How To Calibrate Pressure Gauges

What is a pressure gauge?

A pressure gauge is a device for measuring the pressure generated by a fluid or gas. 

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What is pressure?

Pressure is the force applied perpendicular to a surface, divided by the area of the surface the force is acting on. The SI unit of pressure is the Pascal (Pa). Pressure formula Globally, there are many different units of measurement for pressure which are used in industries that may cause considerable confusion. It is therefore imperative that you understand the correct unit of measurement in which your gauge reads and that of your calibration equipment.  The SI unit for pressure, the Pascal (Pa), is rarely used as it is a very small unit of measurement, therefore generally the prefix of hecto, kilo and mega Pascal (hPa, kPa MPa) is applied. It is possible to convert from one unit of pressure to another and there are a number of online conversions available to do so.  As with anything online, you should use these with care as they are not guaranteed to be accurate.

Types of Pressure

This area of pressure measurement can cause particular confusion and so must be understood thoroughly before specifying gauges or performing a calibration. Pressure chart

Gauge Pressure 

This is the most common pressure that is measured and is referenced against the current barometric pressure. Therefore, it is the difference between the measured pressure and that of the current barometric pressure. Gauge pressure can be positive or negative. 

Absolute Pressure

Absolute pressure is the pressure referenced from an absolute vacuum i.e. there are no air molecules left. Absolute pressure can only be positive as the reference is absolute zero (100% vacuum). 

Differential Pressure 

As the name suggests, differential pressure is the difference between two pressures and can produce a positive or negative result. Common examples of differential pressure gauges are: 
  • Magnehelic Differential Pressure Gauge used in clean room monitoring systems or in filtering systems
  • Tank liquid levels are often measured using a Differential Pressure Gauge
  • Flow meters are often based on a Differential Pressure Gauge and a precision orifice
  • Manometers

Types of Pressure Gauge

Pressure gauges come in a variety of types, sizes and accuracies which can be either mechanical or electronic.

Bourdon Tube Pressure Gauge

The most common type of mechanical pressure gauge is the Bourdon Tube. Bourdon tube   Bourdon tubes are oval in cross-section. The pressure created by the gas of liquid acts on the Bourdon tube and produces a movement at the end of the tube. This is then transferred by the movement to the dial on the front of the gauge. Mechanical pressure gauges such as Bourdon tube gauges are less accurate than digital gauges; a typical mechanical gauge might have an accuracy of 1.6% of full scale but could be between 0.1 & 4% of the measured range. 

Electronic Digital Pressure Gauge or Pressure Sensor

These most commonly work using strain gauges, in a similar manner to load cells. A strain gauge is embedded in the diaphragm of the gauge, as the diaphragm deforms this causes the deformation of the strain gauge which alters the resistance. The resistance is proportional to the applied pressure. All forms of calibration require certification and constant quality checks. We are certified to carry out pressure instrument calibration and regularly update our certifications to provide added peace of mind. One such example can be seen below. AL441825760-17Dec2020-Pressure - Vacuum Gauge-315820-2- With that said we will now get back to explaining the various forms of pressure measurement. A Wheatstone Bridge circuit is used to measure this resistance and by the means of calibration, the resistance values are assigned to a pressure unit of measurement. The pressure sensor can have a digital display attached to it, making a digital pressure gauge or can be manufactured to output its measured pressure as a current or voltage, the most common being a 4 to 20mA current loop. Digital pressure gauges and pressure sensors are generally higher accuracy than a mechanical gauge and do not suffer significantly from hestresus errors. A typical accuracy might be 0.1% of full scale, though there are also digital gauges on the market that are 0.05% of the measured range.

Electronic Digital Pressure Gauge or Pressure Sensor

These most commonly work using strain gauges, in a similar manner to load cells. A strain gauge is embedded in the diaphragm of the gauge, as the diaphragm deforms this causes the deformation of the strain gauge which alters the resistance. The resistance is proportional to the applied pressure. A Wheatstone Bridge circuit is used to measure this resistance and by the means of calibration, the resistance values are assigned to a pressure unit of measurement. The pressure sensor can have a digital display attached to it, making a digital pressure gauge or can be manufactured to output its measured pressure as a current or voltage, the most common being a 4 to 20mA current loop. Digital pressure gauges and pressure sensors are generally higher accuracy than a mechanical gauge and do not suffer significantly from hestresus errors. A typical accuracy might be 0.1% of full scale, though there are also digital gauges on the market that are 0.05% of the measured range. 

