The contouring temperature recorder

INSTRUMENTAL

NOTE

The contouring temperature recorder WILLIAM S. RICHARDSON a n d CHARLES J. HUBBARD

(Received 29 April 1959) (1)

INTRODUCTION

THE bathythermograph (BT) has been the standard instrument for the study of the thermal structure of the upper layers of the ocean for roughly 25 years. This very useful instrument provides a trace of temperature against depth (to either 450 or 900 ft, depending upon the type used) while the ship is under way. For some purposes this trace is the end result o f the experiment, but to study the variations of structure in the horizontal, one commonly reads the BT traces, picks off the depths of various temperatures, and plots these on a curve of depth versus distance. The isotherms are then contoured and the resulting distribution is used as the data from which the physical processes are inferred. It is possible on a very arduous schedule to obtain BT's about every five minutes, but on most trips one is taken every hour or more. This provides sufficient data for many problems, and it may be easily supplemented by continuous surface temperature records to provide an indication of the location of some of the pronounced differences which often occur from one BT to the next. For many purposes the BT data even at its most rapid rate of acquisition is not sufficient; problems in sound transmission, internal waves and microstructure are examples. The Contouring Temperature Recorder to be described in this paper is a device which takes data from temperature sensors (thermistors) located in a chain towed behind a ship and plots on a continuous record the vertical distribution of isotherms. The thermistors are electronically scanned in sequence from the top to the bottom of the chain, in a time interval adjustable from 2 to 20 secs. If it is desired to do so, the scan rate can be adjusted in relation to the speed of the ship so that the information is taken from a vertical column in the water. In the design to be described, the 1° isotherms from --2 ° to 32°C can be contoured along with all 0'1 ° and 0.05°C isotherms between these limits. Also, selection can be made so only the 1°, the 0.1 ° (which of course include the 1°), or the 0.05°C are contoured. It is possible to accentuate the 1°C isotherms with respect to the others to aid in their identification. Provision is made for determining the depth of the tow (which is a function of ship's speed) and also for selecting the fastest possible scan rate consistent with the complexity of the water column. A straightforward method of isotherm identification is provided; this can be quite a difficult problem if there are many inversions within the water column. (2) THE HOIST AND CHAIN-MOUNTING OF THERMISTORS AND DEPTH METER The chain on which the thermistors are mounted is manufactured by Commercial Engineering Company, of Houston, Texas, (Fig. 1). The hoist is diesel-hydraulic powered and carries six hundred feet of chain. The chain consists of links about eleven inches long having a U-shaped cross section in the horizontal plane. A rubber boot is contained inside the links into which a multi conductor electrical harness can be fitted. The rear portion of each link is then closed with a plastic fairing which holds the wire securely in place and completes the streamlining of the link. A 3400 lb ' fish ' is used on the end of the chain; the design of the hoist includes a simple weans of launching and retrieving the fish. The towing characteristics of the chain are shown in Fig. 2. The tow is very steady at speeds up to at least 15 knots and is normally used at about 10 knots to provide measurements to about 400 ft. The electrical harness consists of about 100 leads of A.W.G. 26 hookup wire. This provides two leads to each of 23 thermistors, 3 leads to the depth meter and about 50 spare leads for other

239

240

WILLIAM S. RICHARDSON a n d

CHARLES J. HUBBARD

uses or repairs. The thermistors are mounted 25 ft apart in holes drilled in the plastic fairings and potted in place with an epoxy resin to insulate the electrical connection from the water. The depth meter, which is a commercial Bonrdon type pressure potentiometer with 200 PSI full scale, is mounted in a stainless steel pressure case attached to a fairing 450 ft below the uppermost thermistor. 600

5O0

40(

VS,

:

SHIP VS~EED

,oo

,~

; SHIP SPEED (KNOTS)

FIG. 2. i

I

i-,

Towing depth versus ship's speed. ~

i

f

i

THERMISTOR RESISTANCE VS.

~

,~,

,'~

;o

2'~

;o -

TEMP. (*C)

FIG. 3. Typical curve of temperature versus resistance for type 32A1 Thermistor. (3)

THE THERMISTORS

Type 32A1 Thermistors manufactured by Victory Engineering Company, of Union, New Jersey, are used in this equipment. Fig. 3 shows a typical curve of resistance versus temperature for these units. It is necessary that all units used are nearly identical. This condition is met as carefully as possible by procuring a large stock of thermistors from a single bead mix and culling them at a fixed temperature to find a set with the same resistance at this temperature. The set is then checked at

FIG. 1.

The h o i s t with chain installed

o srA.c

iii¸ ~ :

i

FIG. 8. Vicinity of 39°N l l ° W ; 17 August 1958 ; Course 270~'T ; Speed 10 knots ; I°C and 0.1°C isotherms.

Fio. 7. Vicinity of 39°N 71°W ; 1 October 1958 ; Course 180°T ; Speed 10 knots ; 1°C isotherms.

