Submarine Bathythermograph Type CTB 40079 and CTB 40131, 1943, is the manual for the WW II bathythermograph, a device that records the temperature of the water as the submarine moves through the water. This simple device was a very important innovation during the war and helped submarines evade surface ship sonar.

In this online version of the manual we have attempted to keep the flavor of the original layout while taking advantage of the Web's universal accessibility. Different browsers and fonts will cause the text to move, but the text will remain roughly where it is in the original manual. In addition to errors we have attempted to preserve from the original, this text was captured by a combination of optical character recognition and human typist. Each method creates errors that are compounded while encoding for the Web. Please report any typos, or particularly annoying layout issues to the Mail Feedback Form for correction.

Richard Pekelney


Instructions for the Installation, Care and Use
of the
Types CTB 40079 and CTB 40131
CONTRACT NOs - 98660
Bureau of Ships, Navy Department
Washington, D. C.


This instruction book is furnished for the information of commissioned, warrant, enlisted and civilian personnel of the Navy (and other persons specially designated by the Bureau of Ships) whose duties involve design, instruction, operation and installation of radio and sound equipment. The word "restricted" as applied to this instruction book signifies that this instruction book is to be used only by the above personnel and that the contents of it should not be made known to persons not connected with the Navy.


Instructions for the Installation, Care and Use
of the
Types CTB 40079 and CTB 40131
CONTRACT NOs - 98660

Navy Yard Mare Island, Calif.
Received 19 Mar 1943
Letter File No. NOS-98660 (940-1) 
of 3/5/43


Woods Hole Oceanographic institution
The Bureau of Ships, Navy Department
The Bristol Company, Waterbury, Conn.


  General Location of Equipment 5
  Principal Parts 6
    1. Recorder and Temperature Sensitive Element 6
      a. Thermal Assembly 7
      b. Pen Stylus 8
      c. Pressure Element 8
      d. Card Carriage 9
      e. Base 9
    2. Metal Stuffing Box Fittings 11
    3. The Blister 12
  Auxiliary Equipment 12
  Installing the SBT 13
  Method of Inserting Cards 18
  Adjusting the Bathythermograph 18
  Pressure Element 23
  Temperature Element 23
  Labeling the Cards 26
  Fixing the Cards 27
  Reading the Cards 27



The Submarine Bathythermograph (SBT) is a recording thermometer developed by the Woods Hole Oceanographic Institution. It gives a record of the temperature and depth of the water during a dive. This manual covers the installation and maintenance of the instrument, and a brief description of typical records.


Fig. 1 - Typical Installation 5
Fig. 2 - Miscellaneous Parts of Equipment 6
Fig. 3 - Thermometer Element 7
Fig. 4 - Pen Arm and Stylus 8
Fig. 5 - Pressure Element 8
Fig. 6 - Card Carriage 9
Fig. 7 - The Base 9
Fig. 7a - Mounting Dimensions and Clearance Space 10
Fig. 8 - Stuffing Box 11
Fig. 9 - The Blister (cover off) and Mounting Dimensions 11
Fig. 10 - Tubing Guard for 1/8" Tubing 13
Fig. 11 - Tubing Guard for 1/8" Tubing 14
Fig. 12 - 1/8" Tubing Protected by Superstructure 14
Fig. 13 - Recorder Mounted in Conning Tower 15
Fig. 14 - Capillary Winding on Blister 16
Fig. 15 - Securing Capillary on Blister 17
Fig. 16 - Typical Pressure Connection 18
Fig. 17 - Card Inserted in Recorder 19
Fig. 18 - Pressure and Temperature Adjustments 21
Fig. 19 - Installation of Spare Pressure Element 24
Fig. 20 - Installation of Spare Temperature Element 25
Fig. 21 - Typical Card Properly Marked 26
Fig. 22 - Fixing the Card 27
Fig. 23 - Isothermal Layers 28
Fig. 24 - Negative Gradients 28
Fig. 25 - Positive Gradients 29
Fig. 26 - Daily Heating Effect 29
Fig. 27 - Smoking Gear 31



General Location of Equipment

The Submarine Bathythermograph (SBT) consists of two main elements. The temperature sensitive element is mounted in a blister on the upper forward part of the conning tower fairwater just enough to one side to get good circulation of the water through it. The recorder proper is mounted near the overhead of the conning tower where the record can easily be seen by the conning officer.

