1-15. SURFACE ROUGHNESS ERROR

TM 5-6635-386-12&P to obtain good seating: a. Take a backscatter reading prior to any work. b. Perform a minimum of surface work, take another reading. c. Perform more work, clearing, smoothing, etc, and take another reading. The readings will drop in count rate as the tester is seated better and better. When readings have ceased to change, the tester is seated as well as it can be. This is then an index of the work required to obtain good seating. 1-15. SURFACE ROUGHNESS ERROR Surface roughness error is the error introduced into the tester readings due to the rough surface obtained during seating. Such error was exhibited in the work in paragraph 1-14. The dual position tester backscatter feature provides a means of selecting a shallow backscatter position (AC Position) for best results on thin lift Asphaltic Concrete and a slightly deeper backscatter position (BS Position) for use on soils or thick Asphaltic Concrete. Unfortunately, the thinner the tester attempts to read, the more sensitive it is to surface roughness. The following test will illustrate the potential roughness error and will permit the operator to use the tester to his best advantage with a minimum of error. a. Take a reading in both normal backscatter (BS) and in the shallow backscatter (AC) on a smooth concrete surface. Use the 2 minute count period for this test. b. Raise the tester the thickness of a dime, approximately 0.050-inch, producing a 100% void under the tester. c. Repeat the readings in both AC and BS test. d. The tester will read approximately 3 pounds per cubic feet (PCF) low in BS and will read approximately 7 (PCF) low in AC. This will vary slightly between testers of the same design and between testers of different manufacturers. The error is an index of the amount of effort required by the operator to obtain good seating with any specific gauge. 1-16. DEPTH OF READING (Fig. 1-4.) Certain physics limitations present themselves in depth of reading of nuclear testers. No matter how the design of the tester is made, soil testers using GM tubes and radioactive gamma sources of approximately 0.7 MEV energy will not read more than 5-inches deep for 99% of their response in construction materials from 100 to 170 PCF. If the radioactive source is well collimated as in the BS position, the tester will tend to pick up 90% of its returned radiation from approximately 2.5-inches deep. 1-13