SBE 50 Digital Oceanographic Pressure Sensor
The SBE 50 is a high-accuracy, high-resolution (16 Hz sampling) pressure sensor, intended for use in towed vehicles, ROVs, AUVs, or other autonomous platforms that can supply DC power and acquire data. It is an easy-to-use, light, compact instrument, well suited to even the smallest vehicle. The SBE 50 must be externally powered, and its RS-232C data (and/or 0 - 5 volt output) logged or telemetered by the vehicle to which it is mounted.
The SBE 50’s strain-gauge pressure sensor is offered in eight full scale ranges from 20 to 7000 dbars. Compensation of the temperature influence on pressure offset and scale is performed by the SBE 50’s CPU. Measurements are output as raw data (pressure and pressure temperature) or in engineering units (pressure in decibars or psia, or depth in meters or feet).
Commands sent to the SBE 50 provide status display, data acquisition setup, and diagnostic tests.
- Pressure sensor with temperature compensation.
- Programmable sampling:
- 16 Hz Autonomous sampling (begins sampling when power is applied or on command; can average data if desired), or Polled sampling.
- RS-232 output; voltage output interface (in addition to RS-232 output) available.
- Calibrated over full scale range using a dead weight tester; results tabulated on a certificate furnished with each sensor.
- Compatible with SBE 32 Carousel Water Sampler and Auto Fire Module (AFM) to provide autonomous water sampling on a non-conducting cable (AFM powers SBE 50 and monitors transmitted pressure, closing bottles at pre-defined pressures or depths, or when the system is stationary for a specified period of time).
- Titanium housing; depths to 7000 m.
- Seasoft© V2 Windows software package (instrument setup and data display).
- Five-year limited warranty.
- One of eight full-scale pressure ranges from 20 to 7000 dbars.
- RS-232 output, or voltage output (0 to 5 V) in addition to RS-232 output.
- XSG/AG or wet-pluggable MCBH connector.
|Measurement Range||0 to 20 / 100 / 350 / 600 / 1000 / 2000/ 3500 / 7000 / 10,500 m|
|Initial Accuracy||± 0.1% of full scale range|
|Typical Stability||0.05% of full scale range per year|
|Resolution||0.002% of full scale range
(D/A resolution for optional 0-5 volt output: 16 bit)
|Sampling Speec||16 Hz (16 samples/sec)|
|Input Power||8 - 30 VDC; 51 mA at 8 VDC to 21 mA at 30 VDC (see manual for details)|
0 to 5 V output (in addition to RS-232) if ordered with voltage output interface
|Housing & Depth rating||3AL/2.5V Titanium, 7000 m|
|Weight||0.7 kg in air, 0.4 kg in water|
How should I pick the pressure sensor range for my CTD? Would the highest range give me the most flexibility in using the CTD?
While the highest range does give you the most flexibility in using the CTD, it is at the expense of accuracy and resolution. It is advantageous to use the lowest range pressure sensor compatible with your intended maximum operating depth, because accuracy and resolution are proportional to the pressure sensor's full scale range. For example, the SBE 9plus pressure sensor has initial accuracy of 0.015% of full scale, and resolution of 0.001% of full scale. Comparing a 2000 psia (1400 meter) and 6000 psia (4200 meter) pressure sensor:
- 1400 meter pressure sensor ‑ initial accuracy is 0.21 meters and resolution is 0.014 meters
- 4200 meter pressure sensor ‑ initial accuracy is 0.63 meters and resolution is 0.042 meters
How often do I need to have my instrument and/or auxiliary sensors recalibrated? Can I recalibrate them myself?
- Profiling CTD — recalibrate once/year, but possibly less often if used only occasionally. We recommend that you return the CTD to Sea-Bird for recalibration. (In principle, it is possible for calibration to be performed elsewhere, if the calibration facility has the appropriate equipment andtraining. However, the necessary equipment is quite expensive to buy and maintain.) In between laboratory calibrations, take field salinity samples to document conductivity cell drift.
