Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1312967
ASPHALT CONTENT GAUGE WITH COMPENSATION
FOR SAMPLE TEMPERATURE DEVIATIONS
Background of the Invention
Lowery, et al. U.S. Patent 3,492,479
discloses a portable nuclear gauge which utilizes a
fast neutron source and a thermal neutron detector
for determining the composition of a bulk material,
such as a bituminous paving mix, placed in a sample
pan. This type of gauge relies upon the neutron
moderating characteristics of hydrogen atoms present
in the composition for determining, for example, the
amount of asphalt in a paving mix or the amount of
moisture in a building material. For these
determinations it is known that the amount of
asphalt and the amount of moisture can be related to
the hydrogen content of the material, and the
hydrogen content of the material can be determined
by subjecting the sample to radiation from a fast
neutron source and detecting neutrons which have
been slowed or thermalized as a result of
interaction with the hydrogen nuclei present in the
sample. The number of thermalized neutrons detected
over a period of time is counted, and this "countl'
is utilized in determining the asphalt content of
the sample.
In operating the gauge, it is first
necessary to establish a standard count for
calibration purposes. This is accomplished using a
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standard sample which contains a layer of material
known to produce some standard count, for example, a
block of polyethylene. Then calibration curves are
produced for the particular material being tested,
by using carefully prepared samples of known asphalt
content. After the calibration curves have been
produced, unknown test samples can be placed in the
gauge and counts are taken. By reference to the
calibration curve, the correspondiny asphalt content
for that count can be read.
~ more recent model of this gaug2 has been
produced by applicant's assignee embodying the
principles of the Lowery patent and sold as the
"Model 3241 Asphalt Content Gauge" by Troxler
Electronic Laboratories, Inc. This gauge includes a
microprocessor to facilitate calibration and
computation of the sample asphalt content.
Calibration can be made by taking gauge counts on
two samples of known asphalt content. The
microprocessor then constructs a calibration
equation from these data points, and the gauge
provides a direct readout of the sample composition
(percent asphalt), thus eliminating the necessity of
calculations and reference to external calibration
tables.
In using the gauge for measuring the
asphalt content of hot asphalt samples, it was noted
that the temperature of the sample has an effect
upon the accuracy of the resulting reading and that
variations in temperature from sample to sample can
produce error or inaccuracy in the reading. In
order to overcome this source of inaccuracy, the
aforementioned Model 3241 gauge provided for the
operator to manually enter into the instrument the
temperature of the sample. Then a correction factor
calculated as a function of the temperature was
applied by the instrument to the experimental count
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in order ~o correct for the temperature effect. The
accuracy and reliability of the Model 3241 gauge has
been quite satisfactory and this gauge has gained a
wide acceptance in the industry. However,
applicants have now discovered how to further
improve and enhance the accuracy level of the
measurements made by the gauge.
Summary of the Invention
The present invention is based upon the
recognition and discovery that in measuring samples
such as hot asphalt, the error introduced by
temperature variations in a sample is a function not
only of the temperature of the sample buk also its
asphalt content. Thus, at a relatively low asphalt
content, one correction curve or correction factor
would be utilized to correct for variations in
temperature, while at a higher asphalt content a
different correction curve or correction factor
would be used. Moreover, it has been found that the
percent correction as a function of temperature
varies greatly as a function of the asphalt content.
Thus, it is an object of the present
invention to provide a nuclear measuring gauge which
more accurately compensates for deviations in the
temperature of the sample by employing a correction
factor which is a function of both the temperature
of the sample and its asphalt content.
In accordance with the invention there is
provided an improvement in the method of determining
the hydrogen content of a hydrogen containing
material wherein a sample of the material is
subjected to a neutron source and neutrons which are
thermalized by the presence of the hydrogen in the
material are detected to thus obtain a measurement
of the hydrogen content of the material. The
improvement comprises compensating for inaccuracy
due to variations in the temperature of the sample
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by applying to the measurement a correction factor
which is a function of both the temperature of the
sample and the hydrogen content of the sample. ~ore
particularly, the step of compensating for
inaccuracy by applying a correction factor comprises
a) measuring the temperature of the
chamber containing the sample,
b) obtaining an uncorrected
measurement of the hydrogen content of the sample,
c) generating a correction factor
as a function of the temperature measured in step a)
and the uncorrected hydrogen measurement of step b),
and
d) applying the correction factor
to the uncorrected hydrogen measurement to obtain a
temperature compensated corrected measurement.