'

FtG. 10. Vicinity of 30.5°N 81°W ; 5 October 1958 ; Course 270°T ; Speed 10 knots ; I°C isotherms.

FIG. 9. Vicinity of 6°E 38°N ; 15 July 1958 ; Course 100°T ; Speed 3"3 knots ; I°C and 0-1°C isotherms.

FIG. 11. Vicinity o f 37°N 70°W ; 19 December 1958 ; Course 340°T ; Speed 10 knots ; I°C isotherms.

$11

CONTROL1 I

FIG. 4. System drawing of Contouring

1SCANNING SEED

ALSO INCLUDES: THERMISTOR CABLE P*lCHING. THERMISTOR PIDMNG.LINEARIZING NETWORKS

CHANGE CONMRTED TO VOLTAGE ANALOG OF TEMPERATURE

Temperature

Recorder.

ISOTHERM CONTOUR 1 PLOTTER

1

TEMPERAhRE READOUT PDTENTlOMETER VALANCING POTENTIOMETER

\

DC POLARIZING

WILLIAM S. RICHARDSON

242

and CHARLES

1. HUBBARD

another temperature to determine that the curvatures are sufficiently identical and finally selected numbers of the set are calibrated against a Bureau of Standards thermometer calibrated to 001°C. (4)

GENERAL DESCRIPTION

OF THE

SYSTEM

Fig. 4 is a system drawing of the electronics, slightly simplified for clarity at certain points. The operation is as follows. Each thermistor is connected in a circuit like that shown in Fig. 5. The fixed resistance R, is the lead resistance in the cable to the thermistor. Since this varies with the position of the thermistor in the chain, a second resistor Rp called padding is provided in each circuit. 2K I.05

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PADDING

WIRE

p:$i$G~~;~7j

THERMISTOR

SIGNAL VOLTAGE

FIG. 5. Thermistor

I

circuit.

2.5 -

5 s

VOLTAGE

:

VS. TEMPERATURE

tz ; 5

OIVIOEROUTPUT

2.0-

ul

I.6

;

5

IO

IS

20

25

SO

TEYI!I*CI---L

FIG. 6. Typical curve of temperature

versus signal voltage from circuit shown in Fig. 5.

These are adjusted so that the sum of RT, R,, and Rp has the same value for each circuit at a selected temperature. All these circuits are powered by the same voltage source. These voltage dividers provide a signal output which is very nearly linear with temperature (Fig. 6), and there is one such voltage for each depth where a thermistor is located. The voltage output from each of these dividers is cabled to a large potentiometer (interpolating and scanning potentiometer) where they are connected to equally spaced taps on the potentiometer winding in the order of their depths. The potentiometer has eighty thousand ohms between adjacent taps (total resistance 2.4 megohms). Therefore, each thermistor-voltage divider looks into an apparently open circuit, and the voltages impressed at the taps of the potentiometer remain proportional to temperature. As this potentiometer is rotated, the voltage on the slider is proportional to the temperature at the depth corresponding to its angle of rotation. When the slider is between taps, the voltage on the slider is a linear interpolation between the voltages impressed on the adjacent taps; i.e. the slider voltage varying with time ’ looks like ’ a curve of temperature versus depth.

