Manual revision 001
Section 5: Routine Maintenance and Calibration
SBE 39
plus
-IM
80
Allow the 39
plus
-IM 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 39
plus
-IM to equilibrate before starting will
provide the most accurate calibration correction.
1.
Place the 39
plus
-IM in the orientation it will have when deployed.
2.
In SeatermIM:
A.
Set the pressure offset to 0.0 (
#iiPOffset=0
).
B.
Set the output format to converted decimal (
#iiOutputFormat=1
) with
pressure in decibars (
#iiSetPressUnits=0
).
C.
Send
#iiTP
to take 100 samples and transmit data.
3.
Compare the 39
plus
-IM output to the reading from a good barometer at the
same elevation as the 39
plus
-IM’s pressure sensor port.
Calculate
offset
= barometer reading – 39
plus
-IM reading
4.
Enter the calculated offset (positive or negative) in the 39
plus
-IM’s
EEPROM, using
#iiPOffset=
in SeatermIM.
Offset Correction Example
Absolute pressure measured by a barometer is 1010.50 mbar. Pressure displayed from 39
plus
-IM is -2.5 dbar.
Convert barometer reading to dbar using the relationship: mbar * 0.01 = dbar
Barometer reading = 1010.50 mbar * 0.01 = 10.1050 dbar
The 39
plus
-IM’s internal calculations output gage pressure, using an assumed value of 14.7 psi for atmospheric pressure. Convert
39
plus
-IM reading from gage to absolute by adding 14.7 psia to the 39
plus
-IM’s output:
-2.5 dbar + (14.7 psi * 0.689476 dbar/psia) = -2.5 + 10.13 = 7.635 dbar
Offset = 10.1050 – 7.635 = + 2.47 dbar
Enter offset in 39
plus
-IM.
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 39
plus
-IM’s pressure sensor is an
absolute sensor, so its
raw
output
(
#iiOutputFormat=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 39
plus
-IM outputs pressure
relative to the ocean surface (i.e., at
the surface the output pressure is
0 decibars). The 39
plus
-IM uses the
following equation to convert psia to
decibars:
Pressure (dbar) =
[pressure (psia) - 14.7] * 0.689476
