APPLICATION NOTE NO. 73
Using Instruments with Pressure Sensors at Elevations Above Sea Level
Revised October 2010

Print version

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.

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 and 16plus V2 (RS-232 versions) SEACAT C-T (pressure optional) Recorder, SBE 19plus and 19plus V2 SEACAT Profiler CTD, and SBE 49 FastCAT CTD Sensor
• SBE 16plus and 16plus V2 (RS-485 versions) SEACAT C-T (pressure optional) Recorder, and SBE 16plus-IM and 16plus-IM V2 SEACAT C-T (pressure optional) Recorder
• SBE 37 MicroCAT (all IDO models and all other models with firmware version 3.0 and higher)
• SBE 37 MicroCAT (all models with firmware version less than 3.0, except IDO models)
• SBE 50 Digital Oceanographic Pressure Sensor
• SBE 52-MP Moored Profiler CTD and Optional 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 real-time data acquisition software (Seasave) and post-processing software (SBE Data Processing) use calibration coefficients programmed in a configuration (.con or .xmlcon) file to convert raw data from these instruments to engineering units.

Follow this procedure to correct the pressure:

1. With the instrument in the air, place it in the orientation it will have when deployed.
2. In Seasave, in the .con or .xmlcon file, set the pressure offset to 0.0.
3. Acquire data in Seasave, and display the pressure sensor output in decibars.
4. Calculate offset = (0 – instrument reading).
5. Enter the calculated offset in the .con or .xmlcon file.

Offset Correction Example:
Pressure displayed at elevation is -1.655 db.
Offset = 0 – (-1.655) = + 1.655 db
Enter offset in .con or .xmlcon file.

 

Sea-Bird real-time data acquisition software (Seasave) and post-processing software (SBE Data Processing) use calibration coefficients programmed in a configuration (.con or .xmlcon) file to convert raw data from these instruments to engineering units. These instruments are also able to directly output data that is already converted to engineering units (pressure in decibars), using calibration coefficients that are programmed into the instrument.

Follow this procedure to correct the pressure:

1. With the instrument in the air, place it in the orientation it will have when deployed.
2. In Seasave, in the .con or .xmlcon file, set the pressure offset to 0.0.
3. Acquire data in Seasave, and display the pressure sensor output in decibars.
4. Calculate offset = (0 – instrument reading).
5. Enter the calculated offset in the .con or .xmlcon file.
6. Also enter the calculated offset in the instrument (using the POffset= command in the terminal program*).
   * Note: SBE 16plus V2 and 19plus V2 use SeatermV2 terminal program; the other instruments use Seaterm.

Offset Correction Example:
Pressure displayed at elevation is -1.655 db.
Offset = 0 – (-1.655) = + 1.655 db
Enter offset in .con or .xmlcon file and in instrument.

 

Sea-Bird real-time data acquisition software (Seasave) and post-processing software (SBE Data Processing) use calibration coefficients programmed in a configuration (.con or .xmlcon) file to convert raw data from these instruments to engineering units. These instruments are also able to directly output data that is already converted to engineering units (pressure in decibars), using calibration coefficients that are programmed into the instrument.

Follow this procedure to correct the pressure:

1. With the instrument in the air, place it in the orientation it will have when deployed.
2. In the terminal program*, set the pressure offset to 0.0 (#iiPOffset=0) and set the output format to converted data in decimal form (#iiOutputFormat=3).
   * Note: SBE 16plus V2 and 16plus-IM V2 use SeatermV2 terminal program; the other instruments use Seaterm.
3. Acquire data using the #iiTP command.
4. Calculate offset = (0 – instrument reading).
5. Enter the calculated offset in the instrument (using the #iiPOffset= command).
6. Also enter the calculated offset in the .con or .xmlcon file, using SBE Data Processing.

Offset Correction Example:
Pressure displayed at elevation is -1.655 db.
Offset = 0 – (-1.655) = + 1.655 db
Enter offset in .con or .xmlcon file and in instrument.

The SBE 37 is able to directly output data that is already converted to engineering units (pressure in decibars), using calibration coefficients that are programmed into the instrument. If using SeatermV2 (version 1.1 and later) to upload data, SeatermV2 creates a configuration (.xmlcon) file along with a .hex data file. Sea-Bird post-processing software (SBE Data Processing) uses the calibration coefficients in the .xmlcon file to convert raw data to engineering units.

Follow this procedure to correct the pressure:

1. With the SBE 37 in the air, place it in the orientation it will have when deployed.
2. In the SeatermV2 terminal program, set the pressure offset to 0.0 and pressure sensor output to decibars. *
3. Acquire data. *
4. Calculate offset = (0 – instrument reading).
5. Enter the calculated offset in the SBE 37 in SeatermV2. *
6. If you have already uploaded data, also enter the calculated offset in the .xmlcon file, using SBE Data Processing.

Offset Correction Example:
Pressure displayed at elevation is -1.655 db.
Offset = 0 – (-1.655) = + 1.655 db
Enter offset in the SBE 37.

* NOTE: Commands for setting pressure offset, setting output format, and acquiring data vary:

The SBE 37 is able to directly output data that is already converted to engineering units (pressure in decibars), using calibration coefficients that are programmed into the instrument. These SBE 37s do not use a configuration (.con or .xmlcon) file.