- Thermosalinograph — recalibrate at least once/year, but possibly more often depending on the degree of bio-fouling in the water.
- DO sensor —
— SBE 43 — recalibrate once/year, but possibly less often if used only occasionally and stored correctly (see Application Note 64), and also depending on the amount of fouling and your ability to do some simple validations (see Application Note 64-2)
— SBE 63 — recalibrate once/year, but possibly less often if used only occasionally and stored correctly and also depending on the amount of fouling and your ability to do some simple validations (see SBE 63 manual)
- pH sensor —
— SBE 18 pH sensor or SBE 27 pH/ORP sensor — recalibrate at the start of every cruise, and then at least once/month, depending on use and storage
— Satlantic SeaFET pH sensor — recalibrate at least once/year. See FAQ tab on Satlantic's SeaFET page for details (How often does the SeaFET need to be calibrated?).
- Transmissometer — usually do not require recalibration for several years. Recalibration at the manufacturer’s factory is the most practical method.
We often have requests from customers to have some way to know if the CTD is out of calibration. The general character of sensor drift in Sea-Bird conductivity, temperature, and pressure measurements is well known and predictable. However, it is very difficult to know precisely how far a CTD calibration has drifted over time unless you have access to a very sophisticated calibration lab. In our experience, an annual calibration schedule will usually maintain the CTD accuracy to within 0.01 psu in Salinity.
Conductivity drifts as a change in slope as a result of accumulated fouling that coats the inside of the conductivity cell, reducing the area of the cell and causing an under-reporting of conductivity. Fouling consists of both biological growth and accumulated oils and inorganic material (sediment). Approximately 95% of fouling occurs as the cell passes through oil and other contaminants floating on the sea surface. Most conductivity fouling is episodic, as opposed to gradual and steady drift. Most fouling events are small and mostly transitory, but they have a cumulative affect over time. A severe fouling event, such as deployment through an oil spill, could have a dramatic but only partially recoverable effect, causing an immediate jump shift toward lower salinity. As fouling becomes more severe, the fit becomes increasingly non-linear and offsets and slopes no longer produce adequate correction, and return to Sea-Bird for factory calibration is required. Frequently checking conductivity drift is likely to be the most productive data assurance measure you can take. Comparing conductivity from profile to profile (as a routine check) will allow you to detect sudden changes that may indicate a fouling event and the need for cleaning and/or re-calibration.
Temperature generally drifts slowly, at a steady rate and predictably as a simple offset at the rate of about 1-2 millidegrees per year. This is approximately equal to 1-2 parts per million in Salinity error (very small).
Pressure sensor drift is also an offset, and annual comparisons to an accurate barometer to determine offset will generally keep the sensor within specification for several years, particularly as the sensors age over time.
What are the recommended practices for connectors - mating and unmating, cleaning corrosion, and replacing?
Mating and Unmating Connectors:
It is important to prepare and mate connectors correctly, both in terms of the costs to repair them and to preserve data quality. Leaking connectors cause noisy data and even potential system shutdowns. Application Note 57: Connector Care and Cable Installation describes the proper care and installation of connectors for Sea-Bird instruments. The Application Note covers connector cleaning and cable or dummy plug installation, locking sleeve installation, and cold weather tips.
Checking for Leakage and Cleaning Corrosion on Connectors:
If there has been leakage, it will show up as green-colored corrosion product. Performing the following steps can usually reverse the effect of the leak:
- Thoroughly clean the connector with water, followed by alcohol.
- Give the connector surfaces a light coating of silicon grease.
Re-mate the connectors properly — see Application Note 57: Connector Care and Cable Installation and 9-minute video covering O-ring, connector, and cable maintenance.
- The main concern when replacing a bulkhead connector is that the o-rings on the connector and end cap must be prepared and installed correctly; if they are not, the instrument will flood. See the question below for general procedure on handling o-rings.
- Use a thread-locking compound on the connector threads to prevent the new connector from loosening, which could also lead to flooding.
