MICRO-CTD INSTRUMENT DEVELOPMENT
FOR THE OCEAN SCIENCES

A. Fougere, N.L. Brown, D. Frye and J. Toole
Woods Hole Oceanographic Institution

The Micro-CTD Profiler (Conductivity, Temperature and Depth) is a miniature, state-of-the-art, integrated sensor and communication system. Important features are:

• High Accuracy ± 0.005 mmho (0.0005 S/m) Conductivity
• ± 0.005 Degree Celsius Temperature
• ± 0.15% Full Scale Pressure
• High Stability Inductive Conductivity Sensor
• Platinum Thermometer or Sheathed Thermistor
• Titanium Strain Gauge Pressure Sensor
• Direct Digital Output via RS-485
• No Electrodes to Foul or Alter Conductivity Calibration
• Internal Reference and Self-Calibrating Electronics
• Small Size Sensors with Predictable Responses
• Built-In 7 Channel - 12 BIT High Performance DC Digitizer
• Built-In Real Time Clock with "ON/OFF" Power Control
• Optional 1.0 MB Internal Static Memory - Bi-Directional 10KM FSK Communication
• Inductive Telemetry for Simplified Real Time Data Telemetry for Mooring Applications.

The Micro-CTD is being developed jointly by the Woods Hole Oceanographic Institution and Falmouth Scientific, Inc. of Falmouth, MA. It is presently in the prototype stage with much of the sensor electronics complete, but much of the control hardware and software yet to be done. The Micro-CTD combines newly developed sensors and self-calibrating electronics to achieve high levels of long-term stability and accuracy. These new sensors include an inductively coupled conductivity sensor, a highly stable platinum resistance thermometer or optional high speed sheathed Thermistor and an improved titanium pressure sensor. The sensor electronics use continuous internal calibration against precision fixed references to eliminate errors due to changes in circuit performance. The sensor and electronic technologies were conceived and designed by Neil L. Brown of Woods Hole Oceanographic Institution in conjunction with Falmouth Scientific, Inc., the licensed manufacturer.

The design goal of the Micro-CTD was to optimize dynamic measurement performance and minimize size and cost while enhancing long term stability. The inductive conductivity cell was chosen for the Micro-CTD because it has the capability for long term calibration stability. Through innovative electronic techniques, the negative characteristics of the inductive technique, i.e. large physical size to sensed volume and long flushing lengths, were eliminated. The inductive cell has a 0.5 diameter to length ratio resulting in a geometry with a 6.0 cm flushing length. the small geometry of the sensor enables it to have a low thermal inertia relative to the volume of sea water sensed. All of the Micro-CTD sensors are mounted in the free flow of water past the instrument; no pumps or other devices are used to correct for poor sensor flushing characteristics. Overall size of the Micro-CTD is 4.9 cm diameter by 35.1 cm long.

The Micro-CTD includes a low power microcontroller that collects, scales, and transmits data over the built-in RS-485 interface or optionally via either a two-way telephone modem or an inductive modem developed by WHOI. The Micro-CTD calibration constants are stored in internal nonvolatile memory and can be user updated as required.

CONDUCTIVITY

The conductivity sensor is a new ceramic inductive cell which has excellent long term stability and uses advanced electronic compensation techniques to minimize errors due to changes in circuit performance. The sensor has a resistance of 20 ohms at 65 mmho (ms/cm) resulting in a cell constant near unity. The large, open-bore design allows for high speed flushing without the need for pumps or flow inducing mechanisms. The cell can be coated with antifoulants to minimize biofouling when used near the surface.

The need exists to place CTD measurement equipment in the ocean for extended periods of time with the calibration of conductivity sensors remaining valid. We are investigating various high performance antifoulant coatings to reduce the effects of biological growth on conductivity sensor calibration As the sensor is non-contacting it allows for direct application of coatings over the entire surface area of the sensor. Several candidate coatings have been selected which are low in toxicity, have a high durometer finish, and are compatible with the sensor materials.

TEMPERATURE

The Micro-CTD can be equipped with either a platinum resistance thermometer, or for shorter time response (and lower accuracy) a sheathed 1.3mm diameter Thermistor. The platinum thermometer provides excellent long term stability with a time constant of O.75s. The Thermistor has a time constant of O.1 second. The platinum thermometer is constructed using four helically wound high grade platinum coils which are glass-fused into a ceramic holder. The ceramic holder is mounted in a 2.8mm O.D. titanium sheath. The sheath is filled with ceramic powder to ensure high thermal conduction from the sheath to the element. The end is then sealed with an epoxy plug. The Thermistor is constructed using a high stability 500 ohm Thermistor bead with a diameter of 0.2 mm. This bead is mounted in the sheath using a thermally conductive epoxy.

