1-point vs 2-point calibration: the differences and when to use each

Calibrating the ultrasonic instrument is the procedure that guarantees the accuracy of readings on a given material. Every Dakota NDT gauge supports at least 1-point calibration, and many advanced models also support 2-point calibration. This guide explains the differences, when to use each, and the operating practices that produce repeatable, reliable readings.

Why calibration is necessary

Ultrasonic thickness measurement rests on the formula:

t = v × Δt / 2

where t is the thickness, v is the speed of sound in the material and Δt is the time of flight measured by the instrument. The measurement of Δt is very precise, but the speed of sound depends on:

• the composition of the material (different steels have different velocities)
• heat treatment and microstructure
• temperature of the part
• orientation of the crystal grains (anisotropy)
• any porosity or inclusions

That is why every material has to be calibrated specifically before use. Two pieces of “steel” can have speeds of sound that differ by 2-3 %, which translates into measurement errors of the same order if no calibration is done.

1-point calibration

1-point calibration is the simplest procedure: a block of known thickness is measured and the instrument is corrected so that it reads exactly that value. The system adjusts the internally stored speed of sound (VM) or applies an offset.

Typical procedure:

1. Preparation of a calibration block of the same material as the part to be measured, of known thickness (typically close to the expected thickness)
2. Application of couplant gel to the block
3. Positioning of the probe and reading
4. If the reading differs from the known value, adjustment through the instrument’s 1-point calibration menu
5. Repetition of the reading as a check

When to use 1-point calibration

• Readings on a single uniform thickness, or on thicknesses close to one another
• Field applications calling for quick calibration
• Entry-level instruments such as the Dakota CX2 and ZX1, which support 1-point only
• Routine checks on material that is already well characterised

2-point calibration

2-point calibration uses two reference blocks of different thicknesses: one close to the expected minimum and one close to the expected maximum. The instrument corrects both the offset (static reading) and the gain (linearity of the reading across a range), producing a calibration that is more accurate over wide ranges.

Typical procedure:

1. Preparation of two calibration blocks of the same material, at different thicknesses (for example 2 mm and 20 mm if readings in that range are expected)
2. Reading on the thinner block and correction of the first point
3. Reading on the thicker block and correction of the second point
4. The instrument calculates offset and gain automatically
5. Final check on at least a third, intermediate point to confirm linearity

When to use 2-point calibration

• Readings over wide thickness ranges (for example from 2 to 50 mm)
• Critical applications where accuracy is a contractual requirement
• When the operator wants to verify the linearity of the reading as well as the offset
• Instruments that support it: Dakota CX8-DL, the CMX, MX, PMX and PCX series, and every high-end model

Other supported calibration methods

• Preset materials: the CX, CMX and other instruments carry material lists with pre-programmed velocities (steel, aluminium, copper and so on) — useful as a starting point, but no substitute for calibration on a block
• Velocity Mode (VM): available from the CX4 upwards, it measures the actual speed of sound on a piece of known thickness, useful for characterising non-standard alloys
• Target Velocity: available on the Dakota R9 Sonic Tester, it allows a reference velocity to be entered directly
• Custom velocity storage: the more advanced instruments can save calibration profiles for quick recall

Good operating practice

• Certified calibration blocks: use blocks with a traceable calibration certificate (NIST or equivalent national bodies)
• Same material: the block has to be representative of the material being measured, not merely “generic steel”
• Same temperature: the speed of sound varies with temperature; calibrate at a temperature close to that of the measurement
• Same probe: the calibration is valid only for the instrument-probe combination used
• Periodic re-calibration: on long campaigns, re-calibrate every 2-3 hours or whenever conditions change
• Record keeping: document every calibration with date, blocks used, result and operator
• Check on return: at the end of a campaign, verify that the instrument still reads correctly on the calibration blocks

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For advice on the calibration blocks best suited to Your application and on the periodic verification services for Dakota instruments, contact our technical department.