Room Pressure


Is to verify the capability of the complete installation to maintain the specified room pressure difference between the installation and its surroundings, and between separate spaces within the installation


An electronic manometer, inclined manometer, or differential Room Pressure gauge should be used with all opening and doors closed and all air handling systems operating. Calibrated Digital manometer or differential gauges with a sensitivity of 1Pa ±0.1 Pa, 0 to 100 Pa.


General considerations:

  1. The pressures being measured are very small and incorrect measurement techniques can easily give erroneous readings. The following should be considered:
  2. Installation of permanent measuring points is recommended;
  3. Take measurements near to the middle of the cleanroom and away from any supply air inlets or return air outlet devices which may influence the local pressure at the measuring point.
  1. Pressurization of cleanrooms is used to control contamination, providing resistance to infiltration of external sources of contaminants.
  2. The pressure differential must first be identified on a layout drawing of the facility. Ideally each area pressure should be measured to outdoors. When difficult to do so, it is often acceptable to measure with reference to adjacent rooms.
  3. The area differential pressure is a factor of the quantity of air introduced into a room compared to the quantity taken out of room.
  4. Since the quantity being introduced is a factor of the airflow requirements which already have been set, filter velocities may not be allowed to change.
  5. In order to change the differential between areas in a facility, it is necessary to increase or decrease the return air and increase or decrease the makeup air quantity.
  6. It is often easier to balance from low pressure area to high room pressure area.
  7. This would necessitate starting with less clean areas or lowest pressure areas to more clean areas or higher pressure.
  8. There is certain amount of trial and error in setting required differential between areas. If a system requires greater quantities of outside air for pressurization than the original design, it could affect the temperature and humidity control in the space.
  9. The room pressure differential between adjacent cleanrooms or clean zones of different cleanliness level should lie typically in the range of 5 Pa to 20 Pa, to allow doors to be opened and to avoid unintended cross flows due to turbulence.(Ref: ISO 14644 – 4)


  1. With all doors closed, the room pressure difference between the enclosure cleanroom and any surrounding ancillary environment or between any surrounding ancillary environment and the external environment should be measured and recorded.
  2. If no surrounding ancillary environment is present, the room
  3. pressure difference between the enclosure cleanroom and the external environment should be measured and recorded.
  4. If the installation is subdivided into more than one sub-enclosure or room, the pressure differences between the innermost room and the next adjacent room should be measured. The measurement should be continued until the room
  5. Room pressure difference between the last enclosure and surrounding ancillary environment and against the external environment is measured.


  1. Pressurization levels as specified in the contract documents or as agreed between the customer and the supplier.


By agreement between the customer and the supplier, the following information and data should be recorded:

  1. Identification of apparatus operator or data collection system
  2. Identification of apparatus and their calibration status
  3. Type of tests and measurements, and measuring conditions
  4. Measurement point locations / room name.
  5. Other data relevant for measurements. 

The frequency of the above test should be once a year

For More Post Related to HVAC

  1. SOP For Airflow Velocity and Uniformity Test
  2. SOP For Temperature and Humidity Test