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SBE 35RT Digital Reversing Thermometer

SBE 35RT Digital Reversing Thermometer

Temperature sensor for use with Sea-Bird water sampler systems, taking a measurement each time a bottle closure confirmation is received, for comparison with the water bottle data.

The SBE 35RT is an accurate, ocean-range temperature sensor that is capable of measuring temperature in the ocean to depths of 6800 meters. The SBE 35RT can be used with a CTD / Water Sampler system, eliminating the need for reversing thermometers and providing higher accuracy temperature readings at lower cost.

The SBE 35RT is externally powered, and communicates via a standard RS-232 interface. Measurements are output as raw data or in engineering units (°C).


The SBE 35RT makes a measurement each time a bottle fire confirmation is received, and stores the time, bottle position, and temperature, allowing comparison with CTD and water bottle data.

  • RS-232 output at 300 baud, 8 data bits, no parity.
  • Aluminum housing; depths to 6800 m.
  • Seasoft© V2 Windows software package (instrument setup, data display, and data upload).
  • Five-year limited warranty.


  • XSG or wet-pluggable MCBH connector.
  • Interface Box to connect SBE 35RT to a computer for setup and lab use (100-240 VAC powered, provides 15 VDC to SBE 35RT, and buffers communication lines to minimize interference from external noise).


Temperature is determined by applying an AC excitation to reference resistances and an ultrastable aged thermistor with a drift rate < 0.001 °C/year. Each of the resulting outputs is digitized by a 20-bit A/D converter. The reference resistor is a hermetically sealed, temperature-controlled VISHAY. The switches are mercury wetted reed relays with a stable contact resistance. AC excitation and ratiometric comparison using a common processing channel removes measurement errors due to parasitic thermocouples, offset voltages, leakage currents, and gain errors. Maximum power dissipated in the thermistor is 0.5 µWatts, and contributes < 200 µK of overheat error.

Sensor Output (raw counts) = 1048576 * (NT - NZ) / (NR - NZ)
where NR is reference resistor output, NZ is zero ohms output, NT is thermistor output.

Each measurement acquisition cycle takes 1.1 sec. The number of cycles per measurement is programmable. Increasing the cycles increases acquisition time while reducing RMS temperature noise. In a thermally quiet environment, the temperature noise standard deviation is 82 * sqrt (1/n cycles) [µK].


Measurement Range -5 to +35 °C
Initial Accuracy ± 0.001 °C
Typical Stability 0.001 °C per year
Resolution 0.000025 °C
Calibration -1.5 to + 32.5 °C
Memory 179 samples
Real-Time Clock Watch-crystal type


Input Power 9 - 16 VDC;
On power application (≈ 1 minute) 140 – 160 mA; Operating 60 – 70 mA
Output Signal RS-232 (300 baud, 8 data bits, no parity)


Housing & Depth rating Aluminum, 6800 m
Weight 0.7 kg in air, 0.3 kg in water


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

For older SBE 35RT product manuals, organized by instrument firmware version, click here.

Title Type Publication Date PDF File
SBE 35RT Datasheet Product Datasheet Wednesday, August 19, 2015 PDF icon 35RTbrochureAug15.pdf
SBE 35RT Manual Product Manual Thursday, February 12, 2015 PDF icon 35RT_013.pdf
AN42: ITS-90 Temperature Scale Application Notes Wednesday, May 18, 2016 PDF icon appnote42May16.pdf
AN57: Connector Care and Cable Installation Application Notes Tuesday, May 13, 2014 PDF icon appnote57Jan14.pdf
AN68: Using USB Ports to Communicate with Sea-Bird Instruments Application Notes Friday, October 19, 2012 PDF icon appnote68Oct12.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
File Seaterm_Win32_V1_59.exe for Windows 7/8/10

Please Note:

Seasoft V2 is a single install bundle with the following stand-alone programs (table below) for use with most of our products (especially SBE numbers products like SBE 37 MicroCATs). While most of our instruments uses one or more of these programs, not all instruments use all programs (eg.  Plot39 which is only used by SBE 39 family) .

Each instrument has the list of applicable programs under the product page's software tab. Each software includes Help files with detailed descriptions for use.

Version 2.2.7 released July 26, 2017
File SeasoftV2.2.7-b19.exe for Windows 7/8/10

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.
  • Moored CTD — recalibrate at least once/year, but possibly more often depending on the degree of bio-fouling in the water.
  • 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 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.

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.

