Specifications
4
MINIMIZING ERRORS
Offset Errors
Note: Follow the procedures below for all instruments except the SBE 26plus (see the 26plus manual for details).
The primary offset error due to drift over time can be eliminated by comparing CTD readings in air before beginning
the profile to readings from a barometer. Follow this procedure:
1. Allow the instrument to equilibrate in a reasonably constant temperature environment for at least 5 hours. Pressure
sensors exhibit a transient change in their output in response to changes in their environmental temperature;
allowing the instrument to equilibrate before starting will provide the most accurate calibration correction.
2. Place the instrument in the orientation it will have when deployed.
3. Set the pressure offset to 0.0:
In the .con file, using SEASAVE or SBE Data Processing (for SBE 16plus, 16plus-IM, 19plus, 25, or 49).
In the CTDs EEPROM, using the appropriate command in SEATERM (for SBE 16plus, 16plus-IM, 19plus,
37, 39, 39-IM, 49, 50, or 52-MP).
4. Collect pressure data from the instrument using SEASAVE or SEATERM (see instrument manual for details). If
the instrument is not outputting data in decibars, convert the output to decibars.
5. Compare the instrument output to the reading from a good barometer placed at the same elevation as the pressure
sensor. Calculate offset (decibars) = barometer reading (converted to decibars) instrument reading (decibars).
6. Enter calculated offset (positive or negative) in decibars:
In the .con file, using SEASAVE or SBE Data Processing (for SBE 16plus, 16plus-IM, 19plus, 25, or 49).
In the CTDs EEPROM, using the appropriate command in SEATERM (for SBE 16plus, 16plus-IM, 19plus,
37, 39, 39-IM, 49, 50, or 52-MP).
Note: For instruments that store calibration coefficients in EEPROM and also use a .con file (SBE 16plus, 16plus-IM,
19plus, and 49), set the pressure offset (Steps 3 and 6 above) in both the EEPROM and in the .con file.
Offset Correction Example
Absolute pressure measured by a barometer is 1010.50 mbar. Pressure displayed from instrument is -2.5 dbars.
Convert barometer reading to dbars using the relationship: mbar * 0.01 = dbars
Barometer reading = 1010.50 mbar *0.01 = 10.1050 dbars
Instruments internal calculations and/or our processing software output gage pressure, using an assumed
value of 14.7 psi for atmospheric pressure. Convert instrument reading from gage to absolute by adding 14.7
psia to instrument output: - 2.5 dbars + (14.7 psi * 0.689476 dbar/psia) = - 2.5 + 10.13 = 7.635 dbars
Offset = 10.1050 7.635 = + 2.47 dbar
Enter offset in .con file (if applicable) and in instrument EEPROM (if applicable).
Another source of offset error results from temperature-induced drifts. Because Druck sensors are carefully temperature
compensated, errors from this source are small. Offset errors can be estimated for the conditions of your profile, and
eliminated when post-processing the data in SBE Data Processing by the following procedure:
1. Immediately before beginning the profile, take a pre-cast in air pressure reading.
2. Immediately after ending the profile, take a post-cast in air pressure reading with the instrument at the same
elevation and orientation. This reading reflects the change in the instrument temperature as a result of being
submerged in the water during the profile.
3. Calculate the average of the pre- and post-cast readings. Enter the negative of the average value (in decibars) as the
offset in the .con file.