User`s manual

Manual revision 016 Section 4: Deploying and Operating FastCAT SBE 49

Downcast Data

If performing a single vertical profile from above the surface, and wanting

to obtain downcast data, optimal orientation is horizontal, with the

temperature sting at the bottom. This provides an upward path to the system

plumbing, allowing air to be quickly expelled during a brief soak period just

below the surface, ensuring proper pump operation for the entire cast. If the

FastCAT is oriented so that air cannot be easily expelled, the top 10 meters of

data is suspect because the pump may not be operating properly until the air

bubbles are collapsed due to water pressure.

Upcast or Downcast Data, Many Profiling Cycles

For deployments where the FastCAT will be seeing many profiling cycles

without breaking the surface, the issue of orientation for optimal pump

performance may not be critical. The FastCAT may be taking many tens or

hundreds of profiles, and only the data for the shallow part of the first profile

would be affected by air in the plumbing. Examples of this type of deployment

are a towing operation or a FastCAT deployed on an ROV or AUV.

Directing Exhaust away from Intake vs.

Equalizing Bernoulli Pressures

It is important to direct the exhaust water away from the intake water. Exhaust

water that mixes with / contaminates the intake water will cause errors in the

temperature data, because the pump transfers heat to the exhaust water.

However, there is another consideration in determining how to direct the

exhaust. For fast moving applications, it is important to equalize the Bernoulli

pressures on the intake and exhaust by placing the intake and exhaust on the

same plane, minimizing acceleration of water in the plumbing. Water

acceleration in the plumbing overrides the constant flow provided by the

pump, resulting in temperature and conductivity data that can be difficult to

align because of changing flow rates.

• For fast moving applications (> 0.5 m/sec):

The potential for errors caused by mixing of intake and exhaust water are

small compared to Bernoulli pressure errors at these speeds. The straight

fitting (as shipped) minimizes Bernoulli pressure errors. In addition, at

these speeds the exhaust flow will be blown back as the package moves

forward, reducing the mixing of intake and exhaust waters.

• For slow moving applications (< 0.5 m/sec):

Bernoulli pressure errors are small compared to errors caused by mixing

of the intake and exhaust water at these speeds. The tee fitting (as

provided in the spares kit that ships with the FastCAT) directs the exhaust

path perpendicular to the FastCAT axis, minimizing mixing of the intake

and exhaust water.

FastCAT with exhaust tee fitting

Exhaust

Intake

FastCAT with straight exhaust fitting

Intake

Exhaust