SBE 18 pH Sensor

Pressure-balanced glass-electrode /Ag/AgCl-reference pH probe provides in-situ measurements at depths to 1200 meters.

The SBE 18 pH Sensor uses a pressure-balanced glass-electrode /Ag/AgCl-reference pH probe to provide in-situ measurements at depths to 1200 meters. The replaceable pH probe is permanently sealed and is supplied with a soaker bottle attachment that prevents the reference electrode from drying out during storage. The sensor is a modular, self-contained package that is easy to install, service, and calibrate.

The SBE 18 is intended for use as an add-on auxiliary sensor for profiling CTDs (SBE 9plus; SBE 19, 19plus, and 19plus V2 SeaCAT; and SBE 25 and 25plus Sealogger). Power / signal interface cables and mounting hardware are available separately.

FEATURES

  • Voltage output; interface circuits buffer and offset the differential glass-electrode/reference potential to produce a high-level, pH-dependant, output voltage.
  • Calibrated against precision buffer solutions (4, 7, and 10 pH ± 0.02 pH); results tabulated on a certificate furnished with each sensor.
  • 1200 m housing.
  • Supplied with plastic soaker bottle and KCl soaker solution.
  • Seasoft© V2 Windows software package (computation of pH in engineering units).
  • Five-year limited warranty.

OPTIONS

  • Straight endcap for vertical mounting, or right angle endcap for horizontal mounting (for example, in an extension stand below an SBE 32 Carousel Water Sampler).
  • XSG or wet-pluggable MCBH connector.

Performance

Measurement Range 0 to 14 pH
Initial Accuracy ± 0.1 pH (stated accuracy achievable with frequent field calibrations)
Time Response 1 sec (time to reach 63% of final value following a step change in pH)

Electrical

Input Power 6 - 24 VDC, 15 mA
Output Signal 0 to +5 V

Mechanical

Housing, Depth rating, & Weight Anodized aluminum (6061-T6), stainless steel
1200 m depth rating
0.7 kg in air, 0.4 kg in water

 

  

 

Note: SBE 18 must always be mounted with the pH probe in a vertical orientation, as shown above. A right angle endcap configuration is typically required for applications where the CTD is mounted horizontally (for example, in an extension stand below an SBE 32 Carousel Water Sampler) and the SBE 18 is mounted to the CTD.

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

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.

What are the recommended practices for inspecting, cleaning, and replacing o-rings?

Inspecting and Cleaning O-Rings and Mating Surfaces:

  1. Remove any water from the o-rings and mating surfaces with a lint-free cloth or tissue.
  2. Visually inspect the o-rings and mating surfaces for dirt, nicks, cuts, scratches, lint, hair, and any signs of corrosion; these could cause the seal to fail. Clean the surfaces, and clean or replace the o-rings as necessary.
  3. Apply a light, even coat of 100% silicon o-ring lubricant (Parker Super O Lube) to the o-rings and mating surfaces. For an end cap o-ring, a ball of lubricant the size of a pea is about all that is needed. Too much lubricant can cause the seal to fail as much, if not more, than no grease. Do not use petroleum-based lubricant (car grease, Vaseline, etc.), as it will cause premature failure of the rubber.
    CAUTION: Parker makes another product, Parker O Lube, that is petroleum-based. Do not use this product; verify that you are using Parker Super O Lube.
  4. After lubricating the o-ring, immediately reassemble the end cap or connector, verifying that no hairs or lint have collected on the lubricated o-ring.

Replacing O-Rings:

  • End Cap O-Rings: We recommend scheduled replacement of end cap o-rings approximately every 3 years, to prevent leaks caused by normal o-ring wear.
  • Connector O-Rings: Replacing connector o-rings requires de-soldering and re-soldering the connector wires, which makes it a more difficult task. Therefore, we recommend replacement of connector o-rings when needed, not on a routine, scheduled basis.

Additional Information:

  • 9-minute video covering O-ring, connector, and cable maintenance.
  • Short, silent video of application of lubricant to o-ring.
  • Short, silent video of application of lubricant to o-ring mating surface (note the use of a plastic dental syringe — no sharp points to scratch the housing — to apply the lubricant).

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.

Family Model . Housing Connector Endcap
18   . 1 – aluminum 1 – XSG 1 – Straight
        2 – MCBH 2 – Right angle

Example: 18.111 is an SBE 18 with aluminum housing, XSG connector, and straight endcap. See table below for description of each selection:

PART # DESCRIPTION NOTES
18

pH SENSOR - Modular sensor for use in profiling mode only, 1200 meter depth capability, output 0-5 V. Includes plastic soaker bottle, KCl soaker solution. and complete documentation.

Replaceable pH probe is permanently sealed & is supplied with soaker bottle attachment that prevents reference electrode from drying out during storage. Soaker solution is pH 4 buffer solution saturated with KCL.

SBE 18 can be ordered in two ways:

  • By itself; consult Sea-Bird to order interface cable or mount kit separately, if needed.
  • As an option in SBE 9plus, 19plus V2, or 25plus Auxiliary Sensor Options, complete with sensor, CTD interface cable, & mount kit.

