Industrial Dynamic Measurements: A Best Practice Guide

Dynamic Pressure

In-cylinder pressure: a key dynamic pressure application (AVL List GmbH)

Applications

There are many industrial applications in which the measurement of dynamic pressure is required. Examples of such applications include:

Automotive engine development

Knowledge of the in-cylinder pressure throughout the combustion cycle enables the fuel injection and timing parameters to be optimised, to maximise efficiency and minimise emissions.

Turbomachines

Turbomachines, such as compressors, turbines, pumps, and fans, are important components in steam and gas turbines found in power plants and in aviation jet engines. To find sources of losses and to calculate the efficiency of turbomachines, the measurement of dynamic pressure is vital. In the engineering of turbine engines and rocket propulsion systems, dynamic pressure measurements are used for active control, thrust measurement, and overpressure indication.

Machine control

Hydraulic and pneumatic components, such as engines, pumps, transmissions, actuators, and valves, are often used to control the motion of industrial equipment. Measurement of the dynamic fluid pressure is often critical in developing these components and monitoring their performance.

Materials forming

Dynamic pressure measurements are performed in order to optimise production processes. Examples are injection moulding and extrusion performed in the plastics industry, as well as die-casting.

Blast waves and ballistics

A primary result of the detonation of explosives is the propagation of a pressure pulse known as a blast wave. The measurement of this dynamic pressure is important from two different points of view: firstly, those developing explosives want to achieve maximal and directed destructive capability and, secondly, those developing military and civilian shelters want to achieve constructions which withstand air blast and ground shock loading. Dynamic pressure is also measured when developing weapon systems such as guns, cannons, missiles, and ammunition.

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Existing Standards

Few documentary standards on dynamic pressure calibration currently exist, at least in part due to the fact that there have been no national (non-acoustic) dynamic pressure standards available to provide traceability to the SI. The most applicable existing document is ISA‑37.16.01-2002, A Guide for the Dynamic Calibration of Pressure Transducers, published by ISA, the International Society of Automation.

Under the auspices of ISA, committee 107 (Advanced Instrumentation Techniques for Gas Turbine Engines) has been created to develop standards to ensure measurement accuracy in the operation of gas turbine engines, with subcommittee ISA 107.5 focusing on the measurement of dynamic pressures – this subcommittee is being driven by PIWG, the Propulsion Instrumentation Working Group.

Work is also underway within ISO/TC 108 – the International Organisation for Standardisation’s Technical Committee on Mechanical vibration, shock, and condition monitoring. Within this committee, Working Group 6 (Calibration of vibration and shock transducers) of Subcommittee 3 (Use and calibration of vibration and shock measuring instruments) is developing a standard covering the calibration of dynamic pressure measurement systems.

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Transducers

Transducers capable of measuring dynamic pressure come in a variety of types, each with its own characteristics in terms of e.g., pressure amplitudes, frequency response, working temperature range, shock resistance, linearity, reproducibility, and price. The required instrumentation also varies between transducer types – the following is a brief summary of the more commonly-available designs:

Piezoelectric

The applied pressure causes piezoelectric material to deform elastically, generating a charge between its surfaces. This charge, a function of the applied pressure, is measured by a charge amplifier.

Piezoresistive strain gauge

The applied pressure causes strain within an elastic element – this strain is detected by strain gauges attached to the element experiencing a change in resistance and, usually arranged in a Wheatstone bridge configuration, generating a bridge output voltage proportional to the applied pressure. Strain gauges manufactured from semiconductor material tend to have a significantly greater gauge factor than those made from metals, and are preferred for some pressure transducer applications due to their higher sensitivity, despite being more sensitive to environmental effects such as temperature and vibration.

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Optical

Optical fibre systems can be used to measure the elastic deformation of a diaphragm in response to an applied pressure. Alternatively, the fibre itself can be designed to be strained in response to an external pressure, and a Fibre Bragg Grating technique can be used to monitor this strain, and hence infer the applied pressure.

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Absolute pressure traceability

A number of techniques potentially capable of providing absolute pressure traceability are available, and several NMIs are currently developing standards based on such methods. These methods each tend to cover specific amplitude and frequency ranges, so the choice of NMIs from which an industrial user will be able to gain traceability is likely to be dependent upon the particular pressure measurement application.

