DGS-DIAGRAM: ( Michael Berke )

Present day evaluation methods produce reliable and reproducible results providing that the

testing device meets technical requirements and that the test personnel are accordingly

qualified. The position of a flaw in the workpiece is calculated quickly and very accurately on

the basis of the measured acoustic time of flight of an echo. This means that a flaw location

takes place. The echo amplitude is used for an estimation of the flaw size. However, this is

not quite as easy as flaw location because the echo amplitude is subjected to much more

influences than the acoustic time of flight. Two methods have become generally accepted in

manual ultrasonic testing worldwide:

- Reference block method ( Distance Amplitude Blocks )

- DGS method

Though the two methods differ very much with regard to their application, they are not

different with regard to the physical principles of sound propagation and sound reflection

that they are based on. The reason is that, in both methods, the inspector determines the

size (diameter) of a reference reflector (disk shaped reflector, cylindrical reflector). The size

thus determined is not identical with the actual flaw size and it is therefore termed as the

equivalent circular disk or side drilled hole diameter. The shorter term "equivalent reflector

size" (ERS) has become generally accepted when using disk shaped reflectors. The reason for

the fact that the actual flaw size does not correspond to the equivalent reflector size is

because the sound fractions reflected from a natural flaw are additionally influenced by the

shape, orientation and surface quality of the flaw. In this respect further tests are difficult

and not very practical in manual ultrasonic testing so that most specifications and guidelines

for ultrasonic testing attach the criteria for flaw recording to a defined equivalent reflector

size. This means: the inspector determines whether a detected flaw reaches or exceeds the

equivalent reflector size indicated as a limit value (recording level) in the standard

specifications. In addition to this, the inspector must carry out other tests, e.g. regarding the

recording length, echo dynamics, etc., further details of which are not discussed here.

DGS Diagram

General DGS

Diagram

The 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.

Procedures

To understand this better, let us start by explaining the sequences for both evaluation

methods at this point.

The Reference Block Method

This 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.

The DGS Method

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 .

- 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.

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 (back-wall) as a function of the distance.

- A comparison of the test sequences for the reference block method and DGS method

shows the advantages and disadvantages in next table.

Advantages and Disadvantages of the DGS- and reference block method

Reference Block Method DGS-Method

Advantages

Disadvantages

Electronic DGS Evaluation

The 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 by an optional

evaluation program. There are DGS diagrams for several standard probes stored in the

instrument. However, other probes can also be programmed on the basis of their

parameters and stored in one of the data sets. A flat bottom hole (disk shaped reflector),

side drilled hole or back-wall can be selected as reference reflectors. Owing to the

operational concept, the use of the DGS method 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

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 international standards, but also all other specifications

requiring flat bottom holes as reference reflectors.

The DAC curve contains all

test related influences, i.e.

no time consuming

corrections are required

Easy and reliable

evaluation. No reference

blocks required.

Fabrication or

procurement of a suitable

reference block.

Recording of a DAC curve

for every test application

Measurement and

consideration of different

individual corrections.

Graphical determination

of equivalent reflector

size.