Determining the Density and Unit Weight of Soil in Place by Nuclear Methods (Shallow Depth) - Soil Properties: Testing, Measurement, and Evaluation - page 233

Note 2 —When using the backscatter air-gap procedure, follow
the instrument manufacturer's instructions regarding appa-
ratus set up. Take the same number of readings for the normal
measurement period in the air-gap position as in the standard
backscatter position. Determine the air-gap ratio by dividing
counts per minute obtained in the air-gap position by counts
per minute obtained in standard backscatter position.
(5.1.4) Determine the ratio of the reading to the standard count or
to the air-gap count. From this count ratio and the appropriate
calibration and adjustment data, determine the in-place wet density.
(5.2) Direct Transmission Procedure:
(5.2.1) Make a hole perpendicular to the prepared surface using the
guide and the hole-forming device, or by drilling if necessary. The
hole shall be of such depth and alignment that insertion of the probe
will not cause the gage to tilt from the plane of the prepared area.
The depth of the hole must be deeper than the depth to which the
probe will be placed. The guide shall be the same size as the base of
the gage, with the hole in the same location on the guide as the
probe on the gage. The corners of the guide are marked by scoring
the surface of the soil. The guide plate is then removed, and any nec-
essary repairs are made to the prepared surface.
(5.2.2) Proceed with testing in the following manner:
(5.2.3) Set the gage on the soil surface, carefully aligning it with
the marks on the soil so that the probe will be directly over the
preformed hole.
(5.2.4) Insert the probe in the hole.
(5.2.5) Seat the gage firmly by rotating it about the probe with a
back-and-forth motion.
(5.2.6) Pull gently on the gage in the direction that will bring the
side of the probe against the side of the hole that is closest to the
detector (or source) location in the gage housing.
(5.2.7) Keep all other radioactive sources away from the gage to
avoid affecting the measurement.
(5.2.8) Secure and record one or more readings for the normal mea-
surement period.
(5.2.9) Determine the ratio of the reading to the standard count.
From this count ratio and the appropriate calibration and adjust-
ment data, determine the in-place wet density.
Note 3 —Some instruments have built-in provisions to compute
the ratio and wet density and to enter an adjustment bias.
Additionally, some instruments may have provisions to
measure and compute moisture content and dry density.