# ISO 18436-2: Vibration Analyst Categories I, II, III and IV

> You call in a contractor for vibration diagnostics and get back a sheet of numbers and the phrase “vibration is elevated.” It's hard to tell from the paperwork alone what the person actually knows how to do — everyone carries an instrument these days. ISO 18436-2 splits vibration specialists into four categories and spells out what each one is qualified to handle. Below we walk through the whole scheme, and we will say upfront: AXILINE does not issue these certificates.

**In short:** ISO 18436-2 splits vibration specialists into four categories. Category I collects data along a pre-set route and compares it against fixed setpoints. Category II sets up the measurement, reads the spectrum, recognizes typical defect patterns and balances a rigid rotor in one plane. Category III runs non-routine investigations, works with run-up and coast-down measurements and shaft orbits, and balances in two planes on site. Category IV is responsible for rotor dynamics, modal analysis and reliability programs. The training, experience and exam requirements are set by the current edition of the standard, and the certificate itself is issued by an independent certification body operating under ISO 18436-1. AXILINE is not such a body and does not issue ISO 18436-2 certificates: we explain the scheme and teach the practical side of working with a balancing instrument.

Source: https://axiline.pt/en/articles/iso-18436-2/  
Publisher: AXILINE · Vila Nova de Gaia, Portugal · +351 931 831 229 · axilinegeral@gmail.com

## The instrument measures. The person decides.

Picture a typical situation. Two people show up at the same induced-draft fan with identical instruments. The first one reads 7.2 mm/s RMS (root-mean-square value — the standard way of averaging vibration), writes “above the limit, balancing required” and leaves. The second one reads the same 7.2, looks at the spectrum — vibration broken down by frequency — and sees: the running-speed component (vibration at the rotor's rotational frequency) sits at around 1 mm/s, while most of the energy is at twice running speed, 2x, and its harmonics. He checks the foot bolts, finds shaft misalignment and a loose mounting, and says: balancing won't achieve anything here, this needs shaft alignment and fastener work. The instrument performed identically in both cases. The difference is in the head.

Vibration diagnostics only starts with the measurement. What follows is a chain of decisions, and the person makes every one of them — the instrument doesn't decide for you.

- Where to mount the sensor, in which direction, and how to fix it so the result repeats from one reading to the next.
- Which frequency range to use (the upper frequency, Fmax), how many spectral lines, which window, and how many averages, so the peak you need doesn't smear or merge with its neighbor.
- What a 40% rise over the baseline actually means: a developing defect, a change in operating regime, or a badly mounted sensor.
- Whether to balance, align, or replace a bearing. These are three different repairs, and the symptom can look similar on the overall vibration level alone.
- How far down to bring the numbers, and which document to use to call the result acceptable.

> AXILINE is not a certification body and does not issue ISO 18436-2 certificates. We walk through the scheme below for one reason: so you know who you're bringing onto your site, what you're entitled to ask, and what you're entitled to require in the report.

## The ISO 18436 series: three documents for three different audiences

Categories I–IV don't exist in isolation. They're part of a series of standards on the qualification of personnel who carry out condition monitoring and diagnostics of machines by non-destructive methods. The three parts of the series answer three different questions, and it's worth not mixing them up.

### ISO 18436-1: who certifies, and how

Requirements for the certification body and for the procedure itself: independence from commercial interests, documented procedures, a technical committee for exam content, approved exam centers, examiners and observers, a register of certified personnel, rules for renewing and withdrawing recognition, a code of ethics. For a third-party scheme, the requirements of ISO/IEC 17024 are added on top.

### ISO 18436-2: categories of vibration specialists

The only part that describes the person on the vibration side. It defines four categories, lists the tasks and limits for each, gives the training syllabus in Annex A, and sets the requirements for training, experience and exam format.

### ISO 18436-3: who trains, and how

Requirements for training organizations and for the training process. This part is common to all condition-monitoring technologies, not just vibration: the same logic applies to thermography, ultrasound and lubricant analysis. The specifics on hours and topics come from the relevant technology-specific part — for vibration, that's ISO 18436-2.

