DGS A method using the DGS diagram ( Distance Gain Size) for experessing the high echo from a reflector in terms of the equivalent height of a disc shaped reflector. This techniquie is one of several methods used for the sizing of effects sometimes reffered to a DAC/AVG- method (German term)Sizing Method Advantage :Simple go/no- systemCan be applied to different shapes

Disadvantage:Requires special curves, no indication of vertical extent

DGS diagramThe regularities of sound propagation in material have been theoretically known for a long time and were confirmed in practice by numerous experiments. The development of modern evaluation methods shows two ways. With the reference block method the characteristic curve of the sound field is always determined before carrying out an ultrasonic test, whereas in the DGS method DGS diagrams for probes are applied for this. A DGS diagram shows the echo amplitudes of disk shaped reflectors with different diameters and those of large, flat reflectors (backwall) as a function of the distance Fig.1 DGS Diagram. ProcedureTo understand this better, let us start by explaining the sequences for both evaluation methods at this point. The reference block method requires that a reference block, corresponding to the test object and containing one or more reference reflectors, be available for the test. The distance dependence of echo amplitudes is determined experimentally by means of drilled holes in the reference block, the resulting curve is then transmitted to the screen display of the test instrument (DAC Distance Amplitude Correction). This curve automatically includes all probe (sound field) and material effects. The test object can now be scanned with the probe. An indication recording is made when an echo reaches the DAC curve or exceeds it. A prerequisite with the DGS method is that the corresponding DGS diagram be available for the probe used in the test application. The reference gain of the test instrument, with which the reference echo is at a fixed screen height (reference level), is determined for a specific reflector, i.e. the reference reflector. After this, the instrument gain is increased by a certain value, i.e. the test sensitivity is adjusted. If the reference reflector is a circular arc from one of the standardized calibration blocks, then the instrument gain should be varied in accordance with the correction value given for the angle beam probe: the amplitude correction value is adjusted. With different surface qualities between the test object and the calibration block the transfer correction must be determined experimentally and likewise taken into consideration. The gain difference with regard to the reference echo is determined for the maximum echo from a detected indication. This is followed by a graphic determination of the equivalent reflector size using the DGS diagram. If required, the sound attenuation correction is additionally carried out. This makes it possible to assess whether the indication is to be recorded or not. Nevertheless, by using the DGS scale it is possible to significantly simplify evaluation with the DGS method (Fig.2). Fig. 2 Evaluation using a DGS scale.In this connection, the inspector uses an attachment scale for the screen of the ultrasonic instrument. This scale contains one or several ready made recording curves. The tiresome graphic evaluation with the DGS diagram can thus be omitted. The inspector can directly assess flaw indications by means of the curve.A comparison of the test sequences for the reference block method and DGS method shows the pros and cons in this table.

Pros and cons of the DGS and reference block methodRefernce block methode DGS-method

Pros The DAC curve contains all test-relatedInfluences, i.e. no time-consumingcorrections are recuired.Easy and reliable evaluation. No reference blocks required.

Cons Fabrication or procurement of asuitable reference block.Recording of a DAC curve for everytest application Measurement and consideration of differentindividual corrections.Graphic determination of the equivalent reflector size.

Electronic DGS evaluationThe use of microprocessor controlled ultrasonic instruments considerably simplifies both evaluation methods, resulting in saving of time and higher test reliability. The DGS evaluation now becomes particularly easy in an ultrasonic instrument like the USN 50 by an optional evaluation program (Fig.3): Fig.3 The ultrasonic flaw detector USN 50 with DGS display

There are DGS diagrams for 13 standard probes stored in the instrument. However, other probes can also be programmed on the basis of their parameters and filed in one of the 30 data sets. A flat bottom hole (disk shaped reflector), side drilled hole or backwall can be selected as reference reflectors. Owing to the operational concept, the use of the DGS method in the USN 50 is especially easy and reliable, operating errors by the inspector are largely excluded due to the display of warning messages on the screen. After the input of all parameters necessary for the flaw evaluation, the corresponding recording curve is electronically displayed on the instrument screen (Fig.4). Fig.4 Display contents of the USN 50 with active DGS function

The evaluation program ensures a direct evaluation of a detected indication. All the necessary corrections are taken into consideration in this respect: exceeding of the recording threshold, i.e. the dB value by which the flaw indication exceeds the preset recording curve, is directly displayed on the screen. This type of evaluation meets the practical requirements specified in most of the testing guidelines. For example, these do not only include e.g. the widely known HP 5/3, DIN 54 125, SEL 072, etc., but also all other specifications requiring flat bottom holes as reference reflectors.