Choosing the right pressure control
Absolute pressure or gauge pressure? Which reference should you use when you set up a pressure control process? For many applications, this choice may simply be a matter of continuing with established convention. However, some pressure control applications operate at or just above atmospheric pressure, for example backpressure control for process analyzers or flow characterization for cigarette filters. For applications like these, the choice to use absolute or gauge pressure can be a critical decision that significantly impacts the intended operation of your process.
Absolute vs gauge pressure
Pressure is a measure of the force pushing against a surface. This force comes from the kinetic energy of molecules in motion against the surface. According to the Ideal Gas Law (PV=nRT), pressure increases with temperature and mass, and decreases with volume. Let’s consider a rigid container with a perfect vacuum in it. Because there is no matter, there is no pressure. If we add some gas to the container, the moving gas molecules generate pressure against the container walls (Figure 1).
Doubling the number of gas molecules doubles their pressure against the container (Figure 2). However, if we double the volume, the gas molecules have more room, and the pressure is reduced by half (Figure 3). Increasing the temperature of the gas also increases the pressure because it increases the kinetic energy of the gas molecules and their interactions with the container (Figure 4). Conversely, a drop in temperature reduces pressure, which explains why tire pressures can run low on a winter morning.
This perfect vacuum that we started with in the examples above is the reference for absolute pressure. Measures of absolute pressure cannot have negative values. Gauge pressure is referenced to the local atmospheric pressure, which itself is measured on an absolute scale. In other words, gauge pressure tells you how much above or below local atmospheric pressure your process pressure is. When using a gauge pressure scale, the actual total pressure is the gauge reading plus the current local atmospheric pressure (referenced to an absolute pressure scale).
By convention, processes that cannot dip below atmospheric pressure are typically measured using gauge pressure. Tire pressure, for example, uses a gauge reference because we want to know how much more air is in it than what is already in the atmosphere around it. A flat tire has 0 gauge pressure because its internal pressure is equivalent to atmospheric pressure. Vacuum deposition processes, however, are usually referenced to an absolute scale because they need to keep the process at a specific amount of pressure above absolute vacuum. This, however is not the end of the story.
Pressure, temperature and altitude
Have you ever walked out to your car on a cold morning to find your tire pressure is low? Your car’s Tire Pressure Monitoring System (TPMS) has not gone haywire. According to the Ideal Gas Law (PV=nRT), pressure increases with temperature and mass, and decreases with volume. The cold temperature has reduced the kinetic energy of the molecules of air inside your car’s tire, and thus its pressure has been reduced. This same phenomenon came to light during “Deflategate” at an AFC Championship football game, when cold temperatures may have contributed to a 1.8 psi drop in pressure within the football.
To further complicate matters, atmospheric pressure decreases with altitude, because there is a smaller amount of gas molecules to press against everything else. In the vacuum of space, there is effectively no pressure, but at sea level the pressure is, on average, 14.696 psia (absolute). Thus, cities at sea level have higher atmospheric pressures than cities in the mountains. When changing altitude, measures of absolute pressure will give very different readings than measures of gauge pressure.
For example, let’s say that we tighten the cap on an empty water bottle at Alicat’s Tucson office. At an elevation of 2160 feet, the average ambient air pressure is 13.67 psia (absolute), so the pressure inside the bottle is also 13.67 psi on an absolute scale. On a gauge pressure scale, the pressure inside the bottle is 0 psig, equal to ambient air pressure. We drive the water bottle up to the 9,159-foot summit of Mt. Lemmon, just north of Tucson, where the ambient air pressure is only 10.44 psia. The air pressure inside the sealed bottle remains 13.67 psia, which at this altitude is now equivalent to 3.23 psig (13.67-10.44).
By the 2nd Law of Thermodynamics, fluids move from areas of high pressure to areas of low pressure. If we open the sealed water bottle at the summit, the greater pressure inside will cause some of the air to flow out of the bottle until the internal pressure equals 10.44 psia, or what is now 0 psig. Then, we again seal the bottle at the summit and return down the mountain. The pressure inside the bottle (10.44 psia) is now less than the surrounding atmospheric pressure (13.67 psia), so on a gauge scale the pressure reads -3.23 psig (10.44-13.67). When we open the bottle, the surrounding air rushes into the lower pressure bottle until its pressure has increased to 13.67 psia.
Pressure and weather systems
Weather systems further increase or decrease the local atmospheric pressure by a small amount. Barometric pressure fluctuates throughout the day, typically reaching its peak at about 10 am and its low at about 4 pm. This difference is greatest at the equator, where both the rotation of the earth and daily temperature fluctuations are the greatest. In addition to daily fluctuations, weather systems bring pressures that can be either higher or lower than average. Throughout the year, a single location’s atmospheric pressure might vary as much as 0.3 psi. Locations that see frequent storms, tropical depressions or hurricanes can see even greater variations in a much shorter time.
Again taking the case of Tucson, Arizona, our average atmospheric pressure is about 13.7 psia, with typical highs of 13.8 psia and lows of 13.6 psia. If we intended to control a process at just 0.3 psi above atmospheric pressure, should we use a gauge pressure controller or an absolute pressure controller? Gauge pressure control (the left side in the diagram below) would result in unstable control that rides the waves of the local atmospheric pressure variations. However, these fluctuations would not be visible because the controller would always read a gauge pressure of 0.3 psig. Absolute pressure control (on the right below) provides steady control, regardless of what is happening in the atmosphere, because it is referenced to vacuum, not atmospheric pressure.
Note that the greater your pressure setpoint above atmospheric pressure, the less effect the above fluctuations will have. In Tucson, a process set for 100.0 psig would see fluctuations from 113.6 psia to 113.8 psia, whereas absolute pressure control would provide a steady pressure of 113.7 psia. On the scale of 113.7 psia, a variation of +/- 0.1 psi would likely be insignificant to the process and question and not warrant absolute pressure control.
Choosing the right pressure reference
The examples above illustrate the importance of choosing the right reference scale for measuring or controlling pressure in your processes. If we desire to isolate a specific amount of pressure inside a process, regardless of what happens in the atmosphere, then we should use a system of absolute pressure. If, however, we are concerned with maintaining a certain pressure relative to the current atmospheric pressure, then we should use a system of gauge pressure. A gauge pressure controller will add or remove air as the ambient air pressure goes up and down to maintain the desired pressure differential. As we have seen, applications that require control of low atmospheric pressures will most likely benefit most from absolute pressure control.
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