Pressure gauges provide the easiest, most direct way of measuring and displaying a fluid (liquid or gas) pressure, defined as the force exerted by the fluid on a unit area. The reading on the pressure gauge is measured relative to a reference pressure. For static applications, atmospheric pressure is considered the reference, and the reading is called Gauge Pressure. For dynamic flow applications, reference pressure is taken from another system, and the gauge reading is termed as Differential Pressure. A Bourdon tube is a classic instrument for measuring (static) gauge pressure, whereas a U-Tube manometer is a classic example of measuring (dynamic) differential pressures.
For Aeronautical applications, pressure readings are used to inform the pilot about the aircraft’s condition and flight situations. These Pressure-sensing instruments are typically found in the flight’s engine group and are considered amongst the most critical instruments on the aircraft.
Type and Selection of Pressure Gauges:
The selection of pressure gauges is dependent on the application, external factors (vibration, ingress, humidity), and several other parameters. Broadly the classification is based on:
- Range and Accuracy of measurement
- Type of user interface (Digital or Analog pressure gauges)
- Fluid media (Dry and Liquid pressure gauges)
- Application (Regular and Oxygen pressure gauges)
- Static or dynamic pressure gauges
- Range and Accuracy of Measurement
ASME B40.100 laid out standards for pressure range selection. A pressure gauge with approximately twice the normal operating pressure (of the system) should be selected. The maximum working pressure (of the system) should not exceed 75% of the full-scale range.
For deciding on the gauge accuracy, ASME 40.1 and EN 837 have rated gauges from grade 4A to grade D. Higher grade (4A) pressure gauges often have a higher price tag. So, it is essential to consider the application before deciding on the accuracy. Accuracy is generally defined in terms of the percentage of the full-scale range (or the span). The following guidelines can be referred to (as per ASME 40.1) before deciding on the accuracy:
- ASME Grades 2A, 3A & 4A: (Accuracy – 0.25% to 0.10% of full scale) – Applicable for precise measurements in labs, workshops and certain aeronautical applications.
- ASME Grades A & 1A: (Accuracy – 0.5% of full scale) – Applicable for industrial pressure measurement on machinery and production lines.
- ASME Grades B, C & D: (Accuracy – 1.0% to 2.0% of full scale) – Applicable for simple monitoring applications in industry, without precise requirements.
Type of user interface (Digital or Analog pressure gauges):
Though Analog and Digital pressure gauges fulfill the same objective of pressure monitoring, specific distinctive characteristics differ, such as the operational principle and the application.
- Analog Pressure Gauges: Also called a Dial gauge, they directly respond to changes in pressure by pointing to numbers on a circular scale corresponding to the pressure sensed by the measuring element. Needle movement is referred to while monitoring variations in pressure readings. Purely a mechanical device, these are simple to use and do not require a power source to operate.
- Digital Pressure Gauges: These gauges use sophisticated pressure transducers to display the pressure readings on a digital indicator. Compared to a Dial gauge, Digital gauges are often reliable and provide consistent readings, and are continuously powered by a battery, loop, or solar power source. These gauges are advantageous for integration into automated control systems, eliminating manual measurements.
Fluid media (Dry and Liquid pressure gauges):
Analog (dial) pressure sensors such as the Bourdon Tube, Helical, capsules, bellows, diaphragm, and spring either use dry or liquid fluid media depending on the application type.
- A liquid-filled gauge is selected for heavy load applications, with variations in peak pressures and vibrations of the system. The liquid media dampens and absorbs the needle vibrations to provide an error-free reading. Viscous fluids such as silicone or glycerine are used as a liquid media in pressure gauges. These fluids also lubricate the internal components and prevent air from entering the gauge, resulting in them lasting longer than the Dry gauges. However, care should be taken while selecting the correct fluid media for extremely cold conditions where the liquid can freeze, hampering the gauge performance.
- Dry gauges are more straightforward, and standard gauges are not filled with any liquid. Because of their delicate construction with pivots and pinions, dry gauges are often sensitive to condensation and vibration.
