How often do I need to have my instrument and/or auxiliary sensors recalibrated? Can I recalibrate them myself?
- Profiling CTD — recalibrate once/year, but possibly less often if used only occasionally. We recommend that you return the CTD to Sea-Bird for recalibration. (In principle, it is possible for calibration to be performed elsewhere, if the calibration facility has the appropriate equipment andtraining. However, the necessary equipment is quite expensive to buy and maintain.) In between laboratory calibrations, take field salinity samples to document conductivity cell drift.
- Moored CTD — recalibrate at least once/year, but possibly more often depending on the degree of bio-fouling in the water.
- Thermosalinograph — recalibrate at least once/year, but possibly more often depending on the degree of bio-fouling in the water.
- DO sensor —
— SBE 43 — recalibrate once/year, but possibly less often if used only occasionally and stored correctly (see Application Note 64), and also depending on the amount of fouling and your ability to do some simple validations (see Application Note 64-2)
— SBE 63 — recalibrate once/year, but possibly less often if used only occasionally and stored correctly and also depending on the amount of fouling and your ability to do some simple validations (see SBE 63 manual)
- pH sensor —
— SBE 18 pH sensor or SBE 27 pH/ORP sensor — recalibrate at the start of every cruise, and then at least once/month, depending on use and storage
— Satlantic SeaFET pH sensor — recalibrate at least once/year. See SeaFET page for details (How often does the SeaFET need to be calibrated?).
- Transmissometer — usually do not require recalibration for several years. Recalibration at the manufacturer’s factory is the most practical method.
We often have requests from customers to have some way to know if the CTD is out of calibration. The general character of sensor drift in Sea-Bird conductivity, temperature, and pressure measurements is well known and predictable. However, it is very difficult to know precisely how far a CTD calibration has drifted over time unless you have access to a very sophisticated calibration lab. In our experience, an annual calibration schedule will usually maintain the CTD accuracy to within 0.01 psu in Salinity.
Conductivity drifts as a change in slope as a result of accumulated fouling that coats the inside of the conductivity cell, reducing the area of the cell and causing an under-reporting of conductivity. Fouling consists of both biological growth and accumulated oils and inorganic material (sediment). Approximately 95% of fouling occurs as the cell passes through oil and other contaminants floating on the sea surface. Most conductivity fouling is episodic, as opposed to gradual and steady drift. Most fouling events are small and mostly transitory, but they have a cumulative affect over time. A severe fouling event, such as deployment through an oil spill, could have a dramatic but only partially recoverable effect, causing an immediate jump shift toward lower salinity. As fouling becomes more severe, the fit becomes increasingly non-linear and offsets and slopes no longer produce adequate correction, and return to Sea-Bird for factory calibration is required. Frequently checking conductivity drift is likely to be the most productive data assurance measure you can take. Comparing conductivity from profile to profile (as a routine check) will allow you to detect sudden changes that may indicate a fouling event and the need for cleaning and/or re-calibration.
Temperature generally drifts slowly, at a steady rate and predictably as a simple offset at the rate of about 1-2 millidegrees per year. This is approximately equal to 1-2 parts per million in Salinity error (very small).
Pressure sensor drift is also an offset, and annual comparisons to an accurate barometer to determine offset will generally keep the sensor within specification for several years, particularly as the sensors age over time.