Apparatus, methods, and articles of manufacture for monitoring a condition
of a cavity surface of an elongated member having a cavity therein. In
particular, the example apparatus, methods, and articles of manufacture
emit a first ultrasonic signal that propagates from a first surface of an
elongated member having a cavity therein toward a second surface of the
cavity having a first temperature value. First and second echoes
associated with the first ultrasonic signal are then obtained. At least
one of the first and second echoes is associated with a recess in the
second surface. A condition of the second surface is monitored by
determining a second temperature value of the second surface based on the
first and second echoes and the first temperature value.