What is the principle behind measuring the thermal expansion difference of a steam turbine?

I’ve never seen this type of measurement before; what I’m familiar with are measurements of shaft displacement, vibration, and similar signals. Could you explain what differential expansion is? Is it the thermal expansion and contraction that cause the fitting error?

This post was last edited by phehe77 on 2012-8-16 09:15. Working principle of Bentley 3300/46 slope-type thermal expansion monitoring system: Thermal expansion and displacement monitoring systems both utilize the fact that the output voltage of an eddy current sensor is proportional to the vertical distance from its sensing surface to the metal surface being monitored. The displacement signal is converted into a voltage signal, which is then sent to the monitoring instrument, thereby enabling monitoring and protection functions. During normal operation of the unit, the thermal expansion sensor is fixed to the cylinder body, while the metal surface that the sensor detects is located on the rotor. Therefore, the relative difference in thermal expansion between the cylinder and the rotor is referred to as “thermal expansion difference” (generally, a difference where the rotor’s expansion exceeds that of the cylinder is considered a “positive thermal expansion difference”, while the opposite case is considered a “negative thermal expansion difference”). Based on the principle that the output voltage is proportional to the distance from the surface of the metal being measured, this difference is detected by the eddy current sensor, and the sensing range of the sensor is expanded using the ramps fabricated on the surface of the rotor under test. The expansion differential sensor utilizes the slope of the surface being measured to extend its normal linear measurement range of 4.00 mm to a linear measurement range of 28.74 mm, thereby meeting the requirement to measure the actual expansion differential range of 0–20 mm. The sensor converts the vertical distance between itself and the surface of the slope being measured into a DC voltage signal, which is sent to a preamplifier for shaping and amplification. The resulting 0–24V DC voltage signal is then transmitted to the 3300/46 slope-type expansion displacement monitor. The signals from sensors A and B are combined, and the resulting value is used to display the expansion displacement. Additionally, a digital signal is generated and sent to the protection circuit for alarm activation and tripping protection. Simultaneously output 0–10V DC, 1–5V DC, or 4–20 mA analog signals to the recorder