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AN73: Using Instruments with Pressure Sensors at Elevations Above Sea Level

Document Number: 
73
Publication Date: 
Thursday, May 19, 2016
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appnote73May16.pdf635.19 KB

Background

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 and 25

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

 

SBE 16plus and 16plus V2 (RS-232 versions), 19plus and 19plus V2, 25plus, and 49

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 instrument in air, place it in orientation it will have when deployed.
  2. In Seasave, in .con or .xmlcon file, set pressure offset to 0.0.
  3. Acquire data in Seasave, and display pressure sensor output in decibars.
  4. Calculate offset = (0 – instrument reading).
  5. Enter calculated offset in .con or .xmlcon file.
  6. Also enter calculated offset in instrument (POffset= or SetPOffset= command in terminal program*).
    * Note: SBE 16plus V2, 19plus V2, and 25plus use SeatermV2 terminal program; the other instruments use Seaterm.
    SBE 25plus uses SetPOffset=; the other instruments use POffset=.

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.

 

SBE 16plus (RS-485 versions), and 16plus-IM and 16plus-IM V2

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

 

SBE 37 (all IDO [Integrated Dissolved Oxygen] and ODO [Optical Dissolved Oxygen] models, and all other models with firmware version > 3.0)

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

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

Instrument

Pressure Offset Command

Output Format Command

Command to
Acquire Data **
MicroCATs with Inductive Modem (IM) or RS-485 telemetry
(37-IM, 37-IMP, 37-IMP-IDO, 37-IMP-ODO,
37-SM, 37-SMP, 37-SMP-IDO, 37-SMP-ODO,
37-SI, 37-SIP, 37-SIP-IDO)		 #iiPOffset= #iiOutputFormat=1 #iiTSn:100
(measures and outputs
data 100 times)
MicroCATs with RS-232 telemetry
(37-SM, 37-SMP, 37-SMP-IDO, 37-SMP-ODO,
37-SI, 37-SIP, 37-SIP-IDO)		 POffset= OutputFormat=1 TSn:100
(measures and outputs
data 100 times)

** See MicroCAT manual for location of pressure data in output data string.

 

SBE 37 (all models with firmware version < 3.0, except IDO [Integrated Dissolved Oxygen] and ODO [Optical Dissolved Oxygen] models)

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

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

Instrument

Pressure Offset
Command

Output Format
Command

Command to
Acquire Data
MicroCATs with Inductive Modem (IM) or RS-485 telemetry
(37-IM, 37-IMP, 37-SM, 37-SMP, 37-SI, 37-SIP)		 #iiPOffset= #iiFormat=1 #iiTP (measures and outputs
pressure 30 times)
MicroCATs with RS-232 telemetry
(37-SM, 37-SMP, 37-SI, 37-SIP)		 POffset= Format=1 TP (measures and outputs
pressure 100 times)

 

SBE 39plus

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

Follow this procedure to correct the pressure:

  1. With SBE 39plus in air, place it in orientation it will have when deployed.
  2. In SeatermV2 terminal program, set pressure offset to 0.0 (POffset=0) and pressure sensor output to decibars (SetPressUnits=0).
  3. Acquire data using TP.
  4. Calculate offset = (0 – instrument reading).
  5. Enter calculated offset in SBE 39plus in SeatermV2 (POffset=).

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

 

SBE 39plus-IM

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

Follow this procedure to correct the pressure:

  1. With SBE 39plus-IM in air, place it in orientation it will have when deployed.
  2. In SeatermV2 terminal program, set pressure offset to 0.0 (#iiPOffset=0) and pressure sensor output to decibars (#iiSetPressUnits=0).
  3. Acquire data using #iiTP.
  4. Calculate offset = (0 – instrument reading).
  5. Enter calculated offset in SBE 39plus-IM in SeatermV2 (#iiPOffset=).

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

 

SBE 39-IM

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

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

 

SBE 39

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.
Range
(m or db) +		 Range (psi) =
Range (db) / 0.689476		 0.1 * Range
(psi)		 Atmospheric Pressure (psi) at
elevation at which error occurs =
[14.7 – 0.1 * Range (psi)]		 Approximate
Corresponding
Elevation (m)
20 29 2.9 11.8 psi 1800
100 145 14.5 0.2 psi Nowhere on Earth!
350 507 50.7 ‑ ‑
1000 1450 145 ‑ ‑
2000 2900 290 ‑ ‑
3500 5076 507 ‑ ‑
7000 10152 1015 ‑ ‑
+ Notes:
  • Although decibars and meters are not strictly equal, this approximation is close enough for this Application Note. See Application Note 69 for conversion of pressure (db) to depth (m) for fresh or salt water applications.
  • Equations used in conversions ‑
    As shown above:    pressure (db) = [pressure (psia) – 14.7] * 0.689476;
    Rearranging:          pressure (psia) = [Pressure (db) / 0.689476] + 14.7
    Measuring relative to atmospheric:   pressure (psi; relative to atmospheric pressure) = Pressure (db) / 0.689476

 

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.

 

SBE 50

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

 

SBE 52-MP

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

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

 

SBE 26plus and 53 BPR

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.

 

Application Note Revision History

Date Description
June 2004 Initial release.
July 2005 Add information on SBE 39-IM, 52-MP, 53.
February 2009 - Add information on V2 SeaCATs and newer MicroCATs (version 3 and greater firmware).
- Add information on SeatermV2 software.
October 2010 - Add information on IDO MicroCATs. (37-SMP-IDO, SIP-IDO, IMP-IDO)
- Add information on creation of  .xmlcon file for MicroCATs with data uploaded using SeatermV2 1.1 and later.
- Add information on .xmlcon files for all instruments that use configuration files.
- Update address.
October 2012 - Add information on SBE 25plus and ODO MicroCATs (37-SMP-ODO, IMP-ODO).
- Remove RS-485 version of SBE 16plus V2.
February 2014 - Add information on SBE 39plus.
May 2016 - Add information on SBE 39plus-IM.
- Remove information on SBE 37-SIP-ODO (product never released).

 

Applies To