Explanation: now, when we recognize that gravity is constant at sea level, and density is relatively constant for any liquid, then we can see that, This shows that pressure is a direct function only of the distance (vertical distance) of a liquid, so because the gravity and the density of liquid are constant, pressure can be expressed with linear units of a liquid, such as ft H2O, or mm Hg. 
Explanation: Gauge pressure, or P_{gauge}, doesn't take into account the atmospheric pressure in its readings. In chemical engineering, unless otherwise stated, we want to use absolute pressure in calculations. Converting from P_{gauge} to P_{absolute} is done using the following formula: note: Let me reemphasize, when pressure is initially given in gauge, alway's first convert P_{gauge} to P_{absolute} before any other calculations. Example 1: Perform the following pressure conversions. Assume atmospheric pressure is 1 atm. Unless other wise stated, the pressures are given in absolute.

Explanation: The purpose of this section is to introduce the interrelation that relates the pressure exerted by a fluid or solid to its density. For some fluid or solid, that we'll designate as i, at height h above a referrence point, we have the following interrelation. This pressure is called the hydrostatic pressure. I wanted to formally introduce this interrelation here, and a practicle application of this interrelation is given in the next section. 
Explanation: Typical applications to the interrelations given above are in manometer problems. A typical manometer could be exposed to different pressures at it's openings and is filled with one or more different fluids. A typical manometer problem could as you to find the pressure drop across the two openings. Here, given the following diagram, we derive an expression for the pressure drop. Here, LHS and RHS represent the left and right hand side of the manometer. We begin with writing the pressure representation of each component, then we gradually simplify the expression to get it into a reduced form. or or, with heights known, and L_{T} representing the total length of the manometer, we get, now, we can rearrange the equation, with the total pressure drop written as DP or, This is our expression, given or looking up the quantities on the right side of the equation allows for a determination of the pressure drop. Example 2: The great Boston molasses flood occurred on January 15, 1919. In it, 2.3 million gallons of crude molasses flowed from a 30foot high storage tank that ruptured, killing 21 people and injuring 150. The estimated specific gravity of crude molasses is 1.4 (so the density of the molasses was 1.4 g/ml). What were the mass of molasses in the tank in lb_{m} and the pressure at the bottom of the tank in lb_{f}/in.^{2}? Example 3: PLACE "PRESSURE BMP" HERE. A fluid of unknown density is used in two manometersone sealedend, the other across an orifice in a water pipeline. The readings shown here are obtained on a day when barometric pressure is 756 mm Hg. What is the pressure drop accross the pipeline manometer? 
© 2007 Arizona Board of Regents for The University of Arizona 