Improved Depth and Temperature Conversion Equations for Sippican AXBTs

Abstract

In October 1985, in a region of large-scale thermohaline ”staircases” off the northeast coast of South America, five shallow and seven deep Sippican AXBTs (air deployed expendable bathythermographs) were hand deployed from the research vessel USNS Bartlett during times when CTD (conductivity, temperature, and depth) casts were being conducted. AXBT temperature and depth values calculated from several standard conversion equations were compared with the CTD values and improved conversion equations were derived. Two different elapsed fall time-to-depth conversion equations were found for the shallow and deep AXBTs, but only one frequency-to-temperature conversion equation. The depth equations, accurate to ±5 m over the range 0–700 m, were z = 1.5573t −0.000301t2 for shallow AXBTs and z = 1.6325t −0.000215t2 for deep, with z the depth in meters and t the elapsed fall time of the probe in seconds. The temperature equation, accurate to ±0.2°C between 6–28°C, was T = −38.6045 + 0.0271075F, with T the temperature in degree centigrade and F the transmitted frequency in hertz.