Pressure gauges are instruments designed to measure pressure of media in a system. Measuring the pressure in your system is a critical quality step to ensure consistency of a product and safety check to be aware of leaks or building pressure in a system. Before specifying a gauge, it is important to understand the basic principles of what pressure is. Then the correct pressure gauge can be determined based on selection criteria, to accurately measure the pressure within the system.
Table of Contents
- Basics of Pressure & Pressure Measurement
- Selection Criteria
- Pressure Ranges
- Pressure ranges in bar
- Pressure ranges in mbar
- Vacuum ranges in bar
- Vacuum ranges in mbar
- Combined pressure and vacuum ranges in bar
- Pressure Units
- Nominal Size
- Accuracy Class
- Types of Mounting & Connection
- Safety and Service Life
- Pressure Measurement Techniques
- Additional Information
Pressure represents the amount of force applied perpendicular to a surface per unit area. In a stagnant liquid or gas, this is the amount of force applied to the wall of the container at a given point.
Static pressure is uniform in all directions. However, a moving fluid applies additional pressure in the direction of the flow while having negligible impact on the surfaces parallel to the flow direction (Figure 1). This additional pressure is called dynamic pressure. The total pressure of a flow (also called stagnation pressure) is the sum of static and dynamic pressure in that flow.
If an instrument is facing the flow direction, it measures the total pressure of the flow. Instruments discussed here are designed to measure static pressure in a system.
Figure 1: Static pressure vs. total pressure
Pressure is often measured in three forms:
- Absolute Pressure represents the total amount of force per unit area. A perfect vacuum has an absolute pressure of zero while earth’s atmosphere at sea level has an absolute pressure of 1 atm.
- Gauge Pressure is what most pressure gauges indicate, which is the pressure of the media relative to the atmospheric pressure. This means that gauge pressure is absolute pressure minus atmospheric pressure. If a pressure gauge is designed to measure pressures less than atmospheric pressure (known as vacuum pressure), they may be labeled as “vacuum gauge” and the negative may be omitted. To signify that a pressure measurement is relative to the atmospheric pressure, the letter “g” follows the unit pressure, e.g. 50 psi g.
- Differential Pressure is simply the pressure difference between two given points. Some gauges offer this measurement to provide convenience by eliminating the need to subtract two pressure readings in a system.
Pressure gauges come in a wide variety of designs, each of which serve specific applications and industries. It is important to get familiarised with the factors affecting selection of the right gauge for your system.
Pressure gauges come in a variety of display units. Commonly used units in pressure gauges along with their conversion to their equivalents in Pascals are listed below:
|Pascal (Pa or N/m2)|
|1 Bar||= 105|
|1 at (kg/cm2 or kgf/cm2 or Technical Atmosphere)||= 9.80 665 × 104|
|1 atm (Standard Atmosphere)||= 1.01 325 × 105||= 760 Torr|
|1 Torr ( mmHg or Millimeter of mercury)||= 1.333 224 × 102|
|1 cmH2O (cmWc or Centimeter of water)||= 98.0665||= 10 mmH2O|
|1 mmH2O (mmWc or Millimeter of water)||= 9.80 665|
|1 lbf/in2 (Psi)||= 6.8 948 × 103||= 16 ozf/in2|
|1 oz/in2 (ozf/in2)||= 4.30 922 × 102|
|1 inHg (inch of mercury)||= 3.37 685 × 103|
European norm EN 837 provides standardised procedures, design requirements, testing and installation guides for commonly used pressure gauges. EN 837-1 and EN 837-3 provide information on the design of dials of concentric scales. The preferred unit of pressure is bar, and the pressure ranges are as follows:
In vacuum gauges, the pointer rotates in anti-clockwise direction with increasing vacuum.
The nominal size (NS) of a gauge is the diameter of the gauge. The nominal sizes of gauges according to EN 837 are as follows:
40, 50, 63, 80, 100, 160 and 250 mm
Accuracy classes (KL) determine the maximum margin of error each pressure gauge is allowed to have in terms of the percentage of the maximum scale reading. For example, a pressure gauge with a maximum reading of 10 bar and accuracy class 4 may deviate from the actual pressure by 4% ( 0.4 bar)
|Accuracy Class||Limits of permissible error (Percentage of Span)|
Since pressure gauges use various elements in measuring pressure, it is important to consider the chemical compatibility of materials when choosing the right pressure gauge. Please refer to TAMESON Chemical Compatibility Chart.