Application (Regular and Oxygen pressure gauges):
- Oxygen Gauges: Because of the oxygen’s strong oxidizing property, monitoring the pressure of oxygen requires a unique pressure gauge, which accounts for the behavior of oxygen molecules with changes in pressure and other process parameters. The primary difference of Oxygen gauges lies in the manufacturing process, wherein, use of Oil is strictly banned. Oxygen in contact with oil causes oxidation (also called Rusting), resulting in leakages and brittleness of the gauge. For Aeronautical applications, strict manufacturing, cleaning, and packaging standards are to be followed for Oxygen gauges.
- Regular Gauges: These are conventional gauges used for non-oxygen-related applications.
Static or dynamic pressure gauges:
Depending on the working conditions of process media, a static or dynamic pressure gauge is selected.
- Static Pressure Gauge: As the name suggests, these gauges monitor fluids in rest or static state. Conventional pressure gauges such as Bourdon Tube or diaphragm gauges are used to measure static pressures. Installation of these gauges is independent of the direction and orientation of the pressure needle.
- Dynamic Pressure Gauge: These gauges account for the real-time variations and fluid movement in a process media. For Aeronautic applications, the aerodynamic stress (i.e., stress within a structure subject to aerodynamic forces) experienced by an aircraft depends on the air density and the plane’s velocity. Considering these parameters into account, a dynamic pressure gauge works. Differential pressure gauges are a classic example of a dynamic pressure gauge. However, care should be taken that the gauge is compact and the moving parts or fluid within the pressure gauges are not subjected to external forces.
Pressure Gauges Calibration – Why is it essential:
Pressure gauges are often considered under the critical equipment category for an aeronautical application. Leakages, cabin pressure loss, fuel retention, and lubrication pressure are parameters monitored by pressure gauges in Real-time. Hence, to ensure the pressure gauges are reliable and accurate, scheduled calibration is of prime importance. The following are common errors set into a pressure gauge, and the maintenance Engineer or the Pilot (Captain) has to monitor and calibrate the following errors regularly:
- Accuracy errors: Errors in accuracy set in due to Electro-Mechanical faults in a pressure gauge. For aerospace applications, pressure is corrected with Thermal expansions due to the extreme weather, and due to changes in tolerance limits, accuracy errors are set in.
- Leak and Contamination of fluid media: Due to rapid variations in external factors and improper handling methods, the liquid media is leaked or contaminated with foreign particles, resulting in oxidation and depreciating the service life.
- Linearity errors: Liquid media is prone to have variations in weight due to hydrostatic pressure and can cause errors. The magnitude of this error depends on the density of the liquid and the height difference. So calibration is essential to correct the hydrostatic (altitude) correction and re-scale the pressure gauge.
- Adiabatic corrections: For a closed system with gas as the pressure media, the temperature of the gas effects (increases) the volume of the gas, resulting in pressure variations. The gauge should account for the temperature correction and correct the readings.
- Maintenance and external errors: Post calibration and zeroing (referencing), errors can occur due to a lack of operator expertise in handling precision equipment.
Cumulative defects in a pressure gauge can often result in operational uncertainties leading to Catastrophic failures, downtime, and maintenance losses. Calibration avoids these errors, ensures Pressure Gauges’ accuracy for sustained and repeated, and enhances their service life. Calibration frequency is primarily dependent on the application and the environmental conditions, such as variations in the ambient temperature or physical orientation of the instrument. Following a regular and timely calibration schedule ensures accuracy of measurement and enhances process accuracy.
e2b calibration offers industry-leading ISO-certified Pressure Gauge calibration services. Our labs are ISO/IEC 17025 accredited and operated by a team of qualified calibration experts to test and calibrate your Pressure Gauges. Our verifiable services are unmatched in the industry. We are registered with ANAB. We are also ANSI/NCSL Z540-1-1994 certified. We have the NIST Traceable Wide scope of ISO/IEC 17025 accreditation. Contact e2b calibration for all your equipment calibration needs.
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