PRESSURE MEASUREMENT OF GASES AND LIQUIDS
By definition, pressure (P) is the force (F) applied to an object per unit of area (A), such that P = F/A. The SI unit of pressure is the pascal (Pa); 1 Pa equals 1 newton of force distributed over an area of 1 m2. Pressure can also be defined by other units, such as millimeters of mercury (mm Hg), centimeters of water (cm H2O), pounds per square inch (psi), or atmospheres (atm). These different units are based on the specific way of taking the measurement. For instance, “mm Hg” is the pressure exerted at the base of a 1-mm high column of mercury, whereas “cm H2O” is the pressure exerted at the base of a 1-cm high column of water at 4°C. To convert among the units, it is useful to start with the pressure of the atmosphere at sea level: 1 atm = 760 mm Hg = 988 cm H2O = 14.7 psi.
Clinically, gauges are used to display pressure measurements of both gases and liquids. Examples of gauge pressure include central venous pressure, arterial blood pressure, cylinder pressures, and peak inspiratory pressures. Gauges record pressure above or below the existing ambient atmospheric pressure. “Absolute” pressure is the sum of gauge pressure and atmospheric pressure. For example, a full oxygen E-cylinder has a gauge pressure of about 2000 psi. When the gauge pressure reads 0 psi, the cylinder still contains oxygen at ambient atmospheric pressure (14.7 psi). The absolute pressure of this E-cylinder is 2014.7 psi when completely full.
Manometers are the most common systems used to measure pressure. Manometers contain columns of liquid, usually water or mercury, in an open-ended U-shaped tube. Pressure applied to the end not exposed to atmospheric pressure will displace the fluid column. The column adjusts its height until it achieves equilibrium with the pressure difference between the two ends of the tube. The pressure in the column is the product (ρgz) of the height of the column (z), the density of the liquid (ρ), and the force of gravity (g). Manometers work best for measuring pressures that change slowly. The mass of the liquid column yields significant inertia that works against quick changes in height.
Manometers are not helpful in measuring high pressures because the necessary height of the fluid column would be difficult to achieve. Instead, Bourdon gauges are used. These devices are based on the concept that an elastic tube will deflect when subjected to a given applied pressure. Higher gas pressures will uncoil this tube, which causes the pointer to move on the gauge’s scale.
Pressure regulators, also known as pressure-reducing valves, are used in anesthesia machines to lower pressures and regulate the ...