the calibration signal operation is transferred through decision step 312 which itself tests for the expiration of an overall time out interval anticipating receipt of the calibration signal where the time out interval has not expired operation returns to step 308 steps 308 310 and 312 are continuously executed in a loop until expiration of the time out interval following expiration of the time out interval step 314 sends a calibration time out message to the aboveground receiver followed by step 316 in which operation is transferred into a normal run mode with the new zero offset value an implementation of which is described below returning to the description of step 310 when the calibration signal is received step 318 is entered in which zero tension is applied to the drill string and tension monitoring arrangement step 320 then adjusts the output of digital to analog converter 104 so as to generate the compensation signal on line 112 to produce a target value output from the differential amplifier at the channel 3 input of multiplexer 100 the output of the digital to analog converter is adjusted repetitively until the target value is achieved thereafter settings of the digital to analog converter which achieved the target value are stored by step 322 in nonvolatile memory in step 324 tension applied to the tension monitoring arrangement by the drill string is adjusted to a nonzero value for calibration purposes for example a tension of 40 000 pounds may be applied to the tension monitoring arrangement with this tension applied step 326 is performed wherein microprocessor 92 selects the channel 3 input of multiplexer 100 to read the output of differential amplifier as.