TYPES OF PRESSURE: When and why are they used?

Automation is a key of rapid industrial development in 21^st century and process control is an integral aspect of automation. Process control is extensively used in applications where bulk production is carried out in a consistent manner. It includes wide range of application area such as chemical & mechanical processes, food & goods packaging industry, power plants, oil & petroleum refineries and much more. A fundamental requirement of any of these process environments is an accurate and precise measurement. Good measurement practices along with consistent and reliable measurement systems maintain the defined level of accuracy and ultimately direct to the expected outcome. Clear and systematic way of expressing the entities under measurement is needed to analyze the outcome and take necessary control action.

Most of the process control applications use ‘pressure’ as a core entity of measurement and regulation. Pressure measurement products provide solution to almost every application need.  Pressure sensors find their use in countless number of everyday ordinary applications. They are also used in indirect form for measuring other variables such as fluid level, fluid flow, speed etc. Therefore, it is important to understand the basic concepts of pressure for interpreting various ways of its measurement and related applications.

The simplest definition of pressure says it is a measure of the force applied on an object which is perpendicular to the surface and distributed equally. This definition applies to solids whereas fluid pressure is nothing but force exerted per unit area by fluid. Going further, various units of pressure measurement, their significance and use of certain pressure measurement techniques will be discussed in detail.

Pressure Measurement Units:
Various commonly used pressure measurement units are discussed below. Most of these units can be used in international standards for expressing higher quantities in terms of kilo, mega etc.

BAR: One bar is measure of atmospheric pressure on the earth at sea level. This unit of pressure is commonly used in Europe. Atmospheric pressure is generally given in millibars.

PSI:
One pound of force applied to one square inch of area i.e. Pounds per Square Inch. PSI is a commonly used pressure unit in the United States. 1 bar is approximately equal to 14.7 PSI.

PA (Pascal):
It is the SI derived unit used to quantify stress, internal pressure, tensile strength etc. It is a measure of force per unit area hence defined as one newton per square meter. It is named after a French mathematician physicist Blaise Pascal. Relation between Pascal and Bar is expressed as 1 bar= 100000 Pascal

InHg (Inches of Mercury):
It is the pressure exerted by one-inch tall, one-inch circular column of mercury at gravity and 0°C. It is typically used in the United States for barometric pressure measurement in weather reports, refrigeration, air conditioning and aviation. 1 Bar=29.53 InHg

Torr:
It is unit of pressure on an absolute scale. Traditionally one torr is equal to one millimeter of mercury which is a very small fraction of atmospheric pressure. It is to be noted that the torr is not a part of SI system. This unit is named after an Italian physicist and mathematician Evangelista Torricelli. 1 Torr equals 0.00133322 Bar

InH₂O (Inches of Water): This unit is similar to InHg. It is force exerted by one-inch-tall, one-inch circular column of water at gravity and 4°C (39.2°F). Typical applications of InH₂O are differential pressure measurement, or in low pressure water applications. 1 InH₂O = 0.0024884 Bar.

Referencing Pressure:
Every pressure measurement application requires a certain way of referencing for accurate measurement. Following are the standard references used by pressure sensors.

Absolute Pressure: 
It is the pressure with respect to perfect vaccum. In other words it’s a pressure discounting the effects of atmospheric pressure. Practically it is impossible to pull a full vaccum inside the sensor casing hence it uses a gauge reading with fixed correction factor. Therefore absolute pressure is equal to gauge pressure plus atmospheric pressure. The change in location of measurement changes the atmospheric pressure. To eliminate this effect an absolute pressure sensor can be used which relies only on specific pressure reference.
Absolute pressure is primarily used in research and design purpose. But there are few live applications in which absolute pressure reading is beneficial.

Gauge Pressure:
Atmosphere around the sensor is used as a reference in gauge pressure. It is the most commonly used pressure reference in practice. It measures pressure in one area of the vessel against the atmospheric pressure. Hence it is a type of differential pressure. Pressure sensors that use gauge pressure have a vent which enables the sensor to use the atmospheric pressure as its reference point. Gauge pressure is popular because it ensures reference with instantaneous atmospheric pressure of the location where it is installed, throughout the world.

A suffix “a” or “g” is used to indicate absolute or gauge. When no suffix is used then it is assumed to be gauge.

Differential Pressure:
As the name suggests the differential pressure is the difference of pressure between two media. When determining the difference of pressure between two points is essential than knowing the single entities a differential pressure is used. It eliminates indication of two separate magnitudes and directly gives the difference. Technically, gauge pressure is a differential pressure between process media and atmosphere. Typical applications involve check point of pressure drop or loss of pressure form one point of measuring environment to the other. Differential pressure uses the suffix “d”.

Sealed Pressure: Sealed pressure uses a predefined pressure as a reference point which may not be a vaccum. Sealed sensor is made of a gauge or sensing element that has been sealed to make it air-tight and avoid further changes in the pressure due to change in atmospheric conditions. This chamber is used as permanent internal reference of pressure for the sensor. Thus, mounting sealed pressure sensor at locations that are affected by atmospheric pressure does not disturb sensor working. Also no external venting is required for this sensor. Although sealed pressure is less commonly used it certainly has a few applications. Sealed pressure type is sometimes used to protect pressure transducer from damage.

Conclusion: 
All applications do not require absolute pressure measurement. Most of the applications use gauge or differential type of pressure measurement whereas absolute and sealed pressure types have comparatively lesser but certainly important use in applications. Selecting an appropriate type of pressure sensor is easy after detailed study of application requirements and understanding of available choices. A correct pressure sensor ensures precise measurement and expected outcome in the most efficient and economical way.