What are the characteristics of phased array (PA) probes?

In this article:

• Why Probe Verification Matters: Understand why relying solely on a probe’s designation plate isn't enough—performance must be validated under proper conditions.
• Key Acoustic Parameters to Check: Learn which factors—like sensitivity, frequency, and resolution—are essential for accurate ultrasonic testing.
• Following ISO Standards: Get an overview of ISO 22232 guidelines for verifying instruments, probes, and complete testing systems.
• Recommended Equipment for Reliable Verification: Waygate Technologies’ Krautkrämer USM 100 is ideal for probe validation, offering advanced RF capabilities, analyzer integration, and ISO-compliant performance in one rugged, portable device.

What are the characteristics of phased array (PA) probes?

In ultrasonic testing, the accuracy and reliability of results depend heavily on the condition and performance of the probes and testing system. While manufacturer data may provide baseline specifications, these properties can shift over time due to aging, wear, or environmental factors. This article outlines practical and standardized methods for verifying key parameters such as sensitivity, frequency, and resolution—ensuring that ultrasonic probes and systems operate within expected performance limits. Emphasis is placed on testing under realistic conditions, minimizing signal manipulation, and recognizing common pitfalls that can affect measurement validity.

The acoustical properties of probes can change. If a probe is needed which has certain properties or if the resistance to ageing of the probe has to be checked then simply trusting the data on the designation plate is by no means enough, the properties must be checked. Hereby the following decisive pre-requirements apply: 

1. Probes 

May only be measured under the conditions for which they were de- signed by the manufacturer: For ex- ample, direct steel contact probes only with a direct contact to steel, immersion type probes only with a water delay. 

2. The probes and the equipment 

i. e. the testing system, are always to be checked as one unit. 

3. Data from the sound field (sound beam) 

Should be measured with the least acoustical and electrical correction i. e., if possible, without filtering the signals by large reflectors, without small band amplifier or using other means of signal cosmetics. 

In testing practice the main parameters which have to be checked are: 

4. Testing sensitivity 

To check the sensitivity reference blocks are used with a large reflecting surface. The attenuation in the reference block should be very small. Great care is to be taken with high frequencies when broad band pulses and broad- band amplifiers are involved be- cause the filtering effect of an attenuation reference block cannot be seen from the shape of the signal (the echo amplitude remains almost constant although the reflected pulse only contains the lower frequencies. So, one measures the test sensitivity for a “lower” test frequency than was assumed). 

With special transducer material particular attention has to be paid to see that the echo amplitudes do not become too high. If the amplifier is saturated or if the wave propagation is non-linear in the reference block the measurement will be worthless. 

5. Testing frequency 

The same echo should be used to check the frequency and sensitivity. An exact determination of the frequency is only possible by using analysers. To speed this determination however one can, quite accurately, (with pulses which have more oscillations) count the number of oscillations on screen of an adjusted oscilloscope. (Ultrasonic testing equipment with RF-indication enables a direct reading to be made from the screen). 

Instruments which have a selective amplifier enable very coarse estimations to be made as to whether the pulse embodies the nominal frequency. The frequency control is set in sequence in all the available frequency ranges. That range in which the highest echo amplitude occurs contains the main pulse frequency. This check, is very advantageous with highly attenuating materials (e. g. austenitic steel) in order to confirm whether there have been any frequency shifts in the pulse spectrum. 

6. Resolving power 

The “near resolution” describes the ability of testing equipment to detect reflectors which are very close to the surface. The “far resolution” is the ability of testing equipment to detect reflectors which lie close together at a greater distance from the probe. The resolving power depends upon the pulse length and there- fore is always determined using the smallest pulse strength. The resolving power is measured
by simulation with a suitable measuring pulse instrument or approximately with suitable reference blocks (for direct contact,
TR reference blocks, reference blocks for immersion technique). 

Conclusion

Reliable ultrasonic testing begins with a well-characterized and verified inspection system. By assessing probes and instruments as a unified system and applying rigorous checks on sensitivity, frequency response, and resolution, practitioners can maintain testing integrity across a range of materials and applications. These procedures, closely aligned with ISO 22232 guidelines, help reduce uncertainty and improve flaw detection accuracy. For organizations seeking dependable performance and simplified verification workflows, Waygate Technologies offers advanced flaw detectors such as the Krautkrämer USM 100, purpose-built to meet the demands of modern ultrasonic inspection and verification.

A survey of the checking method is given in: 

[17] ISO 22232-1, 2,3 (2020): Non- destructive testing—Characterization and verification of ultrasonic test equipment 

- Part 1: Instruments,
- Part 2: Probes,
- Part 3: Combined Equipment