Accuracy Isn’t Optional: The Case for Pump Flow in Ocean Research
In oceanographic research, accuracy isn’t just a technical goal – it’s a foundational requirement. Whether studying climate dynamics, monitoring marine ecosystems, or conducting deep-sea exploration, the accuracy of your measurements directly impacts the reliability of your conclusions. One often overlooked factor in achieving high-quality data is how and when measurements are taken, particularly in relation to water flow across sensors.
Why Pump Flow is Critical
Oceanographers rely on accurate measurements of salinity and density to gain a deeper understanding of ocean dynamics. These values are derived from Conductivity-Temperature-Pressure (CTD) data collected by CTD instruments. For accurate salinity and density data, conductivity (C), temperature (T), and pressure (P, often represented as Depth) must be measured in the same parcel of water. If not, errors such as salinity spiking and density inversions may occur.
In the field, CTD instruments struggle to measure the same parcel of water due to two main reasons:
- Sensor physical distance: The physical distance between the temperature probe and the conductivity sensing elements means that they sample slightly different water parcels.
- Sensor response times: While the thermistor has a fixed and known response time, the response time of the conductivity sensing elements depends on the flow rate of the water through it. The flow rate, in turn, depends on the descent/ascent rate of the instrument, which in real conditions is not usually stable due to ship motion.
As a result, many CTDs fail to accurately synchronize C, T, and P measurements. A variable flow rate makes post-processing corrections extremely difficult, if not impossible.
Sea-Bird Scientific CTDs overcome these limitations through a unique controlled flow technology. By actively controlling the flow of water through the sensors, SBE instruments ensure that conductivity and temperature are measured in the same water parcel, regardless of profiling speed or ship motion. This technology enables Sea-Bird Scientific CTDs to deliver the most accurate and reliable field data available that goes beyond lab accuracy. By publishing field specifications, we provide a true measure of how our instruments perform in the harshest environments and most demanding applications.
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How Pump Flow Works
As the CTD descends or ascends, the water enters the system through the duct opening, and its temperature is sensed immediately by the thermistor. The same parcel of water travels through the TC duct and enters the conductivity sensing element. The electronically controlled pump ensures a continuous and steady flow of seawater that matches the response time of both the thermistor and the conductivity sensor.
In the SBE 911plus, the time that it takes for the water parcel to move from the thermistor to the conductivity sensing element is 0.073 seconds. The 0.073-second delay is constant because of the fixed pumping speed and is automatically corrected in real time by the SBE 11plus Deck Unit. In other profiling SBE instruments, such as the SBE 19plus, the delay is not automatically corrected. However, due to the constant and stable flow rate, a post-processing correction can be easily applied using Sea-Bird Scientific software.
In this video, we walk you through the pump flow technology behind the SBE 37-SM, SMP, SMP-ODO MicroCAT, and how it helps eliminate inconsistencies that can compromise your measurements.
Dispelling the Power Myth
There’s a common misconception that pumps are too power-hungry for long-term deployments. Long-term applications refer to either moored instruments deployed at a fixed location to collect data over extended periods or Argo floats, which drift with ocean currents and profile the water column during ascent, typically over a five-year lifespan.
While it is true that pumps consume energy, this does not limit the duration of deployment. In fact, the SBE 41CP-used in over 97% of Argo floats- routinely remains operational in the ocean for five years or more. These floats not only run pumps for accurate conductivity-temperature-depth measurements, but in the case of biogeochemical (BGC) floats, they also power additional sensors that measure fundamental parameters for the global ocean and collectively draw significant energy.
The successful use of pumped CTDs in such long-term missions demonstrates that pumped systems can be both energy efficient and reliable for extended oceanographic research.
The pumped Sea-Bird Scientific CTDs have been central to the unprecedented accuracy in temperature and salinity achieved by the Argo program over the past two decades.
With vs. Without: The Measurable Difference
As mentioned above, the pumped flow ensures that conductivity and temperature measurements are synchronized, resulting in the most accurate salinity data. This is particularly critical in regions with strong thermoclines. The two plots below illustrate this effect. In the first plot, the absence of a pump results in mismatched temperature and conductivity measurements, leading to large salinity spikes that are especially pronounced over the thermocline. In the second plot, the same dataset is shown with the conductivity measurement advanced by just 0.5 seconds. This small adjustment produces a smooth salinity profile, free of spikes, clearly demonstrating the importance of pumped flow for high accuracy salinity data.
To illustrate the impact of pumped flow, consider the following comparisons:
- pH Measurement Lag:
Without pumped flow, pH readings can lag by up to 0.1 pH units due to slow water exchange. With pumped flow, stabilization occurs within seconds. - Conductivity Errors:
In some deployments, conductivity readings without flow control were off by 0.02 S/m due to fouling and inconsistent exposure. Pumped flow reduced this error to near-zero.
Conclusion: Flow Matters
Reliable oceanographic data depends on more than just sensor quality—it depends on the conditions under which those sensors operate. Pumped flow technology addresses a critical variable in the measurement process, offering a practical solution to improve accuracy, reduce uncertainty, and extend deployment longevity.
