Also known as: time base linearity, horizontal linearity ultrasonic, sweep linearity, horizontal linearity in UT, multiple echo linearity check.
What it actually governs
An ultrasonic instrument converts elapsed time into displayed distance. Horizontal linearity is how faithfully it does that across the whole screen rather than at the one or two points used for calibration. Two-point calibration on a V1 block can look perfect while the middle of the sweep is compressed or stretched, and the error only shows up as a reflector reported at the wrong depth.
The consequence is practical. On a weld inspection, depth is what places an indication in the root, the fill or the cap, and what decides whether a beam-path calculation puts a reflector inside or outside the weld volume. On a thickness survey the same error becomes a wrong remaining-wall figure feeding a corrosion rate.
The multiple back-wall echo method
The check exploits a fact that needs no measuring equipment: successive back-wall echoes from a parallel-sided block are, by definition, equally spaced in time.
- Couple a straight-beam probe to a block of known thickness. AS 2083 specifies the 25 mm side of the V1 block.
- Set the range so several multiple back-wall echoes are visible across the graticule. Four or five echoes suit ranges up to 100 mm; ten to twenty suit 100 mm to 500 mm.
- Read the screen position of each echo, taking the reading at a consistent point on each echo, typically the leading edge at a fixed screen height.
- Compute the interval between each pair of successive echoes.
- Compare each interval to the mean of all intervals. The spread is the linearity error, normally expressed as a percentage of full screen width.
Test ranges other than the one checked should be assessed separately, since the error is not necessarily the same at every range setting. This is why procedures often call for the check at the ranges actually used in production.
Reading the result
Plotting echo position against interval makes the failure mode visible in a way a table does not. A horizontally linear instrument gives a flat line: every interval equal to the mean. A drifting line means the sweep speeds up or slows down across the screen. A single outlier usually means a misread echo rather than an instrument fault, and is worth re-reading before the instrument is condemned.
The free linearity tool on this site takes the echo positions, plots them against the mean with tolerance bands drawn in, and produces a chart that can be copied or downloaded straight into an instrument record.
Where the record has to go
A linearity check is only useful if it can be produced later. In practice the chart or worksheet belongs with the instrument, alongside its calibration certificate and next due date, so that any report produced using that instrument can be tied back to evidence that the instrument was verified on the day. In AMPS this sits on the equipment page, and the flag system gives a month of warning before the next check falls due, which is the same mechanism used for personnel certifications.