SBE 38 Digital Oceanographic Thermometer

SBE 38 Digital Oceanographic Thermometer

Standards-level performance of an expensive AC bridge and platinum thermometer at a small fraction of the cost. Real-time temperature data is transmitted in ASCII characters (°C or raw counts) via an RS-232 or optional RS-485 serial interface for display or logging by PC or data logger.

Sophisticated A/D acquisition electronics, ultra-stable thermistor, and state-of-the-art calibration provide the standards-level performance of an expensive AC bridge and platinum thermometer at a small fraction of the cost. The SBE 38 is unaffected by shock and vibration, has high accuracy and stability, and is easy to use. It has a rugged, corrosion-proof, 10,500 m titanium housing. Real-time temperature data is transmitted via the RS-232 or RS-485 serial interface in ASCII characters (°C or raw counts). The SBE 38 must be externally powered, and its data logged or telemetered by a computer, data logger, or instrument.

Applications include calibration baths, oceanographic/aquatic research, and environmental monitoring. The SBE 38 is frequently integrated as a remote temperature sensor with an SBE 21 Thermosalinograph or SBE 45 MicroTSG, to provide accurate sea surface temperature. It can also be integrated as a secondary temperature sensor with an SBE 16plus, 16plus-IM, 16plus V2, 16plus-IM V2, 19plus V2, or 25plus CTD.


  • Programmable sampling:
    • Continuous (begins when power applied or on command);
    • interval between samples (sec) = (0.133 * NAvg) + 0.339        where NAvg is number of acquisition cycles/sample.
    • Polled.
  • Serial output:
    • RS-232 (full duplex) with one SBE 38 connected to the interface;
    • RS-485 (half duplex) with one SBE 38 connected to the interface; or
    • RS-485 (half duplex) with several RS-485 sensors sharing one pair of wires (cannot sample continuously).
  • No batteries or memory.
  • Compatible with Sea-Bird thermosalinographs and some Sea-Bird CTDs.
  • Titanium housing; depths to 10,500 m.
  • Seasoft© V2 Windows software package (instrument setup and data display).
  • Five-year limited warranty.


  • RS-232 or RS-485 output.
  • XSG or wet-pluggable MCBH connector.


The SBE 38 is calibrated in Sea-Bird's state-of-the-art calibration laboratory, which maintains primary temperature standards (water triple point [TPW] and gallium melting point [GaMP] cells), ITS-90 certified and standards-grade platinum resistance thermometers, and a low-gradient temperature bath. Temperature is computed using the Steinhart-Hart polynomial (Steinhart and Hart, 1968; Bennett, 1972). The equation characterizes the non-linear temperature versus resistance response of the sensor. Thermistors require individualized coefficients to the Steinhart-Hart equation, because the thermistor material is an individualized mix of dopants:

t 90L = { [ 1.0 / (a0 + a1*ln(n) + a2*ln2(n) + a3*ln3(n) ) ] - 273.15} * Slope + Offset       [°C]

where: n is SBE 38 output.


Measurement Range -5 to +35 °C
Initial Accuracy 1 ± 0.001 °C (1 mK)
Typical Stability 0.001 °C (1 mK) in six months, certified
Resolution 0.00025 °C (0.25 mK)
Response Time 2 500 msec
Self-heating Error < 200 µK

1 NIST-traceable calibration applying over the entire range.
2 Time to reach 63% of final value following a step change in temperature.


Output Signal RS-232 or RS-485 (half-duplex)
Input Power 8-15 VDC at 15 mA average for RS-232 output;
8-15 VDC at 10 mA average for RS-485 output


Housing & Depth rating Titanium, 10,500 m
Weight 0.9 kg in air, 0.5 kg in water



Example Calibration Data (sensor serial number 80, 02 September 1997):
a0 = -2.809379e-05     a2 = -2.619655e-06    a1 = 2.783484e-04    a3 = 1.598734e-07

(Instrument - Bath)
-1.52985 824162.7 -1.52983 0.00002
1.03108 733633.1 1.03106 -0.00002
4.60520 625547.1 4.60518 -0.00002
8.11169 536776.4 8.11169 -0.00000
11.61533 462132.6 11.61536 0.00003
15.17575 398167.3 15.17574 -0.00001
18.63931 345476.6 18.63934 0.00003
22.14032 300170.8 22.14031 -0.00001
25.66793 261276.6 25.66793 0.00000
29.13948 228549.1 29.13944 -0.00004
32.61481 200420.3 32.61484 0.00003

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

For older SBE 38 product manuals, organized by instrument firmware version, click here.