Sea-Bird does not recommend use of pH sensor for moored applications; see Application Note 76: pH or pH/ORP Sensors and Moored Applications.

SBE 18 Connector Selections — MUST SELECT ONE
18.11x XSG 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.

    

18.12x Wet-pluggable (MCBH) connector
SBE 18 Endcap Selections — MUST SELECT ONE
18.1x1 Straight endcap (for vertical mounting) Photo at top shows straight endcap; drawing at right shows right angle endcap. SBE 18 must always be mounted with pH probe in an approximately vertical orientation, because bulb at bottom is filled with a KCl solution, and needs to stay filled while in use. Deviation from vertical by some degrees will not affect data, as long as bulb remains filled. Right angle probe configuration is typically required for applications where CTD is mounted horizontally (for example, in extension stand below SBE 32 Carousel Water Sampler) & SBE 18 is mounted to CTD.
18.1x2 Right angle endcap (for horizontal mounting)
SBE 18 Spares & Accessories
17474 Interface cable, SBE 18 to 9plus, AG/RMG connectors, 0.8 m (DN 30918) These are cables to interface SBE 18 to Sea-Bird CTDs. Order cable to match CTD and to match connector type.
171827 Interface cable, SBE 18 to 9plus, Wet-pluggable connectors, 0.8 m (DN 32845)
17595 Interface cable, SBE 18 to 19plus / 19plus V2 / 25 / 25plus, AG/RMG connectors, 1.1 m (DN 30918)
171828 Interface cable, SBE 18 to 19plus / 19plus V2 / 25 / 25plus, Wet-pluggable connectors, 1.1 m (DN 32845)
30459 Plastic soaker bottle for pH probe Soaker bottle filled with soaker solution comes with SBE 18; this is spare.
50020 pH sensor O-ring kit Spares kit includes 1 each of:
  • 231004 Backup ring for LS021 L-seal (end cap to housing seal)
  • 30072 O-ring, Parker 2-017 N674-70 (XSG connector seal)
  • 30097 O-ring, Parker 2-111 N674-70 (sensor to end cap seal)
  • 30806 L-seal, LS021 (end cap to housing seal)
24023P pH electrode (prepped for installation, document 67072) Electrode & electrode guard come with SBE 18; these are spares.
23190C pH electrode guard
25001 KCl soaker solution, 500 ml bottle Soaker solution is pH 4 buffer solution saturated with KCL. When SBE 18 is not in use, place plastic soaker bottle over pH electrode to prevent loss of internal electrolyte & resulting change in sensor performance. Soaker bottle filled with soaker solution comes with SBE 18; this is spare solution.
50046 SBE 18 mount kit for SeaCATs  
50042 SBE 18 mount kit for SBE 9plus  
50045 SBE 18 mount kit for SBE 25/25plus  
50326 SBE 18 mount kit for SBE 32 extension stand with 1-inch diameter frame stock  

 

Cables

  • 17474 To SBE 9plus (RMG/AG connectors), 0.76 m, DN 30918
  • 171827 To SBE 9plus (MCBH connectors), 0.76 m, DN 32845
  • 17595 To SBE 19plus, 19plus V2, 25, or 25plus (RMG/AG connectors), 1.1 m, DN 30918
  • 171828 To SBE 19plus, 19plus V2, 25, or 25plus (MCBH connectors), 1.1 m, DN 32845

Mount Kits

To SBE 9plus

  • 50042  DO (SBE 13 or 23), pH, or other 1.9-inch (4.83-cm) diameter housing instrument to SBE 9plus Mount Kit (document 67061)

To SBE 16, 16plus, 16plus-IM, 16plus V2, 16plus-IM V2, 19, 19plus, or 19plus V2

  • 50046  DO (SBE 13 or 23), pH, or other 1.9-inch (4.83-cm) diameter housing instrument to SBE 16/16plus/16plus-IM/16plus V2/16plus-IM V2 or  19/19plus/19plus V2

To SBE 25 or 25plus

  • 50045  DO (SBE 13 or 23), pH, or other 1.9-inch (4.83-cm) diameter housing instrument to SBE 25/25plus Cage Mount Kit (document 67039)

To SBE 32 (for mounting SBE 18 with straight end cap vertically on water sampler frame, when CTD is mounted horizontally in extension stand)

  • 50326  SBE 18/27 to SBE 32 Extension Stand with 1-inch diameter frame stock
  • 50327  SBE 18/27 to SBE 32 Extension Stand with 1.375-inch diameter frame stock

Note: 
Frame parts are 25.4 mm (1 inch) diameter for full size SBE 32 12-bottle through 12-liter bottle size, full size SBE 32 24-bottle through 2.5-liter bottle size.
Frame parts are 34.9 mm (1.375 inch) diameter for full size SBE 32 12-bottle larger than 12-liter bottle size and 24-bottle larger than 2.5-liter bottle size.

Spare Parts

  • 30459 Plastic soaker bottle for pH probe
  • 50020  pH sensor O-ring kit
  • 24023P pH probe for SBE 18, 22, 27, or 30 (document 67072)
  • 23190C  pH electrode guard
  • 25001  KCl soaker solution, 500 ml bottle
  • 23041  Zinc anode ring for end cap