The Most Versatile Tool in Your PdM Toolkit
Airborne ultrasonic instruments pick up sound in the 20 kHz to 100 kHz range — well above human hearing. At these frequencies, sound waves are short, directional, and easy to isolate from background plant noise. This makes ultrasonic instruments uniquely suited for three applications that no other single technology handles as well: leak detection, bearing monitoring, and electrical fault detection.
The equipment is relatively affordable ($3,000-8,000 for a quality handheld unit), training requirements are modest compared to vibration analysis, and results are immediate. An ultrasonic survey typically produces actionable findings within the first hour of use.
Compressed Air Leak Detection: The Fastest ROI in Maintenance
Compressed air is the most expensive utility in most industrial plants. Generating compressed air costs $0.20-0.30 per 1,000 cubic feet, and the U.S. Department of Energy estimates that 20-30% of compressor output is lost to leaks in a typical facility. That’s not a typo. One-quarter of your compressed air — and the electricity it takes to generate it — is blowing into the atmosphere through holes you can’t hear.
An ultrasonic leak detector changes this. Compressed air escaping through even a tiny orifice creates turbulent flow that generates strong ultrasonic energy. The instrument converts this to audible sound through heterodyning, and directional sensors let you pinpoint the leak location precisely.
Conducting a Leak Survey
- Walk the system systematically. Follow main headers first, then branches, then point-of-use connections. Don’t wander randomly.
- Check every fitting, connection, valve, FRL (filter-regulator-lubricator), coupling, and hose end. These are the usual suspects.
- Tag each leak with a unique identifier. Record the location, estimated leak size using the instrument’s dB reading and distance tables, and the component involved.
- Photograph the tag on the leak for documentation.
- Estimate the cost. A 1/4-inch leak at 100 psig wastes roughly 100 CFM and costs about $12,000-15,000 per year in electricity at typical rates. Even 1/16-inch leaks at $500-1,000 per year add up fast when you find 200 of them.
Repair and Verification
Prioritize repairs by leak size. Fix the big ones first — they’re usually the easiest wins. A single cracked hose or failed quick-disconnect can waste more air than fifty tiny fitting leaks combined. After repair, re-survey each repaired leak to verify it’s sealed. Track total leak loss before and after your repair campaign to quantify savings.
Plants that implement a regular leak management program — quarterly surveys with prompt repair — typically reduce compressed air waste by 50-70%. At $50,000-100,000+ per year in compressed air costs for a mid-size facility, the savings fund the entire predictive maintenance program and then some.
Slow-Speed Bearing Monitoring
Vibration analysis struggles with bearings rotating below 100 RPM. The energy generated by defects at low speeds is too small for accelerometers to detect reliably until the bearing is severely damaged. Ultrasonic monitoring fills this gap.
A bearing in good condition produces a steady, smooth ultrasonic signal — a consistent “whooshing” sound. As the bearing deteriorates, the signal changes in characteristic ways.
- Lack of lubrication — Increased dB level, typically 8-12 dB above baseline. The sound quality becomes harsh and crackling. Adding lubricant while monitoring provides immediate feedback — the dB level drops when adequate lubrication reaches the contact surfaces.
- Early-stage defects — Periodic popping or clicking sounds against the smooth background. These repetitive transients correspond to the rolling elements passing over a defect.
- Advanced damage — Erratic, loud signal with no consistent pattern. The bearing geometry is degraded enough that smooth rolling is no longer occurring.
Ultrasound-Assisted Lubrication
Over-lubrication causes as many bearing failures as under-lubrication. Grease guns deliver an imprecise volume per stroke, and applying a fixed number of strokes per schedule ignores the bearing’s actual condition. Ultrasonic monitoring during greasing solves both problems.
Establish a baseline dB reading for each bearing. When the reading rises 8 dB above baseline, the bearing needs grease. Apply grease slowly — one shot, wait 30 seconds, check the reading. Stop when the dB level returns to baseline. If it doesn’t drop, the bearing has a mechanical problem, not a lubrication problem. This approach has been shown to reduce grease consumption by 30-40% while simultaneously reducing lubrication-related bearing failures.
Electrical Fault Detection
Electrical problems generate ultrasonic sound through three mechanisms: corona, tracking, and arcing. Each has a distinctive ultrasonic signature.
Corona
Ionization of air around high-voltage conductors where electric field intensity exceeds the breakdown strength of air. Produces a steady, buzzing ultrasonic signal. Common on insulators, bushings, and cable terminations above 1,000V. Not immediately dangerous but indicates stress that will eventually cause insulation failure.
Tracking
Current flowing across an insulation surface along a carbonized path. Produces an irregular, sputtering ultrasonic sound — like frying bacon. Indicates contaminated or degraded insulation. Left unchecked, tracking paths grow until they bridge conductors and cause flashover.
Arcing
Current jumping across an air gap. Produces sharp, erratic popping sounds ultrasonically. This is the most severe condition. Arcing means insulation has failed and current is flowing where it shouldn’t. It requires immediate attention — the next step is a fault.
Ultrasonic electrical surveys work well as a companion to infrared thermography. Thermography detects resistive heating from loose connections. Ultrasonics detects ionization and discharge that may not produce significant heat. Together, they cover a broader range of electrical failure modes than either technology alone.
Equipment Selection and Training
For general-purpose plant surveys covering all three applications, choose a handheld instrument with both airborne (scanning module) and contact (stethoscope) sensors. The airborne sensor handles leak detection and electrical surveys. The contact sensor handles bearing monitoring and steam trap assessment.
Key features to evaluate: sensitivity range (minimum 20-100 kHz), heterodyne frequency tuning, dB display for quantitative readings, headphone jack for qualitative listening, and data logging capability for trending. Avoid bargain instruments that lack dB display — you need quantitative data for trending and leak sizing.
Training is straightforward. ASNT Level I certification for ultrasonic testing covers the physics and applications. Most technicians become proficient in airborne applications within a few days of hands-on practice. Bearing monitoring interpretation takes longer — expect 2-3 months of regular practice before your technicians develop reliable judgment on bearing condition.
Start with a compressed air leak survey. The savings are immediate, visible, and easy to quantify. This builds credibility for the program and funds expansion into bearing monitoring and electrical surveys. Within a year, ultrasonic technology should be integrated into your routine inspection activities across all three application areas.