The most common type of pressure measurement is gauge  pressure. This is the difference between the pressure to be  measured and the atmospheric pressure. The gauge pressure  inside the deflated balloon mentioned above is therefore  0 pounds per square inch (psi). Gauge pressure is easily  measured and is obtained by ignoring the fact that the  atmosphere is always exerting its pressure on everything. For  example, a tire is filled with air to 32 psi at a sea level location  and checked with a gauge to read 32 psi, which is the gauge  pressure. The approximately 14.7 psi of air pressing on the  outside of the tire is ignored. The absolute pressure in the tire  is 32 psi plus the 14.7 psi that is needed to balance the 14.7  psi on the outside of the tire. So, the tire’s absolute pressure  is approximately 46.7 psi. If the same tire is inflated to 32  psi at a location 10,000 feet above sea level, the air pressure  on the outside of the tire would only be approximately 10  psi, due to the thinner atmosphere. The pressure inside the  tire required to balance this would be 32 psi plus 10 psi,  making the absolute pressure of the tire 42 psi. So, the same  tire with the same amount of inflation and performance  characteristics has different absolute pressure values.  Gauge pressure, however, remains the same, indicating the  tires are inflated identically. It this case, gauge pressure is  more useful in informing us of the condition of the tire. standard temperature of 15 °C at sea level. Specific standard  day values have also been established for air density, volume,  and viscosity. All of these values are developed averages  since the atmosphere is continuously fluctuating. They are  used by engineers when designing instrument systems and  are sometimes used by technicians and pilots. Often, using  a standard value for atmospheric pressure is more desirable  than using the actual value. For example, at 18,000 feet and  above, all aircraft use 29.92 "Hg as a reference pressure for  their instruments to indicate altitude. This results in altitude  indications in all cockpits being identical. Therefore, an  accurate means is established for maintaining vertical  separation of aircraft flying at these high altitudes.  The most important instrument used by the pilot to perceive  the health of an engine is the engine oil pressure gauge.  [Figure 10-13]  Oil pressure is usually indicated in psi. The  normal operating range is typically represented by a green arc  on the circular gauge. For exact acceptable operating range,  consult the manufacturer’s operating and maintenance data. In reciprocating and turbine engines, oil is used to lubricate  and cool bearing surfaces where parts are rotating or sliding  past each other at high speeds. A loss of pressurized oil  to these areas would rapidly cause excessive friction and  over temperature conditions, leading to catastrophic engine  failure. As mentioned, aircraft using analog instruments  often use direct reading Bourdon tube oil pressure gauges.  Gauge pressure measurements are simple and widely useful.  They eliminate the need to measure varying atmospheric  pressure to indicate or monitor a particular pressure situation.  Gauge pressure should be assumed, unless otherwise  indicated, or unless the pressure measurement is of a type  known to require absolute pressure.  In many instances in aviation, it is desirable to compare  the pressures of two different elements to arrive at useful  information for operating the aircraft. When two pressures  are compared in a gauge, the measurement is known as  differential pressure and the gauge is a differential pressure  gauge. An aircraft’s airspeed indicator is a differential  pressure gauge. It compares ambient air pressure with ram air  pressure to determine how fast the aircraft is moving through  the air. A turbine’s engine pressure ratio (EPR) gauge is also  a differential pressure gauge. It compares the pressure at the  inlet of the engine with that at the outlet to indicate the thrust  developed by the engine. Both of these differential pressure  gauges and others are discussed further in this chapter and  throughout this handbook.  Pressure Instruments Engine Oil Pressure Figure 10-13.  An analog oil pressure gauge is driven by a Bourdon  tube. Oil pressure is vital to engine health and must be monitored  by the pilot.  10-8 In aviation, there is also a commonly used pressure known as  standard pressure. Standard pressure refers to an established  or standard value that has been created for atmospheric  pressure. This standard pressure value is 29.92 inches of  mercury ("Hg), 1,013.2 hectopascal (hPa), or 14.7 psi. It is  part of a standard day that has been established that includes a 