Title Type Publication Date PDF File
SBE 38 Brochure Product Brochure Wednesday, August 19, 2015 38BrochureAug15.pdf
SBE 38 Manual Product Manual Monday, June 27, 2016 38_016.pdf
SBE 38 Quick Guide Product Quick Guide Wednesday, March 23, 2011 38_ReferenceSheet_004.pdf
AN38: TC Duct Fundamentals Application Notes Tuesday, July 10, 2012 appnote38Jul12.pdf
AN42: ITS-90 Temperature Scale Application Notes Wednesday, May 18, 2016 appnote42May16.pdf
AN56: Interfacing to RS-485 Sensors Application Notes Wednesday, February 11, 2009 appnote56Feb09.pdf
AN57: Connector Care and Cable Installation Application Notes Tuesday, May 13, 2014 appnote57Jan14.pdf
AN68: Using USB Ports to Communicate with Sea-Bird Instruments Application Notes Friday, October 19, 2012 appnote68Oct12.pdf
AN71: Desiccant Use and Regeneration (drying) Application Notes Wednesday, May 18, 2016 Appnote71May16.pdf
Seaterm© is a terminal program for setup and data upload of a wide variety of older Sea-Bird instruments. Seaterm is part of our Seasoft V2 software suite.
Version 1.59 released October 10, 2007
Seaterm_Win32_V1_59.exe for Windows XP/Vista/7

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 storing sensors at low temperatures, and deploying at low temperatures or in frazil or pancake ice?


Large numbers of Sea-Bird conductivity instruments have been used in Arctic and Antarctic programs.

Special accommodation to keep temperature, conductivity, oxygen, and optical sensors at or above 0 C is advised. Often, the CTD is brought inside protective doors between casts to achieve this.

Conductivity Cell

When freezing is possible, we recommend that the conductivity sensor be stored dry. Remove larger droplets of water by blowing through the cell. Do not use compressed air, which typically contains oil vapor. Attach a length of Tygon tubing to each end of the conductivity cell to close the cell ends. See Application Note 2D: Instructions for Care and Cleaning of Conductivity Cells for details.

There are several considerations to weigh when contemplating deployments at low temperatures in general, and in frazil or pancake ice:

  • Ensure that the instrument is at or above water temperature before it is deployed. If the cell gets colder than 0 to -2 ºC while on deck, when it enters the water a layer of ice forms inside the cell as the cell warms to ocean temperature. If ice forms inside the conductivity cell, measurements will be low of correct until the ice layer melts and disappears. Thin layers of ice will not hurt the conductivity cell, but repeated ice formation on the electrodes will degrade the conductivity calibration (at levels of 0.001 to 0.020 psu) and thicker layers of ice can lead to glass fracture and permanent damage of the cell.
  • For accurate measurements, keep ice out of the sensing region of the conductivity cell. The conductivity measurement involves determining the electrical resistance of the water inside the sensor. Ice is essentially a non-conductor. To the extent that ice displaces the water, the conductivity will register (very) misleadingly low. Some type of screening is necessary to keep ice out of the cell. This is relatively easy to arrange for the Sea-Bird conductivity cell, which is an electrode-type cell, because its sensing region is totally inside a long tube; plastic mesh could be positioned at each end and would have zero effect on accuracy and stability.

The above considerations apply to all known conductivity sensor types, whether electrode or inductive types. 

If deploying at low temperatures but no surface frazil or pancake ice is present, rinse the conductivity cell in one of the following salty solutions (salty water depresses the freezing point) to prevent freezing during deployment. But this does not mean you can store the cell in one of these solutions outside . . . it will freeze.

  • Solution of 1% Triton in sterile seawater (use 0.5-micron filtered seawater or boiled seawater),   or
  • Brine solution (distilled seawater or homemade salt solution that is higher than 35 psu in salinity).

Note that there is still a risk of forming ice inside the conductivity cell if deploying through frazil or pancake ice on the surface, if the freezing point of the salt water is the same as the water temperature. Therefore, we recommend that you deploy the conductivity cell in a dry state for these deployments.

Commercially available alcohol or glycol antifreezes contain trace amounts of oils that will coat the conductivity cell and the electrodes, causing a calibration shift, and consequently result in errors in the data. Do not use alcohol or glycol in the conductivity cell.

Temperature Sensor

In general, neither the accuracy of the temperature measurement nor the survival of the temperature sensor will be affected by ice.

Oxygen Sensor

For the SBE 43 and SBE 63 Dissolved Oxygen sensor, avoid prolonged exposure to freezing temperature, including during shipment. Do not store the with water (fresh or seawater), Triton solution, alcohol, or glycol in the plenum. The best precaution is to keep the sensor indoors or in some shelter out of the cold weather.