> The parts of the series get updated at different times, clause numbers shift between editions, and individual editions get withdrawn and replaced. Before signing a contract, submitting an application, or running an audit, open the current edition of the relevant part and the rules of the certification body you've chosen — not a summary you found online.

## Why the scheme separates roles, and what that means for you

The series is built so that no one checks their own work. Four prohibitions hold the whole structure together.

- A training center doesn't certify its own students. If an instructor does end up present at the exam, their role is limited to an observer with no access to the exam content.
- An examiner holds a higher category than the exam they're administering.
- An instrument vendor cannot act as the third-party certification body for the users of its own products.
- An employer confirms a candidate's training and experience but doesn't interfere in the assessment procedure itself.

> Two things regularly trip people up here. First: a category certificate confirms qualification, but on its own it doesn't grant the right to carry out work — authorization for specific tasks is issued by the employer or the client. Second: a course-completion certificate is not the same as a category. After the course, a person earns the right to apply for the exam, and only a passed exam at a certification body grants the category.

## Roughly what each category can do

The categories follow the natural boundaries of decision complexity, not years of experience or the number of buttons on the instrument. Below are the competence levels in plain terms.

1. **Data collector** — Walks the route on the machines, mounts the sensor at a marked point in the set direction, calls up a pre-set instrument configuration, and downloads the data. Compares the overall level against pre-set alarm setpoints: acceptable, warning, alarm, shutdown. Notices bad data in the readings, reports deviations and whatever else is seen or heard by hand and eye. Doesn't choose the sensor, method or measurement setup. Doesn't diagnose or assess a result beyond comparing it to the setpoint. Works under the guidance of a higher-category specialist.
2. **Analyst** — Defines the scope of measurements for Category I routes. Understands where the spectrum comes from: frequency range, number of lines and frequency resolution, windows, averaging. Sets up data collection deliberately, rather than calling up a template. Recognizes typical patterns in the spectrum: a running-speed component for unbalance, a 2x and axial component for misalignment, a harmonic comb and a raised noise floor for looseness, non-synchronous high-frequency peaks for bearing defects. Runs a basic bump test for natural frequencies. Recommends basic corrective actions, including single-plane balancing of a rigid rotor. Writes the report and instructs Category I personnel.
3. **Diagnostician** — Designs the entire monitoring program: points, routes, frequency of measurement, setpoints, procedures, instrument requirements. Runs non-routine investigations when a routine spectrum doesn't give an answer. Works with an extended set of methods: phase measurements, run-up and coast-down (readings taken while speed rises and falls), orbits (the shaft's path of motion inside the bearing), transfer functions, acceleration envelope, basic structural mode shapes. Initiates corrective actions and verifies the result: two-plane balancing of a rigid rotor in its own bearings, shaft alignment, resonance elimination. Recommends operating restrictions. Understands the fundamentals of rotor dynamics. Trains Category I and II personnel.
4. **Expert** — Directs and audits monitoring strategies. Commands experimental modal analysis — measuring the natural frequencies and mode shapes of a structure — understands rotor dynamics at the calculation level, designs vibration isolation and dampers, works through non-standard cases such as gas pulsation and a compliant machine mounting, and reads and interprets standards as applied to acceptance testing. At this level, individual subjects are no longer broken out separately: equipment knowledge and acceptance testing are treated as already mastered at the earlier categories.

> We deliberately don't give exact requirements here. The minimum training hours per subject, the required practical experience, the exam format and the pass mark are all set out in the standard and vary between editions. Open the current edition of ISO 18436-2 and the rules of the body where you plan to sit the exam.

