BS ISO 29821 — Machine Condition Monitoring and Diagnostics Ultrasound Series

🔊 BS ISO 29821 is an ISO series on ultrasound techniques for condition monitoring and diagnostics of machines, published in the United Kingdom by BSI. The series defines a methodological framework for using ultrasound to monitor equipment condition. Part 1 provides general guidelines, while Part 2 provides procedures and validation. It applies to scenarios such as bearing lubrication, electrical discharge, leak detection, internal valve leakage and gear meshing, and is an important complement to vibration analysis within predictive maintenance programmes. UK participation was coordinated by Technical Committee GME/21/7 (Mechanical vibration, shock and condition monitoring — Condition monitoring).

1. Background and Positioning of the Series 📌

Early defects of rolling-element bearings in rotating machinery, poor gear lubrication, steam/air leaks and partial discharge in electrical equipment often first manifest as abnormal high-frequency ultrasonic energy, developing into perceptible vibration or temperature changes only later. Ultrasound monitoring uses ultrasonic sensors — contact (structure-borne) and airborne types — to capture these high-frequency signals, offering high sensitivity and good localisation capability, complementary to vibration, oil and temperature monitoring. BS ISO 29821 was developed to provide a unified method and operational specification for ultrasonic condition monitoring.

The series belongs to the “condition monitoring and diagnostics” method standards, distinct from ISO 10816 (mechanical vibration evaluation), with ICS classification 17.160 (vibrations, shock and vibration measurements). Part 1 was published in 2011 and Part 2 in 2016, forming a complete ultrasound monitoring methodology.

2. Series Structure and Document List 📋

3. Part 1 — General Guidelines 🎓

BS ISO 29821-1:2011 first explains the principles of the airborne and structure-borne ultrasound methods: the airborne method uses airborne sensors to capture ultrasound propagating through air from leaks and discharges; the structure-borne method uses contact sensors to capture ultrasound generated inside components such as bearings and gears propagating along the structure. The standard then specifies:

• Applications: bearing condition, lubrication state, electrical discharge, gas/steam leakage, internal valve leakage, gear meshing, etc.
• Training requirements: monitoring personnel shall receive training in theory, equipment operation and data analysis, and possess appropriate qualifications.
• Equipment requirements: operating frequency, sensitivity, sensor selection and calibration requirements for ultrasonic detectors.
• Data collection guidelines: including the comparative method and the baseline (quantitative) method.

The comparative method emphasises horizontal comparison of ultrasound levels between similar machines at the same measurement points, while the baseline method requires establishing a “healthy baseline” for each machine and judging deterioration through longitudinal trends. The standard makes clear that “consistency of data collection” is the key to success — measurement points, angles, load and sensor settings must be repeatable.

4. Part 2 — Procedures and Validation 📏

BS ISO 29821-2:2016, within the overall ultrasonic condition monitoring framework, further specifies:

• Monitoring procedures: how to incorporate airborne and structure-borne ultrasound into the overall condition monitoring programme, including monitoring frequency, measurement-point planning and recording conventions.
• Correlation with other technologies: how ultrasound relates to vibration, temperature and oil analysis, and how to interpret combined data to avoid isolated interpretation.
• Equipment selection: choosing the appropriate type of ultrasonic instrument and sensor for different applications (airborne leakage vs. structure-borne).
• Validation methods: validating monitoring results (e.g. against shutdown inspection and teardown findings) to confirm alarm thresholds and the effectiveness of diagnostic conclusions.

The core idea of Part 2 is “procedural and verifiable”: any ultrasound alarm and diagnostic conclusion should be able to be recorded, reviewed and validated through defined processes, supporting reliable condition-based decisions.

5. Engineering and Implementation Practice 🛠️

• 📋 Build a baseline library: establish ultrasonic reference data under normal operating conditions for each critical machine as a starting point for trend judgement.
• 🔁 Standardise measurement points: fix point locations, test angles and equipment load states so successive measurements remain comparable.
• ⚡ Priority scenarios: deploy ultrasonic inspection first at bearing lubrication, motor electrical discharge and flange/valve leakage scenarios.
• 🧪 Multi-technology fusion: combine ultrasound with vibration, thermography and oil analysis for cross-validation to reduce false positives and false negatives.

6. Frequently Asked Questions ❓