PetroStreet

While performing pulse echo testing what is TCG and how it is different from DAC?
Is it something related to fixed gain & the one which can be changed in TCG?

From code perspective, ASME V, and modern phased-array procedures recognize both concepts.
In many refinery PAUT applications, TCG is standard practice while conventional manual weld UT still heavily uses DAC.

This is what I could add here:
You can display DAC without TCG, use TCG without traditional DAC evaluation, or combine both in advanced instruments.
And like Dlew said already, both are recognized by ASME.
And another practical difference is that With DAC, you usually do not alter gain during evaluation. With TCG, gain is intentionally varied automatically throughout the sound path.

TCG is basically an electronic/digital version of DAC.
Instead of drawing a reference curve for comparison, the UT machine automatically changes gain with time (distance).
So deeper signals get automatically amplified more, near-surface signals get less amplification. This makes equal-sized reflectors appear at nearly equal heights on the screen.
So TCG would be having:
Dynamic gain compensation
Gain changes automatically with time/depth
Equalizes reflector amplitudes
Easier sizing and interpretation
Common in PAUT and advanced digital UT
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Therefore, the bottom line is that TCG improves:
Flaw sizing consistency
Interpretation reliability
Scan readability
Phased array imaging quality