Introduction:- The test defined here is an independent check on the accuracy of any continuous monitoring system. It is likely that the test as defined here will not be carried out in-house and will usually be carried out by an external service contractor.
In a controlled environment installation, it is important to ascertain that the correct degree of overpressure can be maintained relative to the adjacent areas of lower classification in order that contamination is not drawn into the controlled environment from its surroundings. It is necessary to ensure that air moves from clean areas to less clean areas and not vice versa. Measurement of room pressure is an indirect means of determining this as it may be assumed that air will flow from an area of high pressure to one with lower pressure. The highest quality clean rooms should therefore have a higher pressure than adjacent clean rooms. The pressure differentials between clean rooms are usually monitored on a continuous basis by either gauge (electronic/magnahelic) or incline manometers.
Equipment:- Instrument of choice is likely to be an electronic manometer (portable and easy to use), incline manometer or magnahelic gauge.
 Sample locations:- It is only possible to measure the differential between adjacent areas connected either by a door or grill. Some systems measure pressure differentials with respect to one point. When this type of system is being checked the differential pressure must be checked against this reference point.
 Frequency of sampling:- Whilst it is recommended that the pressure differential between clean rooms and adjacent areas is monitored continuously by gauges/manometers and recorded on at least a daily basis, it is important that the accuracy of the measuring system be determined independently from gauge/manometer readings on at least an annual basis using the method below.
 Method of sampling:-

1. The test must be carried out with the installation unmanned and operating at its designed flow rate (for both supply and extract) and in a balanced condition. To check the pressure differentials the air supply system must be running at design or as per any modification, and therefore supplying and extracting the correct volumes of air.
2. All doors in the facility/suite must be closed.
3. A tube from the measuring device is passed under the door, or through a by-pass grille or damper connecting the two areas being tested, and into the room.

Note: The tube and manometer device must be well clear of the door such that it is free from obstruction and so that no pressure is registered from air movement along the floor near the door.

4. Following the procedure for operating the equipment record the reading.

Results and interpretation of results:- If the pressure difference is equal to or greater than the appropriate minimum pressure drop, the controlled environment installation shall be deemed to have maintained a satisfactory degree of pressure difference. The pressure difference between clean rooms and adjacent areas shall be equal to or greater than the values specified below:
> 10 Pa between classified area and adjacent area of lower classification
> 15 Pa between classified and unclassified area
The results obtained may be used to check the accuracy of any in-house measuring system of gauges or manometers.
Action:- Where the pressure differential does not meet the requirements an investigation into the problem should be carried out. Low pressure differentials may be addressed by replacing blocked (or partially blocked) HEPA filters, increasing fan speeds, increasing airflows to specific areas by altering damper positions or by rebalancing air supplies. Major changes to the air supply system e.g. alterations of room layouts, addition of extra trunking/HEPA filters, must only be undertaken by authorised personnel. Any corrective action taken as a result of the investigation should be recorded.