It is very important that the length of copper tubing from the recorder to the blister be as short as possible. For this reason the recorder should be mounted near the overhead of the conning tower. The stuffing box must be mounted thru the pressure hull within 3 or 4 feet of the recorder. The stuffing box must be on a level with or higher than the recorder to reduce the length of tubing to the blister. Figure 1 is a typical installation.


Principal Parts

Each complete Submarine Bathythermograph kit consists of:

1 - Recorder and temperature sensitive element 7 - Two boxes unsmoked cards
2 - Blister 8 - Two quarts fixative
3 - Stuffing box parts 9 - Spare thermometer and spare pressure system
4 - Smoking lamp and spare paraffin 10 - Instruction Manual
5 - Wire card holder 11 - Wooden card holder
6 - Two boxes smoked cards

Fig. 2 Miscellaneous Parts of Equipment
Fig. 2 Miscellaneous Parts of Equipment

1) Recorder and Temperature Sensitive Element. The recording instrument is mounted in a metal box with hinged door and transparent window. The scale is illuminated by a 115 volt lamp. Coming from the instrument case is a copper tube 1/8" in diameter and 12 feet long. Inside this tube and protected by it is a temperature sensitive capillary 1/16" in diameter, which extends beyond the protective sheath for about 68 feet and is wound on the blister outside the vessel. The

junction and the tip of the capillary are both silver soldered. The main recording and measuring elements of the SBT consist of the following parts.

a) The thermal assembly (Fig. 3) which consists of approximately 80 feet of 1/16" copper capillary tubing connected to one end of a bourdon tube assembly is filled with liquid xylene The temperature reading represents the temperature of the capillary tubing which is wound on the blister. The bourdon tube element is compensated with a bi-metallic strip so as to approximately offset temperature differences between the inside and outside of the Submarine.

Fig. 3 - Thermometer Element for Submarine Bathythermograph
Fig. 3 - Thermometer Element for Submarine Bathythermograph

The temperature range of the Bathythermograph is 30° to 90°F. Temperatures below 30° will not harm the temperature measuring element; however, temperatures above 150° may result in a permanent set of the bourdon tube. Stops are provided on the bourdon element shaft to stop the motion of the bourdon tube when subjected to temperature below or above the range of the instrument.

Extreme care should be taken to avoid damage to the capillary tube, as the smallest leak in this system will make the SBT inoperative.

b) The pen stylus (Fig. 4) receives its motion from the shaft of the thermal bourdon element by means of a channeled arm and open support, hinged to give the necessary in and out motion to the pen as the arm rotates in an arc. The pen stylus is held against the card by a torsion spring which maintains correct stylus pressure.

Fig. 4 Pen Arm Stylus
Fig. 4 Pen Arm Stylus

c) The pressure element (Fig. 5) consists of a bourdon tube, one end of which is anchored to the supporting base and the other end fastened to a horizontal center shaft. The pressure of the sea water is transmitted to this tube, and an increase in pressure within the bourdon pressure tube tends to straighten the tube, causing the center shaft to rotate. A mechanical stop on the base limits the pressure range of the instrument to a maximum operating depth of either 250 or 450 feet. On the shallow range instrument (Model CTB 40079), the pressure element will stand pressure of 450 feet and on the deep range instrument (Model CTB 40131), the pressure element will stand a pressure of 600 feet. Pressure greater than these limitations will probably produce a set in the bourdon tube, necessitating changing the zero setting.

Fig. 5 Pressure Element
Fig. 5 Pressure Element

d) The card carriage (Fig. 6) consists of a card holder supported by two channeled arms which are attached to the ends of the horizontal center shaft of the pressure element. Changes in pressure cause the center shaft of the element to rotate, and this motion is in turn conveyed to the card carriage causing the unit to move up and down. The card holder head is transparent, and the card is illuminated from the rear. Structurally this carriage is light and delicate, and carefully balanced to permit accuracy in measurement.

