Specifications
5
SBE 39
The SBE 39 directly outputs data that is already converted to engineering units (pressure in decibars), using calibration
coefficients that are programmed into the SBE 39. The SBE 39 does not use a .con file. The SBE 39 is a special case,
because its programmed calibration coefficients do not currently include a pressure offset term. The lack of a pressure
offset term creates two difficulties when deploying at elevations above sea level:
After the data is recorded and uploaded, you must perform post-processing to adjust for the pressure offset.
Sea-Bird software cannot currently perform this adjustment for the SBE 39.
Without adjusting the instrument range, internal calculation limitations prevent the SBE 39 from providing accurate
data at high elevations. Specifically, if (0.1 * sensor range) < (decrease in atmospheric pressure from sea level to elevation),
an error condition in the SBE 39s internal calculations occurs. The table below tabulates the atmospheric pressure and
approximate elevation at which this calculation limitation occurs for different pressure sensor ranges.
Range
(m or db) *
Range (psi) =
Range (db) / 0.689476
0.1 * Range (psi)
Atmospheric Pressure (psi) at
elevation at which error occurs =
[14.7 0.1 * Range (psi)]
Approximate
Corresponding Elevation
(m)
20 29 2.9 11.8 1570
100 145 14.5 0.2 7885
350 507 50.7 - -
1000 1450 145 - -
2000 2900 290 - -
3500 5076 507 - -
7000 10152 1015 - -
* Notes:
Although decibars and meters are not strictly equal, this approximation is close enough for this Application Note.
See Application Note 69 for conversion of pressure (db) to depth (m) for fresh or salt water applications.
Equations used in conversions -
As shown on page 1: pressure (db) = [pressure (psia) 14.7] * 0.689476;
Rearranging: pressure (psia) = [Pressure (db) / 0.689476] + 14.7
Measuring relative to atmospheric: pressure (psi; relative to atmospheric pressure) = Pressure (db) / 0.689476
From the table, it is apparent that the only practical limitation occurs with a 20 meter pressure sensor. To use the SBE
39 in this situation, change the sensor range internally to 100 meters by entering PRANGE=100 in the SBE 39 (using
SEATERM). This changes the electronics operating range, allowing you to record pressure data at high elevations, but
slightly decreases resolution. After the data is recorded and uploaded, perform post-processing to adjust for the pressure
offset. Note that Sea-Bird software cannot currently perform this adjustment for the SBE 39.
CAUTION: Changing PRANGE in the SBE 39 does not increase the actual maximum water depth at which the
instrument can be used (20 meters) without damaging the sensor.
Example 1: You want to deploy the SBE 39 with a 20 m pressure sensor in a mountain lake at 1400 meters
(4590 feet). This is lower than 1570 meters shown in the table, so you do not need to adjust the sensor range.
After the data is recorded and uploaded, perform post-processing to adjust for the pressure offset.
Example 2: You want to deploy the SBE 39 with a 20 m pressure sensor in a mountain lake at 2000 meters
(6560 feet). This is higher than 1570 meters shown in the table, so you need to adjust the sensor range. In
SEATERM, set PRANGE=100 to allow use of the SBE 39 at this elevation. After the data is recorded and
uploaded, perform post-processing to adjust for the pressure offset.
SBE 26plus and 53
Unlike our other instruments that include a pressure sensor, the SBE 26plus and 53 output absolute pressure (i.e., at the
surface the output pressure is atmospheric pressure at the deployment elevation). Therefore, no corrections are required
when using these instruments above sea level. SBE 26plus / 53 software (SEASOFT for Waves) includes a module that
can subtract measured barometric pressures from tide data, and convert the resulting pressures to water depths.