2
RELATIONSHIP BETWEEN PRESSURE AND DEPTH
Despite the common nomenclature (CTD = Conductivity - Temperature - Depth), all CTDs measure pressure, which is
not quite the same thing as depth. The relationship between pressure and depth is a complex one involving water
density and compressibility as well as the strength of the local gravity field, but it is convenient to think of a decibar as
essentially equivalent to a meter, an approximation which is correct within 3% for almost all combinations of salinity,
temperature, depth, and gravitational constant.
SEASOFT (most instruments)
SEASOFT offers two methods for estimating depth from pressure.
• For oceanic applications, salinity is presumed to be 35 PSU, temperature to be 0
o
C, and the compressibility of the
water (with its accompanying density variation) is taken into account. This is the method recommended in
UNESCO Technical Paper No. 44 and is a logical approach in that by far the greatest part of the deep-ocean water
column approximates these values of salinity and temperature. Since pressure is also proportional to gravity and
the major variability in gravity depends on latitude, the latitude is used to estimate the magnitude of the local
gravity field.
¾ SBE 16plus, 16plus-IM, 16plus V2, 16plus-IM V2, 19plus, 19plus V2, 25, and 49 -
Seasave V7 - User enters latitude on the Miscellaneous tab in the Configure Inputs dialog box; the entry is
used if Depth [salt water] is selected as a display or output variable.
SBE Data Processing - User is prompted to enter latitude if Depth [salt water] is selected as an output variable
in the Data Conversion or Derive module. Latitude can also be changed on the Miscellaneous tab in those
modules.
Note: For both Seasave V7 and SBE Data Processing, if the data includes NMEA data, the software uses the
latitude from the NMEA data instead of the user entry for latitude when calculating depth.
¾ SBE 37-SM, 37-SMP, 37-IM, and 37-IMP - User is prompted to enter latitude if Depth [salt water] is selected
as an output variable in SBE Data Processing’s Derive module. Latitude can also be changed on the
Miscellaneous tab in that module.
¾ SBE 37-SI, 37-SIP, and 50 - Latitude is entered in the instrument’s EEPROM using the Latitude= command
in SEASOFT’s SEATERM (terminal program) software.
¾ SBE 39 and 39-IM – User is prompted to enter latitude if conversion of pressure to depth is requested when
converting an uploaded .asc file to a .cnv file in SEATERM.
• For fresh water applications, compressibility is not significant in the shallow depths encountered and is ignored,
as is the latitude-dependent gravity variation. Fresh water density is presumed to be 1 gm/cm, and depth (in meters)
is calculated as 1.019716 * pressure (in dbars). No latitude entry is required for freshwater applications for the
following:
¾ SBE 16plus, 16plus-IM, 16plus V2, 16plus-IM V2, 19plus, 19plus V2, 25, and 49
¾ SBE 37-SM, 37-SMP, 37-IM, and 37-IMP
¾ SBE 50
Notes:
- The SBE 37-SI and 37-SIP firmware does not differentiate between freshwater and saltwater applications when
calculating depth; the Latitude= entry is always used for the depth calculation.
- The Convert utility in SEATERM for the SBE 39 and 39-IM does not differentiate between freshwater and
saltwater applications when calculating depth; the user is always prompted to enter latitude if conversion of
pressure to depth is requested.
SEASOFT for Waves (SBE 26plus SEAGAUGE Wave and Tide Recorder)
SEASOFT for Waves’ Merge Barometric Pressure module subtracts a user-input barometric pressure file from the tide
data file, and outputs the remainder as pressure in psi or as depth in meters. When converting to depth, the
compressibility of the water is taken into account by prompting for user-input values for average density and gravity.
See the SBE 26plus
manual’s appendix for the formulas for conversion of pressure to depth.
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