DACs

The working principle of a DAC is shown in Fig. 1.22. Two brilliant-cut stones are bevelled at the tip (culet) with their large faces (table) resting on flat hard seats. Between the culets, a metal gasket encloses a sample which may be embedded in a suitable pressure medium. The pressure which is generated by a force acting on the anvils is measured by a pressure calibrant which may be the fluorescence of a ruby chip. 

Anvils are usually cut with eight or sixteen facets the latter geometry gives a 

The anvil seats transmit a force to the anvil tables of the order of 10 kN. Thus the seats are subjected to a normal stress of some 2 GPa, for a 3 mm diameter table. This value may well be exceeded for ultrahigh-pressure work. Thus seats are most often made of tungsten carbide with an optical finish. In some X-ray measurements, beryllium seats have to be used, for their transparency. The mechanical performance is then drastically decreased, and the beryllium must be machined in specialized workshops. Single-crystal sapphire has been used, to provide an increased optical aperture.  

The anvil seats are provided with a cylindrical or conical hole for optical access, on which the anvils must be centred. The anvils may be held in place with epoxy glue, which is simple, but is unsuitable for variable-temperature work when the anvils must be set with a soft copper ring which is held in place by screws onto the seat. Since the parallelism and centring of the two anvils is a critical parameter, the shape of the seats usually provides tilt and x-y alignment (Fig. 1.25). Hemispherical shapes are currently the most widely used, although, for moderate pressures, quite 

Strength of the gasket, on the friction coefficient between the gasket and the anvil faces, and most importantly on the compressibility of the sample. No convincing analysis of this problem, which has seldom been addressed theoretically has been made at the present time. 

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Calibration of the ultrasonic instrument, including plotting of a recording curve (DAC), or a reference reflector for a DOS evaluation, or loading of the existing test dataset... 

With the reference block method the distance law of a model reflector is established experimentally prior to each ultrasonic test. The reference reflectors, mostly bore holes, are drilled into the reference block at different distances, e.g. ASME block. Prior to the test, the reference reflectors are scanned, and their maximised echo amplitudes are marked on the screen of the flaw detector. Finally all amplitude points are connected by a curve. This Distance Amplitude Curve (DAC) serves as the registration level and exactly shows the amplitude-over-distance behaviour" of the reference reflector for the probe in use. Also the individual characteristics of the material are automatically considered. However, this curve may only be applied for defect evaluation, in case the reference block and the test object are made of the same material and have undergone the same heat treatment. As with the DGS-Method, the value of any defect evaluation does not consider the shape and orientation of the defect. The reference block method is safe and easy to apply, and the operator need not to have a deep understanding about the theory of distance laws. 

An experimentally recorded DAC from a reference block and a curve calculated from the DGS-diagram are equal in many cases The pre-requirements of this equivalence are ... 

Figure 2a DGS curve (bold) and DAC (with reference points)...

In these cases we will use the required reference block and record the DAC individually ... 

Since any DAC is defined by its coordinates P (Ai,S ) and the instrument sensitivity Gg f (reference gain) during DAC recording, any recalculation of the curve including the consideration of individual corrections (transfer loss, sound attenuation, etc.) is an easy task for modern PC based flaw detectors and does no longer burden the operator. 

Benefits of the DAC-DGS equivalence in modern digital flaw detectors... 

The curve is defined by the Amplitude/Distance pairs P,(Aj,S ), documented in a corresponding DAC table and stored in the instrument s memory. 

Corrections of the reference points of a curve may be made, if required by editing the DAC table or graphically editing the DAC points. 

Figure 3 Multiple DAC with echo amplitude evaluation and screen compare mode (no defect) on the 12 SVGA colour display of the USLT 2000...

The echo amplitude Ar of a reference reflector depends on the type, size (diameter) d f, and distance Sr,., of the reference reflector, and additionally on a possible attenuation in the reference block and finally the absolute gain setting of the instrument G f. In a combined DAC/DGS evaluation program we define the following ... 

Reference echo The maximum reference echo during DAC recording at The maximised reference echo at... 

Absolute instrument gain at which the maximised reference echo reach 80% screen height. After DAC or reference echo recording the instrument gain is automatically... 

We showed that experimentally recorded DACs and synthetically generated DGS curves are equivalent if the described requirements are fulfilled. Consequently an echo amplitude evaluation program has been developed which combines both techniques in one instrument. The software of the new state-of-the-art Ultrasonic Notebook USLT 2000 allows, apart from many other new possibilities, the combined or exclusive use of the reference block and the DGS method for amplitude evaluation. Even in those situations where DACs cannot be re-... 

DAC DAC (distance amplitude control) up to 40 dB optimized for material with high sound damping 0.1 to 20 MHz (-3 dB)... 

Yoo C S, Akella J and Nicol M 1996 Chemistry at high pressures and temperatures in-situ synthesis and characterization of p-SijN by DAC x-ray/laser-heating studies Advanced Materials 96 ed M Akaishi et al (Tsukuba National Institute for Research in Inorganic Materials) p 175... 

Cooling water service failure 3. Same 3. Check and monitor reliability of water DAC 2/93... 

Controller fails and closes valve 4. Same 4. Place controller on critical DAC 1/93... 

This equation was then solved numerieally with known values of erystal growth rate (G) and solubility dc jdO) for seeded solutions. It was further assumed that if G f G(L, 6), dc /d6 = eonstant and dAc/d6 = 0 then the simplified equation for ealeulating transient temperature results (Mullin and Nyvlt, 1971)... 

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