Early profiling CTDs relied on natural flushing to sample new water. As the package descended, its downward motion would flush new water through the CTD’s flow path, exposing the sensors to water from new depths. While this is effective, profiling at 2 m/s certainly pushes more water than profiling at 0.5 m/s, and descent rate is anything but consistent. When foul weather causes the descent rate to heave from -1 to 3 m/s, these flushing inconsistencies can cause a visible decrease in data quality.
Today, almost every Sea-Bird Scientific CTD incorporates a pumped flow path with a carefully designed TC duct to avoid this problem. This constant flow rate provides a consistent response for each sensor, vastly improving real-time data and facilitating robust post-processing. After all, unpumped flow causes unpredictable sensor response, and fine-tuning is difficult on a moving target. So, regardless of the CTD’s speed through the water column or the battering it takes from the sea, a pumped flow path ensures smooth, consistent data quality.