Nuances and peculiarities of DOP-testing and differential pressure measurement
This paper deals with technical aspects of differential pressure measurements, integrity and tightness of HEPA-filter installations (DOP test), and also describes the clean room certification procedures, which include a number of important parameters measurements required by national standards such as particulate matter concentration, temperature, humidity, noise level, differential pressure and others. Particular attention is paid to checking the integrity and tightness of the HEPA-filters.
Clean rooms must be certified periodically according to GOST 14644-4-2002 "Clean rooms and related controlled environments" . During clean room certification (or tests) it is necessary to measure the counting concentration of aerosol particles, temperature, humidity, noise level, air exchange ratio, pressure drop, integrity and tightness of HEPA filter installation and other parameters.
The methodology of clean rooms certification is described in details in GOST 14644-3-2020 .
Differential air pressure control
The purpose of the differential air pressure control is to check the ability of the clean room to maintain the specified differential pressure between the clean room space and the ambient environment. The differential air pressure control should be carried out after acceptance criteria for air velocity or air flow rate, velocity uniformity and other parameters have been confirmed. A detailed description of the methodology for differential air pressure control is given in B.1.
Check of the installed filter system for leakage (particle slippage)
These tests are performed to confirm that the HEPA finishing system is correctly installed by checking that there are no leaks bypassing the filters in the filtration system and that the filters are free of defects (small holes and other damage to the filtering media, leaks in the frame, sealant). These tests are not performed to determine effectiveness of the filter media. Tests are carried out by injecting aerosol upstream the filter and scanning the surface of the filter and its fittings or sampling the duct downstream the filter. The methods to check for leakage are given in B.7.
Tests to measure the differential pressure ΔP
When certifying clean rooms it is not always possible to ensure correct measurement of differential pressure, as fire doors with a threshold and doors with an automatic movable rubber seal do not allow the measurement – it is not possible to pass a tube (or hose) through them. The use of a special ΔP port avoids this problem and enables practical and convenient measurements.
The ΔP-ports are used to measure the differential pressure between adjacent rooms, installed in the envelope of clean rooms and in transfer airlocks. A typical wall-mounted ΔP-port is shown in Fig.1.
Leakage tests of the unit and integrity of the HEPA-filters (DOP test)
The DOP IN ports are used to supply a test aerosol (dioctylsebacinate) when conducting a DOP test to check tightness of the unit and integrity of the filters. To ensure maximum mixing of the test aerosol with air, it is recommended that the DOP IN ports are positioned so that the test aerosol supply port is distanced downstream of each filter at least 7 diameters (ℓ ≥ 7d) of the air duct, which holds the filters, where the tested HEPA filters are installed (see Fig.6).
DOP OUT ports are designed to sample air for counting concentration of test aerosol upstream the filter when checking HEPA filters for tightness of installation and integrity of the HEPA filters. The ports are installed in the duct at a distance of 50–100 mm from the HEPA-filter.
During design and construction, it is required to take into consideration the above described units which are critically important for the certification of clean rooms on the basis of the differential pressure ΔP measurement and leakage tests of the installation as well as integrity of the HEPA-filters (DOP test). Our company  also develops and manufactures some other "useful little things", which will be described in the following publications. ■
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.