Why is Calibration Important?
Why is Calibration Important?
Imagine it’s time to check your blood pressure. You used a machine at the grocery store, and the results were alarming enough that you made an appointment with your doctor. The machine in your doctor’s office also indicated an abnormal blood pressure reading, but this measurement is affected by stress levels, which can be triggered for many of us by a doctor’s visit. So, you purchase a machine to check your blood pressure at home. Unfortunately, despite following the instructions, this machine gives you three very different readings during consecutive measurements. At the end of all of this, you have a lot of data but are no closer to understanding what your blood pressure is and what, if anything, you should do about it.
Ultimately, a sensor’s data is only as good as its calibration. If we think about a CTD as being the tool we use to measure the most vital statistics of the ocean (temperature, conductivity, and depth), then how that tool is calibrated is of utmost importance for collecting high-quality, defensible data that is good enough for decision-making.
Sea-Bird Scientific’s world-class calibration facilities make our sensors exceptionally accurate, and our robust engineering keeps them that way as you use them in the field.
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Calibrating for Accuracy
Your sensor is only as good as its calibration, and your calibration is only as good as the standard to which it is referencing. Where possible, we rely on NIST-traceable laboratory standards to achieve greater accuracy. For example, our primary temperature standards are the Triple Point of Water and Gallium Melt Point, which have been evaluated directly at accredited metrological laboratories. For conductivity, we rely on IAPSO Standard Seawater and a laboratory salinometer to ensure a high level of conductivity accuracy. While not a NIST-traceable standard, the International Association for the Physical Sciences of the Oceans recommends industry standards (such as this seawater) to achieve the best possible results from the physics, chemistry, mathematics, and biogeochemistry that oceanography routinely employs. Finally, for pressure, we rely on a quartz pressure sensor from a manufacturer using a NIST Traceable ISO 9001 Quality System and a hydraulic or deadweight pressure controller. These methods ensure the reliability of the accuracy specification found on your Sea-Bird Scientific datasheet.
Engineering for Precision and Stability
It’s great to have a sensor that can measure very accurately once, but in order to have the highest quality, defensible data that can be used in decision-making, the sensor must also be precise and stable over time. Accuracy refers to how close a given measurement is to the true value, while precision is about ensuring groups of measurements are consistent and reproducible. This is where sensor technology and design come into play, not only of the sensors themselves but also the design of our calibration equipment and processes. Sea-Bird Scientific calibration design coupled with our standards enables calibration bath repeatability of 0.0005 deg C, ensuring we can meet accuracy specifications of 0.001-0.002 deg C depending on the product.
However, a controlled laboratory setting is different from the rigors of field deployment. Most sensor electronics drift over time, meaning that the measurement they take today can be different than the one they took last year, and the one they will take a year from now. Certain environmental conditions can accelerate that drift. Oceanographic sensors, therefore, require a robust design with sources of drift that are minimized and well characterized. Sea-Bird Scientific has designed manufacturing processes to both minimize the amount of electronic drift that a sensor has in the first place and then to characterize the remainder extremely well. This means that the drift specification on Sea-Bird Scientific data sheets is what you can expect a well-functioning sensor to exhibit in the field. This electronic drift is knowable AND linear, meaning it can be used to apply corrections to generate the highest quality oceanographic data found anywhere in the world.
When the stakes are high and the decision-making is critical, Sea-Bird Scientific’s attention to the calibration of each and every CTD is what enables oceanographers to collect reliable, accurate data with confidence.
