Differential probe system
In differential probe systems the same coil arrangement is used as in the comparative measurement method with external reference, only the coils are arranged in such a way that a specific point of the product being tested is compared with another point that is only a short distance away on the same product. In this arrangement the product being tested is compared with itself. Since one can assume that there is no significant change or no change at all in the alloy and structure within the short distance that the two receiving coils are apart from one another, only defects which suddenly occur or other inhomogeneities are indicated this way. As such, material defects of a primarily local nature (e.g. cracks) are found using this method, whereas changes in workpiece properties which occur continuously over the entire length are largely compensated for. The disadvantage of this arrangement is seen in the probe’s dependence on direction. Whereas lengthwise defects such as cracks that run crosswise to the two receiving coils are reliably identified (since only one of the two receiving coils is always affected), as soon as such defects run lengthwise they are either not detected anymore or only to a very limited extent (the lengthwise defects are now covered by both receiving coils simultaneously). This situation can be helped by an arrangement of multiple receiving coils (aka multi-differential arrangements). But these still have their own preferred directions; i.e. defects with certain orientations are still detected only to a limited extent.