ENVIRONMENT power and data cables. Instead, each station is self-contained wind direction and changing ex- perimental needs). The instru- (powered bybatteries and solar panels) and connected to the ments require only minimum maintenance, are stable over periods of weeks, and are easy to calibrate. center by two-way telemetry. The greater extent of the sensor The nature of the dispersing cloud affects instrument requirements. The cold gas condensesessentially all the water vaporin the air close to the pond, forming a thick fog (see front cover) that poses problems not only for optical array imposes severe burdens on the instruments and the measurement stations. Both are light and ‘portable (so that they can be towermounted and easily moved to accommodate shifts in the mean 400 T ] : | | | 300 -— 7 1 instruments but also for sensors that rely upon the chemical or thermodynamic properties of the gas. Hence, each type of instrument must be specifically adapted to function properly within a cold, dense fog. Because of turbulence, hydrocarbon concentrations within the gas cloud fluctuate significantly even over time intervals as short as 0.2 s. Turbulence andits effect on 7 T 1 -f- 1 T O 4 200 -— — _ ¢ 100 ;~- 8 0 04s —4-—_- + 2 a“ —_+ + 4 & © 100-— — © Gas sensor station 2007—- A Anemometer station T . O Turbulencestation S00; COM3 code calculation 400 800 1 l 600 ! TT | 400 J 200 & O > O i | 0 200 | I 400 | 600 t ™ | 800 | 7000 Distance —m Pig. © Far-field dispersion arrayfor 40-m’spill tests at China Lake, with calculated concentration contours for a 40-m7/minspill rate. Gas sensors, anemometers, and thermocouplesare at elevations of I, 3, and &m above the ground. Each of the instrument stations is self-contained (powered by batteries and solarcells) and connected to a central data-processing trailer by two-way telemetry. Each of the stations is portable so that the array can be rearranged in responseto shifis in the average wind direction or changes in experimental conditions. 35