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DESCRIPTION
The SBE 35 is a laboratory standards thermometer with the unique ability to be used both in fixed point cells and at depths up to 6800 meters (22,300 ft). It is unaffected by shock and vibration encountered in shipboard and industrial environments, making it ideal for use in calibration laboratories in the range of -5 to +35° C, and in the thermodynamic method of measuring hydro turbine efficiency. The SBE 35 communicates via a standard RS-232 interface at 300 baud, 8 data bits, no parity.
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DEEP OCEAN APPLICATIONS The SBE 35 can be used with the SBE 32 Carousel Water Sampler and one of the following CTD systems:
The SBE 35 makes a temperature measurement in EEPROM each time a bottle fire confirmation is received, and stores the value in EEPROM. Each stored value contains the time and bottle position in addition to the temperature data, allowing comparison of the SBE 35 record with CTD and water bottle data. Using one SBE 35 eliminates the need for reversing thermometers and provides higher accuracy temperature readings at lower cost. |
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INDUSTRIAL OR LABORATORY APPLICATIONS
The thermometer's digital output can be displayed in real-time and logged to a computer file. The SBE 35 is standardized in water triple point (TPW) and gallium melting point (GaMP) cells, following the methodology applied to a standards-grade platinum resistance thermometer (SPRT). Unlike SPRTs, it does not require an expensive resistance bridge, making the SBE 35 a dramatically cost-effective alternative. For measurements in fixed point cells, the protective guard is removed and a brass and plastic tip bushing is attached to give the SBE 35 the length, diameter, and thermal averaging characteristic of an SPRT. The SBE 35 resolves temperature in fixed point cells to approximately 0.000025° C (25 µK). Accuracy is better than 1 mK.
The SBE 35 is supplied with a small desktop Interface Box that is connected between the SBE 35 and a computer for setup and lab use. The Interface Box is 110/220 VAC powered, provides 15 VDC to the SBE 35, and buffers the communication lines to minimize interference from external noise.
User-selectable operating modes are:
SOFTWARE AND DATA UPLOAD
Seaterm, the terminal program in our SEASOFT© V2 software suite, provides easy SBE 35 setup and data uploading. Calibration coefficients stored in EEPROM allow the SBE 35 to transmit data in engineering units. When configured for a real-time ocean application, the SBE 35 can use the system modem channel for two-way communications; it is not necessary to change cable connections to communicate with and retrieve data from the SBE 35.
MEASUREMENT METHOD
Temperature is determined by applying an AC excitation to reference resistances and an ultrastable aged thermistor with a drift rate of less than 0.001 °C per 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 microwatts, and contributes less than 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, and NT is thermistor output.
Each measurement acquisition cycle takes 1.1 seconds. The number of cycles per measurement is programmable. Increasing the number of 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].
LINEARIZATION AND CALIBRATION
Following the methodology used for SPRTs, the calibration of the SBE 35 is accomplished in two steps:
Characterize the non-linear temperature vs. resistance response of the sensor. Temperature is computed using the Steinhart-Hart polynomial for thermistors (Steinhart and Hart, 1968; Bennett, 1972) which is based on thermistor physics:
t90L = [1.0/(a0+alln(n)+a2ln²(n)+a3ln³(n)+a4ln4(n))] - 273.15
where: n = SBE 35 outputCarried to fourth order, this equation characterizes the SBE 35 output to an error of ± 50 µK.
Unlike SPRTs, where the basic non-linear calibration equation has fixed coefficients that apply to pure platinum, thermistors require individualized coefficients to the Steinhart-Hart equation because the thermistor material is an individualized mix of dopants. The individualized SBE 35 calibrations are performed at Sea-Bird in a low-gradient temperature bath and against ITS-90 certified SPRTs maintained at Sea-Bird's primary temperature metrology laboratory.
Certify the sensor by measurements in thermodynamic fixed point cells. The water triple point (0.0100 °C) and gallium melt point (29.7646 °C) are used because the SBE 35 is designed for ocean range temperatures (-5 to 35 °C). Like SPRTs, the slow time drift of the SBE 35 is adjusted by periodic recertification corrections:
t90 = slope * t90L + offset [°C, ITS-90]
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Note: Temperature sensor guard not shown.
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SPECIFICATIONS
| Measurement range | -5 to +35 °C |
| Initial accuracy | 0.001 °C |
| Typical stability (per year) | 0.001 °C |
| Resolution | 0.000025 °C |
| Sensor calibration | -1.5 to + 32.5 °C |
| Data storage | Up to 179 samples |
| Real-time clock | Watch-crystal type |
| External power | 9 - 16 VDC |
| Current |
On power
application (≈ 1 minute):
140 – 160 mA Operating: 60 – 70 mA |
| Housing materials | Aluminum, rated at 6,800 meters (22,300 feet) |
| Weight | In water: 0.5 kg (1.1 lbs) In air: 0.9 kg (2 lbs) |
ADDITIONAL INFORMATION / LINKS:
Documentation -- manual, photos, technical papers, application notes, etc.
Sales Information -- options, accessories, cables, mount kits, spares, etc.
Software -- components of Seasoft V2
Links to Other Instruments of Interest
Specifications are subject to change without notice.
Sea-Bird Home Phone: (+1) 425-643-9866 Fax: (+1) 425-643-9954 E-mail: seabird@seabird.com
