Comparison of the fall rate and structure of recent T-7 XBT manufactured by Sippican and TSK

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 −bt 2 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.

Heat transport variation due to change of North Pacific subtropical gyre interior flow during 1993–2012

“…From 10 to 700 m, the vertical integration along the z axis was discretized into the summation of the 10-m averaged δ value multiplied by the vertical interval of 10 m. The depth of the XBT measurements was determined using the fall-rate equation proposed by Hanawa et al (1995). Kizu et al (2011) reported that Hanawa et al's correction method yields positive and negative fall rates for TSK T-7 and Sippican Deep Blue probes; in the present case, these depth biases result in temperature biases of up to 0.07 and −0.03 • C, respectively. Furthermore, data collected by Sippican Deep Blue XBT are known to be subject to time-varying biases in both depth and temperature (e.g., Cowley et al 2013), which are reported to cause positive temperature biases of up to 0.1 • C for the study period.…”

The CORA dataset: validation and diagnostics of in-situ ocean temperature and salinity measurements

“…However, other sources of XBT temperature uncertainties and biases exist (Gouretski and Reseghetti, 2010;Gouretski and Koltermann, 2007). For example, it has been known since the use of these probes started that the fall rate should depend on the physical characteristics of seawater like viscosity, temperature, and density (Thadathil et al, 2002;Kizu et al, 2011). It was also suggested early on that the assumption of a terminal velocity might not be always correct, in particular in the surface layer, and, compounded with time constant issues, can result in a depth offset (although the determination of this depth offset is not straightforward, as discussed by di Nezio and Goni, 2011).…”

International Quality-Controlled Ocean Database (IQuOD) v0.1: The Temperature Uncertainty Specification

“…Intercomparison studies on the XBTs and data from TSK and Sippican (Lockheed Martin) have revealed structural differences in probe design, recording system circuitry and wire manufacture practices (e.g., Kizu and Hanawa, 2002;Kizu et al, 2005Kizu et al, , 2011Cowley et al, 2013). Sippican (Lockheed Martin) has indicated a ±0.1 • C accuracy in temperature measurement, which is attributed to the thermistor in the XBT probe (Francis and Campbell, 1965).…”