How to calibrate a pressure gauge?

There are two main methods of pressure gauge calibration.

Comparison to another more accurate and calibrated pressure gauge. 

This could be analogue or digital. The pressure medium could be a fluid, such as oil, or a gas, such as air or nitrogen.  Advantages:
  • Generally a very portable system
  • Faster calibration
  • Generally lower cost, (depending on the range of pressure you wish to calibrate over)
Disadvantages:
  • Less accurate
  • You may require a number of reference gauges to cover the pressure range and accuracies required
  • You will require a means of generating pressure. This could be a hand or bench pump, compressed air or a nitrogen source

Comparison to a calibrated dead weight pressure calibrator. 

A deadweight pressure calibrator is the most accurate type of calibration standard and considered a primary calibration device, they are supplied with a number of weights and a piston. The pistons area is very precisely measured and the weights are highly accurate. Dead weight tester Deadweight testers generally use oil as the calibration medium, however can also be purchased to operate with air or water. Advantages: 
  • High accuracy typically 0.015% of the applied pressure for a standard deadweight tester or 0.008% for a high accuracy version.
  • Inbuilt pump unit.
  • Requires less frequent calibration than a reference gauge.
  • A single deadweight tester is suitable for a comprehensive range of pressures.
  • The longevity of the equipment. Only a few parts will wear or require replacement, providing the correct care is given, in particular being careful not to contaminate the pressure medium.
Disadvantages: 
  • Heavy and less easy to transport.
  • Not as quick to use as a typical pump and reference gauge set-up.

Pressure gauge calibration procedure 

  1. Prepare chosen calibration equipment and ensure set-up is as per the manufacturer's instructions. If using a master test gauge, ensure this meets the accuracy requirements of the gauge under test. This should be 3 to 5 times more accurate than the gauge under test. Remember that both the reference gauges and pressure gauges you're likely to calibrate are usually specified as a % of full scale.
  2. Ensure the pressure medium used is compatible with the gauge under test. For instance, an oxygen service gauge should never be calibrated using oil, as this would cause an explosion!
  3. Mount the gauge in the same orientation on the calibration rig as it would be used in normal service. This is especially important on low pressure, large diameter mechanical gauges.
  4. Apply maximum pressure to the gauge under test 3 times, this exercises the gauges mechanical mechanism or strain gauge. 
  5. Apply 3 or more raising pressures, though we would recommend 5 pressures. Record reading from gauge under test, and reference gauge if applicable. Repeat these same pressures but this time decreasing in pressure. The increasing vs decreasing pressures show any hysteresis in the mechanics or electronics.

How to calibrate a differential pressure gauge

This is a very similar calibration procedure to a standard pressure gauge. 
  1. Ensure both the high and low-pressure ports/connections are vented to the atmosphere.
  2. Connect your reference gauge and pump to the high-pressure port.
  3. Follow steps 1 to 5 in the calibration procedure for pressure gauges.

How to calibrate a pressure sensor 

This is a similar calibration procedure to a pressure gauge.  Follow steps 1 to 5 of the calibration procedure for a pressure gauge. As there is no display with a pressure sensor, the output of the pressure sensor is measured. Depending on the design of the pressure sensor and that of your calibration equipment, it may require a separate power supply. Generally speaking, if it is a 4 to 20 mA output and if you're using a modern multifunction calibrator, such as a Fluke 725 or similar, then this will also power the sensor. 
  1. Connect your calibrator/current meter and/or power supply as necessary in accordance with the manufacturer instructions for the pressure sensor under test. 
  2. From the instruction manual/specification datasheet for the sensor obtaining the scaling for the output, you will need to obtain the 4 mA and 20 mA points. For example, if you are calibrating a 100 bar gauge, the 4mA would most likely be zero and the 20 mA would be 100 bar. 
  3. Calculate the other calibration points which would normally be 8, 12 & 16 mA, so in this example 25, 50 & 75 bar. 
  4. Follow steps 1 to 5 of the calibration procedure for pressure gauges
This post is not intended to replace the guidance listed in any of the national or international standards. The correct calibration of pressure gauges and sensors is a skilled task and the requirements of the relevant international standard should be fully understood and followed. The post doesn't attempt to describe the health and safety implications of working with high-pressure systems which can be deadly if not worked on in a safe manner.

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