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NPL Develops New Dynamic Pressure Sensor Calibration Facility

The National Physical Laboratory (NPL), the UK’s National Measurement Institute, has developed a dynamic pressure sensor calibration facility to provide increased confidence in the high speed pressure sensors used in industrial applications such as gas turbine and internal combustion engine development.

NPL’s facility is based on shock tube techniques and secondary equipment. These can traceably characterise the amplitude and frequency response of pressure sensors up to high frequencies, by exposing them to extremely fast pressure steps of up to 1.4 MPa.

High speed pressure sensors are used to monitor rapid changes in pressure, such as those found in turbines and internal combustion engines. There is a need to ensure that these dynamic measurements are giving true real-time pressure values, as they are essential for optimising the industrial process being controlled.
Many industrial pressure sensors are used to make dynamic measurements, yet have often only been calibrated using static pressures – the pressures exerted by a still liquid or gas. However, the dynamic behaviour of a mechanical sensor deviates from its static characteristics as the frequency increases. So the use of a sensor in a different mode from that in which it was calibrated is a significant factor affecting the reliability and uncertainty of the measurement result.

One major area of need for dynamic pressure measurement is in gas turbine engines. The need to meet emissions targets and improve reliability and performance sees engines undergo many improvement programmes and run with ever-leaner fuel / air mixtures. This can lead to instabilities and excessive pressure pulsations that can result in mechanical failure. Improved dynamic pressure measurement could help lower the costs of mechanical repair, downtime, and environmental fines – potentially saving millions of pounds. (1)

Andy Knott, Principal Research Scientist at NPL, said: “Our new facility can be used to quantify variations between static and dynamic pressure sensitivity and investigate ways in which to compensate for them. Testing of the sensors in more realistic conditions is also available, using either the shock tube or a secondary facility, in which the dynamic pressure is generated within a liquid. We want to encourage businesses to come and test the facility and guide us on the work they need help with; this will ensure that we assist them in developing safer, more reliable, and more effective products.”

A pressure step within the shock tube of up to 1.4 MPa is obtained using different combinations of gases, different initial pressures, and different diaphragm arrangements. The theoretical rise time for these pressure steps is of the order of a few nanoseconds, and consequently the input’s frequency content is sufficient for practically all industrial applications.

The shock tube and secondary equipment can provide a calibration process that extends the measurement traceability of the pressure sensors to the dynamic regime. For example, this could include determination of the sensor’s resonant frequency, or its amplitude and phase response over a range of frequencies. Traceability to the SI is derived from the starting pressures and temperatures, gas species, and shock wave velocity measurements. As well as these standard tests, NPL is happy to use its facilities in collaboration with its customers to help them investigate the dynamic characteristics of their measurement systems.

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