Gas analyzers are essentially molecule counters. When they are calibrated, a known concentration of gas is introduced, and the analyzer’s output is checked to ensure that it is counting correctly. But what happens when the atmospheric pressure changes by five to 10 percent as it is known to do in some climates? The number of molecules in a given volume will vary with the change in atmospheric pressure and, as a result, the analyzer’s count will change. There is a common misperception that atmospheric pressure is a constant 14.7 psia (1 bar.a) but, based on the weather, it may fluctuate as much as one psi (0.07 bar) up or down. In order for the calibration process to be effective, absolute pressure in the sampling system during calibration and during analysis of samples must be the same. Absolute pressure may be defined as the total pressure above a perfect vacuum. In a sampling system, it would be the system pressure as measured by a gauge, plus atmospheric pressure.
To understand the degree of fluctuation in measurement that may be brought about by changes in absolute pressure, let’s refer to the perfect gas law:
PV = nRT
where P = pressure, psia; V = volume, cubic in.; n = number of moles (molecules); R = gas constant; and T = absolute temperature, °F. Rearranging this equation to read
n = PV/RT
shows that as temperature and pressure change, the number of molecules present in the standard volume also changes. Pressure changes are more critical than temperature fluctuations. One atmosphere of pressure is defined as 14.3 psi. Therefore, a 1 psi variation in pressure can change the number of molecules in the analyzer volume by about 7 percent. Temperature, on the other hand, is measured on the absolute scale, keeping in mind that absolute zero is -460°F (-273°C), so a 1°F (0.5°C) temperature variation changes the number of molecules by only about 0.3 percent. In sum, it is probable that one might get a large change in pressure in percentage terms. It is not probable that one would get a large temperature change in percentage terms.
If pressure is so critical, how does one control for it? Some analyzers, especially infrared and ultraviolet, allow atmospheric pressure to affect the reading but then later correct for it electronically. However, many analyzers, including nearly all gas chromatographs, do not correct for atmospheric pressure fluctuations; most systems do not correct for it; and many system engineers or operators are satisfied to ignore it. Some believe that atmospheric fluctuations are not significant. Others maintain that any atmospheric fluctuations are compensated for by other related or unrelated variables affecting the analyzer, and it all comes out in the wash. Nevertheless, atmospheric fluctuations can be extremely significant. Let’s suppose that when you calibrate your analyzer, the atmospheric pressure is X but, later, when you inject the process gas, the atmospheric pressure is X + 1 psi (0.07 bar). The answer may be as much as 7 percent off the measured value.
With environmental regulations, most analyzer systems now vent to flare stacks or other return points. Since pressure fluctuations from such destinations will affect pressure upstream in the analyzer, there are vent systems, equipped with eductors and regulators, designed to control for these fluctuations. Unfortunately, these systems employ regulators that are referenced to atmosphere. As a result, while these systems control for fluctuations from the vent, they do not control for fluctuations in atmospheric pressure which, by far, could be the greater of the two sets of fluctuations. For such a system to control for atmospheric as well as vent pressure fluctuations, an absolute pressure regulator is required. Unlike a normal regulator, an absolute pressure regulator is not comparing pressure inside the system to pressure outside the system, which is itself fluctuating according to the weather. Rather, it is comparing pressure inside the system to a constant set pressure that does not fluctuate at all (or very little). Often, this set pressure is actually 0 psia (0 bar.a).