Abstract
Abstract. The fall rate of recent T-7 expendable bathythermograph (XBT; 760 m) is evaluated based on a series of concurrent measurement with a calibrated Conductivity Temperature Depth profiler (CTD) in the sea east of Japan. An emphasis is placed on comparing the fall rates of T-7 produced by the two present manufacturers, the Lockheed Martin Sippican Inc., and the Tsurumi Seiki Co. Ltd., which have been believed to be identical but had never been compared directly. It is found that the two manufacturers' T-7 fall at rates different by about 3.5%. The Sippican T-7 falls slower than given by the fall-rate equation (FRE) of Hanawa et al. (1995) by about 2.1%, and the TSK T-7 falls faster by about 1.4%. The fall-rate coefficients estimated based on the sea test by applying the equation of traditional quadratic form, d(t)=at−bt2 where d is depth in meters and t is the time elapsed, in seconds, are a=6.553 (m s−1) and b=0.00221 (m s−2) for the LMS T-7, and a=6.803 (m s−1) and b=0.00242 (m s−2) for the TSK T-7. By detail examination of the probes, we found that the two companies' T-7 have different total weight and many structural differences. Because the difference in the fall rate is about twice larger than the difference in weight (about 2%), it is inferred that the structural differences give sizable impact to the difference in their fall rates. Our results clearly show that the recent T-7 of the two companies needs to be discriminated.
Highlights
The expendable bathythermograph (XBT) is a ballisticallyshaped probe instrument for measuring temperature profiles of the upper ocean
A thermistor is fixed near the front of the nose weight and is linked to an on-board data acquisition system via fine coated wire and a launcher unit
When one deploys the probe from a vessel underway, it freely falls in the water column and measures temperature of water as the resistance of the thermistor, which is recorded by the on-board system normally at a constant time interval
Summary
The expendable bathythermograph (XBT) is a ballisticallyshaped probe instrument for measuring temperature profiles of the upper ocean. The probe consists of a metal nose weight with holes that allow water to flow through, and a plastic after-body with three fin stabilizers (Fig. 1). A thermistor is fixed near the front of the nose weight and is linked to an on-board data acquisition system via fine coated wire and a launcher unit. When one deploys the probe from a vessel underway, it freely falls in the water column and measures temperature of water as the resistance of the thermistor, which is recorded by the on-board system normally at a constant time interval. The measurement is terminated at the rated depths, beyond which the manufacturers do not guarantee the quality of measurement, or when the wire runs out and breaks, depending on the acquisition system. A general description of the early system is given by Baker (1981), and a newer review is in Emery and Thomson (2004)
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