Setting up a pressure control process involves choosing between absolute pressure and gauge pressure. For a number of applications the choice could be to stick to an established convention, however some other pressure control applications function at, or just above atmospheric pressure.
A good example of this is the backpressure control for flow characterization for cigarette filters or process analyzers. For such applications, it is a critical decision whether to select gauge pressure or absolute pressure, as the choice will impact in the intended operation of the process.
Figure 1. Fluctuations in barometric pressure
Absolute vs Gauge Pressure
The measure of the force pushing against a surface is called pressure, and it comes from the kinetic energy of moving molecules against the surface (Figure 2). The Ideal Gas Law (PV=nRT) highlights that pressure increases with temperature and mass, and decreases with volume.
Consider a rigid container containing a perfect vacuum, pressure is absent due to no matter. The addition of gas to the container results in the generation of pressure against the container walls by the moving gas molecules (Figure 3).
Figure 2. Pressure is caused by the kinetic energy of molecules pressing against a surface
Figure 3. Adding mass to a rigid container increases the pressure inside it.
When the number of gas molecules doubles, the pressure of these molecules increases against the container. Doubling the volume creates more space for the gas molecules, and reduces the pressure by half (Figure 4).
Increase in the gas temperature also leads to an increase in the pressure, because it increases the gas molecules kinetic energy and their interactions with the container (Figure 5). A temperature drop reduces pressure, which can explain why tire pressure is capable of running low on winter mornings.
Figure 4. Increasing the volume of a rigid container reduces the pressure inside it.
Figure 5. Increasing the temperature of a rigid container increases the pressure inside it
This perfect vacuum, explained above, is the reference for absolute pressure. Measures of absolute pressure do not posses negative values. Gauge pressure is the local atmospheric pressure, which is measured on an absolute scale.
Gauge pressure describes how much below or above local atmospheric pressure the process pressure is. In a gauge pressure scale the actual total pressure is the gauge reading and the current local atmospheric pressure.
Gauge pressure is often used to measure processes that cannot go below atmospheric pressure. A gauge reference is used for tire pressure to discover the amount of extra air in it, besides the already existing atmosphere around it.
Zero gauge pressure is found in a flat tire because the internal pressure is equivalent to atmospheric pressure. However, vacuum deposition processes are generally referred to as an absolute scale because they are responsible for maintaining the process at a particular pressure amount above absolute vacuum.
Pressure, Temperature and Altitude
On cold mornings a car’s tire pressure can sometimes be low, but there may not be any damage to the car’s Tire Pressure Monitoring System (TPMS). The cold temperature can reduce the kinetic energy of the molecules of air present inside the car’s tire, reducing the pressure of the tire.
This kind of a situation occurred during last winter’s “Deflategate” AFC Championship football game. It is assumed that the cold temperatures during this game may have contributed to a 1.8 psi drop in pressure inside the football.
To make matters worse atmospheric pressure decreases with altitude, due to the presence of smaller gas molecules that press against everything else. Pressure is not present in the vacuum of space, but it is present at sea level, on average, 14.696 psia (absolute), so cities at sea level contain increasing higher atmospheric pressures than cities in the mountain regions. When the altitude changes, measures of absolute pressure provide extremely different readings than that provided by measures of gauge pressure.
For instance, an empty water bottle’s cap is tightened at Alicat’s Tucson office. At a height 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 absloute scale.
On a gauge pressure scale, the pressure inside the bottle is 0 psig, the equivilant to ambient air pressure. Repeating the task at 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).
Figure 6. Gauge pressure of a process increases as atmospheric pressure decreases.
According to the 2nd Law of Thermodynamics, fluids move from high pressure areas to low pressure areas. When the sealed water bottle is opened the summit the higher pressure within the bottle causes some of the air to flow out from the bottle, and the pressure on a gauge scale reads - 3.23 psig (10.44-13.67). Once the bottle is opened the surrounding air enters the lower pressure bottle until the pressure increases to 13.67 psia (Figure 7).
Figure 7. Gauge pressure of a process decreases as atmospheric pressure increases.
Pressure and Weather Systems
Weather systems increase and decrease the local atmospheric pressure. Fluctuations in barometric pressure occur throughout the day, it reaches its peak at about 10 am and low at about 4 pm. This difference is higher at the equator, where the daily temperature fluctuations and the earth’s rotation are the greatest.
Along with these fluctuations that occur daily, the weather systems develop pressures that can be either lower or higher than average. The atmospheric pressure of a single location may vary throughout the year, and this variation could be as much as 0.3 psi. Increasing variations also occur within a vey short period of time in locations that frequently experience hurricanes, tropical depressions, or frequent storms.
Considering the case of Tucson, Arizona, the average atmospheric pressure is about 13.7 psia, with typical lows of 13.6 psia and highs of 13.8 psia. If a gauge pressure controller is used to control a process at just 0.3 psi above atmospheric pressure, it results in unstable control that rides the waves of the local atmospheric pressure variations.
However all of these fluctuations are not visible, as the controller always reads a gauge pressure of 0.3 psig. Firm control is provided by absolute pressure control, regardless of what is happening in the atmosphere, because it is referenced to vacuum, not atmospheric pressure (Figure 8).
Figure 8. Absolute pressure control stabilizes the effects of atmospheric pressure variability.
These fluctuations will have a lower effect if the pressure setpoint is greater than atmospheric pressure. In Tucson a process set for 100.0 psig will experience fluctuations from 113.6 psia to 113.8 psia, and a steady pressure of 113.7 psia will be provided by absolute pressure control. On the scale of 113.7 psia, a variation of +/- 0.1 psi is likely to be less important to the process and does not warrant absolute pressure control.
Choosing the Right Pressure Reference
The examples above illustrate the significance of selecting the right reference scale to control or measure pressure in the processes. A system of absolute pressure should be used to isolate a certain amount of pressure within a process, regardless of what is happening in the atmosphere.
A system of gauge pressure should be used to maintain a specific pressure relative to the current atmospheric pressure. A gauge pressure controller helps to add and remove air based on the variations in the ambient air pressure varies, and this further helps to maintain the desired pressure differential. Applications requiring control of low atmospheric pressures could benefit a lot from absolute pressure control.
This information has been sourced, reviewed and adapted from materials provided by Alicat Scientific.
For more information on this source, please visit Alicat Scientific.