Xn accordance with a further aspect of the
present invention there is provided an apparatus for
measuring the hydrogen content of a material,
wherein the apparatus comprises a measurement
chamber for receiving a sample of the material, a
source of fast neutrons mounted adjacent to the
chamber, a detector for thermal neutrons mounted
adjacent to the chamber, counting means cooperating
with the thermal neutron detector for obtaining a
count of the thermal neutrons, and means for
compensating for deviations in the temperature of
the sample as function of both the measured
temperature of the chamber containing the sample and
the measured hydrogen content.
More specifically, the means for
compensating for deviations in the temperature
includes a temperature sensor means mounted adjacent
to the chamber for measuring the temperature of the
chamber containing the sample, means for storing a
correction factor which varies as a function of both
temperature and hydrogen content of the sample,
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means for calculating the correction factor based
upon the measured temperature and upon the
uncorrected measurement of sample hydrogen, and
means for applying the correction factor to the
uncorrected measurement of hydrogen content to
thereby obtain a corrected measurement.
Brief Description of the Drawinqs
Some of the features and advantages of the
invention having been stated, others will become
apparent from the detailed description which
follows, and from the accompanying drawings, in
which ~-
Figure 1 is a perspective view showing an
asphalt content gauge in accordance with the present
invention;
Figure 2 is a schematic cross sectional
view of the gauge; and
Figure 3 is a block diagram showing the
operations performed by the gauge.
Detailed Description of the Drawinqs
Referring now more particularly to the
drawings, the asphalt content gauge is indicated
generally in Figure 1 by the reference character 10
and comprises a generally rectangular housing 11
having a door 12 in the lower portion thereof
providing access to a sample chamber . When the
door 12 is opened, a removable sample pan ~3 can be
placed in the sample chamber as shown in Figure 2.
After the closing of the door, the gauge may be
activated so as to measure the composition of the
~aterial in the sample pan. As illustrated, the
upper portion of the gauge includes a control panel
14 including various controls for controlling the
functions of the gauge and a display 15, which may
be of any suitable known construction, such as a
liquid crystal display device.
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Referring now to Figure 2, located in the
interior upper portion of the gauge is a source 21
o~ fast neutrons. The source 21 may for example
suitably comprise a Am-241:se source. In the lower
portion of the gauge beneath the sample pan are a
series of detector tubes 22 for detecting thermal
neutrons. While any suitable thermal neutron
detector device can be employed, the illustrated
embodiment employs He3 detector tubes. The gauge
also includes a data processor module 23 containing
a microprocessor programed to perform various basic
instrument functions including a measurement
function, a calibration function and a test
function. These same basic functions are carried
out in commercially available asphalt content
gauges, such as the Troxler 3241 gauge referred to
earlier. Reference may be made to this gauge and
its accompanying operation manual for a more
complete description of the`various functions and
details of the operations which are carried out in
these functions.
As shown in Figure 2, the instrument is
also provided with upper and lower temperature
sensors 24, 25 located above and below the sample
pan, respectively. The purpose of these temperature
sensors is to determine the temperature of the
sample so that an appropriate temperature correction
can be made, as explained more fully hereinafter.
The gauge illustrated in Figures 1 and 2
is especially suited for rapidly measuring the
asphalt content of bituminous paving mixes. Fast
neutrons emitted by the Am-241:Be source are
moderated by the hydrogen present in the asphalt
sample. These slowed or thermalized neutrons are
then detected by the thermal neutron detectors 22.
The number of counts detected by the detectors is
proportional to the asphalt content of the sample.
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Since the gauge detects neutrons moderated
by hydrogen in the total mix, i.e. the asphalt, the
aggregate and moisture present in the aggregate, a
careful calibration to isolate changes in asphalt
content is importan~. Calibration will vary with
the following factors:
1) Different brands of asphalt will
affect the calibration because of varying hydrogen
concentration per weight of asphalt.
2) Changing the aggregate mix, or source
of supply, will change the amount of hydrogen
present in the aggregate.
3) Changes in the density and
homogeneity of the mix will also affect calibration.
4) The temperature of the sample and its
effect on the ambient temperature inside the gauge
will also affect the measurements. An increase in
temperature increases the average speed of the
neutrons and thus the average energy of the
neutrons. This in turn will cause a decrease in the
number of counts detected by the thermal neutron
detectors 22.