- If the cell guard must be removed to open the instrument, take extra care not to break the glass conductivity cell.
Can I use a pressure sensor above its rated pressure?
Digiquartz pressure sensors are used in the SBE 9plus, 53, and 54. The SBE 16plus V2, 16plus-IM V2, 19plus V2, and 26plus can be equipped with either a Druck pressure sensor or a Digiquartz pressure sensor. All other instruments that include pressure use a Druck pressure sensor.
- The overpressure rating for a Digiquartz (as stated by Paroscientific) is 1.2 * full scale. The sensor will provide data values above 100% of rated full scale; however, Sea-Bird does not calibrate beyond the rated full scale.
- The overpressure rating for a Druck (as stated by Druck) is 1.5 * full scale. The sensor will provide data values above 100% of rated full scale; however, Sea-Bird does not calibrate beyond the rated full scale.
Note: If you use the instrument above the rated range, you do so at your own risk; the product will not be covered under warranty.
What is the maximum cable length for real-time RS-232 data?
Cable length is one of the most misunderstood items in the RS-232 world. The RS-232 standard was originally developed decades ago for a 19200 baud rate, and defines the maximum cable length as 50 feet, or the cable length equal to a capacitance of 2500 pF. The capacitance rule is often forgotten; using a cable with low capacitance allows you to span longer distances without going beyond the limitations of the standard. Also, the maximum cable length mentioned in the standard is based on 19200 baud rate; if baud is reduced by a factor of 2 or 4, the maximum length increases dramatically. Using typical underwater cables, allowable combinations of cable length and baud rate for Sea-Bird instruments communicating with RS-232 are shown below:
|Maximum Cable Length (meters)||Maximum Baud Rate*|
*Note: Consult instrument manual for baud rates supported for your instrument.
|50||.||1 – 7000 m (titanium)||1 – 20 m strain gauge||1 – XSG/AG||0 – RS-232|
|2 – 100 m strain gauge||2 – MCBH||1 – RS-232 & analog|
|3 – 350 m strain gauge|
|4 – 600 m strain gauge|
|5 – 1000 m strain gauge|
|6 – 2000 m strain gauge|
|7 – 3500 m strain gauge|
|8 – 7000 m strain gauge|
Example: 50.1820 is an SBE 50 with 7000 m housing, 7000 m strain gauge pressure sensor, MCBH connector, and RS-232 interface. See table below for description of each selection:
DIGITAL OCEANOGRAPHIC PRESSURE SENSOR - Includes 7000 m titanium housing, power/interface cable, Seasoft software, & complete documentation.
|SBE 50 pressure sensor, with no memory or internal batteries, is designed to be incorporated into oceanographic sensing systems. SBE 50 provides RS-232 output; for pressure sensor with 0 - 5V output, see SBE 29 Strain Gauge Pressure Sensor.
SBE 50 can be incorporated into several Sea-Bird CTD systems:
|SBE 50 Pressure Sensor Range (depth) Selections — MUST SELECT ONE|
|50.11xx||20 m strain gauge pressure sensor||Pressure sensor is installed in end cap, & is not field replaceable / swappable. While highest pressure rating gives you most flexibility in using SBE 50, it is at expense of accuracy & resolution. It is advantageous to use lowest range pressure sensor compatible with your intended maximum operating depth, because accuracy & resolution are proportional to pressure sensor's full scale range. For example, comparing 2000 & 7000 m sensors:
|50.12xx||100 m strain gauge pressure sensor|
|50.13xx||350 m strain gauge pressure sensor|
|50.14xx||600 m strain gauge pressure sensor|
|50.15xx||1000 m strain gauge pressure sensor|
|50.16xx||2000 m strain gauge pressure sensor|
|50.17xx||3500 m strain gauge pressure sensor|
|50.18xx||7000 m strain gauge pressure sensor|
|SBE 50 Connector and Interface Selections — MUST SELECT ONE|
|50.1x10||RS-232 digital output (on 4-pin XSG connector, includes 2.4 m power/digital interface cable PN 801385)||
SBE 50 with RS-232 output only has 4-pin connector; SBE 50 with RS-232 & 0-5 volt output has 6-pin connector.