- Standard threaded connection: This type of gauge is simply screwed into an available thread. The threads are sealed using a compression seal for tapered threads and O-ring for parallel threads.
- Integrated pressure gauge: This pressure gauge mounting is accommodated by a female thread.
- Flange pressure gauge: This kind of mounting is offered for those who wish to install the pressure gauge onto a control cabinet.
According to EN 837-2, for safety purposes, a pressure gauge shall be selected with a range such that the maximum working pressure does not exceed 75% of the maximum scale value for steady pressure or 65% of the maximum scale value for cyclic pressure.
When using hazardous pressure media such as oxygen, acetylene, combustible substances and toxic substances, it is necessary to choose a pressure gauge with additional safety measures such as a blow-out device on the rear. These safety measures ensure that any leaks or bursting of pressurised components will not injure anyone at the front of the scale.
The entire case of gauges that are prone to constant mechanical vibrations are often filed with oil or glycerin.
In rapidly pulsing pressures, such as gauges placement by reciprocating pumps, an orifice restriction is commonly used to even out the pressure fluctuations and provide an average reading. This increases the life span of the gauge by omitting unnecessary wear on the gears of the gauge.
Many techniques have been developed to measure pressure in a system, which accordingly have been used as the main mechanism in numerous pressure gauges available today. Among these techniques, aneroid gauges also known as mechanical gauges are the most widely adopted technology.
Aneroid gauges measure pressure using a metallic pressure responsive element. This element takes different forms, but its main functioning principle remains the same which is flexing elastically under application of a pressure differential. The deformation of this element can then be measured and converted into the rotation of a pointer on a scale display. The three main aneroid gauges are the bourdon tube, diaphragm and capsule element.
A bourdon tube is a flattened thin-wall closed-end tube formed into a C shape or a helix, as shown in figure 2. When fluid pressure is applied to the inside of this tube, the oval cross section of the tube becomes more circular and that straightens the tube. The tube will regain its shape when fluid pressure disappears. The change in the shape of this tube creates a motion pattern at the free end of the tube which is converted into a pointer rotation with links and gears.
A bourdon tube measures gauge pressure (relative to atmospheric pressure). The bourdon tube is the most commonly used pressure gauge type because of its excellent sensitivity, linearity and accuracy.
Bourdon tube gauges come in a variety of designs and specialties to serve a diverse range of applications. The pressure range of bourdon tube gauges varies from 0 … 0.6 to 0 … 1600 bar with an accuracy class of typically between 0.1 and 4.0.
Figure 2: Bourdon Tube Gauge
A diaphragm pressure gauge uses the deflection of a flexible membrane that separates two environments, as shown in figure 3. One side of the diaphragm can be exposed to atmosphere in which case gauge pressure is measured, or it can be sealed against a vacuum in which case absolute pressure can be measured. The diaphragm is often metallic or ceramic which can be clamped between two flanges or welded. As the pressure builds, it flexes the diaphragm, which through the use of gears and linkages can turn this into a dial measurement.
Diaphragm pressure gauges are suitable for corrosive gases, liquids or highly viscous media. This type of gauge is widely used in chemical/petrochemical, power stations, mining, on and offshore, and environmental technology industriesThe measuring range of this type of gauge lies between 0 … 2.5 mbar and 0 … 25 bar with an accuracy class of typically between 0.6 and 2.5.
Figure 3: Diaphragm Gauge
Capsule element pressure gauges are developed to measure air and dry gases at low pressures. This gauge consists of two circular membranes joined along their outer edge, as shown in figure 4. One of the diaphragms has an opening in the middle that lets media to enter. The expansion or contraction of the chamber due to pressure differential between the outer and inner media allows for pressure measurement.
These pressure gauges are almost exclusively used for precise pressure measurements in gaseous media. They are most prevalent among low pressure pneumatic systems, breather valves, overpressure monitoring, filter monitoring and vacuum pumps. The range most these gauges measure is usually from 0 … 1 mbar to 0 … 600 mbar with an accuracy class of typically between 0.1 and 2.5.
Figure 4: Capsule Element Gauge
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