RBR Data Logger & Sensor Technology
Sensors: Sensors (RBR and Third Party) Conductivity Temperature Depth   General: Memory and Power Physical Construction Software Calibration

Sensors

The table below includes those sensors RBR uses regularly. Links to each company's web site are provided so you can examine their sensor specifications and determine exactly which suits your needs.

RBR is also ready to accommodate other sensors. Contact us if you have a particular sensor you would like installed.

Conductivity

RBR manufactures two type of conductivity sensors; one for the marine range, and one for the freshwater range.

• The normal freshwater conductivity range of 0 to 2 mS/cm is measured by a three electrode cell, with an accuracy of ± 0.003 mS/cm and resolution to better than 0.0001 mS/cm. The maximum depth rating of this conductivity cell is 2,000m.

• The normal oceanographic conductivity range of 0 to 70 mS/cm is measured by an inductive cell to an accuracy of ±0.003 mS/cm. RBR uses a three-coil system with a closed loop feedback for superior temperature compensation. The titanium housing provides a depth rating for the cell of up to 6,600m. It also provides shielding and stable cell geometry. The response time of the inductive cell is less than 95 milliseconds when profiling.  An axial cell will be supplied for all profiling applications.

Salinity and speed of sound are calculated by the logger software.  RBR uses the internationally accepted method for deriving salinity from conductivity, temperature and depth. (1978 Practical Salinity Scale). RBR software gives the user the choice between the Chen and Millero UNESCO algorithm (default) , the older Wilson algorithm, and the more recent Del Grosso algorithm for computing the speed of sound (sound velocity). Download the RBR technical brief (.pdf format) on Seawater Salinity Calculation.

Temperature

RBR data loggers are calibrated to an accuracy of ± 0.002°C (ITS-90) over the range -5 to +35°C.  

RBR maintains primary standards for

•  - 38.8344K mercury triple point
•  +  0.0100K water triple point
•  +29.7646K gallium freezing point

Extended temperature calibration, down to -40°C and up to +50°C is available. Beyond +35°C the platinum resistance thermometer is used to provide the extrapolation, and the accuracy is about ± 0.005°C

Drift of the thermistors has been measured to be better than 0.002°C/year.

Calibration constants are stored in the logger, together with a complete calibration history.

The standard model thermistor is made by Thermometrics. The thermistor is aged and hermetically sealed in the tip of a shock resistant solid glass rod. It has a time constant of 85 msec and is housed in a durable 3mm 316 stainless steel externally mounted housing. In this housing the thermistor has a time constant of less than 3 seconds. An optional fast thermistor is also available with a 0.9mm OD housing and a time constant of less than 0.095 seconds.

Temperature measurements are made to 24-bits. Temperature measurements are taken by the XR-420 at a resolution of <0.00005°C. The measured rms noise floor is ±0.000046°C.

RBR fits the temperature measurements to the Steinhart-Hart Equation:

1/T = a + b(LnR) + c(LnR)³

This equation is an empirically developed polynomial which best represents the resistance-temperature relationships of NTC thermistors. The reference resistor is specified to 5 ppm/year and 0.6 ppm/°C.

Depth

The available depth ranges for an RBR depth sensor are:

* maximum depth rating for loggers with Delrin housing

Measurement resolution is <0.001% full scale, and accuracy is 0.05% full scale.

The XR-series, the TDR-2050 and DR-1050, all use an internal Keller piezo-resistive strain gauge sensors. The sensors are absolute pressure sensors. Atmospheric pressure (nominally 10.1325 dbar at sea level) must be subtracted from the absolute pressure to obtain water depth.  An alternate pressure gauge available for the XR-series instruments is the 8CDP series from Paroscientific.  These offer an accuracy of 0.015% and a resolution approaching 1 part in 10^-8.

The standard depth sensors are calibrated at RBR to better than ±0.05% full scale using an NIST traceable deadweight pressure generator. Using reliable semiconductor technology and a 316 stainless steel housing, the depth sensors are developed for long-term stability in harsh environments. This is also reflected in their operating temperature range of -40°C to 125°C. Typical depth hysteresis of the depth sensor is ±0.015%FS, and it has a response time of 0.1 ms.

