The dynamic changes in seafloor volcanoes can be observed with seafloor geodetic methods. Seafloor pressure measurements serve as a proxy for detecting vertical deformation (Bürgmann & Chadwell, 2014). Axial Seamount is an active submarine volcano and is the focus of many earth, ocean and life sciences investigations. Fox (1993) initiated the first pressure observation; Chadwick et al. (2006) and Nooner and Chadwick (2009) performed repeated geodetic pressure surveys. Chadwick et al. (2012) hypothesized that the magmatic system was repeatable and predicted a time range for the next eruption; their forecast was validated in 2015.Seafloor geodetic pressure measurements are adversely affected by sensor drift (Polster et al., 2009). The drift cannot reliably be removed with on-shore calibration, averaging multiple instruments or extrapolating from past performance. One approach to estimate and remove the drift is to collect reference pressure measurements at stable seafloor site(s) during an epoch survey, using portable sensors carried by remotely operated vehicles (ROV); Chadwick et al., 2006, andStenvold et al., 2006 discuss pressure surveys in detail. This method is analogous to terrestrial optical leveling or relative gravimeter surveys. Geodetic pressure surveys are effective but require considerable ship time and ROV assets, which limit the temporal sampling frequency.Our drift mitigation approach uses in-situ calibration. First proposed by Munk et al. (1990), a deadweight tester (DWT) is used to apply a known and in principle, absolute reference pressure to a pressure sensor. The DWT is conceptually quite simple; a cylindrical piston of known area A and mass M is inserted into a closely fitting