = 133.32 Pa =  1.3332 mbar = 0.01933 psi

In everyday applications, pressure is commonly expressed as a depth of a particular fluid because of its ability to displace a column of the fluid in a manometer at defined conditions. The conditions that most dominantly change this pressure are the temperature and its affect on the density of the fluid. The most common units are inches of water, centimeters of water, millimeters of mercury and inches of mercury (Hg).

1 mm of Hg = 1 Torr = 133.32 Pa = 0.01933 psi
1 inch of H2O (@60 °F)= 248.84 Pa =  0.036 psi

## Pressure Media

Pressure media is the fluid on which the pressure is exerted in an application. The fluid could be either pneumatic or hydraulic. Pneumatic media is a compressible fluid due to a gas under pressure, while hydraulic media is a relatively incompressible fluid due to a liquid or oil under pressure.

Pneumatic media is used in low to intermediate pressure applications and is typically dry air, nitrogen or other non-corrosive gases. Applications requiring safe, contamination-free and portable operation use pneumatic pressure media.

Hydraulic media is used for high pressure applications because of their ability to generate a higher force with less area displacement. Water, along with a variety of oils depending on the application, can be used as hydraulic pressure fluid. Contamination and cleaning is always a risk when using hydraulic media. However, applications requiring stable high pressure with reduced temperature effects on the system pressure use hydraulic media. Typical fluids used in high pressure calibration equipment are water and sebacate oil.

## Altitude and Airspeed

Absolute pressure transducers with a range that corresponds to atmospheric pressure (static pressure) at any altitude are used in Mensor's CPA8001 Air Data Test Set and the CPA2501 Air Data Indicator. The output from these pressure transducers is converted to altitude using a formula based on the 1962 U.S. Standard Atmosphere. The "Standard Atmosphere" assumes a known relationship between pressure, temperature and atmospheric density. Pressure has a non-linear, inversely proportional relationship to altitude. As altitude increases, pressure decreases.

Differential pressure transducers with a range corresponding to expected airspeed are also used in the CPA8001 and the CPA2501. The output from these transducers is converted to airspeed (the motion of the aircraft through the surrounding air mass). The low side of the transducer is connected to the static pressure (atmospheric pressure at altitude) and the high side is connected to the impact pressure of the air mass.

The following equation defines the relationship between static pressure, impact pressure and total pressure:
Qc = Pt - Ps

Where:
Qc = Impact Pressure
Pt = Total Pressure
Ps = Static Pressure

The diagram below shows a pitot tube configuration and the pressure generated across a differential sensor. The total pressure includes the effect of the existing atmospheric pressure (static pressure) and the impact pressure. The differential sensor in the pitot tube measures the difference between Pt and Ps, giving the impact pressure Qc, using the equation above.

## High and Low Pressure

The terms high pressure and low pressure can be vague and misleading. While a product may be considered high pressure in one industry, it may in fact be considered standard or low pressure in another. So, how are they generally classified?

## High Pressure

In most cases, if an instrument is labeled high pressure, its range most likely spans above 6,000 to 10,000 psi. While it’s not uncommon for products to range this high, it is a significant amount of pressure that not all instruments can handle. Pneumatic systems at these high pressures store a large amount of energy in the compressed gas. If there is a leak or breakage, the decompression can cause a violent release of energy.

High pressure controllers and calibrators fall into a wide scope of pressure ranges, from pneumatic instruments that are often capped at 10,000 to 20,000 psi, to hydraulic instruments that can exceed hundreds of thousands of psi. The ceiling for pneumatic pressure instruments, even those labeled as high pressure, tends to be lower than hydraulic instruments that are more stable, even at high pressures. Since they use liquid or oil molecules, hydraulic instruments tend to leak or spray for a moment when there is a failure, as opposed to the more energetic failure a pneumatic instrument could cause.

## Low Pressure

When a gauge pressure instrument is labeled as low pressure, its range is often 10 psig or less. This low gauge pressure is often measured in inches of water (in. H2O) or millibar (mbar) units in order to get the most resolution in the reading. Low absolute pressure is usually considered to be sub-atmospheric pressure below approximately 14.7 psia. However, psi is less common when you get into pressures that are significantly lower than atmosphere. Very low absolute pressure is difficult to work with because molecules at these extremes transition to a non-laminar flow. This white paper explains this concept and other challenges in low pressure calibration. For low gauge pressure, small adjustments in the reference pressure and even temperature have a large impact on the pressure observed.

When dealing with low gauge pressure, a degree difference in temperature, or the pressure change at the reference port before a thunderstorm, or even someone opening a door in the building could be enough to affect results on a low gauge pressure reading.

When controlling these pressures, there is an added challenge to find valves with a small enough resolution to control the flow, and calibration standards with low enough minimum values to calibrate these devices. These factors lead to the limits we currently face in measuring, controlling and calibrating low pressure instruments. Advancements such as snubbers that delay the effects of environmental changes on gauge pressure sensors and specially designed valves with a resolution even smaller than what can be displayed by the instrument are some of the advancements that make low pressure readings more stable and attainable.

## Need to know more about pressure?

A Mensor sales associate or systems engineer can discuss any application. We specialize in pressure calibration and testing equipment in laboratory, production or instrument test cells.