## Where the line falls: the task and the minimum category

The most practical way to apply the scheme to your own situation is to look at specific jobs. The table below shows the logic behind the boundaries; it doesn't quote the standard.

| Job on site | Level | Why |
| --- | --- | --- |
| Route walk, reading at a marked point, downloading data | I | the settings are already fixed, the decision comes down to comparing against the setpoint |
| Screening out bad data: noise pickup, a detached magnet, a lost tacho signal | I, deeper into II | spotting bad data is a Category I task; finding the cause is often already analysis |
| Choosing Fmax, number of lines and averaging for a specific machine | II | requires understanding frequency resolution and its link to defect frequencies |
| Diagnosis from the spectrum: unbalance, misalignment, looseness, bearing | II | recognizing typical defects is exactly the subject matter of fault analysis |
| Single-plane balancing of a rigid rotor | II | basic correction, one plane, a well-established procedure |
| Two-plane balancing of a rigid rotor in its own bearings | III | two planes, mutual interaction, assessing the result under actual operating conditions |
| Run-up and coast-down measurements, hunting for resonance, orbits | III | transient regimes and an extended set of methods |
| Revising setpoints and writing procedures for the plant | III | managing the program, not a single reading |
| Rotor dynamics calculation, modal analysis, vibration-isolation design | IV | goes beyond measurement and standard correction |

> The term “rigid rotor” here is used in the ISO 21940 classification sense: at running speed, rotor bending doesn't noticeably change the distribution of unbalance. A flexible rotor is a separate task and a separate level of difficulty. The category boundaries are about the complexity of the decision, not the brand of instrument: a two-channel instrument in the hands of someone who can't tell couple unbalance from resonance produces a nice-looking report and the same vibration as before.

Sources: [ISO 21940-11:2016](https://www.iso.org/standard/54074.html) · [ISO 21940-12:2016](https://www.iso.org/standard/50429.html)

## Knowledge map: the subject areas of Annex A

Annex A of the standard contains the training syllabus, broken into subject areas. The same list is covered at every category; what changes is the depth and volume. For a client, this is a ready-made list of topics you can question a contractor about and, in five minutes, understand who you're talking to.

- Vibration fundamentals: the sine wave, period and frequency, amplitude, RMS and peak values, phase, units, the relationship between displacement, velocity and acceleration.
- Data collection: sensor types, mounting, choosing points and directions, the tachometer signal, repeatability, building routes.
- Signal processing: time waveform and spectrum, FFT, frequency range and number of lines, frequency resolution, windows, averaging, leakage and aliasing.
- Condition monitoring: maintenance strategies, baseline levels, trends, measurement frequency, setting warning and alarm setpoints.
- Fault analysis: unbalance, misalignment, mechanical looseness, shaft bow, rubbing, bearing defects, gear meshes, electromagnetic causes, hydraulic phenomena.
- Corrective actions: balancing, shaft alignment, bearing fitting and replacement, resonance elimination, isolation and damping.
- Equipment knowledge: rolling-element and plain bearings, couplings, gearboxes, pumps, fans, electric motors, variable-frequency drives.
- Acceptance testing: what and how to measure when accepting a new or repaired machine, and how to record the measurement conditions.
- Equipment testing and diagnostics: bump test, run-up and coast-down, phase measurements, orbits, mode shapes.
- Reference standards: where to find assessment criteria and how to choose the applicable part and edition.
- Reporting and documentation: what a report needs to contain so a third party can verify it.
- Severity assessment: how bad it is, how much longer the machine will run, what to address first.
- Rotor and bearing dynamics: critical frequencies, shaft mode shapes, support stiffness and damping.

> Two areas get skipped most often. The first is equipment knowledge itself: the standard separately requires training on machine construction and operation, otherwise a person can describe a spectrum but can't connect the symptom to the physics of the defect. The second is reporting. A diagnosis that can't be checked against the report is useless to a reliability department.

## How to choose a training center, and what to look at

The requirements for training organizations are formal and verifiable. You don't need to be an auditor to ask eight questions and tell from the answers whether you're looking at a real course or a paper sale.

A word on advertising. “Guaranteed exam pass” is a promise a training center has no business making: a different organization administers the exam, and promises like this directly contradict the scheme's code of ethics.

And a word of advice back to the client. Holding a category is necessary but not sufficient. A specialist with a high category who has never balanced an overhung induced-draft fan in its own bearings is going to learn on your site, at your expense. Ask about both the category and hands-on experience with similar machines.

