1-18. DRILL TRANSMISSION HOLE

TM 5-6635-386-12&P Seating area requirements for the transmission mode comprise only a 2 x 6-inches wide surface under the GM tube, thus the tester can be used on very difficult surfaces where a backscatter tester would be impossible to use. Transmission surface roughness error is approximately 1/10th that of the backscatter errors. 1-18. DRILL TRANSMISSION HOLE The transmission hole must be vertical to the smoothed surface and must not be damaged in the process of drilling and retraction of the drill. Place the Cast Guideplate on the surface and use the guide to support the drill pin. Hammer the drill pin into the ground, tap it lightly to loosen it, twist it to further loosen it, and pull it out of the ground. WARNING Do not use the cast guideplate as a retraction device. It will damage the top of the guide hole. Figure 1-6. Transmission Hole NOTE During drilling and retraction, the operator’s weight on the guideplate confines the soil to prevent eruption or puffing. The operator’s weight on the plate confines the soil in drilling and in retraction to produce a neat, undamaged hole for the readings. On heavily compacted material above 95%, cracks may appear in the soil. These will not affect the readings if the tester is placed so no crack extends from the source directly back to the detector tubes. If the drill pin "walks away" as a result of striking a rock in the soil, try another location. If repeated tries cannot produce a satisfactory hole, then the test should be aborted. 1-19. SEATING THE TESTER IN TRANSMISSION From a radiation exposure standpoint, it is undesirable to have to spend a great deal of time "playing" with the tester in seating it. Locating the hole for insertion of the source rod in transmission is frequently a problem and CPN has made this easy for the operator. Figure 1-7. Positioning Source in Prepared Hole A small cast lip (Fig. 1-7) is on the front of the bottom casting to provide for insertion ease. 1-15