Follow this procedure to correct the pressure:

1. With the SBE 37 in the air, place it in the orientation it will have when deployed.
2. In the Seaterm terminal program, set the pressure offset to 0.0 and pressure sensor output to decibars. *
3. Acquire data. *
4. Calculate offset = (0 – instrument reading).
5. Enter the calculated offset in the SBE 37 in Seaterm. *

Offset Correction Example:
Pressure displayed at elevation is -1.655 db.
Offset = 0 – (-1.655) = + 1.655 db
Enter offset in the SBE 37.

* NOTE: Commands for setting pressure offset, setting output format, and acquiring data vary:

The SBE 50 is able to directly output data that is already converted to engineering units (psia, decibars, or depth in feet or meters), using calibration coefficients that are programmed into the instrument. The SBE 50 does not use a configuration (.con or .xmlcon) file.

Follow this procedure to correct the pressure:

1. With the SBE 50 in the air, place it in the orientation it will have when deployed.
2. In the Seaterm terminal program, set the pressure offset to 0.0 (POffset=0) and set the output format to the desired format (OutputFormat).
3. Acquire data using the TS command a number of times.
4. Calculate offset = (0 – instrument reading).
5. Enter the calculated offset in the SBE 50 (use POffset= in Seaterm). The offset must be entered in units consistent with OutputFormat=. For example, if the output format is decibars (OutputFormat=2), enter the offset in decibars.

Offset Correction Example:
Pressure displayed at elevation with OutputFormat=2 (db) is -1.655 db.
Offset = 0 – (-1.655) = + 1.655 db
Enter offset in the SBE 50.

 

The SBE 52-MP is able to directly output data that is already converted to engineering units (decibars), using calibration coefficients that are programmed into the instrument. The SBE 52-MP does not use a configuration (.con or .xmlcon) file.

Follow this procedure to correct the pressure:

1. With the SBE 52-MP in the air, place it in the orientation it will have when deployed.
2. In the Seaterm terminal program, set the pressure offset to 0.0 (POffset=0).
3. Acquire data using the TP command.
4. Calculate offset = (0 – instrument reading).
5. Enter the calculated offset in the SBE 52-MP (use POffset= in Seaterm).

Offset Correction Example:
Pressure displayed at elevation is -1.655 db.
Offset = 0 – (-1.655) = + 1.655 db
Enter offset in the SBE 52-MP.

 

The SBE 39-IM directly outputs data that is already converted to engineering units (pressure in decibars), using calibration coefficients that are programmed into the SBE 39-IM. The SBE 39-IM does not use a configuration (.con or .xmlcon) file.

Follow this procedure to correct the pressure:

1. With the SBE 39-IM in the air, place it in the orientation it will have when deployed.
2. In the Seaterm terminal program, set the pressure offset to 0.0 (#iiPOffset=0).
3. Acquire data using the #iiTP command.
4. Calculate offset = (0 – instrument reading).
5. Enter the calculated offset in the SBE 39-IM (use #iiPOffset= in Seaterm).

Offset Correction Example:
Pressure displayed at elevation is -1.655 db.
Offset = 0 – (-1.655) = + 1.655 db
Enter offset in the SBE 39-IM.

 

The SBE 39 directly outputs data that is already converted to engineering units (pressure in decibars), using calibration coefficients that are programmed into the SBE 39. The SBE 39 does not use a configuration (.con or .xmlcon) file. The SBE 39 is a special case, because its programmed calibration coefficients do not currently include a pressure offset term. The lack of a pressure offset term creates two difficulties when deploying at elevations above sea level:

• After the data is recorded and uploaded, you must perform post-processing to adjust for the pressure offset.
  Sea-Bird software cannot currently perform this adjustment for the SBE 39.
• Without adjusting the instrument range, internal calculation limitations prevent the SBE 39 from providing accurate data at high elevations. Specifically, if (0.1 * sensor range) < (decrease in atmospheric pressure from sea level to elevation),
  an error condition in the SBE 39’s internal calculations occurs. The table below tabulates the atmospheric pressure and approximate elevation at which this calculation limitation occurs for different pressure sensor ranges.

From the table, it is apparent that the only practical limitation occurs with a 20 meter pressure sensor. To use the SBE 39 in this situation, change the sensor range internally to 100 meters by entering PRange=100 in the SBE 39 (using Seaterm). This changes the electronics' operating range, allowing you to record pressure data at high elevations, but slightly decreases resolution. After the data is recorded and uploaded, perform post-processing to adjust for the pressure offset. Note that Sea-Bird software cannot currently perform this adjustment for the SBE 39.

CAUTION: Changing PRange in the SBE 39 does not increase the actual maximum water depth at which the instrument can be used (20 meters) without damaging the sensor.

Example 1: You want to deploy the SBE 39 with a 20 m pressure sensor in a mountain lake at 1400 meters (4590 feet). This is lower than 1800 meters shown in the table, so you do not need to adjust the sensor range. After the data is recorded and uploaded, perform post-processing to adjust for the pressure offset.

Example 2: You want to deploy the SBE 39 with a 20 m pressure sensor in a mountain lake at 2000 meters (6560 feet). This is higher than 1800 meters shown in the table, so you need to adjust the sensor range. In Seaterm, set PRange=100 to allow use of the SBE 39 at this elevation. After the data is recorded and uploaded, perform post-processing to adjust for the pressure offset.

 

Unlike our other instruments that include a pressure sensor, the SBE 26plus and 53 output absolute pressure (i.e., at the surface the output pressure is atmospheric pressure at the deployment elevation). Therefore, no corrections are required when using these instruments above sea level. SBE 26plus / 53 software (SEASOFT for Waves) includes a module that can subtract measured barometric pressures from tide data, and convert the resulting pressures to water depths.

Last modified: 23 Feb 2011

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