Positive vs negative pressure laboratory design
A positive-pressure room tends to send air towards a lower-pressure adjoining space; a negative-pressure room tends to draw air in. The required direction follows the purpose and risks of the work. Neither arrangement is a universal choice for every laboratory.
Room pressure is a difference between two locations, not an absolute label. State which corridor or adjoining room is the reference and which operating condition applies. Air tends to move from higher pressure to lower pressure through available openings.
The purpose may be to limit outward migration from a space, but the required pressure relationship comes from the process and risk assessment. A negative room is not automatically suitable for every laboratory activity, and a pressure reading alone does not demonstrate chemical containment at a work point.
Use the fume hood design guide to connect room planning with the local extraction serving the work.
Supply, extract and transfer air
In a steady operating condition, mechanical extract greater than mechanical supply must be made up by inward transfer air through planned openings and building leakage. The pressure difference that results depends on the resistance of those paths. There is no universal airflow offset that produces the same room pressure everywhere.
For an illustrative balance, 500 L/s of extract and 450 L/s of mechanical supply leaves 50 L/s to enter through transfer paths at steady state. That arithmetic does not predict a pressure in pascals. The door gaps, transfer grilles and other openings determine the relationship.
Our make-up air guide explains why the replacement air needs a planned route. Avoid introducing a strong draught across a cupboard opening while solving the room balance.
Doors and changing operating modes
Opening a door changes the airflow path and can reduce or disrupt the intended pressure relationship. Other doors, lift movement, wind and changes in neighbouring systems can also affect the result. The design needs to consider real use rather than one closed-door measurement.
List the operating modes: occupied and unoccupied periods, cupboard sash changes, equipment shutdowns and any emergency sequence. Variable extraction and variable supply need coordinated control. Define what the system should do when one fan stops or a sensor signal is lost.
Do not choose a pressure setpoint from a generic online example. The responsible designer should establish the requirement, tolerances and response appropriate to the laboratory.
Monitoring and verification
Identify the sensing locations and reference space, the display or alarm arrangements and the procedure for responding to a fault. Sensor tubing, calibration and maintenance need to be included in the system record. The sign of the displayed value should be clear to users.
- Measure the relevant supply and extract flows.
- Check the intended pressure relationship in defined operating modes.
- Review door operation and transfer-air paths.
- Verify the local extraction and its alarms separately.
- Record the control sequence and agreed acceptance criteria.
Our air changes per hour calculator describes another ventilation measure. ACH, room pressure and local containment answer different questions and should not be substituted for one another.
A coordinated chemical-extraction brief
Send a room plan, the process description, proposed cupboard duties and the adjoining spaces. Include the existing supply and extract information, door positions and the intended operating schedule. Identify any pressure relationships already required by the building's design.
We can discuss the cupboard, fan and duct components alongside the building-services designer's room ventilation work. WorkSafe's LEV guidance focuses on capturing contaminants at source; room pressure complements that control rather than replacing it.
Your questions
Does negative pressure prove a fume cupboard is working?
No. Room pressure describes a relationship between spaces. The cupboard's airflow, containment and alarms need their own assessment.
Can I calculate room pressure from the supply-extract difference?
Not from that difference alone. You also need the characteristics of transfer openings, leakage paths and the connected building conditions.