It is important to consider how pump performance is affected by the relationship between atmospheric pressure and altitude. Atmospheric pressure is caused by the weight of the air pressing down on Earth and on the subsequent air below. Since the pressure depends on the amount of air above the point where the pressure is being measured, the pressure decreases as the altitude increases. The air's pressure is related to its density, which is affected by its temperature, the amount of water vapor it contains and the height above the Earth's surface. The lower the temperature, the slower the molecules that comprise air are moving which means they push less against their surroundings causing lower pressure. A decrease in air pressure therefore has the effect of reducing the air’s density and its respective mass for a set volume of flow.
Diaphragm pumps have a set compression stroke that produces a fixed volumetric flow regardless of altitude. Since air is less dense at higher altitudes, the mass of the volumetric flow and therefore the ability to attain maximum sea level pressure or vacuum for a diaphragm pump is reduced. For example, the same 78° F (26° C) saturated air at sea level takes 14.010 ft³/lb dry air versus 16.953 ft³/lb dry air at an altitude of 5,000 feet (1,524 m), or an increase of 21%. In a similar manner, the density is reduced from 0.0765 lb/ft³ (1.225 kg/m³ ) to 0.0659 lb/ft³ (1.056 kg/m³), or 13.8% less, thereby reducing the air pressure drop for the same air flow volume. This correlates to an approximately 13.8% reduction in pressure or vacuum performance from an air pump at an altitude of 5,000 feet (1,524 m) compared to if it was being operated at sea level.
Fluidic system designers need to be aware of the full range of altitudes their system may operate to ensure proper performance integrity at different locations. The Standard Atmosphere Table lists the percentage loss on a standard day for pressure and vacuum levels at varying altitudes. Air pumps should therefore be sized with the necessary additional capacity at the maximum possible altitude. Either a pressure relief valve or pressure regulator should be incorporated to bleed off the extra capacity at the lower altitudes.
Consider what would happen if a diaphragm pump is designed for a system without regard to possible high altitude locations where it might be operated. For example, a pump is required to compress air to 20 psig (1.38 bar) and provide 10 LPM flow for a product to function properly. If this system was tested and verified at sea level and the pump was not sized with additional capacity, then the product would not operate properly if operated at higher altitudes. For instance, should this product be sold to operate in a large market such as Mexico City, it would be advisable to determine the respective pressure loss at this high altitude. Using the Altitudes of World Cities Table, it is noted that this city is located at an approximate altitude of 7,400 feet (2,256 m). Referencing the Standard Atmosphere Table, the effective pressure is determined to be approximately 24.5% less than the pump’s specified pressure at sea level. Instead of producing the necessary 20 psig (1.38 bar) of pressure, the diaphragm pump would only be able to produce approximately 15 psig (1.03 bar). The diaphragm pump should be sized at 26.5 psig (1.83 bar) to have enough capacity to compensate for the pressure loss at an altitude of 7,400 feet (2,256 m).