Determining the Density and Unit Weight of Soil in Place by Nuclear Methods (Shallow Depth) - Soil Properties: Testing, Measurement, and Evaluation - page 231

count rate to a count rate made on a reference standard or to an
air-gap count [for the backscatter air-gap technique, see (5.1.3) in
the “Procedure” section]. The reference count rate should be of
the same order of magnitude as the measured count rate over the
useful density range of the instrument.

(2) Standardization of the gage shall be performed at the start of each
day's work, and a permanent record of these data shall be retained for
a sufficient time to ensure compliance with subsections 2.3 and 2.3.1.
Perform the standardization with the gage located at least 8 m (25 ft)
away from other sources of radioactive material, and clear of large
masses or other items, which may affect the reference count rate.

(2.1) If recommended by the instrument manufacturer to provide
more stable and consistent results: ( 1 ) turn on the gage prior to use
to allow it to stabilize, ( 2 ) leave the power on during the use of the
gage for that day.

(2.2) Using the reference standard, take at least four repetitive
readings at the normal measurement period and determine the
mean. If available on the gage, one measurement period of four or
more times the normal period is acceptable. This constitutes one
standardization check.

(2.3) If the value obtained above is within the limits stated below,
the gage is considered to be in satisfactory condition, and the value
may be used to determine the count ratios for the day of use. If the
value is outside these limits, allow additional time for the gage to
stabilize, make sure the area is clear of sources of interference, and
then conduct another standardization check. If the second stan-
dardization check is within the limits, the gage may be used, but if
it also fails the test, the gage shall be adjusted or repaired as
recommended by the manufacturer. The limits are as follows:

| N s N o | 1.96 N o / F ( 16-1)

where:

N s value of current standardization count

N o average of the past four values of N s taken for prior usage

value of prescale. [The prescale value ( F ) is a divisor that
reduces the actual value for the purpose of display. The
manufacturer will supply this value if other than 1.0.] Some
instruments may have provisions to compute and display
these values
(2.3.1) If the instrument standardization has not been checked
within the previous three months, perform at least four new stan-
dardization checks, and use the mean as the value for N o .
(3) Use the value of N s to determine the count ratios for the cur-
rent day's use of the instrument. If for any reason the measured

F