Method and apparatus for measurement of moisture

A bulk material of grain, lump or any other form is irradiated with neutron and gamma radiation emitted from preferably a single radiation source, and preferably a single radiation detector detects the neutrons and gamma radiation transmitted through the bulk material, so that the moisture content of the bulk material can be accurately measured without being affected by the bulk density of the bulk material.

BACKGROUND OF THE INVENTION 
This invention relates to a method and an apparatus for measuring the 
moisture content of bulk material in grain or lump form utilizing the 
scattering and absorption of radiation. 
Control of moisture in a bulk material, for example, coke or sinter mix 
materials used in the steel industry is indispensible for the successful 
production of steel of good quality, since the coke ratio and the quality 
of the steel are dependent upon how the moisture content is controlled. 
For this purpose, a moisture meter or gauge of the scattering type 
utilizing the scattering of neutrons has hitherto been widely used for the 
measurement of the moisture content. 
It is known that, in a method of moisture measurement in which the 
scattering of neutrons is only resorted to for the measurement of the 
moisture content of a bulk material, the rate of scattering of neutrons is 
affected not only by the moisture content of the bulk material, but also 
by the bulk density of the bulk material. The bulk density varies 
depending on the grain size distribution or moisture distribution of the 
bulk material, and it is therefore impossible to measure the actual bulk 
density. U.S. Pat. No. 3,786,251 discloses a method which reduces the 
influence of the bulk density on the measurement of the moisture content 
of a bulk material. In this U.S. patent, gamma radiation is employed in 
addition to the neutrons, and the measured value of gamma radiation is 
used to compensate the measured value of neutrons so as to reduce the 
influence of the bulk density on the result of moisture content 
measurement. 
However, due to the fact that a neutron detector and a gamma detector are 
provided independently of each other in the system disclosed in U.S. Pat. 
No. 3,786,251, and the position measured by the neutron detector differs 
from that measured by the gamma detector, the value of the moisture 
content obtained by compensation has not always been accurate. 
SUMMARY OF THE INVENTION 
It is therefore a primary object of the present invention to provide a 
method and an apparatus for moisture measurement by which the moisture 
content of a bulk material can be measured more accurately than hitherto. 
The present invention is featured by the fact that a radiation source 
radiating neutron and gamma radiation is employed, and a radiation 
detector capable of detecting both the neutrons and the gamma radiation is 
employed. Thus, since a single detector detects both the neutrons and the 
gamma radiation, the moisture content and the bulk density at the same 
position of a bulk material can be measured, and the result of measurement 
is therefore highly accurate. When independent radiation detectors are 
provided for detecting the neutrons and gamma radiation respectively as 
described hereinbefore, it is impossible to make measurement on the same 
point, and it is also necessary to maintain a considerable distance 
between the two detectors so as to prevent mutual interference. It is 
therefore preferable to prepare a single radiation source capable of 
emitting both the neutrons and the gamma radiation. As an alternative, the 
neutrons and the gamma radiation may be derived from independent radiation 
sources, respectively. In this case, however, it is necessary to dispose 
the neutron radiation source and the gamma radiation source in such a 
relation that they are as close to each other as possible. As a radiation 
source capable of emiting both the neutrons and the gamma radiation, 
.sup.252 Cf is an example. As a radiation source emitting the neutrons 
only, .sup.241 Am-Be is an example, and as a radiation source emitting the 
gamma radiation only, .sup.137 Cs and .sup.60 Co are examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1 showing an embodiment of the moisture measuring method 
according to the present invention, a radiation source 14 emitting fast 
neutron and gamma radiation is disposed on one side of a hopper 12 charged 
with a bulk material 10 whose moisture content is to be measured, and 
radiation detectors 16 capable of detecting both the fast neutrons and the 
gamma radiation are disposed on the other side of the hopper 12 opposite 
to the radiation source 14. The radiation source 14 is mounted in a box 18 
having a sufficient shielding ability, and the fast neutrons and the gamma 
radiation emitted from the radiation source 14 pass through the hopper 12 
and the bulk material 10 to be detected by the radiation detectors 16. 
Each of these radiation detectors 16 is composed of a stilbene 
scintillator or a liquid scintillator and a high-resolution 
photomultiplier so that it can detect both of the neutrons and the gamma 
radiation at any desired sensitivity. Employment of .sup.252 Cf, which is 
a spontaneously fissionable nuclide, as the radiation source 14 in the 
system of radiation transmission measurement type is most suitable since 
it emits a very large number of neutrons and the energy of the gamma 
radiation is high. It is the sole defect of .sup.252 Cf that its half life 
is as short as about 2.7 years, and therefore, compensation for the decay 
becomes necessary. This decay can however be automatically compensated in 
a manner as described later. 
