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Glossary

Vertical linearity (ultrasonic testing)

Vertical linearity is the extent to which the displayed height of an ultrasonic echo stays proportional to the amplitude of the received signal, so that a reflector returning twice the signal is displayed twice as high.

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Also known as: screen height linearity, amplitude linearity, vertical linearity ultrasonic, vertical linearity in UT, vertical linearity echo.

Why amplitude is the one that decides

Ultrasonic acceptance criteria are written in amplitude. An indication is evaluated when it exceeds a reference level. A flaw is sized by traversing until the echo drops 6 dB or 20 dB. A weld is rejected when an indication exceeds a percentage of a distance amplitude correction curve. Every one of those is a comparison of echo heights, which makes the linearity of the amplitude display the property that sits underneath accept and reject decisions.

An instrument whose screen response compresses at the top of the graticule will under-report a large reflector. One that exaggerates at the bottom will produce indications that are evaluated but should not have been. Neither fault announces itself during a routine two-point calibration, because calibration sets one echo to one height and never asks what happens at the others.

The two-echo method

The check in AS 2083 is designed so no signal generator is needed. It uses two echoes whose true amplitude ratio is fixed by physics and cannot change during the test.

  1. Choose a centre echo h(n) and the adjacent echo h(n+1).
  2. Adjust the gain so h(n) sits at 10% of full screen height. Read h(n+1).
  3. Add gain to bring h(n) to 20% of full screen height. Read h(n+1) again.
  4. Repeat at 30%, 40%, 50% and so on up to 100% of full screen height.
  5. Calculate the ratio h(n)/h(n+1) at every step and compare each to the mean.

Because the two reflections come from the same reflector geometry, their real ratio is constant. Any variation in the measured ratio as the pair is walked up the screen is instrument error, and plotting the ratio against h(n) with tolerance bands shows immediately whether the error is a gradual compression, a step, or a single misread point.

The ASME criterion

ASME BPVC Section V, Article 5 states the requirement as a pass or fail rather than a plot. Under Mandatory Appendix I, two reflectors are positioned to give a 2 to 1 amplitude relationship, the larger set to 80% of full screen height and the smaller to 40%. The larger indication is then stepped by the gain control from 100% down to 20% of full screen height in 10% increments, and at every step the smaller indication must read half the larger within 5% of full screen height. Mandatory Appendix II then checks the gain control itself, confirming that a stated decibel change produces the change in indication height it should.

Article 4, which governs weld examination, points back to these checks and sets the interval: at the beginning of each period of extended use, or every three months, whichever is less.

Producing the evidence later

The check is worth nothing to an assessor if the record cannot be found. The plot or worksheet belongs with the instrument, next to its calibration certificate and its next due date, so any report generated with that instrument can be traced to proof it was verified. The free linearity tool on this site produces a chart that can be copied or downloaded for exactly that purpose, and in AMPS the result is filed on the equipment page where the flag system warns a month before the next check falls due.

Frequently asked questions

What is vertical linearity in ultrasonic testing?
Vertical linearity is the accuracy of the instrument's amplitude display. It is the degree to which the height of an echo on the screen stays proportional to the strength of the signal the probe received. Because ultrasonic acceptance criteria are written in terms of amplitude, a reflector at or above a reference level, or a percentage of distance amplitude correction, an instrument that is not vertically linear will size flaws wrongly and can accept a defect or reject sound material.
How is vertical linearity checked?
The method in AS 2083 uses two adjacent echoes. Adjust the gain so the chosen centre echo h(n) sits at 10% of full screen height and read the height of the adjacent echo h(n+1). Add gain to bring h(n) to 20% and read the adjacent echo again. Repeat at 30%, 40% and so on up to 100% of full screen height. Since the physical ratio between the two echoes never changes, any change in the measured ratio across the range is instrument error. Plotting h(n) against the ratio h(n)/h(n+1) with tolerance bands makes the deviation visible.
What is the ASME acceptance criterion for screen height linearity?
ASME BPVC Section V, Article 5, Mandatory Appendix I positions two reflectors so that the larger indication is at 80% of full screen height and the smaller is at 40%, a 2 to 1 relationship. The larger indication is then adjusted with the gain control from 100% down to 20% of full screen height in 10% steps. At each step the smaller indication must read 50% of the larger within 5% of full screen height. Readings are recorded on the instrument's calibration record.
What is the difference between screen height linearity and amplitude control linearity?
Screen height linearity, ASME V Article 5 Mandatory Appendix I, tests the display and receiver: whether two signals in a known ratio keep that ratio at every screen height. Amplitude control linearity, Mandatory Appendix II, tests the calibrated gain control: whether adding or subtracting a stated number of decibels moves the indication by the amount it should. Both are required, because a fault in either one corrupts amplitude-based sizing.
How often is vertical linearity 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 requires gain control accuracy to be qualified at intervals not exceeding two months. Written procedures commonly add a check after repair, after a drop or when results look inconsistent.
How much vertical linearity error is acceptable?
Take the number from the code or written procedure the work is performed to, because there is no universal figure. The most widely applied criterion is the ASME BPVC Section V, Article 5, Mandatory Appendix I limit: with two indications set in a 2 to 1 relationship, the smaller must remain at 50% of the larger within 5% of full screen height as the larger is stepped from 100% down to 20% of full screen height. For amplitude control linearity, Mandatory Appendix II gives permitted ranges for each decibel change rather than a single tolerance. The AS 2083 graphical method instead plots the ratio between two adjacent echoes across the screen and compares the deviation against decibel tolerance bands, which shows the shape of the error rather than only whether it passed.
Why does vertical linearity matter more than horizontal for sizing?
Because ultrasonic acceptance is written in amplitude terms. Codes call for an indication to be evaluated when it exceeds a reference level, or a percentage of a distance amplitude correction curve, and flaw length is often determined by traversing to a 6 dB or 20 dB drop. Every one of those decisions is a comparison of echo heights, so an instrument whose amplitude response is not linear moves the accept or reject boundary. Horizontal linearity affects where the indication is reported, which matters for location but not for the sizing threshold itself.

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