A CTD or sound velocimeter that is not in the field is not earning its keep.  Nevertheless, users of oceanographic equipment have been packing up their instruments and shipping them back for annual recalibration for over 50 years.

Every ocean-focussed organization knows the story of kit made unavailable because of recalibration.  Recal almost always takes longer than originally expected.  The diagram at right paints a clear picture of how complicated and cumbersome the recalibration process can be.  End-to-end downtime is often six weeks.

In contrast, field-swappable sensor heads ensure that instruments stay in the field, where they belong.  Instead of removing an instrument from a vessel for recalibration, recently calibrated replacement sensor-heads can be sent directly to the ship.  Because any sensor-head can work on any instrument, sensor-heads are maintained in inventory and can be couriered same- day or next-day.  Upon their arrival, old sensor-heads are unscrewed from the instrument and are replaced by the recently arrived, newly recalibrated sensor-heads.    Recalibration is complete.

At the end-user’s leisure, the old sensor-heads are sent back to the factory for their own recalibration.   Upon arrival at the factory, they can be recalibrated and returned to the field for use on another instrument.  Alternatively, they can be warehoused for future, just-in-time recalibration and delivery.

This change – recalibration travels to the instrument, instead of the instrument travelling for recalibration – is a game changer for all users of oceanographic instrumentation.  For companies operating in countries where import / export regulations are complicated, it is even more attractive.

On a large profiling CTD, transportation costs – shipping, couriers, duties, and brokerage fees – for the return trip to a recalibration centre can easily reach $1,000.  In contrast, field-swappable sensor-heads are small.  Three such sensors – conductivity, temperature, and pressure - can easily fit in the palm of a hand.  Together, they weigh next to nothing and can be delivered via courier with no customs broker.  In summary, they can be delivered for a fraction of the cost. 

At its best, this means that the organization’s total expenditure on instrumentation and its recalibration will drop.  At worst, monies previously spent on transportation & brokerage– pure cost - are invested in spare field swappable sensor-heads, assets that can be used time and time again.   Asset versus expense: this approach makes sense in any economic climate.

Of all the benefits of field-swappable sensor-heads, the most difficult to quantify – and yet perhaps the most important - is the increased flexibility that asset or service managers obtain with this approach. 

It is not at all unusual for survey companies to await contract signature before choosing to recalibrate kit.  Upon contract signature, short field seasons force deployment to begin immediately and thus instruments require expedited recalibration.  On occasion, mistakes are made – by the user, by the recalibration centre, by transportation companies - and the right kit isn’t available at the right time.  In the worst case, sailing dates are delayed.  At best, service managers dedicate extra time and energy to ensuring recalibrated instruments are in the right place at the right time. 

Instead, service managers could choose to order next-day delivery of replacement sensor-heads.  These recalibrated sensor-heads can be installed on any instrument, regardless of intended function: vertical profiler, in-situ data logger, or real-time instrument designed for AUV or ROV integration.  

With field-swappable sensor-heads, a recalibration becomes an asset that can be moved from instrument to instrument, depending upon application requirement and deployment urgency.  This flexibility is worth its weight in gold to service managers under pressure.

Field-swappable sensor heads make multi-range instruments possible.  In older generations of oceanographic instrument, an instrument is sold with one pressure sensor capable of functioning over one chosen range.  For example, a CTD with a 200 dbar pressure sensor is used on deployments from 0 to 200m.  A separate instrument – equipped with a 2000 dbar pressure sensor – is used on a deeper, 2000m deployment.  

Field-swappable pressure sensors change those rules.  One instrument can be used for multiple ranges – 200m or 2000m - by simply unscrewing one sensor-head and replacing it with another.  These changes can be made in the field, on the fly.

This approach is most compelling for field-swappable pressure sensors, where sensors can be damaged or destroyed due to over-ranging.  That said there are also advantages to multi-range instruments in terms of conductivity, temperature, or sound velocity.

The ability to dynamically change range has significant consequences for larger owners of CTD instrumentation, such as rental companies.  Previously, multiple flavours of the same instrument were required – one CTD with a 200m pressure range, and additional units with 2000, 4000, and 6000 meter ranges.  All of these units had to be recalibrated on a regular basis.  Now, those four instruments can perhaps be consolidated to two instruments supported by four field-swappable pressure sensors of varying ranges.

The ability to dynamically change pressure range also has implications in terms of absolute accuracy.  Normally, survey vessels have one profiling instrument equipped with a pressure sensor that covers the maximum range of all potential deployments.   For example, a CTD might occasionally be deployed to 4000m, even if 80% of deployments are to depths of less than 1500m.  In the past, such a profiler would be equipped with a 4000 dbar sensor to match the maximum theoretical deployment depth. 

However, absolute accuracy is determined by percentage of full scale.  In the above scenario, full scale is 4000 dbar, even if the actual deployment is only to 1500 dbar.  Shallow deployments of pressure sensors with a wider range offer less accuracy and resolution than pressure sensors whose range matches the deployment depth.  With field-swappable sensor heads, full scale – and hence accuracy – can be adapted on the fly.