The moisture content M of the bulk material 10 can be calculated from the 
equations described presently. 
The measurement value of the neutrons is given by 
EQU N.sub.n =N.sub.no 
e.sup.-(.mu..sbsp.n1.sup..rho..sbsp.1.sup.R+.mu..sbsp.n2.sup..rho..sbsp.2. 
sup.x) (1) 
where 
N.sub.n : counted value of transmitted neutrons 
N.sub.no : counted value of transmitted neutrons in the absence of bulk 
material 
.mu..sub.n1 : mass absorption coefficient of bulk material (neutrons) 
.mu..sub.n2 : mass absorption coefficient of water (neutrons) 
.rho..sub.1 : bulk density of bulk material 
.rho..sub.2 : density of water 
R: thickness of bulk material 
x: moisture content expressed in terms of thickness of water 
The measured value of the gamma radiation is given by 
EQU N.sub.65 =N.sub.65 o 
e.sup.-(.mu..sbsp..gamma.1.sup..rho..sbsp.1.sup.R+.mu..sbsp..gamma.2.sup.6 
5 .sbsp.2.sup.x) (2) 
where 
N.sub.65 : counted value of transmitted gamma radiation 
N.sub..gamma.o : counted value of transmitted gamma radiation in the 
absence of bulk material 
.mu..sub.65 1 : mass absorption coefficient of bulk material (gamma 
radiation) 
.mu..sub.65 2 : mass absorption coefficient of water (gamma radiation) 
The value of x is calculated from the equations (1) and (2) as follows: 
##EQU1## 
The value of .rho..sub.1 is calculated from the equation (2) as follows: 
##EQU2## 
Therefore, the moisture content M of the bulk material 10 is expressed as 
##EQU3## 
It will be apparent from the above equations that the moisture content M 
of the bulk material can be calculated on the basis of the value of the 
bulk density .rho..sub.1 calculated at a constant thickness R of the bulk 
material. By the way, the value of transmitted neutrons N.sub.no and the 
value of transmitted gamma radiation N.sub.65 o counted in the absence of 
the bulk material very depending on a decay of the radiation source 14 or 
a variation of the wall thickness of the hopper 12 due to wear, deposition 
of matters on the hopper wall, etc. However, the value of neutrons 
N.sub.no and the value of gamma radiation N.sub.65 o transmitted through 
the hopper 12 before the bulk material 10 is charged into the hopper 12 
are to be previously measured so as to facilitate the compensation of 
N.sub.no and N.sub.65 o due to the decay of the radiation source or the 
variation of the wall thickness of the hopper. 
In another embodiment of the present invention shown in FIG. 2, the 
position of moisture measurement is changed from that shown in FIG. 1. In 
FIG. 2, the moisture content is measured at a position of a discharge 
chute 22 of a hopper 12 by a single radiation detector 16 disposed 
opposite to a radiation source 14. A gate 20 opened to discharge a bulk 
material 10 through the discharge chute 22 onto a belt conveyor 24 which 
conveys the bulk material 10 toward the next station. Although the length 
of time required for a batch of the bulk material 10 to be discharged 
completely onto the belt conveyor 24 is limited by the moving speed of the 
belt conveyor 24, the required moisture measurement can be successfully 
carried out within the above length of time which is as long as about 30 
seconds. The method shown in FIG. 2 is advantageous over that shown in 
FIG. 1 and can economically attain the desired object. It is a first 
advantage that the amount of radiation to be emitted from the radiation 
source 14 can be reduced since the distance between the radiation source 
14 and the radiation detector 16 is smaller than that in FIG. 1 and the 
path of the radiation absorbed by the bulk material becomes shorter than 
that in FIG. 1. It is a second advantage that the single radiation 
detector 16 can participate in the moisture content measurement of the 
batch of the bulk material charged into the hopper since the entirety of 
the bulk material moves through the position of moisture measurement. 
Practical means for putting the method shown in FIG. 2 into practice will 
be described with reference to FIG. 3. 
