# Horizontal linearity (ultrasonic testing) > Horizontal linearity is the extent to which an ultrasonic instrument displays echo positions along the time base in true proportion to their distance, so that equally spaced reflections appear equally spaced on the screen. - Source: https://amps.global/glossary/horizontal-linearity/ - Type: Glossary - Published: 2026-09-10 - Tags: NDT, ultrasonic testing, calibration, linearity ## Key facts - Horizontal linearity is a distance check: it governs whether a flaw is reported at the right depth, not how big it looks - Usual method: display a series of multiple back-wall echoes from a known thickness and confirm the spacing between successive echoes stays constant across the screen - ASME BPVC Section V, Article 5, T-534 evaluates horizontal linearity in accordance with ASTM E317 (SE-317) - AS 2083 uses the V1 calibration block on its 25 mm side; ASME V instrument linearity checks are required at the start of each period of extended use or every 3 months, whichever is less - AWS D1.1 requires horizontal linearity to be qualified at intervals not exceeding two months - A common acceptance band is a deviation within 1% of the full screen width, but the applicable code or written procedure sets the limit 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. 1. Couple a straight-beam probe to a block of known thickness. AS 2083 specifies the 25 mm side of the V1 block. 2. 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. 3. 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. 4. Compute the interval between each pair of successive echoes. 5. 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](/ndt-tools/linearity/) 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. ## Frequently asked questions ### What is horizontal linearity in ultrasonic testing? Horizontal linearity is the accuracy of the instrument's time base. It is the degree to which the position of an echo along the horizontal axis of the display is proportional to the true distance the sound travelled. If an instrument is horizontally linear, back-wall echoes from a 25 mm block appear at 25, 50, 75 and 100 mm on the screen with equal spacing. If it is not, a reflector will be reported at the wrong depth even though the calibration at one point looked correct. ### How is horizontal linearity checked? Place a straight-beam probe on a block of known thickness, commonly the 25 mm side of a V1 calibration block, and set the range so that several multiple back-wall echoes appear across the screen. Use four or five echoes for ranges up to 100 mm and ten to twenty for ranges of 100 mm to 500 mm. Record the screen position of each echo, then calculate the interval between each pair of successive echoes. Those intervals should all be equal; the deviation of each interval from the mean is the horizontal linearity error, expressed as a percentage of the full screen width. ### What is the acceptance criterion for horizontal linearity? It is set by the applicable code or the written procedure, not by a universal number. A deviation within 1% of the full screen width is a commonly applied band, with 2% used in some procedures. ASME BPVC Section V, Article 5, T-534 refers the evaluation to ASTM E317, and the equipment standards ISO 22232-2 and its predecessor EN 12668-2 define how the measurement is made. Always take the limit from the document your work is being performed to. ### How often does horizontal linearity have to be checked? ASME BPVC Section V requires instrument linearity checks at the beginning of each period of extended use, or every three months, whichever is less. AWS D1.1 sets a maximum interval of two months for horizontal linearity qualification. Many written procedures additionally require a check after an instrument is repaired, dropped or otherwise disturbed. ### How is horizontal linearity different from vertical linearity? Horizontal linearity is about position and therefore about depth: it tells you whether a reflector is where the screen says it is. Vertical linearity is about amplitude and therefore about sizing and acceptance: it tells you whether an echo that is twice as strong is displayed twice as high. An instrument can pass one and fail the other, so both are checked. ## Related pages - [Free UT linearity tool (horizontal and vertical)](https://amps.global/ndt-tools/linearity/) - [Vertical linearity](https://amps.global/glossary/vertical-linearity/) - [Ultrasonic testing (UT)](https://amps.global/glossary/ultrasonic-testing/) - [Reference block](https://amps.global/glossary/reference-block/) - [ASME BPVC Section V](https://amps.global/glossary/asme-bpvc-section-v/) - [Calibration certificate](https://amps.global/glossary/calibration-certificate/) ## Sources - [ASTM E317 Standard Practice for Evaluating Performance Characteristics of Ultrasonic Pulse-Echo Testing Instruments and Systems](https://www.astm.org/e0317-21.html) - [ASME Boiler and Pressure Vessel Code](https://www.asme.org/codes-standards/bpvc-standards) - [ISO 22232-2 Characterization and verification of ultrasonic test equipment: receivers](https://www.iso.org/standard/71353.html)