SBE 29 Strain-Gauge Pressure Sensor

SBE 29 Strain-Gauge Pressure Sensor

Mechanical strain-gauge pressure transducer with thermistor temperature compensation, used on SBE 25 CTD (not in production since 2012).

The SBE 29 pressure sensor, used on our SBE 25 Sealogger CTD (not in production since 2012), is a modular sensor consisting of a mechanical strain-gauge pressure transducer with thermistor temperature compensation. The SBE 29 measures absolute pressure in one of eight full-scale pressure ranges from 20 to 7000 meters depth. The sensor elements and their interface electronics are modular and self-contained, providing easy installation, service, and calibration.

FEATURES

  • Pressure-dependent output voltage (+5 to -5 V) and a temperature-compensation voltage of 0 to +5 VDC (30K ohm thermistor at 25 °C).
  • Calibrated over full scale range using a dead weight tester; results tabulated on certificate furnished with each sensor.
  • Temperature [T] and temperature-corrected pressure [Pnum] in engineering units can be computed with:
    T = temperature [°C] = -Vt * 21.02373 + 40.293
    where Vt = TC signal voltage.
    Pnum = Vp * 819 - (TC * T / 1024)
    where Vp = pressure signal voltage, TC = compensation value (from SBE 29 calibration sheet).
  • Five-year limited warranty.

OPTIONS

  • One of eight full-scale pressure ranges from 20 to 7000 m.
  • XSG or wet-pluggable MCBH connector.

Performance

Measurement Range 20/100/350/1000/2000/3500/7000 meters (meters of deployment depth capability)
Accuracy ± 0.1% of full scale range
Typical Stability 0.05% of full scale range per year

Electrical

Input Power +10 and -10 VDC at 15 mA
Output Signal Pressure: -5 to +5 VDC
Temperature: 0 to +5 VDC

Mechanical

Housing & Depth Rating Depths < 3500 m: Anodized aluminum (6061)
Depths > 3500 m: Anodized aluminum (7075)
Weight in air 0.7 kg; in water 0.3 kg

 

 

The list below includes (as applicable) the current product brochure, manual, and quick guide; software manual(s); and application notes.

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?

General recommendations:

  • 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.

Profiling CTDs:

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:

  1. Thoroughly clean the connector with water, followed by alcohol.
  2. 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.

Replacing Connectors:

  • 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 function of the zinc anode on some instruments?

A zinc anode attracts corrosion and prevents aluminum from corroding until all the zinc is eaten up. Sea-Bird uses zinc anodes on an instrument if it has an aluminum housing and/or end cap. Instruments with titanium or plastic housings and end caps (for example, SBE 37 MicroCAT) do not require an anode.

Check the anode(s) periodically to verify that it is securely fastened and has not been eaten away.

PART # DESCRIPTION NOTES
29 STRAIN GAUGE PRESSURE SENSOR - Modular sensor for use with SBE 25 CTD (not 25plus). Includes complete documentation.

SBE 29 provides 0 - 5V output, and is the modular pressure sensor used on the SBE 25 CTD (replaced by the 25plus in 2012). It can be ordered to replace an existing pressure sensor on an SBE 25, or for custom applications. consult Sea-Bird to order interface cable or mount kit separately, if needed.
For pressure sensor with RS-232 output, see SBE 50 Digital Oceanographic Pressure Sensor.

SBE 29 Pressure Sensor Range (depth) Selections — MUST SELECT ONE
29-1a 20 m strain gauge pressure sensor

Pressure sensor is installed in end cap, & is not field-replaceable / swappable. While highest depth rating gives most flexibility in using SBE 29, 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 sensor's full scale range. For example, comparing 2000 & 7000 m sensors:

  • 2000 m sensor — initial accuracy is 2 m (= 0.1% * 2000 m)
  • 7000 m sensor — initial accuracy is 7 m (= 0.1% * 7000 m)
29-1b 100 m strain gauge pressure sensor
29-1c 350 m strain gauge pressure sensor
29-1h 600 m strain gauge pressure sensor
29-1d 1000 m strain gauge pressure sensor
29-1e 2000 m strain gauge pressure sensor
29-1f 3500 m strain gauge pressure sensor
29-1g 7000 m strain gauge pressure sensor
SBE 29 Wet-Pluggable Connector Option
29-2 Wet-pluggable (MCBH) connector instead of AG 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.

    
Note: SBE 29 uses 6-pin AG-306 or MCBH-6M connector; photos are for illustration of differences between XSG and MCBH connectors.

 

Many cables, mount kits, and spare parts can be ordered online.

Cables

  • 17168 To SBE 25 (AG connectors), 1.1 m, DN 30568
  • 171798 To SBE 25 (Wet-pluggable connectors), 1.0 m), DN 32758

Mount Kits

To SBE 25

  • 50376 SBE 5T (or 5P), 23, and 29 Mount Block Kit (holds 3 instruments) (drawing 20846 for 23831 mount block; kit also includes hardware)
  • 50064 Pump and Pressure Sensor Mount Kit
  • 50045 DO (SBE 13 or 23), pH, or other 1.9-inch (4.83-cm) diameter housing instrument to SBE 25 Cage Mount Kit (document 67039)

Spare Parts

  • 50025 Pressure sensor oil refill kit (document 67066)
  • 23041 Zinc anode ring for end cap