Shock tube-based dynamic pressure standards
Many industrial pressure measurements are made while the pressure is rapidly changing. The transducers used to make these measurements are, in general, only calibrated statically, due to the lack of dynamic calibration facilities. The assumption is made that the statically-determined sensitivity is valid under the dynamic conditions experienced in the measurement applications.
To establish dynamic pressure calibration facilities, a method of generating a known dynamic pressure, traceable to the SI, must be employed. One of the only methods available for giving traceability at high frequencies employs the extremely fast and calculable pressure step generated across a shock front in a shock tube. NPL is therefore developing dynamic pressure standards based on shock tube techniques.
One of the more demanding of these applications is in-cylinder pressure measurement for automotive engine development, in which an uncertainty in pressure measurement of better than 1 % at frequencies of up to 30 kHz would offer engine developers significant support in their efforts to maximise fuel efficiency and minimise NOx emissions. Huge challenges are being faced by the industry to meet new EU emission regulations being phased in over the next few years.
Internal combustion engines provide an environment with extreme operating conditions for reliable sensor operation. Engine development requires high frequency and amplitude measurements - for example, 'knocking' produces in-cylinder pressure fluctuations with a period of the order of 0.05 ms.
The accuracy of current pressure sensors is limited by not having the means to calibrate them under conditions that match those that will be encountered in use and, in particular, by only calibrating the sensors at static pressures. Parameters such as the resonance frequency of the sensors and associated fittings (e.g. mounts, connectors and pipework), and damping and rise-times have to be estimated through computer modelling, increasing uncertainty in the sensor output under normal working conditions.
The engine developers need to be confident that sensors:
- are reproducible and consistent
- provide accurate measurements of peak pressures
- have, when connected with pipe-work and processing electronics, a flat (or known) response up to frequencies significantly higher than the encountered frequency
To address these requirements, NPL has developed shock tube facilities that are able to calibrate these sensors under the dynamic pressure conditions that they experience in real-world conditions. Dynamic calibration requires a source with known characteristics in both amplitude and frequency. A shock wave generated in a shock tube has a rise time of the order of one nanosecond, and the amplitude of the pressure step generated upon reflection of the wave from the end face of the tube can be calculated. This makes it an ideal candidate for a pressure calibration standard if it can be verified that the magnitude of the pressure step can be determined accurately from ideal gas theory using readily measured parameters such as shock wave velocity and static temperatures and pressures.
NPL has manufactured and characterised two shock tubes, of 1.4 MPa and 7 MPa capacity, investigating the effect of diaphragm material, thickness and configuration, and driven section length, on their performance - development of the 1.4 MPa shock tube is detailed in Phil. Trans. R. Soc. A. Current testing is demonstrating that ideal gas theory does seem to be applicable over the tubes' pressure ranges, leading to an expected expanded uncertainty in generated step pressure of the order of 1 % - this work was reported on at the IMEKO World Congress in 2015.
The facility is now operational and providing traceable dynamic calibrations for pressure sensors. It has already been used by a major transducer manufacturer to investigate the dynamic characteristics of a range of their pressure sensors and associated instrumentation. In addition to the applications in the development of automotive engines, the facility has the ability to help investigate the performance of gas turbines and to calibrate instruments used in blast studies. It has also been used to determine the sensitivity of pressure transducers to high frequency vibration, work which was reported at the IMEKO TC22 Conference in 2014.
For further information, please contact Andy Knott (T: 020 8943 6180).
Please note that the information will not be divulged to third parties, or used without your permission