Atmospheric pressure can be expressed in three terms, PSIA (PSI Absolute), inches of mercury, and feet of water (abbreviated feet of head or just feet). The three terms are interchangeable using the following conversions factors: To convert PSIA into feet, multiply by 2.31, and to go from PSIA to inches of mercury, multiply by 2.04. At sea level, atmospheric pressure equals 30 inches of mercury, 34 feet of water or 14.7 psig. The term you use depends on what you need to do with the information. The only time we are likely to see it expressed in terms of inches of mercury is reporting barometric (atmospheric) pressure. From this point on, we’ll talk in terms of PSIA and feet of head.
Absolute Pressure vs. The Gauge PressurePressure is a relative term, meaning it is measured and identified relative to some basic pressure. Absolute pressure is measured relative to a perfect vacuum. At the edge of Earth’s atmosphere, the absolute pressure is zero. The deeper into the atmosphere we get, the higher the pressure. From the chart above, (see Figure 1) at 8,000 feet altitude, it measures 10.9 psia (25.2 feet). At sea level, we’ve said it is 14.7 psia (34 feet). At 1,000 feet below sea level, it is 15.2 psia (35.1 feet). PSIA is an important term for ground water professionals to understand because it is encountered in several formulas used in our industry.
Gauge pressure, or PSIG, on the other hand, is measured relative to atmospheric pressure. Pressure gauges used in our industry use a bourdon or “U” tubes with one side open to atmosphere. A pressure gauge at sea level with a reading of 50 psig therefore would be equal to an absolute pressure of 64.7 psia, 50 plus 14.7. The term PSI can be used as an abbreviation for PSIG. PSIA can not be abbreviated.
How Pumps Lift WaterPicture a large pan of water with a 35 foot straw sticking out of the water. We hook a vacuum pump to the top of the straw. What happens? Does the vacuum pump suck the water up into the straw? No, the vacuum pump removes most of the air (atmosphere) from inside of the straw, and the weight of the atmosphere pushing down on the water in the pan forces the water up into the straw. How high? If it were possible to remove all of the air from the straw, 34 feet at sea level by definition.
In a water well, the weight of the atmosphere is acting on the surface of the water down in the well. The pump at ground level acts as the source of vacuum, and has a theoretical lifting capability of about 30 feet (It would lift 34 feet if it could create a perfect vacuum). In practice, 25 feet is all you should design a centrifugal or jet pump to lift in order to have adequate capacity left for water usage. The pump performance tables supplied by pump manufacturers show a pump’s capacities at differing lift levels.
In Figure 2, the third column shows the suction lift level in feet, and the next few columns show the GPM performance of the pump at various discharge pressures. For instance, if your water level in the well is 15 feet below the inlet of this particular shallow-well jet pump, and you require a discharge pressure of 40 psig, you would get 8.5 gpm from the pump.
Next month, we will take a look at just how jet pumps work. ’Til then ….