Fig. 6 Card Carriage
Fig. 6 Card Carriage

e) The base (Fig. 7) is a suitable mounting plate for the thermal and pressure assemblies. In addition there is a 115 volt lamp with suitable electrical connection for internal illumination, a pen lifter, a zero adjustment for the thermometer system, and a zero adjustment for the pressure system. See also Figs. 7a. and 18.

Fig. 7 The Base
Fig. 7 The Base


Fig. 7a Mounting Dimensions and Clearance Space
Fig. 7a Mounting Dimensions & Clearance Space

2) Metal Stuffing box Fittings (Fig. 8) for passing the 1/8" tubing through the pressure hull. There are three brass parts and they fit a Navy type XLVI steel stuffing tube size A with an inside hole of 0.406". Note that one jam nut and the bushing are already on the thermal element. The other jam nut is slid over the capillary tubing from the outside of the submarine.

Fig. 8 Stuffing Box
Fig. 8 Stuffing Box

Fig. 9 The Blister (cover off) and Mounting Dimensions
Fig. 9 The Blister (cover off) and Mounting Dimensions

3) The Blister or coil form housing (Fig. 9). This consists of a bronze plate with 6 holes for mounting on a curved surface. On the plate are 8 small fluted studs with notches in them for the tubing to be wound around. The four corner studs are shorter than the others. Two studs on opposite corners contain a clamp for the 1/16" capillary tubing. There is a hole in the back plate through which the tubing passes.

A bronze cover with screened ends fits over these studs to prevent damage to the copper tubing.

Auxiliary Equipment:

5) Two boxes each containing 50 smoked cards
6) Two small filing boxes each containing about 500 graduated unsmoked cards.
7) Two cans of fixative solution for fixing smoked records.
8) One paraffin smoking lamp.
9) One wire card holder for holding card while smoking.
10) One spare cake of paraffin for the smoking lamp.
11) One spare pressure assembly.
12) One spare thermal assembly.
13) Wooden card holder.



Installing the SBT

1) Examine the instrument, read the instructions and decide upon location of all parts before starting the installation. All topside gear such as tube guard,

stuffing box and blister should be made and fitted before the tubing is brought out through the stuffing box. This will prevent the danger of having to drill, tap, or bend metal near the tubing.

2) Weld the steel stuffing box for the thermal element through the pressure hull above and as close to the recorder as possible. The maximum permissible distance along the 1/8" tubing is 4 feet. In many cases there will be blank stuffing boxes thru the pressure hull. These must not be used unless they are in the position specified.

3) The location of the blister should be on the forward side of the fair-water with the screen ends pointed fore and aft to permit a free flow of water around the capillary and through the housing when the submarine is in forward motion. When locating the blister, bear in mind that the lead from the stuffing box should be as short and well protected as possible. Figure I shows a typical location and installation.

The blister or capillary tube housing is bolted to the fairwater by means of 6 - 5/16" or 3/8" non-corrosive bolts and the back plate is spaced from the fairwater about 3/8". 1/2" brass nuts make convenient spacers. Drill a 3/8" or 1/2" hole thru the fairwater to line up with the hole in the blister back plate to bring the 1/8" tubing through. Be sure to debar the hole to prevent chafing of the copper tubing.

4) Mount some sort of protection for the 1/8" copper tubing leading from the stuffing box to the blister. Leave as much water circulation about this copper tube as possible. The details of this guard will be left to the installation man. However, the following three methods have been used with success.

a) Bronze or copper channels (Fig. 10) were made in about 2 foot lengths. The 1/8" copper tube is then centered in this channel by means of corks, rubber stoppers with holes in them or by tying in with copper wire. The 1/8" tubing should be secured about every 6" to 8". As many of the channels as are needed are then put end to end from the stuffing box to the blister. The channel strips can then be screwed to the fairwater.

Fig. 10. 1/8 inch Tubing Guard
Fig. 10. 1/8" Tubing Guard

b) A 1/8" x 2" brass or bronze strip was run from the blister to below the deck as in Fig. 11. The strip is drilled about every 12" to 15" along each edge so that it may be bolted to the fair water. Pipe spacers about 1" or 2" long will permit water to flow past the tubing and permit the assembly to clear small obstacles. The 1/8" tubing is then secured every 6" or 8" to the under side of the strip by copper wire or marline. A pair of 1/4" holes about 1/2" apart at every securing point makes a very easy method for installation.