The contouring

temperature

recorder

243

In fact, this voltage may be impressed on one axis of an oscilloscope, the other axis of which is driven synchronously with the interpolating potentiometer, and a BT type trace occurs. Fig. 9 shows three such traces as insets. The interpolating potentiometer is directly coupled to the shaft of a helical facsimile recorder and is driven by the helix drive motor at a steady (but selectable) rate. This recorder utilizes the rotation of a helix, mounted on a drum, to produce a uniform motion of a ‘ contact point ’ between the helix and a straight edged electrode across the emerging chart paper. A voltage between the helix and this ‘ blade ’ darkens the paper electrochemically. The position of the contact point is then equivalent to the angle of rotation of the potentiometer, which is equivalent to depth, and it remains only to sample the potentiometer slider voltage and mark the record when this voltage corresponds to an integral temperature (or tenths or five hundredths thereof). This is accomplished by balancing the slider voltage against the voltage from another potentiometer which is driven to balance by a servo amplifier/motor combination (similar to a standard self-balancing potentiometer). This in effect converts the scanning potentiometer voltage (which is proportional to temperature) to a shaft rotation of the servo system. By proper design of the servo gearing and adjustment of the balance voltage it is possible to make one shaft rotation of the servomotor exactly equal to one degree centigrade. A disc is then attached to the servomotor shaft which has slots corresponding to the rotation equivalent to l”C, O.l”C and O.OY’C (Fig. 4). Light passing through these slots illuminates one of three phototransistors which provide pulse signals when isotherms are passed. These are amplified and used to mark the recorder. The servomotor which drives the balance potentiometer is of special construction having very low inertia, with the result that it can attain full speed from a stopped condition in 0.05 sets. The associated servo amplifier is also of special design, as it must have a balanced differential input and present a very high impedance to the scanning potentiometer. The balance potentiometer is a three-turn, high resolution, high linearity unit. It is energized by the same voltage source that polarizes the thermistor circuits (Fig. 5). Therefore, voltage drifts do not affect the accuracy of the system. The slight nonlinearity of the voltage divider output as a function of temperature is corrected in the balance circuit by adding four taps to the balancing potentiometer. By impressing certain voltages on these taps the voltage distribution along the winding of the balance potentiometer is forced to agree more closely to Fig. 6 than does a linear voltage distribution. While this correction is small it permits the servomotor shaft angle to follow the temperature distribution with an error of less than 0.02%. The servo balance system must ‘ snap back ’ from the voltage equivalent to the temperature at the bottom of the chain to that of the surface, at the end of each sweep. Time for this ‘snap back’ is provided by using only 270” of the interpolating potentiometer winding for the thermistor taps and having the recorder helix made to record the full width of the paper with 270” of drum rotation. The speed at which the servomotor must turn to follow a particular temperature structure is determined by the steepness of the structure and the selected speed of the recorder drum, the scan rate. There is, however, a maximum speed at.which a servomotor can turn, and this determines the maximum allowable scan rate for a given temperature structure. A synchronous rectifier is provided that operates on the servomotor control voltage. This yields a d.c. voltage whose amplitude is proportional to motor speed and whose polarity is determined by direction of rotation. This voltage is used to light a neon bulb if the motor is near full speed, indicating that the scan speed must be reduced. The same voltage is used to tint the recorder paper any time the servo system is moving in the ‘ hotter ’ direction. Thus, a band appears on the chart that indicates the presence of the temperature inversion in the water column. With this feature, it is only necessary to know the identity of one isotherm appearing on the record. Both the scanning potentiometer and the servo balance system drive ‘ slave ’ potentiometers whose outputs are put on meters. Thus, the scan can be stopped at the position on the paper corresponding to any line (isotherm) and the meters turned on to identify that isotherm and its depth. (5)

EXAMPLES

OF DATA

equipment described above (or some of its prototypes whose operation was essentially identical with it but lacked automatic inversion detection) has been towed extensively over the last two years. Many trips have been made along the East Coast of the United States and in the Caribbean The

244

WILLIAM

S. RICHARDSON

and

CHARLES J. HUBBARD

__-

Sea. An Atlantic crossing from Newport, Rhode Island to Gibraltar via the Azores was made in July of 1958, and the equipment was used in the Mediterranean Sea during the summer of that year. The return crossing from Belfast, Ireland to Argentia, Newfoundland and thence to Boston, Masasachusetts was made in September, 1958. Following this, several other trips have been made in the Western Atlantic including a New England to Bermuda passage and return in December, 1958. Fig. 7 shows a typical record for the slope water south of New England in the fall. Such a record is also typical of the Sargasso Sea (with, of course, different temperatures) during the months when a pronounced seasonal thermocline exists there. It may be noted that the isotherms are quite level and uncomplicated, which is in sharp contrast to the picture shown in Fig. 8. This recording was made about fifty miles east of Lisbon, Portugal, but is typical of most of the eastern North Atlantic during the summer and early fall. In this figure there are pronounced indications of periodicity which may be internal waves. However, without further data it is impossible to tell whether these are actually moving waves or stationary temperature structure. Fig. 9 shows a recording taken near the centre of the Western basin of the Mediterranean Sea, which is typical of the complexity encountered there. The three insets on the record are photographs of the BT-like trace obtained on an oscilloscope as mentioned above. It may be noted that a pronounced deepening of the isothermal layer and sharpening of the thermocline occurs at about three miles, and this is followed at five miles by a shallowing and further sharpening of the thermocline. Fig. 10 is a short section showing a passage out of the Gulf Stream into slope water east of Jacksonville, Florida. The slope of the warm water overlying cooler coastal water is apparent. The series of inversions which occur throughout this record at about 175 ft are caused by a thermistor which is slightly off calibration being towed at that depth. Fig. I I is a similar crossing of the edge of the Gulf Stream south of New England in December. In this case the slope water to the north was isothermal to about 350 ft. The complex thermal structure which occurs just before leaving the stream presents a problem in isotherm identification and contouring which the prototype instrument could not handle.

Acknowledgements-The authors wish to acknowledge their indebtedness to their colleagues at the Woods Hole Oceanographic Institution who have assisted in the design, construction and use of this equipment. In particular, we are indebted to Mr. Charles H. Wilkins, who sorted and calibrated the thermistors and assisted in construction of the equipment. The work of Mr. Charles P. Cook in preparation of the figures is gratefully acknowledged. The patience and assistance of the officers and men of the U.S. Coast Guard Cutter Yamacraw and the USNS Chain are greatly appreciated. Woods Hole Oceanographic Institution Woods Hole, Massachusetts. Woods Hole Oceanographic Institution Contribution No. 1046.