Errors due to factors 1 and 2 above can be
minimized by preparing calibration curves for each
different brand of asphalt and type of aggregate
mix. Errors due to variations in the density and
homogeneity of the mix can be minimized by careful
and consistent sample preparation.
Temperature correction is achieved in
accordance with the present invention by means of
the two temperature sensors 24, 25 inside the gauge.
The two temperature readings from the sensors are
averaged and are used in the temperature
compensation of the asphalt measurement.
In accordance with the present invention
it has been found that the effect of temperature on
the asphalt content gauge varies with the amount of
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asphalt in the mix and appropriate compensation has
to be made. This is achieved by preparing a
calibration curve or calibration equation relating
the error as a function of both temperature and
asphalt content. Calibration curves or equations
are obtained using samples of known asphalt content
covering the entire usable range of asphalt content
to be measured by the gauge. Then these correction
factors are later used to correct the asphalt
content measurement based both upon the initial
asphalt content measurement and the tempera~ure.
Thus the operations which are carried out
by the processor unit 23 in obtaining an accurate
measurement of asphalt content are represented in
the block diagram of Figure 3, and involve first
measuring the temperature by use of the temperature
sensors 24 a~d 25 (as indicated at 31), next taking
a hydrogen count (32) over a predetermined period of
time and calculating an uncorrected asphalt content
based directly upon the hydrogen count (33). A
correction factor is then generated based upon both
the uncorrected asphalt content and the measured
temperature as indicated at 34. A temperature
compensated measurement is then obtained by applying
the correction factor to the uncorrected asphalt
content measurement as shown at 35.
The procedures of the present invention
will be understood more fully from the following
description of the calibration procedure and the
temperature compensation procedure.
Temperature Calibration Procedure
The temperature compensation parameters
are determined at the factory before the gauge is
put into field use. The procedure is as follows-
1. Select samples of known asphalt
content covering the usable range of the gauge (e.g.
2 percent to 10 percent asphalt content).
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2. Measure these samples at room
temperature and several different chamber
temperatures (Please note all ~emperature readings
are averages of the two sensors).
3. Find "percent difference" between
counts at room temperature and all other
temperatures.
4. Perform a straight line fit between
temperature and "percent difference". So,
Percent Difference = A1 ~ Bl T = temperature
A1,B1 = constants
Repeat this for all the samples tested.
Therefore at each percent asphalt there will be an
equation such as the one above. Now a series of A1's
and B1's are calculated for all the samples used.
5. Fit the Al's, and B1's against percent
asphalt using a second order equation.
Al = al + a2%Ac + a3%Ac2
Bl = bl + b2%Ac + b3%Ac2
% Ac = % Asphalt
%Ac is known for each sample
a1, a2, a3 = Constants
b1, b2, b3 = Constants
Gauqe % Asphalt Calibration
The field calibration for % Asphalt
Content (% AC~ can be accomplished by the use of the
linear equation
% AC = E + F x Count
where E and F are calibration constants calculated
by using at least two known % AC samples in the
gauge. Each sample is measured and the counts are
corrected for temperature effect by using the known
% AC's along with the factory derived valued of a1,
a2, a3, b1, b2, and b3 to calculate A1 and Bl. With
the known temperature value one can then calculate
the percent difference in the counts. The counts
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are then corrected for temperature effect and the
calibration constants E and F are calculated.
Field Measurements With TemPerature Com~ensation
With the calibration constants thus known,
temperature compensation can be readily applied.
The ~ollowing relationship is used for calculating
the percent asphalt content:
%AC = E + F x (Count) (1)
where E and F are the calibration
constants derived above ~or the particular asphalt
mix. The gauge measurement procedure invol~es the
following steps:
1) Take a hydrogen count. (Count)
2) Take a temperature reading.
3) Calculate %Ac from
equation (1). This
number is an estimated
%Ac not corrected for
temperature.
4) Use %Ac to calculate A1 and Bl by
Al = al + a2%Ac + a3%Ac2
Bl = bl + b2%Ac + a3%Ac2
5) Use temperature
reading to calculate
percent difference.
Percent Difference = A1 + B1T
6) Correct the measured count by percent
difference.
Adjusted Count = Count (1 + %
difference)
7) Finally, the temperature corrected
%AC is corrected %Ac = E + F x
(Adjusted Count)
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