- [x] The link to a certification body. Ask directly: whose exam will you sit after the course, which edition of ISO 18436-2 the program is built on, and who can confirm that link.
- [x] The instructor's category. For the vibration discipline the bar is higher than the series' general rule: the instructor must hold at least Category III, and no lower than the category being taught. A Category III course from a Category II instructor isn't a Category III course.
- [x] Match to the syllabus. The training plan must cover every subject area of Annex A for the target category, at the stated volume. A cut-down two-day intensive in place of the full program devalues the certificate when you apply for the exam.
- [x] Hands-on practice and a lab. The standard requires lectures, demonstrations and practical exercises, so a course made up entirely of recorded videos doesn't meet it. Ask about the rigs: an unbalanced rotor, a misaligned coupling, bearings with seeded defects, workstations with analyzers.
- [x] Training on the machines themselves. Alongside the vibration content, the program must cover equipment construction and operation: bearings, couplings, gearboxes, pumps, fans, drives.
- [x] Independence from a single manufacturer. Demonstrating material on a specific analyzer is fine; pushing a specific product within the training course is not. If half the sessions amount to comparing instruments in favor of one brand, you've walked into a marketing pitch.
- [x] Records and access to your own file. The training organization is required to keep a personal file for each student and hold it confidentially for years. Ask how to get a copy of your documents if you change employer or the center closes.
- [x] The internal exam and what the certificate must state. The training center's own exam confirms that the material was covered — it is not a certification exam. The certificate should name the student, the date, the category trained, the technology, the hours, a certificate number, and the issuing organization.

> If someone offers you an ISO 18436-2 certificate without an exam at an independent certification body, they are selling you paper, not qualification.

## What to ask a contractor, and what to require in the report

The category scheme is useful to you for exactly one thing: understanding who you've called in, and whether their level is sufficient for the task. Six questions worth asking before the site visit.

- Who exactly is coming, and what category does that person hold. Categories are issued to people, not companies, and the phrase “our company is certified” doesn't answer this question.
- Whether the recognition is currently valid. Recognition is issued for a limited term and gets renewed. Ask for the expiry date, not just whether a document exists.
- What they'll do if the running-speed component turns out to be a small share of the overall vibration. “We'll balance it” is the wrong answer to this question.
- How they'll tell resonance apart from unbalance. The right answer is about run-up or coast-down measurements and a bump test, not about experience and intuition.
- Which document they'll use to assess the result: which part and edition of ISO 20816, which frequency band, which points, and which operating regime of the machine.
- What happens if it turns out on site that balancing won't help. An honest answer sounds like: we'll stop, show you the data, and name the real cause.

- [x] Machine, unit, rotational speed, regime and load at the time of the reading.
- [x] A point diagram with numbers, measurement directions, sensor mounting method.
- [x] Instrument and sensor types and serial numbers, calibration details where available.
- [x] Quantity, units and frequency band: vibration velocity in mm/s RMS over the 10–1000 Hz band for general-purpose machines.
- [x] Overall vibration and the 1x running-speed component with phase, reported separately. Without that split you can't check whether balancing made sense in the first place.
- [x] Spectra and time waveforms as an attachment, stating Fmax, the number of lines and the averaging.
- [x] For balancing: the masses and radii of the trial and correction weights, angles or fixed-position numbers, vibration before and after at every bearing support.
- [x] The assessment criterion and the result as figures. Not “within limits,” but “2.1 mm/s RMS, zone B under the applicable part of ISO 20816,” stating the edition.
- [x] A conclusion and recommendations with priorities and deadlines, not “monitoring recommended.”

> You can check a contractor's category once; you'll be checking reports every time. A good report reads like a repeatable experiment: a different person with a different instrument should get the same numbers at the same points.

Sources: [ISO 20816-1:2016](https://www.iso.org/standard/63180.html) · [ISO 21940-11:2016](https://www.iso.org/standard/54074.html) · [ISO 13373-3:2015](https://www.iso.org/standard/40840.html)

## What we teach, and what we don't do

Let's repeat this plainly and without hedging: AXILINE is not a certification body, is not an accredited ISO 18436-3 training center, and does not issue ISO 18436-2 certificates. We are engineers who design and manufacture the Balanset instruments and balance with them on site ourselves. That shapes what our training is: not exam preparation for ISO 18436-2, but a hands-on course in operating the instrument and in measurement technique on live equipment.

