INTRODUCTION TO SONAR 



The interior of the BT should be rinsed 

 weekly with half a cupful of corrosion pre- 

 ventive compound (specification MIL-C-16173, 

 grade 3 (stock number 8030-244-1296 for 1 

 gallon can)). Place the BT in a clean bucket 

 with the tail fins down. Slide the sleeve forward 

 toward the nose, pour in the compound, and 

 close the sleeve. Cover the four openings in 

 the body tube, shake the BT, and turn it over 

 on its nose and back several times so that 

 every part is rinsed thoroughly. Then let the 

 compound drain out. The compound can be re- 

 used several times. Do not oil the BT; fresh 

 water or rust-preventive compound is the only 

 lubrication necessary. 



Inspecting New Slides 



Do not remove the water and airtight cover 

 from around the box of slides until ready to 

 use the slides from that box. Samples of slides 

 in a new box should be inspected before using. 

 If the coated surface of a new slide is in bad 

 condition, or if the surface shows spots after 

 lowering the BT, all the slides in the box 

 should be tested by holding each slide under 

 a moderate stream of water. Do not use slides 

 on which spotting or flaking of the coated sur- 

 face appears. If more than 25 percent of the 

 slides are unusable as a result of spotting or 

 flaking, report the failure in accordance with 

 current Naval Ship Systems Command instruc- 

 tions. 



CHECKING CALIBRATION 



Expected accuracy of the BT is O.lT for 

 temperature and 10 feet or less for depth. 

 If, for any reason, the instrument is suspected 

 to be in error, the following emergency pro- 

 cedures can be used aboard ship to check its 

 accuracy. 



Temperature 



To check the calibration of the thermal 

 unit, insert a new slide in the BT and leave 

 the sleeve up so that the stylus does not touch 

 the slide. Immerse the BT, tail first, in a 

 bucket containing water of about 40°F, and deep 

 enough to cover the sleeve. Insert a bucket 

 thermometer into the water and stir for 30 

 seconds. Push down the sleeve, engaging the 

 stylus, and take the bucket temperature reading. 

 Raise the sleeve and trip the stylus, leaving 

 the BT in the water. Add hot water to bring 



the bucket temperature up to about 60°F and 

 repeat the measuring process, and again at a 

 water temperature of 80°?. Next, subtract alge- 

 braically each bucket thermometer temperature 

 reading from its corresponding BT temperature 

 reading. Add all the differences and determine 

 the average temperature difference. On future 

 lowerings, apply the average difference to each 

 temperature reading. 



Depth 



Immerse the BT thermal element, with the 

 sleeve down, in a bucket of water of about 40°F, 

 then in a bucket containing water of about 85°F. 

 This temperature difference will cause a zero 

 depth line to be drawn on the slide. Place the 

 slide in a viewer and read the difference between 

 the trace depth line and the zero depth line of 

 the grid. For future slide readings, apply the 

 depth error to each temperature change point 

 to obtain correct depth at that point. 



REPLACEMENT 



Turn in the BT for recalibration or repair 

 if any of the following conditions exist: BT 

 temperature consistently differs 4°F or more 

 from bucket thermometer temperature; depth 

 error of more than 10 feet for the 200-foot model, 



71.81 

 Figure 5-8.— BT grid. 



70 



