An in-situ experiment identifying flow effects on temperature measurements using a pumped CTD in weakly stratified waters

A simple experiment shows that the tubing leading to and from the pumped duct of temperature T and conductivity C-sensors of a Sea-Bird Electronics 911plus CTD can cause artificial T-effects as a function of the instrument package vertical velocity. This artifact is due to a pressure difference between inlet and exhaust tubes of the pump-system, even when they are mounted at precisely the same height (pressure level). The vertical velocity dependent pressure difference causes an estimated internal flow speed variation of ±0.5 m s−1 inside the pumped duct that generates artificial temperature variations of ±0.5 mK due to sensor frictional heating. First, this effect is demonstrated in weakly stratified waters by precise but horizontal mounting of the two tubes (at the same height), which leads to similar amplitudes of erroneous T and opposite sign as erroneous T observed using the standard vertical mounting. Secondly, the use of identical surface area tubes, mounted (at the same height) in the vertical downward direction, successfully removes the unwanted pressure gradient and hence the temperature dependence. This second mounting, acting as a hardware filter, can effectively replace a recently proposed software filter.