A metal blade on a rotary cleaning tool for cleaning heat exchanger tubes, is insulated from companion cutting blades circularly spaced on the tube. A direct current voltage of low level is applied through the flexible cable and central shaft of the cleaning tool and is a part of a circuit including a current amplifier input and the heat exchanger tube being cleaned. As the tool removes foreign matter from the tube, the resistance to current flow from the insulated blade to the tube decreases, and the current level increases. The current signal, which is thereby an input signal analog of the thermo conductance from the sensor blade to the tube, is converted to a voltage analog. The voltage analog signal is then rectified to smooth out the effect of sensor bounce and rotational effects of mechanical drive system for the tool. The output signal from the rectifier is monitored by a zero to one volt DC meter movement and displayed on the meter face, which may include a color coding thereon from red to white or to green, whereby a red or low scale reading is indicative of a dirty or low thermo conductance tube, whereas a reading in the green at the high end of the scale is indicative of a clean or high thermo conductive tube. The visual observation provides the servicing mechanic a measurement base by which he can determine the required cleaning tool insertion rate and cleaning time cycle, and optimize the efficiency of the service provided.