LVDT Signal For Tundish Slidegate Position

LVDT's are pretty insensitive to temperature variations affecting the electrical circuit. Since both secondary windings are in close proximity they will be affected the same way by any environmental change, and the difference will not change.

Is the low-temperature error always in the same direction? If so, I suspect expansion is the culprit. The rise in temperature is may be expanding the hydraulic fluid in the cylinder. If this is the case, the slide gate should actually move during warmup.

If the slide gate is not moving, the rod which connects the LVDT core to the cylinder rod may be expanding excessively.

There is a third possibility. The LVDT measures the position of the moving core relative to the stationary windings. If the equipment to which the LVDT is mounted expands, the stationary windings would move relative to the core, resulting in a change in output.

One last question: If the output is correct during normal operation, is this really a problem? I have a lot of equipment on high pressure, high temperature boilers, which grow several centimeters during warmup. Since the instrumentation is only needed during operation, we only calibrate the devices after the boiler is hot. (Yes, it's uncomfortable for the maintenance technician, but that's the only way to be accurate.)

Yes,Low temperature readings always give an under reading. I am not sure of the accuracy of the reading during operation but we are currently looking for an opportunity to check the calibration just after they finish casting.

My money is on you completely.....!

pwr2thepeople has covered it pretty well.

I would add that if the LVDT reading change is not having an adverse effect on operations and is simply an anomaly then perhaps you might ignore it.

If you want to narrow down the source of movement it you should be able to get a ballpark displacement by checking the expansion of the related material with the temp rise you have. You can find the approximate length change of the cylinder or its piston and correlate that to the gate displacement and see if they agree to a reasonable amount. Likewise you can toss in some numbers on the hydraulic fluid expansion.

From what you said--looks like(99%):

it is the expansion of the metals due to heat from the tundish heater.