Family . Housing Connector Communications Miscellaneous (factory use)
38 . 1 – 10,500 m (titanium) 1 – XSG 0 – RS-232 x
      2 – MCBH 1 – RS-485  

Example: 38.120x is an SBE 38 with 10,500 m housing, MCBH connector, and RS-232 communications. See table below for description of each selection:


DIGITAL OCEANOGRAPHIC THERMOMETER - Standard measurement range (-5 to +35 C), certified stability of 0.001 C in six months. Includes 10,500 m titanium housing, real-time RS-232C or RS-485 interface, 2.4 m data I/O cable, Seasoft software, & complete documentation.

SBE 38 has no memory or internal batteries. SBE 38 is frequently integrated as a remote temperature sensor with an SBE 21 or 45 thermosalinograph, to provide sea surface temperature. It can also be integrated as a secondary temperature sensor with an SBE 16plus, 16plus-IM, 16plus V2, 16plus-IM V2, 19plus V2, or 25plus CTD.
SBE 38 Connector Selections MUST SELECT ONE
38.11xx XSG connector on instrument and data I/O cable PN 801376

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.

XSG connector on left, Wet-pluggable (MCBH) connector on right

38.12xx Wet-pluggable (MCBH) connector on instrument and data I/O cable PN 801263
SBE 38 Communications Selections MUST SELECT ONE
38.1x0x RS-232 serial interface

Sea-Bird supplies SBE 38 wired to match selected serial interface option. However, SBE 38 can be user-configured for other serial interface by modifying wiring from SBE 38’s bulkhead connector to PCB & reprogramming SBE 38 to match wired configuration.

Two-wire, RS-485 interface provides communication with individual SBE 38 or with all SBE 38s attached to RS-485 interface. This allows for coordination of sampling among many instruments. However, note that half-duplex communication is one-direction at a time (i.e., you cannot send commands & receive data at same time).

38.1x1x RS-485 (half duplex ONLY) serial interface
SBE 38 Spares & Accessories
801376 Data/Power interface cable, RMG-4FS to DB-9S & 9V battery snap, 2.4 m (DN 32604)

These cables are compatible with SBE 38 if 38.11xx (XSG connector) selected.

  • 801376 is included with standard shipment; listing here is spare.
  • 801385 & 801355 have twisted wire leads in place of battery snap, & can be used in place of 801376.
801385 Data/Power interface cable, RMG-4FS to DB-9S & red/black twisted wire leads, 2.4 m (DN 32277)
801355 Data/Power interface cable, RMG-4FS to DB-9S & red/black twisted wire leads, 20 m (DN 32277)


Data/Power interface cable, Wet-Pluggable, MCIL-4FS to DB-9S & 9V battery snap, 2.4 m (DN 32490)

These cables are compatible with SBE 38 if 38.12xx (MCBH connector) selected.

  • 801263 is included with standard shipment; listing here is spare.
  • 801206 & 801476 have twisted wire leads in place of battery snap, & can be used in place of 801376.
801206 Data/Power interface cable, Wet-Pluggable, MCIL-4FS to DB-9S & red/black twisted wire leads, 2.4 m (DN 32366)
801476 Data/Power interface cable, Wet-Pluggable, MCIL-4FS to DB-9S & red/black twisted wire leads, 25 m (DN 32366)
17031 4-pin pigtail cable, RMG-4FS with lock sleeve, 2.4 m (DN 30581) For applications where not connecting directly to computer.
20200 USB to Serial Port Adapter, FTDI UC232R-10 (connects computers with USB ports to RS-232 instruments) Many newer PCs & laptop computers have USB port(s) instead of RS-232 serial port(s). USB serial adapter plugs into USB port, & allows a serial device to be connected through adapter. Multi-port adapters are available from other companies; see Application Note 68.



  • 801376 To computer COM port with 9V connector (from XSG connector),2.4 m, DN 32604
  • 801385 To computer COM port with power leads (from XSG connector), 2.4 m, DN 32277
  • 17031 pigtail (from XSG connector), 2.4 m, DN 30581
  • 801263 To computer COM port with 9V connector (from Wet-pluggable connector),2.4 m, DN 32490
  • 801206 To computer COM port with power leads (from Wet-pluggable connector), 2.4 m, DN 32366
  • 171368 pigtail (from Wet-pluggable connector), 2.4 m, DN 32363

Mount Kits

Mount to seawater intake pipe leading to SBE 21 or SBE 45

  • 50244 Thermosalinograph Stainless Remote Temperature Sensor Mount Kit (document 67071)