Specifications
3
CHOOSING THE RIGHT SENSOR
Initial accuracy and resolution are expressed as a percentage of the full scale range for the pressure sensor. The
initial accuracy is 0.1% of the full scale range. Resolution is 0.002% of full scale range, except for the SBE 25
(0.015% resolution). For best accuracy and resolution, select a pressure sensor full scale range to correspond to no more
than the greatest depths to be encountered. The effect of this choice on CTD accuracy and resolution is shown below:
Range
(meters)
Maximum Initial Error
(meters)
SBE 16plus, 16plus-IM, 19plus, 37, 39, 39-IM,
49, 50, and 52-MP -
Resolution (meters)
SBE 25 -
Resolution (meters)
0 20 0.02 0.0004 0.003
0 100 0.10 0.002 0.015
0 350 0.35 0.007 0.052
0 600 0.60 0.012 0.090
0 1000 1.0 0.02 0.15
0 - 2000 2.0 0.04 0.30
0 - 3500 3.5 0.07 0.52
0 - 7000 7.0 0.14 1.05
Note: See the SBE 26plus manual or data sheet for its resolution specification; 26plus resolution is a function of
integration time as well as pressure sensor range.
The meaning of accuracy, as it applies to these sensors, is that the indicated pressure will conform to true pressure to
within ± maximum error (expressed as equivalent depth) throughout the sensors operating range. Note that a
7000-meter sensor reading + 7 meters at the water surface is operating within its specifications; the same sensor would
be expected to indicate 7000 meters ± 7 meters when at full depth.
Resolution is the magnitude of indicated increments of depth. For example, a 7000-meter sensor on an SBE 25
(resolution 1.05 meters) subjected to slowly increasing pressure will produce readings approximately following the
sequence 0, 1.00, 2.00, 3.00 (meters). Resolution is limited by the design configuration of the CTDs A/D converter.
For the SBE 25, this restricts the possible number of discrete pressure values for a given sample to somewhat less than
8192 (13 bits); an approximation of the ratio 1 : 7000 is the source of the SBE 25s 0.015% resolution specification.
Note: SEASOFT (and other CTD software) presents temperature, salinity, and other variables as a function of depth or
pressure, so the CTDs pressure resolution limits the number of plotted data points in the profile. For example, an
SBE 25 with a 7000-meter sensor might acquire several values of temperature and salinity during the time required to
descend from 1- to 2-meters depth. However, all the temperature and salinity values will be graphed in clusters
appearing at either 1 or 2 meters on the depth axis.
High-range sensors used in shallow water generally provide better accuracy than their absolute specifications indicate.
With careful use, they may exhibit accuracy approaching their resolution limits. For example, a 3500-meter sensor has
a nominal accuracy (irrespective of actual operating depth) of ± 3.5 meters. Most of the error, however, derives from
variation over time and temperature of the sensors offset, while little error occurs as a result of changing sensitivity.