The important thing about any such protection is that the 1/8" copper tube must be secured so that it cannot chafe, while at the same time precautions are taken against corrosion and electrolytic action.

Fig. 11 1/8 inch Tubing Guard
Fig. 11 1/8" Tubing Guard

c) On the newer submarines it has been possible to run the 1/8" tubing from the stuffing box to the blister entirely behind the fairwater under the bridge. In this case it is only necessary to clamp the tubing about every 18" to prevent flowing water from harming it. Over any parts of the tubing exposed to passways, it is advisable to make guards to prevent injury. See figure 12.

Fig. 12 1/8 inch Tubing Protected by Superstructure
Fig. 12 1/8" Tubing Protected by Superstructure

5) Mount the recorder box in the conning tower. If possible it should be mounted so that the conning officer can easily see the record and in a convenient location for servicing. The main casting or mounting plate has lugs suitable for fastening to angle brackets or other rigid members, and should be mounted with the cover housing down, and with sufficient room to permit removal of the four screws and dropping the housing to facilitate servicing.

When the instrument is mounted in this position the pen arm extends upward. Figure 13 illustrates a typical mount for the bathythermograph. With the instrument so mounted, the card record gives a visual picture of the variation of temperature with depth, with the top of the card representing the surface of the water. A shock type mounting is advisable for best operation.

Fig. 13 Recorder Mounted in Conning Tower
Fig. 13 Recorder Mounted in Conning Tower

Note the plug in the pressure connection to the recorder. This plug should not be removed until the pressure line is connected. The pressure element is filled with oil at the place of manufacture.

The distance from the recorder box to the stuffing box should be kept as small as possible.

6) Pass the thermal copper tubing from the recorder box to the stuffing box keeping it back of pipes, etc., so that it cannot get dented or pulled.

7) While installing the temperature sensitive element great care should be exercised so that the tubing is not kinked or injured. The tubing is shipped wound on a form such that by removing the tubing in consecutive layers kinking will be avoided. Pass the free end of the tubing through the submarine hull's stuffing gland, slip on the outer jam nut and pull the copper tube through until the 1/8" tubing extends as far as possible outside the hull. Screw the jam nut into the stuffing gland after packing has been inserted. From the inside place the bushing in the center of the stuffing gland, insert packing and tighten the inside jam nut into the gland. The final assembly should look like Fig. 8.

8) Pass the 1/8" tubing along the guard as in paragraph 4.

9) Wind the capillary around the studs of the coil form in a fore and aft direction to avoid stresses on the tubing due to flowing water as illustrated in Fig. l4.

Fig. 14 Capillary Winding on Blister
Fig. 14 Capillary Winding on Blister

Secure chafing material around the 1/8" tubing where it comes thru the back plate. Wind the remainder of the 1/8" tubing around the bases of the studs on the blister. Secure the 1/16" capillary in the clamp on the bottom corner stud and wind the two short bottom studs full. Then cross over to the bottom of the next pair of studs as in Fig. 14. Wind second pair of studs full, noting that winding is in reverse direction. Similarly for 3rd and 4th pair of studs. Wind tubing with a tension of 5 or 10 pounds, but do not stretch it.

10) The end of the 1/16" capillary can best be secured by the method shown in Fig. 15. The 1/16" capillary can be secured on the studs by soldering occasionally if necessary. Use only soft solder and avoid heating the tube unnecessarily. Soldering must be done with an iron. Do not use torch.

Fig. 15 Securing Capillary on Blister
Fig. 15 Securing Capillary on Blister

11) Place the cover on the blister and secure 1/8" tubing to guard. Take care that the tubing from the blister to the end of the guard is secured against chafing or vibration due to the water rushing past it. Any holes through which tubing is passed should be provided with grommets to prevent chafing.

12) Connect one side of a 1/4" valve into the main depth gauge line. This valve should then be shut. The plug in the recorder pressure connection can now be removed. A 1/4" line is then run from the recorder connection to the valve. This line should have a "T" inserted in it about 1/4 of the way from the recorder to the valve. The tube should also be bent to form an oil trap. See Fig. 16.

