Developing systems or components to be operated at high temperature requires a technology mix that challenges materials, design skills and certain manufacturing capabilities. It’s definitely not for everyone. This statement can be also extended to measurement instruments such as accelerometers and associated electronics.
So, how can we be sure we are using the right product for high temperature vibration measurement?
Very few manufacturers worldwide are capable of providing a vibration measuring solution for high temperature engine component vibration testing. Even fewer are capable of providing high-temperature-range accelerometers (above 400ºC), and there is only one that is capable of continuous operation at 850ºC (1500ºF) with a 7 kHz bandwidth, that’s Meggitt.
We are proud to introduce our 6245 EHTPE accelerometer and 1772-3 remote charge converter: Meggitt’s highest temperature vibration measurement chain. It’s a powerful pairing, innovative and state-of-the-art.
The 6245 piezoelectric (PE) unit is designed specifically for use in extremely high temperature environments such as aircraft and ground-based gas turbines and incorporates our new MC2 shear mode crystal, a unique element capable of minimising pyroelectric-induced noise.
In addition, the crystal can withstand temperature transitions, meaning the unit is insensitive to thermal velocity changes and doesn’t generate pyro-spikes that may impact the output signal. This is essential for the integration of the acceleration signal with velocity, enabling undesired very low frequency content to be filtered out. Something that is difficult to achieve.
The sensor is single ended (signal ground is connected to an inner shield) and sensor output is isolated from the outer case. It is packed into a classic form factor with a single point mount and the sensitivity axis is located in line with the mounting screw. The unit features an integral hardline cable with a default length of 120 inches (3m) or custom length. The connector is designed to operate in an environment up to +900˚F (+482˚C).
What’s particularly interesting is the small size and light weight (30 gm) of these accelerometers, which facilitates their installation in narrow locations with minimal structural support, making the sensor perfect for aircraft and ground-based gas turbine installation. Moreover the unit provides a 3 pC/g with good signal-to-noise ratio over the temperature range.
The recently introduced 1772-3 two-wire output charge converter unit is part of a series of remote charge converters (RCC) developed to be paired with specific units, providing extreme precision and an extended frequency bandwidth operational range.
A remote charge convertor (RCC) converts a charge signal received from a PE pressure transducer into a voltage output signal. The unit is connected to the accelerometer with a high temperature coaxial cable, allowing us to place the RCC outside of the “hot zone”. Meaning the unit can be fitted with a charge amplifier without risking damage.
Focusing on the RCC, its inner electronic circuit transforms the PE transducer’s high impedance into the amplifier’s low impedance allowing the voltage signal (mV) to be transmitted over long cable lines, making it possible to use a more cost effective cable to send the signal to the front end.
The correct sizing of the filtering configuration, specifically within the 1772 RCC, allows electronic sensor resonance suppression (TRS) of around 20 dB, permitting the maximum frequency range of the measurement chain, and increasing the HTPE transducer’s measuring frequency by ± 3 dB, reaching a usable frequency of 7 kHz with transducer’s nominal resonance frequency of 11 kHz.
Additionally the 1772 unit can work with HTPE sensor impedances as low as 10 kΩ at high temperature. Considering the environment in which these units are meant to operation, this feature is a fundamental requirement.
HTP accelerometers start working from room temperature and thus reach temperatures well above 600° or 700°C. One severe effect of this is the drop of the crystal inner impedance that may occur in an order of x10 every 100°C gradient. Therefore a 1 GΩ inner impedance unit at 24°C will become 10 kΩ impedance one at 650°C.
Benefits of the 1772 unit:
– RCC conversion of electrical charges to low impedance voltage signal. The converter can resist 100°C, allowing it to be placed relatively close the machine, resulting in reduced length of high temperature cable and lower noise signal.
– A value of 1.0 MRads (integrated Gamma) radiation resistance that permits the RCC unit to be placed within NPP safe area.
*residual low- frequency charge developed across the crystal mass due to strains and extreme temperature variations.