Manual revision 041
Section 5: Routine Maintenance and Calibration
SBE 37-SM RS-232
67
Pressure Sensor (optional) Calibration
The optional strain-gauge pressure sensor is a mechanical diaphragm type,
with an initial static error band of 0.05%. Consequently, the sensor is capable
of meeting the MicroCAT’s 0.10% 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 MicroCAT pressure output to readings from a barometer.
Allow the MicroCAT 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 MicroCAT to equilibrate before starting will
provide the most accurate calibration correction.
1.
Place the MicroCAT in the orientation it will have when deployed.
2.
In Seaterm232:
A.
Set the pressure offset to 0.0 (
POffset=0
).
B.
Set the output format to converted decimal (
OutputFormat=1
), so the
pressure output will be in decibars.
C.
Send
TSN:100
to take 100 samples and transmit data.
3.
Compare the MicroCAT output to the reading from a good barometer at the
same elevation as the MicroCAT’s pressure sensor port.
Calculate
offset
= barometer reading – MicroCAT reading
4.
Enter the calculated offset (positive or negative) in the MicroCAT’s
EEPROM, using
POffset=
in Seaterm232.
Offset Correction Example
Absolute pressure measured by a barometer is 1010.50 mbar. Pressure displayed from MicroCAT 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 MicroCAT’s internal calculations output gage pressure, using an assumed value of 14.7 psi for atmospheric
pressure. Convert MicroCAT reading from gage to absolute by adding 14.7 psia to the MicroCAT’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 MicroCAT.
For demanding applications, or where the sensor’s air ambient pressure
response has changed significantly, calibration using a dead-weight
generator is recommended. The pressure sensor port uses a
7
/
16
-20 straight
thread for 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 MicroCAT’s pressure sensor is an
absolute sensor, so its
raw
output
(
OutputFormat=0
) 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
MicroCAT outputs pressure relative to
the ocean surface (i.e., at the surface
the output pressure is 0 decibars). The
MicroCAT uses the following equation
to convert psia to decibars:
Pressure (db) =
[pressure (psia) - 14.7] * 0.689476
