If "A" were open to the atmosphere, as shown in the illustration, the pressure gage would read zero and the levels in the "U" tube would be equalized. If "D" is manufactured to indicate inches of water, what will be indicated reading of the diaphragm gage if the atmospheric pressure is 14.7 psia? See illustration GS-0154.
• Relationship between psia (absolute pressure) and a column of water head in inches • The fact that a diaphragm gage like D can be calibrated to read absolute pressure referenced to a vacuum, not gauge pressure • Standard conversion: 1 psi ≈ 2.31 feet of water ≈ 27.7 inches of water at standard conditions
• Is gage D measuring pressure above atmosphere (gauge pressure) or above a perfect vacuum (absolute pressure) in this setup? Look closely at how it is connected compared with gage A and the open leg of the manometer. • Once you decide it is absolute or gauge, ask: what pressure is actually being applied to D when the only pressure present is the atmosphere (14.7 psia)? • Convert the pressure in psi to an equivalent water column in inches, then compare that value with the options given.
• Be clear whether 14.7 psia corresponds to zero gauge pressure or a positive pressure on an absolute scale. • Use the correct conversion factor: 1 psi ≈ 27.7 inches of water (fresh water at standard conditions). • After converting, check whether your result is closer to a very small number, a few tens of inches, or several hundred inches of water and match that to the choices.
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