What unit is commonly used to express the differential pressure between the OR and surrounding spaces?

• A. Inch water gauge
• B. Pascal
• C. psi
• D. millimeters of mercury

Answer: (A)

Smaller differential air pressure in operating rooms is expressed in inches of water gauge because this unit directly reflects the tiny pressure differences that HVAC systems create to control airflow between spaces. It’s a practical, intuitive scale for measuring how much pressure one space has above another, and it aligns with the sensors and gauges used in building management and cleanroom/OR ventilation.

In contrast, pascals are the SI unit for pressure but aren’t as convenient for the very small pressures typical in room-to-room differential measurements; pounds per square inch is far too large for these tiny differentials; millimeters of mercury is used mainly for medical gas pressures or clinical measurements, not for HVAC room differentials.

So the commonly used unit for expressing this differential pressure is inches of water gauge, with 1 inch of water approximately equal to 249 pascals or about 0.036 psi.