Upon completion of weighing, the gate 20 is opened, and the batch of the 
bulk material 10 charged in the hopper 12 is discharged within a length of 
time of about 30 seconds. After the bulk material 10 has been completely 
discharged, the gate 20 is closed again. The next batch of the bulk 
material 10 is charged into the hopper 12, and weighing is done again. The 
bulk material is processed according to such a batch system in which the 
above cycle is repeated. 
In the first step, the neutron and gamma radiation emitted from the 
radiation source 14 is directed toward the radiation detector 16 in the 
state in which the gate 20 is in its closed position and the bulk material 
10 is not present in the discharge chute 22. In a neutron-gamma 
(n-.gamma.) separating circuit 30 which is, for example, a pulse shape 
discrimination circuit, the neutrons and the gamma radiation are countably 
separated to be counted by a neutron (n) counter 402 and a gamma (.gamma.) 
counter 404 respectively in an arithmetic processor 40. The counted values 
are stored in a neutron (no) memory 406 and a gamma (.gamma.o) memory 408 
respectively. These values are counted and renewed for every batch to 
accomplish the function of so-called automatic zero calibration, so that 
the factors due to the variation of the wall thickness of the hopper, the 
variation of the amount of deposits on the hopper wall and the decay of 
the radiation source can be fully compensated. A timing signal generating 
circuit 410 generates a timing signal in response to the opening and 
closure of the gate 20 and applies the timing signal to various blocks. 
Then, when the gate 20 is opened to start discharge of the bulk material 
10, the neutron and gamma radiation which is transmitted through the bulk 
material 10 enters the radiation detector 16. The amount of the neutrons 
transmitted through the bulk material 10 is dependent upon the moisture 
content and bulk density of the bulk material 10, while that of the gamma 
radiation is dependent upon the bulk density of the bulk material 10. The 
values of the neutrons and gamma radiation counted until the complete 
discharge of the bulk material 10 are stored in a measured neutron (n) 
memory 412 and a measured gamma (.gamma.) memory 414 respectively. The 
data stored in the measured n memory 412 and the auto-zero data stored in 
the no memory 406 are supplied to a moisture content calculating circuit 
416 which calculates the mean moisture content M.rho.. Similarly, the data 
stored in the measured .gamma. memory 414 and the auto-zero data stored in 
the .gamma.o memory 408 are supplied to a bulk density calculating circuit 
418 which calculates the mean bulk density .rho.. Then, on the basis of 
the value of .rho., the bulk density is compensated in a bulk density 
compensation calculating circuit 420 which calculates the true mean 
moisture content M which is displayed on a display unit 50. 
According to the embodiment shown in FIGS. 2 and 3, the radiation source 
emitting the neutron and gamma radiation and the associated radiation 
detector are disposed on the opposite sides of the discharge chute of the 
hopper, so that the entirety of the batch of the bulk material charged 
into the hopper can be completely measured as the bulk material moves down 
past the point of moisture measurement, so that the mean moisture content 
of the entire batch of the bulk material can be calculated. Further, the 
neutrons and the gamma radiation transmitted through the discharge chute 
of the hopper in the absence of the bulk material in the discharge chute 
are measured to store the so-called auto-zero values which are used as the 
reference data during later calculations. Therefore, variations of the 
hopper wall thickness and hopper deposit amount can be automatically 
compensated, and the results of measurement are free from the influences 
of the wall thickness of the hopper and the amount of deposits on the 
hopper wall. 
FIGS. 4 and 5 show other embodiments of the present invention. As shown in 
FIGS. 4 and 5, the radiation source 14 may be inserted into the hopper 12, 
and the neutron and gamma radiation emitted from the radiation source 14 
may be detected by radiation detectors 16 disposed outside the hopper 12. 
In this case, the total moisture content of the bulk material 10 is 
measured while the bulk material 10 stays within the hopper 12. A 
plurality of radiation detectors 16 may be disposed outside the hopper 12 
as shown in FIG. 4, or a single radiation detector 16 may be disposed to 
replace each set of three detectors 16 shown in FIG. 4 and may be arranged 
to be guided along a guide rail 60 as shown in FIG. 5. The arrangement 
shown in FIG. 5 reduces the required number of radiation detectors 16. 
It will be understood from the foregoing description that the present 
invention comprises a radiation source emitting fast neutron and gamma 
radiation and a radiation detector capable of detecting both the neutrons 
and the gamma radiation, so that the neutrons and the gamma radiation can 
be directed toward the same point of a bulk material whose moisture 
content is to be measured, and the moisture content can be accurately 
measured without being affected by the bulk density of the bulk material.