4. Paste on standard mailing label NODC 

 3167/11 (9-61) giving ship's name, date (s) 

 of cruise, and BT log sheet number. 



5. Pad box well and pack in cardboard box. 



6. Wrap securely and label clearly repeating 

 information in item 4. 



7. Fold and staple the bathythermograph 

 log so that the mailing format printed on 

 the reverse side is displayed. 



8. Mail BT slides and log sheets to : 



National Oceanographic Data Center 

 Washington, D.C. 20390 U.S.A. 

 All grids from BT's lost during operations 

 at sea shall be forwarded to the NODC on re- 

 turn to port or at the end of a survey cruise. 



C-8 BT Maintenance.— The BT requires 

 very little maintenance, but careful handling 

 is essential to maintain the accuracy of the deli- 

 cate internal mechanisms. 



After survey operations, the BT should be 

 rinsed with fresh water. Never store a BT that 

 is being withdrawn from use without thorough- 

 ly rinsing it. 



Do not disassemble the BT. It is a precision 

 instrument with delicate internal mechanisms, 

 and even with the greatest care possible it is 

 difficult to avoid damage if disassembling is at- 

 tempted aboard ship. If for any reason the BT 

 fails to operate satisfactorily, it should be turned 

 in for repair with a report indicating the symp- 

 toms to aid the repair facility in correcting the 

 trouble. A replacement BT can be obtained by 

 filling out the standard forms and mailing them 

 to Naval Ship Systems Command Headquarters 

 Washington, D.C. 20360. This also applies to 

 those BT's lost during operations. Standard 

 failure reports also should be submitted in ac- 

 cordance with current directives. 



NAVOCEANO BT's should be replaced after 

 6 months or after 200 drops, whichever occurs 

 first. These BT's should be returned to : 

 Supply Officer 



U.S. Naval Oceanographic Office 

 Washington, D.C. 20390 



C-9 Malfunctions.— The BT normally is a 

 very reliable instrument; however, the operator 

 should be alert to several common malfunctions. 

 Shocks which occur to the instrument during 

 the handling aiad lowering may cause hyster- 

 esis, temperature error, and/or depth error. 



(1) Hysteresis. — The stylus scratches its 

 trace while the BT is diving and as it rises to the 

 surface. Water conditions where it dives may be 

 slightly different from where it rises. These con- 

 ditions ai'e usually negligible; however, the in- 

 strument may have liysteresis ; i.e., tliere may be 

 a slight lag in the movement of its thermal and 

 depth elements. If the up and down traces are 

 essentially similar, a slight divergence of the 



C-8 



traces usually is immaterial. // the traces differ 

 ividely, change to another BT. The temperature 

 reading at the given depth (if the water condi- 

 tions are not changing) would be a point mid- 

 way between the two traces. Nothing can be done 

 aboard ship for hysteresis. Note : Closely spaced 

 traces (less than 0.5" F.) and double traces in 

 strong gradients (layers of rapid change of 

 temperature) are not considered as hysteresis. 



(2) Temperature Error. — It is advisable to 

 make frequent comparisons between the BT 

 surface temperatures and the sea surface refer- 

 ence temperatures. These temperatures should 

 be approximately the same. If they differ 

 slightly, the difference should remain constant 

 over a long period of time. If this difference 

 changes and if the amount of the difference 

 then found continues for subsequent lowerings, 

 it is an indication that the calibration has 

 shifted. A shift in calibration, sometimes called 

 a "shift in the zero points," should not affect the 

 shape of any given trace. The operator should 

 make a note on the log sheet showing the slide 

 number and time at which this shift m calibra- 

 tion was detected. 



If the zero shift is more than 4° F., or if it 

 shifts from one lowering to another, the BT 

 needs adjustment and should be turned in for 

 repair. If the instrument must be used, the fol- 

 lowing procedures can be used to determine the 

 amount of temperature correction to apply. 



Load the BT with a slide and leave the brass 

 sleeve up so the stylus does not rest on the slide. 

 Immerse the tail fins, thermal element, and the 

 sleeve in a bucket of water for several minutes. 

 Then push the sleeve down to bring the stylus 

 in contact with the slide. At that instant obtain 

 the water temperature in the bucket with a 

 thermometer. Then raise the sleeve and trip the 

 automatic stylus lifter without taking the BT 

 out of the bucket. Add hot water to raise the 

 temperature a few degrees. Stir the water and 

 allow time for the BT to come to temperature 

 and then make another mark as before and read 

 the thermometer. Repeat the process several 

 times to establish a series of temperature points 

 across the slide, along the zero depth line. The 

 values of the points are read with the viewer 

 and may be plotted on a graph against the tem- 

 peratures obtained by the thermometer. 



(3) Depth Error.— The BT, when on deck, 

 usually has a different temperature than when 

 in the water. The BT thermal element assembly 

 moves the stylus assembly along the zero depth 

 line to the surface water temperature position 

 during the period the BT is being towed at the 

 surface (see par. C-5, Step 4). Thus, the top 

 of the trace is almost always a horizontal line 

 which should be on the zero depth line of the 

 grid when the slide is viewed. If the trace ap- 

 pears more than 3 feet above or below the zero 

 line, the depth readings must be corrected by 