From there the choice is simple. If you need an ISO 18436-2 category, go to a training center and a certification body, and use the scheme in this article as a checklist when choosing. If you need your own technician to confidently take vibration readings and balance a fan in its own bearings without calling in a contractor for every case — that's us: the Balanset-1A instrument, training on how to use it, and advisory support afterward. If you need balancing done right now and there's no time to train — we'll come and do it ourselves, and leave you a report built so it can be checked.

### Measurement you can trust

Where to mount accelerometers on the bearing supports, how to fix them, where to aim the laser phase sensor, how to get a repeatable result. We cover the difference between overall vibration and the running-speed component and teach you to work out the 1x share, so you can decide whether balancing is worth doing at all.

### Single-plane and two-plane balancing

The trial weight and the criterion for a valid trial run, measuring the angle from the trial-weight location, fixed-position mode for blades and holes, splitting the mass between neighboring positions, the drilling-correction calculation, trim balancing until you're inside tolerance.

### Influence coefficients and batch rotors

How influence coefficients calculated and saved once save you trial runs on identical rotors, and where the trick stops working: different supports, a different regime, changed geometry.

### Reports and the archive

What to record in the results archive, how to use the polar diagram and the spectrum as attachments, how to put together a document that the client's reliability department can verify.

### The instrument as a machine's measuring system

Balancing batches of rotors on a soft-bearing, above-resonance machine (with compliant supports)? We cover working in this mode, calculating arbor eccentricity, and recalculating weights for other planes.

> One more time, so there's no ambiguity: training on the Balanset instrument does not grant, and does not substitute for, any ISO 18436-2 category.

Sources: [Balanset-1A manufacturer specification](https://vibromera.eu/product/balanset-1/) · [Balanset-1A operation manual](https://vibromera.eu/balanset-1a-operation-manual/) · [ISO 21940-11:2016](https://www.iso.org/standard/54074.html) · [ISO 20816-1:2016](https://www.iso.org/standard/63180.html)

## Frequently asked questions

**Does AXILINE issue an ISO 18436-2 certificate?**

No. We are not a certification body and don't issue certificates like that. An ISO 18436-2 category is awarded by an independent certification body operating under ISO 18436-1, and only after an exam. We manufacture the Balanset instruments, balance with them on site, and teach the practical side of operating the instrument and measurement technique.

**What category do you need to balance a rotor in the field?**

The standard places single-plane balancing of a rigid rotor within Category II competencies, and two-plane balancing in the rotor's own bearings within Category III. If the rotor is flexible, resonance is nearby, or the machine runs on plain bearings, the task moves up a level and calls for an extended set of methods.

**I completed the course — does that mean I hold Category II?**

No. A course-completion certificate confirms that training took place, not qualification, and it doesn't grant the right to carry out measurements and analysis. After the course you earn the right to apply for the certification exam. The category only exists once you've passed the exam at a certification body.

**How many hours of training and experience does the standard require for each category?**

The standard does set minimums for each subject area and requirements for practical experience, but the exact figures depend on the edition and are interpreted together with the certification body's own rules. Don't take them from articles, including this one. Open the current edition of ISO 18436-2.

**Is a category certificate valid indefinitely?**

No. Recognition is valid for a limited term and gets renewed on confirmation of continuous practical work in the field; after a long break in practice, renewal isn't available. The exact terms and conditions are set by the current edition and the certification body. Ask a contractor for the expiry date, not just whether the document exists.

**Can an instrument manufacturer be a training center?**

As a training center, yes, that's allowed, but with a direct ban on promoting a specific commercial product within the course. What a manufacturer cannot be is the certification body for the users of its own products: the scheme requires independence from commercial interests. We manufacture instruments, which is exactly why we don't certify anyone.