APPLICATION NOTE NO. 73
Using Instruments with Pressure Sensors at
Elevations Above Sea Level
July 2005
Application note in pdf format
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.
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:
Sea-Bird software (SEASAVE or SBE Data Processing) uses calibration coefficients programmed in a configuration (.con) file to convert raw data from these instruments to engineering units.
Follow this procedure to correct the pressure:
Offset Correction Example:
Pressure displayed at elevation is -1.655 db.
Offset = 0 – (-1.655) = + 1.655 db
Enter offset in .con file.
Sea-Bird software (SEASAVE or SBE Data Processing) uses calibration coefficients programmed in a configuration (.con) 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:
Offset Correction Example:
Pressure displayed at elevation is -1.655 db.
Offset = 0 – (-1.655) = + 1.655 db
Enter offset in .con file and in instrument.
Sea-Bird software (SEASAVE or SBE Data Processing) uses calibration coefficients programmed in a configuration (.con) 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:
Offset Correction Example:
Pressure displayed at elevation is -1.655 db.
Offset = 0 – (-1.655) = + 1.655 db
Enter offset in .con 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. The SBE 37 does not use a .con file.
Follow this procedure to correct the pressure:
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:
Instrument |
Pressure Offset Command |
Output Format Command |
Command to Acquire Data |
SBE
37-IM and 37-IMP,
and RS-485 version of SBE 37-SM, 37-SMP, 37-SI, and 37-SIP |
#iiPOffset= | #iiFormat=1 or #iiFormat=2 | #iiTP (measures and outputs pressure 30 times) |
RS-232 version of SBE 37-SM, 37-SMP, 37-SI, and 37-SIP |
POffset= | Format=1 or Format=2 | TP (measures and outputs pressure 100 times) |
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 .con file.
Follow this procedure to correct the pressure:
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 .con file.
Follow this procedure to correct the pressure:
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 .con file.
Follow this procedure to correct the pressure:
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 .con 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:
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 | 1570 |
100 | 145 | 14.5 | 0.2 psi | 7885 |
350 | 507 | 50.7 | ‑ | ‑ |
1000 | 1450 | 145 | ‑ | ‑ |
2000 | 2900 | 290 | ‑ | ‑ |
3500 | 5076 | 507 | ‑ | ‑ |
7000 | 10152 | 1015 | ‑ | ‑ |
+ Notes:
|
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 1570 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 1570 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: 06 Apr 2007
Sea-Bird Home Phone: 425-643-9866 Fax: 425-643-9954 E-mail: seabird@seabird.com