PRESSURE

The Micro-CTD uses a newly developed miniature strain gauge pressure transducer. It is mounted inside the instrument so that it can be protected when used for extended deployments. The strain gauge is a high output 2OmV/V excitation output bridge with a 1500 ohm impedance. Thermal effects on the transducer are corrected using an internal temperature sensor which also monitors the operating temperature of the reference networks for all three high accuracy channels. The pressure transducer was selected for its small size, high accuracy, and availability in a large number of pressure ranges.

CONDUCTIVITY

This sensor employs electronic compensation techniques to correct for measurement errors in the magnetic properties of the inductor. The electronic compensation technique makes it possible to use a small inductive sensor, resulting in both small spatial measurement resolution and low thermal contamination. Inductive conductivity sensors can maintain measurement stability as they have no wetted electrodes to foul which can alter the geometry and thus the calibration of the sensor.

A block diagram of the inductive conductivity cell drive circuit is shown in Figure 1 below.

Figure 1

INDUCTIVE CONDUCTIVITY ELECTRONICS BLOCK DIAGRAM

The inductive sensor is driven from an oscillator located at the left of the diagram. The oscillator directly drives the drive core winding Wi and the drive error sensor winding W2. A high gain feedback amplifier Al supplies additional voltage to the drive winding Wi, until the voltage across winding W2 is equal and opposite the oscillator drive level. This results in a voltage input to Al equal to zero. As a result, Al supplies additional drive current to fully correct the forward voltage errors of the drive core and the sense core. The seawater forms a one turn winding around both the drive core and the sense core. The drive core induces a current in the seawater (Isw) whose magnitude is directly proportional to the sea water resistance path (Rsw). The sense core winding, W4, senses the difference between the induced current and the drive current flowing through sense core winding W3. The conductivity detection circuit utilizes active feedback to adjust the current flowing in winding W3 until it is equal and opposite the current flowing in the sea water, Isw, (times the turns ration). In effect the input current to high gain amplifier A2 is driven to zero. This results in a zero flux level in the sense transformer at balance. As a result, the measured current, Iref, is independent of the sense core magnetic properties and any changes in them. This results in a conductivity detection circuit output, which is directly proportional to the oscillator level and the resistance of sea water, Rsw. This output forms the input to the conversion circuitry for the AID converter.

TEMPERATURE

Micro-CTD sensor electronics are based on technology developed by Neil Brown. This new approach has demonstrated its ability to attain performance equal or superior to existing technologies at a fraction of the cost. The technique offers excellent long term stability through the use of continuous internal calibration using fixed reference resistors. The interface in the Micro-CTD is multiplexed between all three sensors and the corresponding calibration network which represents the parameter in question.

To maintain the smallest possible sensor size while providing a standard serial data stream, a new approach to high accuracy temperature measurements was required. Existing AC ratiometric techniques as used in previous sensors are too large and complex to mount in a small self-contained instrument. Alternate Wien bridge oscillator techniques are inappropriate due to their inherent temperature instability and reliance on high output, low stability Thermistor sensors. In the Micro-CTD, the use of four terminal measurement techniques coupled with frequent internal calibration against fixed resistors has simplified and reduced the size of the electronics. This approach relies on internal logic to select calibration elements and sensor output, measure the circuitry's response to them, and then compute the actual measured value from the sensor. The microcontroller used in conjunction with the auto-calibrating electronics allows for the direct conversion of the output into engineering units in ASCII format.

An illustration of the calibration circuit for temperature is shown in Figure 2.

Figure 2

BLOCK DIAGRAM ELECTRONIC CIRCUITRY FOR TEMPERATURE SENSOR

The basic temperature bridge is made up of precision resistors Ri, R2 and Rf and the thermometer Rt. The self calibration circuit consists of precision resistors Ra, Rb, Rc, and Rd. A secondary circuit consisting of temperature sensor Rt' is used to measure and correct for thermal shifts in both the reference and calibration resistors. This secondary temperature measurement removes the requirement to tie the reference and calibration resistors thermally to the temperature source being measured. The large physical structures associated with such thermal connection result in long thermal stabilization times between the reference resistor and the outside temperature.

A polynomial fit is used to calibrate the PRT such that:

T = Ac + Bc * X + Cc * X²

Ideally, the reference network Ra, Rb, Rc, Rd and Rf would have near zero temperature coefficients and be absolutely stable. However, only the absolute stability of the resistors is required. To ensure adequate stability of the resistors, high precision Vishay type devices are used. These resistors exhibit only 1 to 2 PPM drift/year after elevated temperature burn-in stabilization. This approach, i.e. using a precision resistor network to accurately simulate a sensor output at known values of the measured parameter allows simple electronics to be used with negligible requirement for electronic stability. This results in a dramatic reduction in the size, cost, and power consumption associated with the temperature measurement.

