Apparatus, methods, and articles of manufacture for monitoring a condition of a material are disclosed. In particular, the example apparatus, methods, and articles of manufacture emit a first acoustic signal into a wall of the tubular member having a first temperature value and obtain a first propagation time associated with the first acoustic signal. In addition, a second acoustic signal is emitted into the wall having a second temperature value and a second propagation time associated with the second ultrasonic acoustic signal is obtained. The second temperature value is determined based on the first temperature value and the first and second propagation times.