placed in the viewer care must be taken to keep 

 the slide always tight against the stop and 

 against the groove opposite the spring. Other- 

 wise incorrect temperature and depth readings 

 will result. 



2-52 Reading the Slide. — The trace scratch- 

 ed by the stylus is a temperature-depth rec- 

 ord. Each point on the trace represents a 

 value of temperature and depth which can be 

 read off the appropriate line of the grid. The 

 lines on the grid are established by actual test 

 of the instrument. Each BT has its own grid. 

 Serial numbers of both grid and BT must 

 agree. Temperatures should be read to tenths 

 of a degree and depth to within 10 feet or 

 better for the 900-foot tvpe, 5 feet or better for 

 the 450-foot type, and 2 feet for the 200-foot 

 type. 



2-53 Checking for Malfunctions.— The sty- 

 lus scratches its traice both while the BT is 

 diving and as it rises to the surface. The 

 water conditions where it dives may be slightly 

 different from where it rises. These conditions 

 are usually negligible. However, the instru- 

 ment may have hysteresis; that is, there may 

 be a slight lag in the movement of the element. 

 If the up and down traces are essentially similar, 

 a slight divergence of the traces is usually 

 immaterial. // the Iraces differ widely, change 

 to another BT. The temperature reading at the 

 given depth (if the water conditions are not 

 changing) would be a point midway between 

 the two traces. 



The BT, when on deck, usually has a different 

 temperature than when in the water. The 

 thermal element assembly will move the stylus 

 assembly along the zero depth line to the sur- 

 face water position during the period it is being 

 towed at the surface. Thus, the top of the 

 trace is almost always a horizontal line which 

 should fall on the zero depth liiie of the grid 

 when the slide is viewed. If the trace appears 

 more than 3 feet above or below the zero line, 

 the depth readings must be corrected by the 

 amount of this error for accurate results. 



The surface temperature is read from the 

 BT slide by noting the temperature of the 

 point at which the trace starts downward from 

 the surface trace. 



It is advisable to make frequent comparisons 

 between the BT surface temperatures and those 

 obtained by the bucket tliermometer. For a 

 series of slides taken with tlie same BT, the 

 slide and thermometer surface temperature 

 readings should be approximately the same, or 

 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 



24 



difference then found continues for subsequent 

 lowerings, it is an indication that the calibra- 

 tion has shifted. A shift in calibration, some- 

 times 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 or time at which 

 this shift in calibration 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 BT strikes bottom or an underwater 

 object, the depth at which it struck usually can 

 be determined by reading the depth of the hori- 

 zontal lines across the trace, made where the 

 stylus arm vibrated with the shock. Always 

 check the depth juat before making each lowering. 

 Shocks which occur to the instrument during 

 the handling and lowering may cause hysteresis, 

 temperature error, and depth error. Nothing 

 can be done aboard ship for hvsteresis. In 

 order to determine the amount of correction to 

 apply to temperature or depth errors for accu- 

 rate work the following procedures can be 

 used — in an emergency. 



2-54 Temperature Error. — Load the BT 

 with a smoked slide and leave the brass sleeve 

 up so the stylus does not rest on the slide. Im- 

 merse the tailfins, 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 the 

 bucket 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 temperatures obtained by the 

 bucket thermometer. A shift in surface tem- 

 perature values may be corrected by adjusting 

 the eccentric stop screw, on the slide holder 

 of the viewer, to line up the grid values cor- 

 rectly with the recorded values. 



2-55 Depth Error.— With the sleeve all the 

 way back, immerse the thermal element in cold 

 and then in warm (less than 105° F.) bucket of 

 water. This will cause a long zero depth line 

 to be drawn across the slide. The slide is then 

 placed in the viewer and the difference, in feet, 

 of the trace above or below the zero depth line 



H. O. 607 



