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Functional Testing: What It Checks, and How It Differs from Performance Testing
Functional testing asks a plain question of every function: does it do what the spec says? Each control, sequence and interlock gets checked, pass or fail. In commissioning it gets muddled with performance testing, which measures how well the system copes under load rather than whether a function works at all.
What does functional testing check?
One question per function: does it behave the way the spec says? Typical checks:
- Control actions – each command gives the right move, state or output.
- Sequences and interlocks – steps fire in order, unsafe states get blocked.
- Modes and setpoints – manual, auto and fault modes all respond correctly.
- Alarms – a simulated fault raises the right alarm and action.
- Integration points – signals cross correctly to and from linked systems.
Functional testing vs performance testing – what’s the difference?
Different questions. Functional testing asks whether a function works. Performance testing asks how well it works under real or peak load. A pump can pass functional testing by starting and stopping on command, then flunk performance testing because it can’t hold its rated flow. Functions first – there’s no sense measuring the performance of something that doesn’t work yet.
Where does functional testing fit into commissioning?
It’s the first live stage of commissioning, once pre-commissioning has proved the system is safe to switch on. On installed equipment it usually runs as part of the site acceptance test.
Functional vs performance testing at a glance
Two tests, two questions, one steady mix-up.
| Aspect | Functional Testing | Performance Testing |
|---|---|---|
| Question | Does the function work? | How well does it perform? |
| Measures | Correct behavior | Speed, capacity, throughput, stability |
| Order | First | After functions pass |
| Passes when | It meets the spec | It hits the target under load |
| Example | Pump starts on command | Pump holds rated flow at pressure |
Free Functional Test Matrix
It lists every function with its expected result and a pass or fail, so no check gets skipped and the results form clean test documentation. Download the checklist (PDF). You can also complete the checklist digitally in flowdit on your phone, tablet or PC.
Related Terms
- Commissioning Process – the sequence this sits in.
- Factory Acceptance Test (FAT) – functional checks before shipment.
- Machine Commissioning – where functions get proven live.
- Operational Readiness – the state these checks build toward.
- Punch List – catches the failed checks.
Further Reading
- Commissioning Management Software – commissioning end to end
