The other is more scientific and is based on the work of Drs Krautkramer in 1959. Curves are made up for specific ranges and specific probes and specific instruments. The Echo Dynamic method uses the drop of a maximised signal to a specified amplitude difference rleated to a probe displacement. This is often done using Distance Amplitude Correction Curves generated from side drilled holes at different distances. This method is again filled with unaccountable variables such as flaw shape, flaw lateral extents, flaw orientation effects, reflectivity variations, etc., etc.
Any sizing method is subject to limitations.
When a UT technician is given the task of Sizing a flaw, the people using this information must be cognisant of the REAL tolerance of the method. Both the Amplitude Sizing and the Echo dynamic sizing methods are subject to considerable variations and results from these must be treated with caution and suitable tolerance must be stated up front. I am researching flaw detection using ultrasonic inspection. Many of the items have appeared in previous writings on the NDT.net so a search of the website might help. John Drury's text, Dr. Harumi book on Tip Diffraction, Mark Davis' book on Sizing and the ASNT Handbook are just some of the texts more information can be found in.