13) Diesel oil is then poured into the tee until the pipe is filled up to the opening. A plug is then inserted in the tee and the valve can then be opened.

Any air remaining in the line will act as a cushion against sudden increases in pressure due to depth charges. Too much air in the line may cause a decided pressure lag. (See Hints for Trouble Shooting, part 2e, page 22)

14) Connect the lamp which illuminates the card to the ship's 115 volt supply through a suitable switch.

15) Remove the recorder cover and remove any shipping bands (Fig. 18) that were used for protecting parts during shipment.

16) Mount the small wooden card holder in the conning tower where it will be

easily visible to the conning officer. This holder is to hold the SBT record of the last dive. In most cases the water conditions will change so little in a few hours that the previous record will show the conning officer what to expect.


Fig. 16 Typical Pressure Connection
Fig. 16 Typical Pressure Connection


17) When inserting the card, note that it is centered in the card holder. The side edges of the holder are made the same length as the cards. Care must be taken that all cards are properly inserted. In actual service it will be found advisable to always leave a new smoked card in the recorder and have the pen lifter up. In case of a quick dive it will only be necessary to release the pen lifter to get a record.


18) Insert a calibrated card in the SBT and adjust the pressure zero until the depth indicated on the card agrees with the keel depth as read on the main depth gauge. Fig. 18 illustrates this adjustment. Loosen the locking screw (A) and move arm (B) to rotate card carriage until the pen stylus is at the correct depth. Be sure to tighten screw (A) securely after making this adjustment. All submarine commanders who have used the instrument, strongly prefer this choice of the zero setting of the depth scale.


Fig. 17 Card Inserted in Recorder
Fig. 17 Card Inserted in Recorder

19) Determine and record the height of the blister above the keel. This is a correction which must be applied to the records before detailed calculations are made.

20) Adjust the temperature reading (Fig. 18) until it coincides with the temperature at the blister on the conning tower. Loosen locking screws (C) approximately 1/2 turn. Insert screw driver in pinion adjustment slot (D) and set stylus to correct value. Tighten locking screws (C) securely and check whether the set value is correct.

There are several ways to set the correct temperature:

a) If the temperature in the top 30 or 50 feet of the sea is constant, as indicated by the SBT during a dive, then the temperature reading should be adjusted while the blister is a few feet beneath the surface until it agrees with the surface water temperature as read by a thermometer or by the intake water temperature thermometers.

It must be remembered that it is the thermal gradient that is important. An error of several degrees in the actual temperature will not be of serious consequences.

b) If no other method is available, wait until the blister is in the shade. The tubing is then at the same temperature as a thermometer laid across a few of the turns in the middle of the blister.

c) It a surface model bathythermograph type CTB40080 or CTB40120 is available it is desirable to secure it near the blister in order to obtain comparison records for the first few dives. These will probably be available at base stations, such as New London or Pearl Harbor. Such records may be used for setting the zero of the thermal element if desired.

21) Test for pressure hysteresis by displacing the card holder gently upward and then downward with the finger and note if it returns to the same position in both cases. If the difference in position is more than 2 feet, refer to Hints for Trouble Shooting (page 22).

22) Test for thermal hysteresis by displacing the pen arm first to the right and then to the left noting if it returns to the same temperature in both cases. If it does not return to within 2/10° the hysteresis is objectionable. Refer to Hints for Trouble Shooting (page 22).

23) Test the response of the thermal assembly by throwing a bucket of either cold or warm water (not hot) into the blister. The response should be almost instantaneous.


Fig. 18 Pressure and Temperature Adjustments
Fig. 18 Pressure and Temperature Adjustments



1. Failure to respond to temperature changes.

a) Shipping band on thermal element not removed.

b) Capillary tube sharply bent and constricted or open. This can only be prevented by care in installation.

c) Bearings binding on thermal assembly, or shafts bent in shipment. Adjust these parts to correct faults.

d) If the thermal assembly still does not operate the spare should be installed.

e) The thermal element of the SBT is a compromise to get the greatest speed of response with the greatest possible mechanical ruggedness. Therefore, it will not be unusual if the thermal assembly should spring a leak after six months or so, and need replacement. An element which leaks after installation is easily detected because the stylus is clear over to the cold side of the scale and it will not respond to temperature changes.

