This page provides a brief summary of why time-of-flight sound velocity sensors are a better choice than CTDs for determining sound velocity.  The relative pros, cons, and specifications of Applied Microsystems three different time-of-flight sound velocity sensors are also discussed.  Finally, a sound velocity reference library is provided at the bottom of the page.

CTD & SV Comparison

	CTD	SV
Total Accuracy	+/- 0.25 to 0.50 m/s	+/- 0.025 m/s
Number of Sensors	Three	One
Equation Errors	± 0.19 m/s, although equations may differ by as much as ±0.30 m/s	No equation of state is used and hence there is zero equation error
Sensor Errors	Depends on the manufacturer; Total sensor error of ± 0.06 m/s likely	+/- 0.025 m/s
Response Times	Market leading temperature sensors have response times of 100 milliseconds	47 microseconds
Sound Velocity Spiking	Significant, given multiple sensors with varying response times	Zero
Acclimatization	20 minutes	5 seconds (SV Xchange sensor)

Sound speed - also called sound velocity - can be measured directly using a sound velocimeter or it can be calculated from salinity temperature and depth measurements, assuming standard ocean salt ratios.

The direct measuring devices are known as sound velocimeters. These instruments measure the actual sound speed of the water at the location of the instrument.  Sound velocimeters measure sound velocity using a time-of-flight methodology.  A single acoustic pulse is transmitted into the water.  The pulse travels a fixed, calibrated distance to a reflector plate and then returns through the water to the transducer. The fixed distance is achieved using rods which are thermally stable and have zero thermal response time. A high resolution, high stability timing circuit is used to measure the acoustic travel time.  With travel distance and elapsed time known, sound velocity is determined.  Applied Microsystems sound velocity sensors offer accuracies of up to ±0.025 m/s.

Calculation of sound velocity based on CTD results is a second, less accurate approach to sound velocity.  The CTD uses three separate sensors to measure the conductivity, temperature and pressure at the location of the instrument. The conductivity, temperature and pressure are used to compute salinity using an equation of state. Then the salinity, temperature and pressure are used to calculate the sound speed of the water, using another equation of state.  Thus CTD based sound speed data is a combination of three sensor measurements and two equation of state calculations.  This pyramid like approach - calculations based on calculations based on measurements - has a dramatic impact on overall accuracy, as seen in the table to the right.

When considering whether to measure sound velocity with time-of-flight sound velocity sensors or to calculate using conductivity, temperature, depth and salinity, two other factors should be taken into account: 1) the possibility of sound velocity spiking; and 2) acclimizatization times.

For a more detailed discussion of the comparative advantages of time-of-flight sound velocity, please read our white paper on Sound Velocity: Calculated or Measured?

Applied Microsystems offers three choices of time-of-flight sound velocity sensor: Invar, Composite, and Xchange.  We also manufacture CTDs, although the CTDs have a sound velocity accuracy of anywhere between 10 and 20 times poorer as compared to sound velocimeters.

Sound Velocity Sensor Types

	Release	Pros	Cons	Photo
SV Xchange Sensor SV • Xchange™ is the latest innovation from AML.  SV • Xchange™ is the industry’s only field swappable time-of-flight sound velocity sensor. The sensors store all calibration data within the sensor, allowing any sensor to be used with any Xchange enabled instrument.  In addition to being field-swappable, SV•Xchange™ is more accurate and offers better results in high turbidity environments.	2008	Field swappable Highest accuracy Best performance in turbid environments
Composite Sensor The Composite sensor was introduced in 2001.  This 2nd generation sensor incorporated radical advances that permitted a dramatic shrinking of the length of the sensor.  Hi-tech composite sensor materials eliminate the need for screws and bolts.  With less susceptibility to vibration and corrosion – and a shorter path-length - the composite SV provides improved durability and resistance to G-force shock or impact.  Other advances include dramatic improvements in the temperature response times, resulting in greater profiling accuracy.	2001	No zincs required Smaller overall size Greater shock resistance Better thermal stability	Sensor and boards must be returned for recal
Invar Sensor The Invar sensor was the first ever direct measured Time-of-Flight (ToF) field sensor.  Developed by Applied Microsystems and released to the market in 1996, the Invar sensor provided significant new advantages over the outdated sing-around sensor technique.  The Invar sensor improved field accuracies to 0.05m/s, compared to the best case of 0.25 m/s of sing-around or CTD sound velocity calculations.	1996	Proven technology	Rods require zinc anodes Longer path length Sensor and boards must be returned for recal

The following table summarizes the accuracy, precision, resolution, and response times of Applied Microsystems' three time-of-flight sound velocity sensors.  Keep in mind that CTDs normally calculate sound velocity to an accuracy of between ¼and ½  metre per second.

Specifications by Sensor Type

	Accuracy (m/s)	Precision (m/s)	Resolution (m/s)	Response Time
SV Xchange	0.025	0.006	0.001	47 microseconds
Composite	0.05	0.03	0.015	145 microseconds
Invar	0.05	0.03	0.015

Many vendors of oceanographic instrumentation refer to accuracy and precision interchangeably.  They are not interchangeable.  In effect, accuracy refers to how well a sensor performs against a known third party standard.  For example, a temperature sensor may be +/- 0.001 C, as compared to a Black Stack themistor module.  Precision refers to the repeatability of the readings of a given sensor.   A sensor is precise when it repeatedly provides the same reading, regardless of how accurate that reading is.

A good analogy is a dart board.  The thrower of darts is accurate when he or she is able to reach the target, the bulls-eye.  He or she is precise if, having thrown three darts, all three land in the same location, irrespective of whether or not that location is the bulls-eye.

For a detailed explanation of how we calibrated Applied Microsystems' time-of-flight sound velocity sensors - including a listing of Frequently Asked Questions - please click here.

Field swappable sound velocity sensors offers significant advantages over their non-swappable competitors.  Key benefits include the following:

• Increased Instrument Field Time: At time of recalibration, spare calibrated sensors can be shipped to the instrument, allowing the instrument to stay in the field.

• Lower Cost of Ownership: Tiny sensors – not heavy instruments – travel to and from the factory, generating dramatic savings in shipping costs and brokerage fees.

• Greater Convenience: Recalibrated sensors can be scheduled for just-in-time delivery, eliminating the headache of managing calibration expiry dates.

• Increased Flexibility: Damaged or out-of-calibration sensors can be swapped for field-ready spares on board the survey vessel.

In 1999 AML developed an equation for computing salinity from sound speed, temperature and pressure data.  Once salinity has been calculated the standard EOS80 equation can be used to compute density.  Both equations are embedded into the SV Plus Xchange and the Micros SVTP Xchange instruments.  In older instruments the equations are implemented in Applied Microsystems SmartTalk sofware. The accuracy of the Applied Microsystems salinity equation is 0.035 ppt (rms) with respect to Chen and Millero's sound speed equation.

Applied Microsystems has a long history of innovating in the field of time-of-flight sound velocity sensors.    In 1996, we developed the first ever time-of-flight sensor for the hydrographic market.  In 2001, we launched the first composite sensor.  And in 2008, we came out with the industry's first field-swappable sound velocity sensor.

Put simply, sound velocity is our business.  We don't make tide and wave gauges, or single beam echo sounders, or wave recorders, or current meters.  Our business is sound velocity.  Our focus equals your success.