Top Oil Thermometers
These thermometers are typically sealed, spiral-bourdon-tube dial indicators with liquid-filled bulb sensors. The bulb is normally inside a thermometer well, which penetrates the tank wall into the oil near the top of the tank. As oil temperature increases in the bulb, liquid expands, which expands the spiral tube. The tube is attached to a pointer that indicates temperature. These pointers may also have electrical contacts to trigger alarms and start cooling fans as temperature increases. An extra pointer, normally red, indicates maximum temperature since the last time the indicator was reset.
This red pointer rises with the main pointer but will not decrease unless manually reset; thus, it always indicates the highest temperature reached since it was reset. See the instruction manual on your specific transformer for details.