2. Serious Pressure Hysteresis.

a) Examine the pen pressure and adjust if necessary. The lightest pressure of the stylus is sufficient to mark the smoked card.

b) Lubricate possible friction points with light oil.

c) Check for bent shafts or binding bearings.

d) Dull stylus a little if it seems to be digging into the paper.

e) If the SBT seems to have too much hysteresis when in actual use, fill the 1/4" tube from the main depth gauge line to the recorder completely full of oil and see if this corrects the trouble. Too much air space in this tube will definitely give noticeable pressure hysteresis.

f) To make sure if pressure hysteresis is present or not, let the cover down when making a dive and tap the pen arm to the right every 10 or 20 feet as read off the depth gauge. When surfacing, tap the pen to the left at the same depth points. If these points agree, pressure hysteresis is not present.

3. Serious Temperature Hysteresis.

a) Check points a, b, c, and d of the previous paragraphs.

b) Make sure that the temperature of the blister is not changing during the test.

c) If all these methods fail, remove the thermal assembly from the instrument and install the spare. Test for hysteresis before removing the capillary tubing of the spare system from the winding form on which it is shipped. Dip the capillary alternately in buckets of cold and warm water and see whether the temperature points repeat.



Installation of Spare Pressure Element.

The valve from the main depth gauge should be shut off before disconnecting the pressure line from the recorder. Referring to Figure 19, loosen the screws (A) securing the card carriage to the pressure element shaft and then spring the side arms of the card carriage slightly to remove the carriage from the instrument. Care should be exercised not to injure the stylus arm. Loosen screw (B) and remove screw (C) to disconnect the temperature capillary from the pressure mounting. Remove the two pressure element base screws (D) and connection screw (E) and the pressure element will slip sidewards from around the main center support shaft. Screws (D) are under plate (F).

Slide the new pressure element in place and fix in position with the proper screws in reverse procedure of removal. Set the zero adjustment approximately in the center and position the card carriage on the pressure element shaft at about zero and tightens screws (A) securely.

The pressure line may now be refilled with oil as in Part 13 Section II and the pressure zero adjusted as in Part 18 Section II. Finally the recorder should be checked for pressure hysteresis as in Part 21 Section II.

Installation of Spare Thermal Element.

To install a new temperature element, the card carriage should be taken off the recorder. Referring to Fig. 20, unlock the capillary from the pressure element mounting by removing plate (A). The bourdon element may then be unlocked from its mounting by removing screws and clamp washers (B).

The spare temperature element is then easily set in place and locked in position by the clamp washers and locking screws. The pen arm should be removed from the original temperature mounting. By removing plate (C) and stop arm (E) the pen arm may be slipped off the center shaft by unlocking screws (D). The pen arm then is placed on the new temperature element shaft and assembled in the reverse procedure. The stop should be set to limit the pen arm travel to that of the card range. Care should be taken to adjust the pen arm height to proper setting for full card carriage travel without any interference of the moving parts.

The card carriage should be returned to the pressure shaft and properly checked and adjusted. The instrument should be checked for hysteresis and response by the method described in parts 21 to 23, Section II.


Fig. 19 Installation of Spare Pressure Element
Fig. 19 Installation of Spare Pressure Element


Fig. 20 Installation of Spare Temperature Element
Fig. 20 Installation of Spare Temperature Element



Labeling the Cards

Fig. 21 shows a typical card properly marked.

Fig. 21 Typical Card Properly Marked
Fig. 21 Typical Card Properly Marked

The smoked cards are very accurately printed upon a special grade of paper. The groove in the card carriage for the card to slide in is made very close to size so the cards will fit quite firmly.

The front of the card not only carries the graduated grid but there are the following extra entries that should be filled in.

Ship - The name of vessel.
G. C. T. -Greenwich Civil Time.
Card No. -This is the serial number of records for this particular ship. These can be made the same as the dive number for the submarine.
Date - Day, month, year.
Lat. - The approximate latitude of the dive.
Long. - The approximate longitude of the dive.

