Specifications

1
APPLICATION NOTE NO. 73 Revised July 2005
Using Instruments with Pressure Sensors at Elevations Above Sea Level
This application note covers use of a Sea-Bird instrument that includes a pressure sensor at elevations above sea level,
such as in a mountain lake or stream.
Background
Sea-Bird pressure sensors are absolute sensors, so their raw output includes the effect of atmospheric pressure. As
shown on the Calibration Sheet that accompanies the instrument, our calibration (and resulting calibration coefficients)
is in terms of psia. However, when outputting pressure in engineering units, most of our instruments output pressure
relative to the ocean surface (i.e., at the surface the output pressure is 0 decibars). Sea-Bird uses the following equation
in our instruments and/or software to convert psia to decibars:
Pressure (db) = [pressure (psia)  14.7] * 0.689476
where 14.7 psia is the assumed atmospheric pressure (based on atmospheric pressure at sea level).
This conversion is based on the assumption that the instrument is being used in the ocean; the surface of the ocean
water is by definition at sea level. However, if the instrument is used in a mountain lake or stream, the assumption of
sea level atmospheric pressure (14.7 psia) in the instrument and/or software can lead to incorrect results. Procedures are
provided below for measuring the pressure offset from the assumed sea level atmospheric pressure, and entering the
offset in the instrument and/or software to make the appropriate correction.
Perform the correction procedure at the elevation at which the instrument will be deployed. Allow the
instrument to equilibrate in a reasonably constant temperature environment for at least 5 hours before starting.
Pressure sensors exhibit a transient change in their output in response to changes in their environmental
temperature. Sea-Bird instruments are constructed to minimize this by thermally decoupling the sensor from the
body of the instrument. However, there is still some residual effect; allowing the instrument to equilibrate before
starting will provide the most accurate calibration correction.
Inclusion of calibration coefficients in the instrument itself or in a file used by our software to interpret raw data varies,
depending on the instrument. Commands used to program the instrument vary as well. Therefore, there are variations in
the correction procedure, depending on the instrument. These instruments are addressed below:
SBE 9plus CTD and SBE 25 SEALOGGER CTD
SBE 16plus (RS-232 version) SEACAT C-T (pressure optional) Recorder, SBE 19plus SEACAT Profiler CTD,
and SBE 49 FastCAT CTD Sensor
SBE 16plus (RS-485 version) SEACAT C-T (pressure optional) Recorder and
SBE 16plus-IM SEACAT C-T (pressure optional) Recorder
SBE 37 MicroCAT (all models  IM, IMP, SI, SIP, SM, SMP)
SBE 50 Digital Oceanographic Pressure Sensor
SBE 52-MP Moored Profiler CTD and DO Sensor
SBE 39-IM Temperature (pressure optional) Recorder
SBE 39 Temperature (pressure optional) Recorder
SBE 26plus SEAGAUGE Wave and Tide Recorder and SBE 53 BPR Bottom Pressure Recorder
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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