Sea-Bird Electronics SBE 38 Manual de usuario descargar pdf (Pagina 102)

Manual revision 006 Section 5: Routine Maintenance and Calibration SBE 19plus V2

102

Pressure Sensor

The SBE 19plus V2 includes a standard strain-gauge pressure sensor or

optional Quartz pressure sensor. These sensors are capable of meeting the

19plus V2 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

19plus V2 pressure output to readings from a barometer.

Allow the 19plus V2 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 19plus V2 to

equilibrate before starting will provide the most accurate calibration correction.

1. Place the 19plus V2 in the orientation it will have when deployed.

2. In Seaterm232:

A. Set the pressure offset to 0.0 (POffset=0).

B. Send TP to measure the 19plus V2 pressure 100 times and transmit

converted data in engineering units (decibars).

3. Compare the 19plus V2 output to the reading from a good barometer at the

same elevation as the 19plus V2 pressure sensor.

Calculate offset = barometer reading – 19plus V2 reading

4. Enter the calculated offset (positive or negative) in two places:

• In the 19plus V2 EEPROM, using POffset= in Seaterm232, 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 19plus V2 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 19plus V2’s internal calculations and our processing software output gage pressure, using an assumed

value of 14.7 psi for atmospheric pressure. Convert 19plus V2 reading from gage to absolute by adding

14.7 psia to the 19plus V2 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 19plus V2 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. The end cap’s

/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 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 19plus V2

outputs pressure relative to the ocean

surface (i.e., at the surface the output

pressure is 0 decibars).

The 19plus V2 uses the following

equation to convert psia to decibars:

pressure (db) =

[pressure (psia) - 14.7] * 0.689476

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Sea-Bird Electronics SBE 38 Manual de usuario Pagina 102

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