An encoded phased array scan stores a complete A-scan for every law at every position. The size is therefore samples × bytes per sample × laws × positions × groups. Samples come from the digitising frequency and the time the displayed range represents: 2 × range / c microseconds multiplied by the sampling rate in MHz.

The numbers get large quickly. A 100 mm shear range digitised at 100 MHz is about 6200 samples per A-scan; multiply by 101 laws and by 1001 positions along a metre of weld and the raw volume is over half a gigabyte for a single pass at 8 bit resolution.

The levers are the same ones that govern frame rate. Trim the displayed range to what is actually being inspected, reduce the digitising frequency to the usual four to ten times the probe frequency, use 8 bit storage unless the procedure requires more, and do not carry redundant laws. Where the instrument offers averaging of the stored A-scan or compression, the saving is real but so is the loss of raw data for re-analysis.

Compression or shear

Worked example

Displayed sound path range	100 mm
Wave mode	shear
Material velocity	3240 m/s
Digitising frequency	100 MHz
Sample size	1
Laws per acquisition point	101
Scan length	1000 mm
Scan axis resolution	1 mm
Groups stored	1
Time represented by the range	61.73 µs
Samples per A-scan	6173
Stored positions along the scan	1001
File size (MB, 10⁶ bytes)	624.1
File size (GB, 10⁹ bytes)	0.624

A 100 mm shear path represents 2 × 100/3240 = 61.73 µs, which at 100 MHz is 6173 samples. One metre at 1 mm resolution is 1001 positions. 6173 × 1 byte × 101 laws = 623 473 bytes per position, times 1001 positions = 624 096 473 bytes, i.e. 624.10 MB.

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