Wet-pluggable connectors may be mated in wet conditions. Their pins do not need to be dried before mating. By design, water on connector pins is forced out as connector is mated. However, they must not be mated or un-mated while submerged. Wet-pluggable connectors have a non-conducting guide pin to assist pin alignment & require less force to mate, making them easier to mate reliably under dark or cold conditions, compared to XSG/AG connectors. Like XSG/AG connectors, wet-pluggables need proper lubrication & require care during use to avoid trapping water in sockets.
|50.1x11||0-5 Volt analog output and RS-232 digital output (on AG 6-pin connector, includes
2.4 m power/digital/analog interface cable PN 801386)
|50.1x20||RS-232 digital output (on 4-pin MCBH connector, includes 2.4 m power/digital interface cable PN 801206)|
|50.1x21||0-5 Volt analog output and RS-232 digital output (on MCBH 6-pin connector, includes
2.4 m power/digital/analog interface cable PN 801387)
|SBE 50 Interface Cables (4-pin)|
|801385||Data/Power interface cable, RMG-4FS to DB-9S & red/black twisted wire leads, 2.4 m (DN 32277)||Test cables are used for setting up system. Applicable cable (XSG/AG or wet-pluggable connector) with red/black twisted wire leads included with SBE 50 if RS-232 digital output only ordered; these are spares.|
|801376||Data / Power interface cable, RMG-4FS to DB-9S & 9V battery snap, 2.4 m (DN 32604)|
|801206||Data/Power interface cable, Wet-Pluggable, MCIL-4FS to DB-9S & red/black twisted wire leads, 2.4 m (DN 32366)|
|801263||Data/Power interface cable, Wet-Pluggable, MCIL-4FS to DB-9S & 9V battery snap, 2.4 m (DN 32490)|
|SBE 50 Interface Cables (6-pin)|
|801386||Data/Power interface cable, AG-206 to DB-9S & red/black twisted wire power leads, blue/black analog signal leads, 2.4 m (DN 32737)||Test cables are used for setting up system. Applicable cable (XSG/AG or wet-pluggable connector) included with SBE 50 if voltage output and RS-232 digital output ordered; these are spares.|
|801387||Data/Power interface cable, Wet-Pluggable, MCIL-6FS & red/black twisted wire power leads, blue/black analog signal leads, 2.4 m (DN 32738)|
Many cables, mount kits, and spare parts can be ordered online.
- 801385 To computer COM port with power leads (from XSG 4-pin connector), 2.4 m, DN 32277
- 801376 To computer COM port with 9V connector (from XSG 4-pin connector), 2.4 m, DN 32604
- 17031 pigtail (from XSG 4-pin connector), 2.4 m, DN 30581
- 801386 To computer COM port with power leads (from 6-pin AG connector), 2.4 m, DN 32737
- 17032 pigtail (from 6-pin AG connector), 2.4 m, DN 30582
- 801206 To computer COM port with power leads (from Wet-pluggable 4-pin connector), 2.4 m, DN 32366
- 801263 To computer COM port with 9V connector (from Wet-pluggable 4-pin connector), 2.4 m, DN 32490
- 171368 pigtail (from Wet-pluggable 4-pin connector), 2.4 m, DN 32363
- 801387 To computer COM port with power leads (from Wet-pluggable 6-pin connector), 2.4 m, DN 32738
- 171513 pigtail (from Wet-pluggable 6-pin connector), 2.4 m, DN 32515
Mount to AFM
- 50126 SBE 5T, 5P, 5M, 43, or other 1.5-inch (3.81-cm) diameter housing to SBE 16/16plus/16plus-IM/16plus V2/16plus-IM V2, 19/19plus/19plus V2 or other 3.9-inch (9.9-cm) diameter housing Mount Kit
- 233186 High-head pressure port plug for muddy/biologically productive environments (Application Note 84)