The first measurement is worth as much as your ability to repeat it

In brief

• The baseline is a condition, a stable and known machine, not a minimum number of measurements.
• A machine with several regimes wants one baseline per state.
• Two criteria: the absolute level and the change, which is 25% of the B/C boundary, not of the baseline.
• Programs fail on who looks at the data, not on the measurement.

The baseline is a condition, not a number of measurements

Baseline means one thing only: the data collected when the machine’s operation is known to be acceptable and stable (ISO 13373-1:2002, clause 7.2.1; ISO 17359:2018, clause 8.10).

Many people expect a prescription like twelve measurements in thirty days. There is none: none of the standards cited sets a number of samples or a minimum duration. They set a condition: normal and stable operation, after run-in, with the operating conditions documented.

A condition is more serious than a number: a number can be met by measuring twelve times a machine that is already running badly. On a new or overhauled machine the vibration changes in the first days: that is run-in, not degradation. The baseline is taken afterwards, at steady state, recording speed, load, flow and pressure together with the measurement.

A machine with several states wants several baselines

A machine that works in several regimes does not have one normal vibration: it has one per state. An inverter-driven fan at 40% and the same fan at 90% are two machines, from the measurement’s point of view. ISO 20816-3 (clause 4.5) requires measuring in all the conditions in which the machine runs for long periods, taking the highest value as the severity.

The mistake that empties the program: mixing different states into a single baseline. Out comes an average that describes no real state, and the system sounds when the machine has merely changed the way it works.

Two evaluation criteria

ISO 20816-1 separates the absolute level (clause 6.3.2) from the change relative to a reference (clause 6.3.3). The second sees what the first does not: a machine can go from 1.6 to 2.6 mm/s while staying in zone B, but the change says something has shifted.

Zone	Group 2, rigid support	How to read it
A	up to 1.4 mm/s	typical of a newly commissioned machine
B	up to 2.8 mm/s	can run like this for a long time
C	up to 4.5 mm/s	should not run like this continuously
D	above 4.5 mm/s	can damage the machine

Broadband RMS velocity, group 2 (above 15 kW and up to 300 kW) on a rigid support; other classes have other numbers. These are guidelines, not a report card: manufacturer and customer can agree on different values. One requirement, however, is mandatory: two measurements are comparable only if taken at the same point, with the same orientation, under similar operating conditions and at steady state.

25% of what

The 25% threshold is not 25% of the baseline or of the previous measurement: it is 25% of the boundary between zone B and zone C (ISO 20816-3, clause 6.3).

Group 2 on a rigid support: B/C boundary at 2.8 mm/s, hence 0.7 mm/s. It is an absolute quantity, which changes with the machine class and not with your baseline:

• baseline 0.5 mm/s rising to 1.3: change 0.8, threshold exceeded;
• baseline 2.0 rising to 2.4: change 0.4, below threshold.

In percentage terms they would be +160% and +20%, but the criterion is applied in mm/s. The 25% remains a guideline: the standard allows other values based on experience, and asks to investigate evident changes even below threshold.

Alert and action

The alert flags a value reached or a significant change: the machine can keep running while you investigate. The action level marks the point beyond which continuing can cause damage.

Alert at the B/C boundary and action at the C/D boundary is common practice, not a rule of the standard. What the standard says instead:

• alarm above the baseline by 25% of the upper limit of zone B, and in any case no more than 1.25 times that limit (ISO 20816-3, clause 6.5.2); with a low baseline it can fall below the B/C boundary, and that is intended;
• new machine with no baseline: start from experience on similar machines or from the agreed acceptance values, then correct; after a major overhaul it is redone;
• the action limit concerns mechanical integrity and depends on the design: it falls in zone C or D, normally no more than 1.25 times the upper limit of zone C (clause 6.5.3);
• initial thresholds are a starting point: ISO 17359 recommends reviewing them over time (clauses 8.9 and 11).

Who looks at the data

Programs do not fail on the measurement. They fail on four questions with no written answer: who looks at the data, how often, what is done when a threshold trips, who stops the machine. Answer with people’s names, before buying anything.

The enemy is the false alarm: a few weeks of useless notifications and the system gets ignored. Three defenses: compare only measurements taken under similar conditions; apply a statistical filter before the change criterion, as ISO 20816-3 suggests for machines with variable behavior; validate the quality of the measurement (failed transducer, machine stopped, vibration from a neighboring machine).

The data that stops arriving. A dead sensor and a healthy machine look alike: neither reports anything. Look at the age of the last measurement, not just the value; cross-check with running hours or electrical draw; every now and then go and touch the sensor, mountings loosen.

Start-up checklist

1. Which machines are in and why: by criticality, not by ease of access.
2. Which failures you expect, and whether they have a measurable symptom.
3. Where you measure: points on the bearings, marked and named in writing.
4. How you mount the instrument: method, calibration and sensitivity never change.
5. Which operating states you distinguish, and what you record: at least speed and load.
6. When run-in ends and the baseline starts.
7. Which initial thresholds you use, where they come from, when you review them.
8. Who looks at the data, how often, what they do if a threshold trips.
9. Who stops the machine, and who stands in for them.
10. How you notice that an instrument has stopped talking.