Memory and Power

The amount of memory available depends upon the model. All RBR Data Loggers are equipped with 8MB of non-volatile flash. This provides sufficient memory for 2,400,000 readings at 3 bytes per reading per channel.

The data retention of the non-volatile flash is considered to be 20 years at normal temperatures.

Power consumption can vary significantly depending on the complement of sensors, sampling rate, temperature of the water and whether averaging is used. The RBR Logger Software includes a calculator that will automatically estimate the power consumption and memory usage for the deployment settings that you choose. You may download, install, and run the software without a real instrument attached to your PC, and simulate these calculations for any logger/sensor combination.

Construction

RBR data loggers are available in two casings: plastic, using the Delrin^® acetal polymer, or titanium. The depth rating of the plastic housing is 740m; and that of the titanium housing is 8,500m (10,500m for the 1050 and 2050 series cases).  Loggers with inductive conductivity cells are limited to 6,600m depth, and with freshwater conductivity cells to 2,000m Other sensors may also restrict the operating depth -- see the above table. Both ends of all XR-series loggers utilize an integral pressure release for safe opening. The O-ring seals at both ends are situated above the threads to ensure a safety pressure release before the end-cap threads fully disengage from the body.

The battery carousel and data port are easily accessed from the non-sensor end. One can access the data port without having to remove the battery carousel. only the end cap needs to be removed. Download time is ~150,000 samples/minute. Batteries are easily changed; simply untwist the end cap and then unscrew the mounting bolt of the carousel.

The TR-1050 has CE marking EMC Directive 89/336/EEC. This EMC Directive governs conformity with regulations concerning electromagnetic emissions and immunity to electromagmnetic emissions.

Calibration

Temperature

RBR uses a Hart Scientific 4012 or 4037 stirred bath and a Model 1575 or 1590 Platinum thermometer. The calibration of the system is verified using our own primary reference standards: a -38.8344°C (±0.0001°C) triple point of mercury, a 0.0100°C triple point of water cell (±0.00008°C) and a 29.7646°C freezing point of gallium reference (±0.0001°C). All of the instruments are supplied with NIST traceable calibration. Standard calibration of each unit is carried out at eight temperatures from -5° to +35°C. Temperature readings are taken at ten second intervals, and an average of 100 stable samples is used to obtain each point. This produces a fit with a typical uncertainty of <0.2 mK on each point.

Sample temperature calibration graph:

The NIST claims an uncertainty for thermistor calibration of little better than ±2 mK, and only suggests ±0.3 mK for a TPW cell. The NRC of Canada currently will only accredit secondary standards laboratories to an accuracy of ±6 mK.  We offer calibration to our own primary standards to avoid any problems of transfer of calibration.

Pressure

A Druck Pressurements hydraulic deadweight tester is used to give a direct NIST traceable standard. Pressures throughout the range of 0 to 4000PSI (0 to 2700m depth) are available to an accuracy of 0.015%. For each pressure range offered, an eight point calibration is carried out and a cubic fit to the data is used to give the sensor characteristic. The gauges used are all absolute pressure. The conversion from absolute pressure to depth requires a knowledge of the local atmospheric pressure and water density. These may be entered into the logger software at the time of download of data to give water depth. Depths beyond this range are calibrated against a Paroscientific 10,000dBar gauge which has an accuracy of 0.015% full scale.

Sample pressure calibration graph:

Conductivity

We use a Guildline Autosal 8400B (±0.002), which is standardized using IAPSO Standard Seawater. All calibrations are carried out using eight points, and a cubic polynomial curve fit. All loggers are supplied with a calibration certificate showing the deviation from the curve at each point. Below is a graph of residuals for the freshwater range of 0 to 2 mS/cm (graph in µS/cm).

Residuals vs conductivity for the 0 to 70 mS/cm range are as follows:

Other Sensors

Our other sensors are third party equipment. For Turbidity, DO, pH and ORP we offer an in-house calibration using standard techniques.  In all other cases we rely upon the manufacturer's calibration.

• To see our list of third-party sensor, click here.
• For pricing enquiries please e-mail: info@rbr-global.com