TCG is a time-corrected DAC so that equal dimension reflectors give equal amplitude responses for all sound path distances. Used for PAUT Sectorial scans where it would be otherwise impossible to set every angle and sound path to the same sensitivity level using DAC's.
ASTM E-1316: DGS (distance gain size-German AVG)distance amplitude curves permitting prediction of reflector size compared to the response from a back surface reflection.
The probe manufacturer supplies data sheet diagams for each probe which shows the amplitude response curves from the backwall and a range of diameters of flat-bottom holes along the length of the soundfield.
Have a look at EN 583-2:2001 Sensitivity and range setting for excellent authoritative descriptions of DAC/TCG and DGS. You'll have to look at AWS D1.1. for instance for knowledge of their sensitivity setting requirements.
Knowledge of these techniques is desirable but will such knowledge really improve your inspection method? You use DAC because the Codes and standards you work to require you to assess indications to those DAC's. A report that a reflector was 3,5 mm equivalent FBH size to DGS would most probably be rejected.
I always liked "Adjust gain until 2mm grass apears acrosss the timebase." and let the technician do the rest in terms of indication characterisation. In my opinion techs raised on this method interrogated the indication envelope (that phrase is almost an anachronism now) the most and built up the best picture of an indication's size, shape and reflecting surfaces. Note even CIT's such as PA and TOFD still often require manual UT characterisation.
Ah Nigel, no nostalga for this boy. After I convert a nice, clean DAC curve into a TCG flat-line, life gets very simplified.
"Does the indication break the [flat, straight] reference line?"
if answer is no:
"Does the indication break the nice, straight -12/14dB line?"
I can perform those evaluations 12 hours into a tough day, whiling hanging in a harness, reaching to the back side of a big pipe. Today, most of the tough work is performed during the calibrations. Makes the field measurements & evaluations so very much easier.
The other applications where TCG has major advantages:
1. automated system - just detect a gate being broken.
2. Highly attenuative materials such as composites or cast iron, where the signal amplitude variation in the region of interest exceeds the 15 dB or so it is practical to evaluate on a constant gain A-scan.
From a code point of view TCG and DAC are effectively equivalent, so you can generally make things a lot easier without a downside. Most codes date back from the days before TCG was such a low cost option, so its rare to find it actually specified.
I have seen instruments that implement a Time controlled gain version of DGS, but I'm not sure about that one.
This is exactly why the ASME Code requests a TCG, not a DAC. With a TCG, you can just set your thresold at one pre-defined colour, and everything above it is rejectable; if you want to go -12/-14 dB down from there, you can set your palette accordingly. That way, one can tell at a glance if there is a rejectable defect or not, or a defect that requires further investigation.
Naturally, it is possible to make a DAC (or multiple DACs) for AUT and phased arrays, but there is a LOT more work involved, and a much higher chance of error.
i'm a beginner UT Tech and have a very small experience in Phased array and other advance method. I need your explanation upon your post.
as your post above ;
"This is exactly why the ASME Code requests a TCG, not a DAC."
can you give me the ASME codes for my referrences.
"With a TCG, you can just set your thresold at one pre-defined colour, and everything above it is rejectable; if you want to go -12/-14 dB down from there, you can set your palette accordingly. That way, one can tell at a glance if there is a rejectable defect or not, or a defect that requires further investigation"
I understand but i have a question on +dB and -dB which will represent colour after we set the threshold, is it the +dB and -dB actually represent the amplitudes of the echo because of the setting on Full wave rectifier.
If the answer is NO, what is it?
If the answer is YES, some says that the electric sinusodial graph is not same as ultrasonic pulse wave, is it true ?
If i have wrong information, please correct me.
and lastly, is it true that any equipment that using the colour coding (thresold at one pre-defined colour )for their references need to use TCG???
With pleasure. ASME currrently has five Code Cases in Section V Article 4, covering manual and encoded phased array scanning: 2541, 2557, 2558, 2599 and 2600. If you wait until the summer of 2010, we should have a Mandatory Appendix on phased arrays.
There are related practices in ASTM: E-2491 on setting up phased arrays, and E-2700, a procedure for contact weld inspections.
I am with Olympus. My colleague and friend Michael has passed on, so I will answer your query on Olympus' behalf.
I recommend you get a demonstration of the multiple units out in the market and confirm your needs (environment, ease of use, features etc.). We could go on and on about perceived advantages. and another manufacturer will do the same on a forum like this. We do benefit from making the OmniScan platform and being in the phased array game for a long time. You can see the OmniScan page (and our other phased array instruments) here and also sign up for some one to contact you or find your representative on our web page.