Problems
Since XBTs do not measure depth (e.g. via pressure), fall-rate equations are used to derive depth profiles from what is essentially a time series. For a considerable time, these equations were relatively well-established, however in 2008 a meeting of experts exposed a systematic bias in the fall-rate equations. A major implication of this is that a depth-temperature profile can be integrated to estimate upper ocean heat content; the bias in these equations lead to a warm bias in the heat content estimations. The introduction of Argo floats has provided a much more reliable source of temperature profiles than XBTs, however the XBT record remains important for estimating decadal trends and variability and hence much effort has been put into resolving these systematic biases.
Read more about this topic: Expendable Bathythermograph
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