35RT DIGITAL 'REVERSING' THERMOMETER – same as SBE 35 but with short probe; not usable in fixed-point cells. Power & communication supported by CTD / SBE 32 Carousel system for in-situ use. Includes Seasoft software & complete documentation. (Note: Laboratory power supply/interface box & associated cables, & CTD/Carousel interface cable sold separately)  
SBE 35RT Wet-Pluggable Connector Option
35RT-1 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 (exception: 6-pin connectors) & 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.
SBE 35 Spares & Accessories
80555 Lab data cable, SBE 35/35RT to 90248, MS to AG206, 2.4 m (DN 31116) 80555 (standard connector) or 801534 (wet-pluggable connector) cable to interface with 90248 Power Supply/Interface Box included if optional 90248 SBE 35 Power Supply/Interface Box ordered (see below).
801534 Lab data cable, SBE 35 / 35RT to 90248, Wet-pluggable,  MS to MCIL-6FS, 2.4 m (DN 32962)
171220 Carousel interface Y-cable, AG connectors, SBE 35/35RT to SBE 9 & SBE 32 (DN 32208) SBE 35RT can record temperature with each SBE 32 Carousel Water Sampler bottle closing, for comparison to data from CTD’s temperature sensor. SBE 35RT can be used with any size Carousel (full size 32, compact 32C, or sub-compact 32SC).
  • 171220 (AG connectors) or 171995 (wet-pluggable connectors) Y-cable connects SBE 35 to:
    — SBE 9plus CTD & Carousel, when Carousel used with 9plus CTD with SBE 11plus Deck Unit for real-time operation.
    — SBE 17plus V2 & Carousel, when Carousel used with 9plus CTD & SBE 17plus V2 Searam for autonomous operation.
    — AFM & Carousel, when Carousel used with SBE 19, 19plus, 19plus V2, 25, or 25plus CTD & AFM for autonomous operation.
  • 171221 (AG connector) or 171996 (wet-pluggable connector) cable connects SBE 35RT to Carousel, when Carousel used with SBE 19, 19plus, 19plus V2, 25, or 25plus CTD & SBE 33 Deck Unit for real-time operation.
171995 Carousel interface Y-cable, Wet-pluggable, SBE 35 / 35RT to SBE 9 & SBE 32 (DN 32963)
171221 Carousel interface cable, AG connectors, SBE 35/35RT to SBE 32, used with SBE 33 Deck unit, 2.4 m (DN 32209)
171996 Carousel interface cable, Wet-pluggable, SBE 35 / 35RT to SBE 32, used with SBE 33 Deck Unit, 2.4 m (DN 32964)
90248 SBE 35/35RT Power Supply/Interface box (includes 80555 sensor to box cable, 171887 box to computer cable, & AC power cable) For laboratory use.
50309 SBE 35RT Mounting Kit for Compact or Sub-Compact Carousel Mounts SBE 35RT on SBE 32C Compact or 32SC Sub-Compact Carousel Water Sampler.
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.


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


  • 80555 To Power Supply / Interface Box (from 35/35RT AG connector), 2.4 m, DN 31116
  • 801534 To Power Supply / Interface Box (from 35/35RT Wet-pluggable connector), 2.4 m, DN 32962
  • 171887 To computer COM port (from Power Supply / Interface Box), 3 m
  • 17015 To AC power supply from Power Supply / Interface Box (U.S. Standard)
  • 171220 To SBE 32 and SBE 9plus or 17plus V2 or AFM (AG connectors), Y-cable, DN 32208
  • 171995 To SBE 32 and SBE 9plus or 17plus V2 or AFM (Wet-pluggable connectors), Y-cable, DN 32963
  • 171221 To SBE 32 (AG connectors) (when SBE 32 used with SBE 33 Deck Unit), 2.4 m, DN 32209
  • 171996 To SBE 32 (Wet-pluggable connectors) (when SBE 32 used with SBE 33 Deck Unit), 2.4 m, DN 32964

Mount Kits

Mounted to 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

Mounted to SBE 32 (see note below)

  • 231561/231562 SBE 35RT Bottle Position Mount Kit
  • 50326 SBE 35RT Mount Kit to SBE 32 extension stand with 1-inch (2.54-cm) diameter post
  • 50327 SBE 35RT Mount Kit to SBE 32 extension stand with 1.375-inch (3.49-cm) diameter post

Mounted to SBE 32C/32SC
50309 SBE 35RT to SBE 32C or 32SC Mount Kit (document 67120)

Note: When ordering kits for mounting an instrument on SBE 32 Carousel frame, note that SBE 32 comes in 2 frame sizes:

  • Small Frame (1-inch diameter) for full size SBE 32 12-place (through 12-liter bottle size) and 24-place (through 2.5-liter bottle size), and all sizes of compact SBE 32C and sub-compact SBE 32SC.
  • Large Frame (1.375-inch diameter) for full size SBE 32 12-place (larger than 12-liter bottle size) and 24-place (larger than 2.5-liter bottle size).

Spare Parts

  • 23041 Zinc anode ring for end cap