User`s manual

Manual revision 016 Section 5: Routine Maintenance and Calibration SBE 49
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Pressure Sensor Calibration
The FastCAT’s strain-gauge pressure sensor is capable of meeting the
FastCAT’s error specification with some allowance for aging and ambient-
temperature induced drift.
Pressure sensors show most of their error as a linear offset from zero. A
technique is provided below for making small corrections to the pressure sensor
calibration using the offset (POffset=) calibration coefficient term by comparing
FastCAT pressure output to readings from a barometer.
Allow the FastCAT to equilibrate (with power on) 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 FastCAT to equilibrate
before starting will provide the most accurate calibration correction.
1. Place the FastCAT in the orientation it will have when deployed.
2. In Seaterm:
A. Set the pressure offset to 0.0 (POffset=0).
B. Send TP to measure the FastCAT pressure 100 times and transmit
converted data in engineering units (decibars).
3. Compare the FastCAT output to the reading from a good barometer at the
same elevation as the FastCAT’s pressure sensor.
Calculate offset = barometer reading – FastCAT reading
4. Enter calculated offset (positive or negative):
• In the FastCAT’s EEPROM, using POffset= in Seaterm, and
• In the configuration (.xmlcon or .con) file, using Seasave or
SBE Data Processing.
Offset Correction Example
Absolute pressure measured by a barometer is 1010.50 mbar. Pressure displayed from FastCAT is -2.5 dbars.
Convert barometer reading to dbars using the relationship: mbar * 0.01 = dbar
Barometer reading = 1010.50 mbar * 0.01 = 10.1050 dbar
The FastCAT’s internal calculations and our processing software output gage pressure, using an assumed
value of 14.7 psi for atmospheric pressure. Convert FastCAT reading from gage to absolute by adding
14.7 psia to the FastCAT’s 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 dbars
Enter offset in FastCAT and in .xmlcon or .con file.
For demanding applications, or where the sensor’s air ambient pressure
response has changed significantly, calibration using a dead-weight generator
is recommended. This provides more accurate results, but requires equipment
that may not be readily available. The end cap’s
7
/16-20 straight thread permits
mechanical connection to the pressure source. Use a fitting that has an O-ring
tapered seal, such as Swagelok-200-1-4ST, which conforms to MS16142 boss.
Note:
The FastCAT’s pressure sensor is an
absolute sensor, so its raw output
includes the effect of atmospheric
pressure (14.7 psi). As shown on the
Calibration Sheet, Sea-Bird’s
calibration (and resulting calibration
coefficients) is in terms of psia.
However, when outputting pressure in
engineering units, the FastCAT
outputs pressure relative to the ocean
surface (i.e., at the surface the output
pressure is 0 decibars). The FastCAT
uses the following equation to convert
psia to decibars:
Pressure (db) =
[pressure (psia) - 14.7] * 0.689476