string to be monitored is easily determined during installation of the pipe strings as is the final location of each of the monitoring points for example if the individual sections of pipe are 34 l 34 feet in length and monitoring for corrosion conditions at the deepest portion of the well is to be at intervals of 3 l feet then a modified joint 50 is used to join each third section of pipe to the next as the string descends into the well in a further preferred embodiment the apparatus of the invention includes a reference block 60 such as that schematically illustrated in fig7 the reference block is fabricated from the same material as or a material similar to the tubing or casing string being monitored and as its names indicates will provide reference or comparative data on one or more thicknesses of material the reference block is stepped and is provided with a plurality of transducers 62 affixed to its stepped surfaces and is installed so that it is isolated from the source of corrosion in the embodiment of fig7 the step wedge reference block 60 is provided with transducers for three different thicknesses the data received from each pair of transducers 62 39 and 62 34 and 62 39 34 corresponds to the signal passed through sound metal i e unaffected.