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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Industrial Maintenance › Predictive Maintenance Technologies for Industrial and Marine Systems › Lesson 7
MFG-INDU16-07 · Lesson 7 · 15 clock hours · lab

Ultrasonic Testing: Airborne and Structure-Borne Techniques

Objective. Students will use an ultrasonic detector in both airborne and contact modes to locate compressed-air leaks, evaluate bearing lubrication condition, and assess steam-trap operation, recording decibel baseline and fault readings.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

What Is Ultrasonic Testing?

Ultrasonic testing (UT) in predictive maintenance uses high-frequency sound — typically in the 20 kHz to 100 kHz range, well above human hearing — to detect mechanical and fluid-related faults. Portable ultrasonic detectors convert these signals into an audible tone through headphones and display a decibel (dB) reading on screen. The two main modes you will use are airborne and structure-borne (contact).

Airborne Mode: Finding Compressed-Air Leaks

When compressed air escapes through a crack, worn fitting, or faulty valve, turbulence at the leak point generates a cone of ultrasonic energy. The scanning cone (also called the parabolic or airborne probe) is held 2–12 inches from the surface and swept slowly along distribution lines, fittings, and connections. The detector's dB reading rises as you approach the source. Move side-to-side to pinpoint the peak — that is the leak.

Procedure overview:

1. Set background dB with the system pressurized but away from any suspected leak. Record this as your baseline.

2. Sweep systematically: headers first, then branch lines, then end-use connections.

3. When a rise of 6 dB or more above baseline is noted, slow down and isolate. Mark the location.

4. Consult the manufacturer's reference chart to estimate leak rate (standard cubic feet per minute, SCFM) from the dB level and probe distance. Larger leaks produce higher dB and wider ultrasonic plumes.

A common rule of thumb is that every 3 dB increase roughly doubles the intensity of the signal, but leak-size estimates must come from your instrument's chart, not guesswork.

Contact Mode: Bearing Condition Monitoring

The contact probe (sometimes fitted with a rubber focusing disk or magnetic tip) is pressed firmly to a bearing housing, gearbox casing, or pipe surface. Mechanical defects — surface pitting, spalling, lack of lubrication — create friction and impact events that radiate structure-borne ultrasound.

Three bearing conditions you will recognize:

Re-greasing technique: Add grease in small increments while monitoring dB in real time. Stop when the dB levels off or drops; adding excess grease can over-pressurize seals. After re-greasing, re-test and document the new dB value to confirm improvement.

Contact Mode: Steam-Trap Assessment

Steam traps remove condensate without passing live steam. A functioning trap opens briefly to discharge condensate, then closes — on ultrasonic survey you hear intermittent activity. A failed-open trap passes steam continuously, producing a loud, steady, high-dB signal. A failed-closed (plugged) trap is silent and cool; no flow of any kind.

Pair ultrasonic data with surface temperature measured by infrared thermometer or thermocouple:

Record both dB reading and temperature on the steam-trap survey form, then write a pass/fail with your reasoning.

Reading the Decibel Scale

The dB scale is logarithmic. A 3 dB increase means roughly twice the acoustic power. A 6 dB increase means roughly four times. This matters when you compare readings across sessions or use the manufacturer's leak-size chart — a small instrument position change can shift your reading several dB, so keep probe distance and orientation consistent.

Safety Requirements

Lockout/tagout (LOTO): Ultrasonic surveys are typically performed on energized equipment; this is a non-contact, online inspection method. You must never defeat guards, open pressurized components, or reach into rotating equipment during scanning. Know the machine's energy sources before you approach.

PPE for this lab:

Compressed-air hazards: Never use compressed air to clean skin or clothing. Even low-pressure leaks can inject air into tissue if held directly against skin. Mark leak locations with chalk; do not place a hand over the leak to feel it.

Steam hazards: Steam-trap test points can be extremely hot. Use insulated gloves and stand to the side — not directly in front — of any vent or drain when steam equipment is energized.

Documentation and Trending

A single dB reading has limited value. Its power comes from trending: compare today's reading to last month's baseline and last year's baseline. A bearing reading that rises 8 dB over 90 days warrants scheduling a replacement before failure. Good records — date, equipment ID, probe type, probe location, dB, and any qualitative notes — are the foundation of a predictive maintenance program.

Key terms8

Airborne ultrasonic probeA scanning cone or parabolic transducer that detects ultrasonic energy radiating through air from leak points or electrical discharge sources, used without direct contact with the surface.
Contact (structure-borne) probeA probe pressed directly against a solid surface — bearing housing, pipe, or valve body — to detect ultrasonic vibrations transmitted through the material from mechanical defects or fluid flow.
Baseline readingA dB measurement recorded on known-good equipment under normal operating conditions, used as a reference point for detecting future changes.
Decibel (dB)A logarithmic unit used to express the ratio of one sound intensity to another; on an ultrasonic instrument, higher dB indicates stronger ultrasonic signal from a defect or leak source.
Failed-open steam trapA steam trap stuck in the open position, allowing live steam to pass continuously into the condensate return line, wasting energy and indicated by constant high dB and elevated outlet temperature.
TurbulenceChaotic fluid flow that occurs as pressurized gas or liquid escapes through a restriction or orifice, generating broad-spectrum ultrasonic energy that the detector captures.
TrendingThe practice of recording and comparing periodic measurements over time to identify gradual deterioration before it reaches a failure threshold.
Logarithmic scaleA scale in which equal intervals represent multiplication rather than addition; on the dB scale, each 3 dB increase represents roughly a doubling of acoustic power.

