I think we've got an unit conversion error in that calculation. One foot of gauge water pressure (or head), disregarding absolute pressure since the atmosphere is working on all surfaces, is 0.433psi. 6*0.433=2.598psi. Since you got me thinking, I pulled out my dual pressure differential manometer and stuck it in a bottle of water to be sure-I didn't bother with many decimals but 12" of air in the tube below the surface read 12" of water column and 0.43psi.
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