If there is any other pertinent information it can be put on the back of the card. The sound ranging conditions, the keel depth in feet, the sea bottom depth in fathoms, the air and sea surface temperature, the state of the weather and wind and sea conditions are all particularly useful in interpreting the observations and should be recorded where possible.

The card shown in Figure 21 is used with the Model CTB 40079 Bathythermograph. For the Model CTB 40131 instrument, a card reading to 450 feet is employed.

Fixing the Cards.

The cards are preserved for permanent record by fixing. This is done by dipping in a quick drying fixative as shown in Fig. 22.

If it is desirable to write on the back side of the card, this may be done after the card has been fixed and dried. The fixative must be kept covered when not in use or it will quickly evaporate.

Fig. 22 Fixing the Card
Fig. 22 Fixing the Card

Reading the Cards.

The record as in Fig. 21 is scratched on the smoke covered rectangular card. The horizontal direction is temperature with the scale running from 30° to 90°F. with vertical lines every 2°. If detailed computations are to be made from the record, temperatures should be read to the closest 2/10°.

The vertical scale is depth with horizontal lines every 10 or 20 feet. Note that this is keel depth. The distance from the blister to the keel must be known and recorded in case detailed computations are to be made. Also the depth should be read to the closest 2 feet.

The stylus scratches its curve as the submarine sinks and as it rises. In a short dive the up trace will usually agree with the down trace although the water conditions may change rapidly from place to place.

The variation of temperature with depth can be very complicated and a large number of different cases will be represented by different card records. It is often convenient to consider the water as consisting of layers in each of which the temperature changes in a simple manner from the top to the bottom.

The observer should be able to picture in his mind the location of these layers from the appearance of the card record and the manner in which the temperature is changing. The following examples should be studied until the observer can identify each kind of layer.

1. An Isothermal Layer is a layer (not necessarily at the surface) in which the temperature is the same all the way from the top to the bottom. "Isothermal" may be remembered as meaning "same temperature". The water must be well mixed from top to bottom of the layer for the temperature to be the same throughout, -- hence the name "mixed layer" is also used. Mixed layers are very important and the operator should be able to read accurately the top and bottom depths of such a layer. Traces for typical isothermal layers are shown in Fig. 23; the isothermal layer in each case is shown by diagonal hatching.

Fig. 23 Isothermal Layers
Fig. 23 Isothermal Layers

Vertical sections of a trace indicate layers of water in which the temperature is the same from top to the bottom. Such layers are known as "Isothermal" or "Mixed" layers.

2. Next in importance to an isothermal layer is a layer in which the temperature DECREASES with increasing depth. In this case the trace slopes to the LEFT as we look down from the top of a layer towards the bottom. (Such a decrease of temperature with depth is sometimes referred to as a "negative gradient"). Diagrams showing this type of layer are given in Figure 24; the regions of decreasing temperature are indicated by diagonal hatching.

Fig. 24 Negative Gradients
Fig. 24 Negative Gradients

Sections of a trace which slant downward and to the left indicate layers of water in which the temperature decreases as the depth in- creases. Such layers, indicated by diagonal hatching, are said to have

negative temperature-depth gradients. (Between the negative gradient layers are shown, in (A), a layer having a positive gradient and, in (B), an isothermal layer.

3. Layers are sometimes found in which the temperature INCREASES with depth. Here the trace slopes down and to the RIGHT (sometimes called a "positive gradient"). This state of affairs is usually restricted to coastal waters, or waters off the continental shelf, but may be found anywhere. Diagrams for this case are given in Fig. 25. The layers in which the temperature increases with depth are shown by diagonal hatching.

Fig. 25 Positive Gradients
Fig. 25 Positive Gradients

Sections of a trace which slant downward and to the right indicate layers of water in which the temperature increases as the depth increases. Such layers, indicated by diagonal hatching, are said to have positive temperature-depth gradients. Reading downward from the surface in (A) are: an isothermal layer, then a positive, a negative, a positive, and a positive gradient layer. Reading downward from the surface in (B) are; a negative, a positive, a positive, and a negative gradient layer.

