The differential height of the left hand side of the tube from the zero mark to the open end for device "C" is 4.4". If the barometric pressure is 14.7 psia, and the "U" tube contains mercury, what minimum pressure applied to "A" would just force the liquid out of the tube? See illustration GS-0154.
β’ Hydrostatic pressure from a mercury column β how many psi per inch of mercury? β’ Difference between psia (absolute pressure) and psig (gauge pressure), especially when barometric pressure is given as 14.7 psia. β’ Understanding what the 4.4 inch differential height in a U-tube actually represents β is it one leg or the total difference between the two legs?
β’ First, convert the 4.4 inches of mercury into pressure (psi). What standard conversion factor (psi per inch of Hg) should you use? β’ Once you know the pressure from 4.4" of Hg, decide: is this pressure added to atmospheric pressure or measured above atmosphere? How does the drawing of device C help you decide? β’ Look at the answer units: some are psia and some are psig. Based on how a U-tube manometer open to atmosphere works, should the required applied pressure be expressed as absolute or gauge?
β’ Calculate ΞP from 4.4" Hg using the correct conversion (donβt forget to keep track of units). β’ Decide whether the applied pressure at A must be above atmospheric or is being measured from zero absolute β match this to psia vs psig. β’ Verify whether the 4.4" height already represents the full difference between the two liquid levels or if you would wrongly double it. This will eliminate one of the numerical choices.
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