EVT / PVT Ductwork Cleanliness Verification
Independent ventilation hygiene assessors · No cleaning services
The vacuum test measures how much dust is on a defined area of duct surface, by weight. It gives a gravimetric result in grams per square metre that can be compared with TR19 Air criteria, and it works on surfaces where deposit thickness testing is not reliable.
Established 19992,500+ assessmentsSafeContractor approvedBOHS Affiliate MemberLondon & South East
EVT, PVT — what the names mean
The test was originally called the Preferred Vacuum Test (PVT). Current guidance refers to it as the vacuum test, commonly written EVT. Both names describe the same approach: dust is drawn from a template area of duct surface onto a pre-weighed filter, and the filter is re-weighed to give the mass of dust per square metre.
How EVT differs from DTT
The two methods measure different things.
| Vacuum test (EVT) | Deposit Thickness Test (DTT) | |
|---|---|---|
| What it measures | Mass of dust per area (g/m²) | Thickness of dry deposit (microns) |
| Result | After weighing | On site, immediately |
| Surfaces | Wider range, including where DTT is unreliable | Suitable flat, rigid metal surfaces |
| Typical use | Quantitative verification, pre- and post-clean | Condition surveys and pre/post-clean comparison |
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When EVT is used
EVT is the right choice in several situations.
- Pre-clean surveys where a gravimetric baseline is needed
- Post-clean verification of cleaning works
- Surfaces where DTT cannot give a reliable reading
- Where a client, specification or insurer requires a gravimetric result
- Disputed results, where an objective weighed measurement helps
How an EVT survey works
An EVT survey follows six steps.
Agree purpose and scope
We confirm whether the survey is pre-clean, post-clean or both, and which systems are included.
Select sample locations
We choose representative locations using the same principles as for DTT: system layout, air velocity, bends, filters, fan sections and visible condition.
Take samples
At each location a template is fixed to the duct surface and the dust within it is drawn onto a pre-weighed filter cassette.
Record and track
Each sample is labelled, photographed in place and recorded on a sample schedule so the chain from location to result is clear.
Weigh and calculate
Filters are re-weighed and the dust mass is converted to grams per square metre.
Compare and report
Results are compared with the TR19 Air criteria for the system and reported with conclusions and recommendations.
Standard applied
We sample and report in line with BESA TR19® Air (Version 2, 2026), and state the method and criteria used for each system in the report.
What EVT cannot tell you
- It does not identify what the dust is or whether it contains micro-organisms
- It does not measure ventilation performance
- It describes the sampled locations; unsampled and inaccessible sections are recorded, not assumed
- Results are not available on site; they follow weighing
What your EVT report includes
- Purpose, scope and system boundaries
- Sample location plan and schedule
- Photographs of each sample location
- Results in g/m² for each sample
- Summary by system and comparison with TR19 Air criteria
- Pre- and post-clean comparison where applicable
- Limitations and inaccessible areas
- Conclusions and recommended actions
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