Worked examples3

1. You record a background dB of 24 dB in the area of a compressed-air manifold with no leaks present. As you scan a fitting, the reading climbs to 42 dB. How many dB above baseline is this reading, and how does the acoustic power at the fitting compare to the background?
  1. Step 1 — Calculate the dB rise: 42 dB − 24 dB = 18 dB above baseline.
  2. Step 2 — Determine power ratio. Every 3 dB doubles acoustic power, so 18 dB = 6 doublings. Start at 1× and double six times: 1 → 2 → 4 → 8 → 16 → 32 → 64.
  3. Step 3 — Interpret: the signal at the fitting is 64 times the acoustic power of the background level, a strong indicator of a significant leak warranting immediate investigation.
Answer: The reading is 18 dB above baseline, representing approximately 64 times the acoustic power of the background — a strong positive indication of a compressed-air leak.
2. A bearing housing reads 31 dB at initial inspection. Three months later it reads 37 dB. Is this change significant, and what does it suggest?
  1. Step 1 — Calculate rise: 37 dB − 31 dB = 6 dB increase over 90 days.
  2. Step 2 — Interpret magnitude: a 6 dB rise represents roughly four times the acoustic power (two doublings of 3 dB each), which is outside normal measurement variation.
  3. Step 3 — Assess trend: a 6 dB rise over 90 days in a bearing suggests progressive mechanical deterioration (early pitting or lubrication breakdown) and typically triggers a maintenance alert for inspection and possible re-lubrication or replacement planning.
Answer: A 6 dB increase (≈ 4× acoustic power) over 90 days is a significant trend indicating bearing deterioration; a maintenance action such as inspection, re-lubrication, or scheduling replacement should be initiated.
3. During re-greasing of an under-lubricated bearing, you start at 44 dB. After the first pump of the grease gun the reading drops to 39 dB. After a second pump it drops to 36 dB. After a third pump it stays at 36 dB. What should you do and why?
  1. Step 1 — Observe the trend: 44 → 39 → 36 → 36 dB. The reading dropped with the first two pumps but did not drop further with the third pump.
  2. Step 2 — Apply the stop rule: stop adding grease when the dB levels off or stops decreasing. Adding more grease beyond this point risks over-pressurizing the bearing seal, which can cause seal failure and introduce a new defect.
  3. Step 3 — Document: record the final dB of 36 dB as the post-greasing reading, note the quantity of grease added, and compare to the bearing's manufacturer specification to confirm the quantity is reasonable.
Answer: Stop at two pumps (final reading 36 dB). The reading has plateaued, meaning the bearing is adequately lubricated; additional grease risks seal damage. Document 36 dB as the new post-maintenance baseline.

The math in this lesson1

Learners compute dB rise above background and baseline to judge leaks and bearing lubrication.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Students use the airborne scanning probe to survey a compressed-air manifold and distribution loop on the training rig that contains two seeded leaks (plugged with removable defect inserts by the instructor); students mark leak locations with chalk, record dB levels, and estimate leak size using the instrument manufacturer's reference chart. · Students use the contact probe (rubber focusing disk) on three bearings — one normal, one with induced surface pitting, one with insufficient grease — recording dB levels and waveform pattern on a data sheet; instructor demonstrates proper re-greasing technique and students re-test to confirm dB reduction. · Students evaluate two steam traps (one functioning, one simulated failed-open) using the ultrasonic contact probe and supplemental temperature measurement, record findings on a steam-trap survey form, and write a pass/fail determination with supporting data.
Materials: Ultrasonic detector with airborne scanning cone and contact probe; compressed-air training manifold with seeded leaks; bearing-defect trainer with three bearing conditions; steam-trap demonstration unit; grease gun; dB data sheet; steam-trap survey form; safety glasses; hearing protection
Check: Students must locate both seeded air leaks (required), correctly classify all three bearing conditions (pass 2 of 3 minimum), and produce a correct steam-trap pass/fail determination with supporting dB data; overall pass requires 80% of scored items correct.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Industrial Maintenance — Instrumentation and Testing
1. When using an airborne ultrasonic probe to locate a compressed-air leak, what indicates you have found the precise leak point?
MSSC CPT — Maintenance Awareness
2. A bearing contact probe reading rises from a baseline of 28 dB to 34 dB over two inspection intervals. Which statement best describes this situation?
NCCER Industrial Maintenance — Predictive and Preventive Maintenance
3. Which combination of ultrasonic and temperature data best indicates a failed-open steam trap?
NCCER Industrial Maintenance — Bearings
4. Why should you stop adding grease to a bearing when the ultrasonic dB reading levels off during re-lubrication?
MSSC CPT — Safety
5. Which PPE item is specifically required because ultrasonic detector headphones can produce dangerously loud output when the instrument encounters a strong signal?
NCCER Industrial Maintenance — Safety
6. A student is preparing to scan a pressurized air manifold with an ultrasonic detector. The manifold has no guards removed and all connections are intact. Which action is correct regarding lockout/tagout?
MSSC CPT — Maintenance Awareness
7. What is the primary reason ultrasonic testing relies on trending (comparing readings over multiple inspection intervals) rather than a single measurement?
NCCER Industrial Maintenance — Instrumentation and Testing
8. When scanning for compressed-air leaks in a noisy manufacturing environment, a technician notices the background dB fluctuates by ±4 dB while walking along the line. Which threshold rise above background should the technician use as the minimum indicator to stop and investigate a potential leak?
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