The above description has been predicated on the use of electronics and sensors with a simple transfer function. However, if the transfer function is more complex, a 3rd or 4th order polynomial fit can be performed using the same techniques. A calibration resistor network would be required to simulate four or five different values of the measured parameter with the appropriate correction algorithms. The additional cost and complexity of accommodating a more complex transfer function is trivial. Overall accuracy is thus dependent only on the stability of the sensor and the simulation network of resistors. As an example, the standard platinum thermometer transfer function has been accurately described by a 2nd order polynomial. Thus, the simulation network consists of only four precision resistors. Both pressure and conductivity sensors are essentially linear and can be represented by a 1st order polynomial. For the sake of simplicity and commonality, 2nd order fits are used in all three channels.

FIRMWARE

Micro-CTD data processing and control hardware is based on the Intel 8051 series micro-controller. The 87C51FC controller has on-board program, memory, and interfaces allowing it to control the various measurement functions while maintaining communication with the user and/or to an optional 1.0 Mbyte static memory. The processor is programmed in "C". The FC version controller was selected due to its expanded internal static memory which allows for higher speed floating point operations. The CPU is supported with an additional 32K of SRAM for storage of temporary values. The CPU also serially accesses an 8 channel - 12 bit DC digitizer which used to monitor the temperature of the calibration references, with the remaining 7 channels available to the user for optional sensor inputs. Operating conditions and calibration constants are held in a 2K EEPROM allowing the user to download and save either new operating parameters or new calibration coefficients. The CPU has a real-time clock with independent battery back-up allowing for timed sleep/data collection operations, data, or event time tagging.

COMMUNICATIONS

The Micro-CTD supports RS-485 communication. This IEEE standard is becoming more popular due to its ability to support multi-drop, long wire, and low power communication. Two additional interfaces are planned. The first is a Bell 212 modem allowing direct communication with standard PC plug in modems. The Bell 212 modem allows for communication over standard phone lines or over long hydrographic cables. Second, we plan to develop an inductive modem interface to allow telemetry on standard, plastic jacketed mooring wires.

POWER

The present instrument design has power provided from an external source. The pressure housing diameter will be increased to accommodate a battery in autonomous versions of the Micro-CTD. Instrument power consumption is 600 milliwatts while actively making measurements and less than 5 milliwatts at idle.

TEMPERATURE

Shown in Figure 3 are the deviations between a Neil Brown Instrument Systems ATB-1250 Temperature Bridge and the FSI Temperature module with response essentially similar to the Micro-CTD. The measurements were made in a Tronac 25-liter temperature controlled bath at the WHOI Calibration Facility.

Shown in Figure 4 are the residual errors from a 3rd order polynomial regression fit to the data given in Figure 3.

Figure 5 shows the residual errors found from an up-down calibration cycle of a 1000 dBar pressure transducer. This data was collected one month after its initial calibration. The test was performed at 22°C with the original calibration having been performed at 0°C.

Figure 3

INITIAL RAW OTM OUTPUT ERROR VS. ATB-1250 STANDARD

(REGRESSION FIT OVERLAID)

Figure 4

RESIDUAL ERRORS FROM 3^RD ORDER POLYNOMIAL FIT

Figure 5

UP-DOWN CALIBRATION LOOP MCTD VS. RUSKA

AIR DEADWEIGHT RAW DIFFERENCE

RANGE: 0 - 65 mmho (0 - 6.5 S/m)

ACCURACY: +1- 0.005 mmho (+1- .0005 S/m)

STABILITY: +1- 0.001 mmho/month (+1- .0001 S/m/month)

RESOLUTION: 0.0004 mmho @ 10 Frames/second (16 bits)

0.0001 mmho @ 1 Frames/second (18 bits)

RESPONSE: 6.0 cm Flushing length

RANGE: 20 - 320 Celsius

ACCURACY: +1- 0.005⁰C PRT (+1- O.010⁰C Thermistor)

STABILITY: +/- 0.001 ⁰C/month

RESPONSE: 500 - 800 milliseconds PRT

SELF HEATING: <O.0003⁰C @ I meter/second flow

RANGE: 0- 600 dBar to 0 - 7000 dBar

ACCURACY: +1- 0.15% of Full Scale

HYSTERESIS: +1- 0.01% of Full Scale

REPEATABILITY: +1- 0.015% of Full Scale

STABILITY: +1- 0.01% of F.S./month

OVER-PRESSURE: 25% of Range

MATERIAL: Titanium 6AL-4V

RESOLUTION: 16 Bits @ 10 Samples/second

DC CHANNELS Type: 8 Unipolar

Resolution: 1.22 mV

CONNECTOR:

DATA FORMAT: Conductivity in mmho/cm IPS-78

Temperature in Celsius ITS-90

Pressure in Decibars (SNNNNN.NN)

Baud rates: 9.6, 19.2, 38.4 KBPS

FSK Data Bits: 8

Stop Bits: 1

Format: ASCII

CLOCK: Crystal 32.768 KHz

Input Connector Jumper Instrument Wake-Up

WARM-UP: 3.0 Seconds after Power-Up

Funded by the Office of Naval Research under Contract N00014-86-0751.

Outline, Micro CTD, Drawing #C162-004