4. An important effect which should be recognized by the observer is the daily heating of the surface water layer. When the surface is heated by the summer sun and the winds are not strong enough to keep the surface water well mixed, then the water close to the surface may be several degrees warmer than the water twenty to thirty feet down. The effect reaches a maximum late in the afternoon, and usually disappears at night as the surface cools off. Successive stages in the development of daily heating in the top of an isothermal layer at the surface are shown in Fig. 26.

Fig. 26 Daily Heating Effect
Fig. 26 Daily Heating Effect

This is a very simplified illustration, showing how the traces on a series of slides may indicate daily heating or cooling near the surface. An isothermal layer is shown at 0600 (local time) of a calm, clear day. From 0600 to 1600, the surface is shown gaining heat from the sun faster than it can dissipate the heat back into the atmosphere. The surface temperature increases, and a negative gradient layer, shown by diagonal hatching, is established. This layer deepens as the heat penetrates and diffuses downward by conduction during the time when the surface is gaining heat.

After 1600, the surface is shown cooling off by radiation or evaporation faster than it receives heat from the declining sun. The cooler and denser surface water mixes by convection downward and may destroy the negative gradient layer. Sufficient cooling may occur during the night to reestablish a single isothermal layer, as shown at 0400.

The water conditions will not as a rule vary a great deal in several hours run in the open ocean. Near shore, water conditions may change more rapidly from place to place.

As soon as the record is taken from the SBT it should be slipped into the wooden card holder furnished, so that the officers will at all times have before them, the water conditions during the last dive.

The above simplified description of the temperature conditions near the surface of the ocean is elementary and incomplete. For a more thorough understanding of the significance of SBT records and of the factors determining the temperature in the sea, the observer should study the Instruction Manual for Submarine Bathythermograph Observers, which will be furnished to each vessel equipped with SBT apparatus.

After the BT stylus has reached the lower edge of the calibrated card (250 or 450), the instrument behaves like an ordinary indicating thermometer. If the temperature decreases below 250 or 450 feet, the pen moves to the left parallel to the 250 or 450 foot graduation. When the pen is observed to move to the left note the depth gauge reading at regular intervals. By this method the temperature gradient below 250 or 450 feet may be determined.



Two boxes of smoked cards are furnished for a temporary supply and also to show how they should be smoked.

Equipment is provided for smoking more cards in the field. It will be found handier to mark each card with the ship's name before it is smoked.

Select a place to work that is as free from drafts as possible. Light the small paraffin lamp. Hold the card in the wire card holder.

Fig. 27 Smoking Gear
Fig. 27 Smoking Gear

Pass the card over the flame several times until a suitable smoke is deposited. This process requires a little practice but one soon acquires the knack. Some individuals may prefer a darker or lighter smoke than the originals. Spare paraffin for the lamp is furnished. The wick may require occasional trimming with a pocket knife.

In practice it will be advisable to always keep a fresh card in the instrument. Then in case of a quick dive the only thing to be done is to release the pen lifter.



At times, special instructions may be given for forwarding complete card records. When such special instructions have not been received, at the end of each cruise, or when a quantity of card records has accumulated, send in the card records and any special notes to the address below.

The Hydrographic Office,
Navy Department,
Washington, D. C.
During the current hostilities, observers' reports, card records, etc., shall be handled and transmitted as CONFIDENTIAL to avoid disclosure as to ship locations.

When practicable, the observer should prepare a brief report to include:

a. General description of water temperature conditions, current systems and meteorological conditions encountered, with references made to the corresponding card records or times.

b. Comments on the installation and suggested improvements, noting particularly any failures and the cause thereof. A rough sketch of the installation used should be sent in with the first report.

c. Comments on the operation of the instrument in general, showing the instrument number where an instrument fails to give proper results, and stating where the defective instrument is located or where it has been sent.

d. Cards, special studies, etc., prepared by the operator should accompany the report.



The Bathythermograph is an accurate measuring instrument and the mechanism, while reasonably rugged, is delicate so that care in handling is essential. The only special attention that should be given is to oil the bearings of the pressure and thermometer elements with a light oil at intervals of every month or two.

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