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Shock tubes

Within a shock tube, the diaphragm separating two volumes of gas at different pressures is ruptured, and a shock wave propagates through the lower pressure section. If the end of the tube is blanked off, the shock wave reflects from the end wall and a pressure transducer mounted flush to the end face will experience a nearly instantaneous step change in applied pressure to a value that remains constant for a period of time. Analysis of the output of the transducer system during this period can yield significant information about its dynamic characteristics. The value of the pressure step is calculable from gas theory, using measurements of the shock wave velocity (obtained from sensors mounted in the side wall of the tube) and the initial temperature and pressure of the driven gas, together with its chemical composition. For further details, see Matthews et al, 2014.

Although the pressure rise-time is of the order of nanoseconds, enabling very high frequency (> 1 MHz) characteristics to be determined, the pressure amplitude achievable in shock tubes is limited by their physical construction and associated safety requirements.  Within the EMRP project, NPL have developed a 1.4 MPa plastic shock tube and a 7 MPa steel one is under construction; SP have a steel tube capable of generating pressure steps of up to approximately 0.6 MPa.

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Drop-weight systems

Drop-weight systems are used for calibrating high-amplitude dynamic pressure sensors. A free-falling mass falls along a guide system and, at the end of its travel, strikes a piston located at the top of a chamber full of hydraulic fluid. The force transmitted through the piston increases the fluid pressure which acts on pressure sensors located in the walls of the chamber. The generated pressure signals typically resemble a half-sine or ‘squared’ sine wave. The magnitude and duration of the pressure pulse is varied by using different masses released from different heights.

With maximum pressure values of hundreds of megapascals, drop-weight systems have much higher pressure capacities than shock tubes. However, signal rise times for drop-weight systems are of the order of 1 ms to 2 ms, leading to much reduced calibration bandwidths (less than 1 kHz) in comparison to shock tubes.

Two of the partners in the EMRP project, MIKES and PTB, are developing drop‑weight systems as new primary measurement standards for dynamic pressure calibrations. Traditionally, drop-weight systems have been used as secondary standard, or comparison, systems - the attempt to use them as primary standard systems is novel. The systems being developed at the two NMIs are mechanically similar and work over similar pressure intervals, but traceability is achieved in different ways. The MIKES system achieves metrological traceability by calculating the acceleration of the piston from a traceable interferometric measurement of the mass displacement, and independent measurements of the mass of the drop-weight and the cross-sectional area of the piston, whereas the PTB approach is based on measuring the time variation of the pressure-dependent refractive index of the fluid under compression.  More information about the two systems can be found in Matthews et al, 2014 and Bruns et al, 2013.

The bulk modulus of the fluid in the drop-weight system chamber is an important parameter that directly influences the performance of the drop-weight calibration system. A model for bulk modulus of hydraulic oil is proposed in Shang and Kong, 2011.

One further consideration which needs bearing in mind is the sensitivity of the transducer being calibrated to the pressure transmission medium. If the device is calibrated using pressures generated in a liquid but then used to measure gas pressures, the effect of this sensitivity will need to be corrected for or, more realistically, incorporated within the uncertainty budget.

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Step generators

Step generators are pieces of equipment capable of exposing the transducer being calibrated to a known static reference pressure by means of a fast‑acting valve.

 

Periodic generators

Periodic pressure calibrators, such as pistonphones, typically generate a defined sine wave pressure for calibrating microphones and other low pressure acoustic sensors.

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Online literature

Dynamic Measurement of Pressure - A Literature Survey, Hjelmgren, SP Swedish National Testing and Research Institute, 2002

Design of a shock tube, Teichter, SP Swedish National Testing and Research Institute, 2005

Introduction to Piezoelectric Pressure Sensors, PCB website

Development of a New Piezoelectric Dynamic Pressure Generator for High Pressure Periodic and Aperiodic Calibration, Tomasi et al, IMEKO World Congress, 2003

Uncertainty of Measurement of Transient Pressure, Zhang et al, IMEKO World Congress, 2009

Step Response of Vacuum Sensors – A Preliminary Study, Arrhén, IMEKO World Congress, 2012

High Dynamic Pressure Standard using a Step Pressure Generator, Choi et al, IMEKO World Congress, 2012

Dynamic Calibration Methods for Pressure Sensors and Development of Standard Devices for Dynamic Pressure, Diniz et al, IMEKO World Congress, 2006

Dynamic High-Pressure Calibration of the Fiber-Optic Sensor Based on Birefringent Side-Hole Fibers, Nawrocka et al, IEEE Sensors Journal, 2005

Dynamic Impulse Calibration of 100 MPa Blast Pressure Transducers, Marian et al, Australian Government, 2003

Linking dynamic to static pressure by laser interferometry, Bruns et al, Metrologia, 2013

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