Specifications

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APPLICATION NOTE 27Druck Revised July 2005
Minimizing Strain Gauge Pressure Sensor Errors
The following Sea-Bird instruments use strain gauge pressure sensors manufactured by GE Druck:
SBE 16plus and 16plus-IM SEACAT (not 16*) with optional strain gauge pressure sensor
SBE 19plus SEACAT Profiler (not 19*)
SBE 25 SEALOGGER CTD, which uses SBE 29 Strain-Gauge Pressure Sensor (built after March 2001)
SBE 26plus SEAGAUGE Wave and Tide Recorder with optional strain gauge pressure sensor in place of
Quartz pressure sensor
SBE 37 MicroCAT (-IM, -IMP, -SM, -SMP, -SI, and -SIP) with optional pressure sensor (built after September
2000)
SBE 39 Temperature Recorder with optional pressure sensor (built after September 2000) and 39-IM Temperature
Recorder with optional pressure sensor
SBE 49 FastCAT CTD Sensor
SBE 50 Digital Oceanographic Pressure Sensor
SBE 52-MP Moored Profiler CTD and DO Sensor
* Note: SBE 16 and SBE 19 SEACATs were originally supplied with other types of pressure sensors. However, a few
of these instruments have been retrofitted with Druck sensors.
The Druck sensors are designed to respond to pressure in nominal ranges 0 - 20 meters, 0 - 100 meters, 0 - 350 meters,
0  600 meters, 0  1000 meters, 0  2000 meters, 0  3500 meters, and 0  7000 meters (with pressures expressed in
meters of deployment depth capability). The sensors offer an initial accuracy of 0.1% of full scale range.
DEFINITION OF PRESSURE TERMS
The term psia means pounds per square inch, absolute (absolute means that the indicated pressure is referenced to
a vacuum).
For oceanographic purposes, pressure is most often expressed in decibars (1 dbar = 1.4503774 psi). A dbar is 0.1 bar; a
bar is approximately equal to a standard atmosphere (1 atmosphere = 1.01325 bar). For historical reasons, pressure at
the water surface (rather than absolute or total pressure) is treated as the reference pressure (0 dbar); this is the value
required by the UNESCO formulas for computation of salinity, density, and other derived variables.
Some oceanographers express pressure in Newtons/meter
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or Pascals (the accepted SI unit). A Pascal is a very small
unit (1 psi = 6894.757 Pascals), so the mega-Pascal (MPa = 10
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Pascals) is frequently substituted (1 MPa = 100 dbar).
Since the pressure sensors used in Sea-Bird instruments are absolute types, their raw data inherently indicate
atmospheric pressure (about 14.7 psi) when in air at sea level. Sea-Bird outputs pressure in one of the following ways:
CTDs that output raw data (SBE 16plus, 16plus-IM, 19plus, 25, and 49) and are supported by SEASOFTs
SEASAVE (real-time data acquisition) and SBE Data Processing (data processing) software  In SEASOFT, user
selects pressure output in psi (not psia) or dbar. SEASOFT subtracts 14.7 psi from the raw absolute reading and
outputs the remainder as psi or converts the remainder to dbar.
SBE 26plus  Real-time wave and tide data is output in psia. Wave and tide data stored in memory is processed
using SEASOFT for Waves Convert Hex module, and output in psia. Tide data can be converted to psi by
subtracting a barometric pressure file using SEASOFT for Waves Merge Barometric Pressure module.
SBE 50  User selects pressure output in psia (including atmospheric pressure) or dbar. Calculation of dbar is as
described above.
All other instruments that can output converted data in engineering units (SBE 16plus, 16plus-IM, 19plus, 37,
39, 39-IM, 49, and 52-MP)  Instrument subtracts 14.7 psi from the raw absolute reading and converts the
remainder to dbar.
Note: SBE 16plus, 16plus-IM, 19plus, 49, and 52-MP can output raw or converted data.
Sea-Bird Electronics, Inc.
1808 136th Place NE
Bellevue, WA 98005
US
A
Phone: [phone]
Fax: [phone]
E-mail: [e-mail]
Web: www.seabird.com
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