neous data, they can generate measurements that are averaged over periods of a few seconds to a few minutes and produce a record of consecutive short-term averages that span the entire period of interest. Integrated monitoring relies on instruments that collect a sample of air or of a contaminant in air over a longer period (several minutes to more than an hour, depending on instrument design); the resulting concentration is the integrated, or time-weighted average, concentration over the period of sample collection.

Air temperature is measured and controlled in all commercial aircraft for the comfort of passengers and crew and to help provide cooling capacity to maintain appropriate operating temperatures for electronic and mechanical equipment. Because thermal loads are not the same in all parts of the aircraft, control zones are used. Each zone has an independent temperature sensor and adjustable supply of conditioned air. For example, thermal conditioning in the cockpit is controlled separately from that in the passenger cabin, which may be divided into two or more control zones. The latter subdivision helps to minimize longitudinal movement of air in the cabin (Lorengo and Porter 1986; Stevenson 1994; Hunt et al. 1995).

The location of air temperature sensors varies, but the temperature generally appears to be measured in the supply air as it enters a zone. In some instances, the temperature is measured in the cabin or cockpit air after the supplied air mixes with the resident air. Although air temperature is automatically controlled, the set point can be changed by cockpit crew in response to reports of thermal discomfort from cabin occupants. (See Chapter 2 for additional details on temperature control in aircraft).

Barometric pressure in the pressure hull of the fuselage is measured continuously and is under precise control of an automatic system. The supply of compressed air from the environmental control system (ECS) and release of air through an exhaust valve are balanced automatically to maintain cabin pressure. The system is designed to operate so that the pressure difference across the pressure hull does not exceed a specified limit and to ensure that, at least under routine conditions, the barometric pressure in the pressure hull does not fall below a cabin pressure altitude of 2,440 m (8,000 ft) (ASHRAE 1999a). Those two requirements limit the maximal altitude of the aircraft (Stevenson 1994; Hunt et al. 1995; ASHRAE 1999a).

The PO2 in the cabin is not measured routinely. However, because the aircraft ECS does not alter the fraction of oxygen (O2) in outside air, and human occupants in a plane do not reduce the O2 concentration by an amount that is physiologically important, the PO2 in the cabin will be a fixed fraction



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