Cleanroom static air pressure differentials are very small. The conventional units of measurement (imperial units) are inches of water. This has many abbreviations, including in. w.c., in. H2O, “w.c., “w.g. “H2 O, in w.g., w.c., and w.g., amongst others.
All these abbreviations stand for the same thing. In the real world, they refer to an air pressure equivalent to the pressure exerted at the bottom of a column of water that many inches high. If G is the magnitude of the gauge reading in inches of water, then G is often in the range of 0.01 or 0.001.
Gauge readings are the total atmospheric pressure plus gauge pressure at point P2 minus the total atmospheric pressure plus gauge pressure at point P1. Standard atmospheric pressure at sea level (29.9212 inches of mercury column) measured in this unit is 406.792 in. H2 O.
So that implies that a measured difference in air pressure between two cleanrooms of 0.001 in. H2 O, representing a sensitivity of = 0.00000246. Or, 0.000246% or less than 3 parts per million.
The atmospheric pressure of the air varies with elevation. At a higher elevation, there are fewer molecules of air above you pressing down.
Alternatively, rather than considering air pressure, if we take a look at the effect of actual columns of water, the gravitational phenomenon becomes more pronounced. Unlike air, water, of course, is a liquid and is much denser. When we are measuring water system pressure in a pipe, we have to account for the difference in elevation between points P2 and P1. Water balancers are very familiar with this phenomenon.
A water pressure gauge inserted at point P2 will read 7 feet, 6 inches (90 inches) higher than that same gauge inserted at point P1. That is the literal meaning of inches of water column.
When the water balancer has to calculate the effect of the pump on the system pressure and energy, she must account for the elevation at which the pressure measurement was taken. It is a common practice to equilibrate all system pressure readings to the elevation of the pump impeller. However, any arbitrary, consistent elevation will suffice.
The relationship between the units of inches of water and pounds per square inch (psi) is such that a column of water 27.68 inches (2.31 feet) high exerts a pressure of 1.0 psi at its bottom surface. As shown, 90 inches of water column means that the pressure measured at P1 will be 3.25 psi lower than that measured at P2.
This realization naturally raises the question: Is there an elevation-dependent difference in room static air pressure measurements? We know that the air pressure at cruising altitude outside of the airplane is measurably different than at sea level, but what about the air static pressure at the cleanroom ceiling elevation relative to the floor?
The formula for air pressure as a function of altitude is known as the barometric formula. One version of this formula can be expressed as follows:
Where the negative exponent indicates a decreasing pressure with increasing elevation, and where: Ph = the resultant pressure at the elevation of concern (Pa) Pb = the barometric pressure at sea level 101,325 (Pa) e = the base of the natural logarithm (2.71828) g = the acceleration due to gravity 9.80665 (m/s^2) h = height of elevation of concern (m) hb = height of sea level 0 (m) R = universal gas constant 8.3144598 [(J)/(mol*K)] Tb = standard temperature 288.15 (°K) M = molar mass of Earth’s air 0.0289644 (kg/mol)
When we calculate Ph for a height of 7 feet 6 inches (2.286 m), which is the elevation of the suspended ceiling above my desk, we end up with something like this:
Ph = 90 inches = 101,297.542 Pa, which is 27.458 Pa less than 101,325 or equivalent to 0.110 in H2 O. This result seems to indicate that we have a serious problem in how we specify and measure room pressure differentials! It doesn’t appear that the barometric formula is useful for very small elevations.
To check this result empirically, I used a Shortridge ADM-880C data logging multimeter configured as shown. Two hundred individual samples were taken, with one hundred of those configured as shown above and the other hundred taken at floor level without the 7 feet 6 inches extension tube.
The readings clearly indicate a measurable difference in pressure when measured at the 7 feet 6 inches elevation (p = 0.005). The average (n=100) pressure difference was 0.00012 in. w.c. So, the answer to the question of whether there is a significant, measurable difference is yes. A ceiling-mounted room pressure tap will definitely read at least 0.00012 in H2 O less than the tube inserted under the door. However, the answer to whether it matters for our purposes is probably no.
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