Method for measuring the flow of fluids

The disclosed method of measuring the flow of a fluid with a porous particulate ceramic tracer and an optical instrument is characterized in that spherical particles having diameters in the range of 0.5 to 150 .mu.m are used as the tracer. Inasmuch as the tracer particles for flow measurement are spherical, the sectional area of scattered light to be detected by an optical sensor means is constant regardless of the orientation of particles. Furthermore, spherical particles have no surface irregularities that might cause concatenation so that individual particles are not agglomerated in tracking a fluid flow, thus contributing to improved measurement accuracy.

TECHNICAL FIELD 
The present invention relates to a method for measuring the flow of fluids, 
herein after referred to as "flow measurement". It should, however, be 
understood that the term "flow measurement" as used throughout this 
specification means not only a measurement of the flow velocity of a gas, 
such as air, fuel gas, etc., or a liquid, such as water, liquefied gas, 
etc., but also a topological visualization of the distribution of such gas 
or liquid. 
BACKGROUND OF THE INVENTION 
Prior Art 
The particles heretofore used as tracer particles in optical flow 
measurements are porous particles made of SiO.sub.2, TiO.sub.2, SiC or the 
like which are obtainable by a coprecipitation process or from a natural 
material such as the mineral ore. These particles generally have a mean 
particle diameter of about 0.5 to 150 .mu.m. 
In a measurement of the flow velocity using a laser device such as a laser 
Doppler velocimeter, a phase Doppler velocimeter or the like, tracer 
particles somewhere between 0.5 and 10 .mu.m in mean diameter, in 
particular, have so far been employed. 
In technologies involving a visualization of a flowing fluid by 
photographing the distribution of tracer particles in the fluid with the 
aid of an instantaneous, powerful light source, such as a flash-light or a 
pulse laser, and a determination of the flow pattern from the resulting 
picture, particles somewhere between about 5 .mu.m and about 150 .mu.m in 
mean diameter are generally employed. 
Electron microphotographs of the representative tracer particles which are 
conventionally employed are presented in FIGS. 3 through 14; viz. white 
carbon in FIGS. 3 and 4, TiO.sub.2 in FIGS. 5 and 6, talc in FIGS. 7 and 
8, TiO.sub.2 -talc in FIGS. 9 and 10, particles from kanto loam, and white 
alumina in FIGS. 13 and 14. 
However, as apparent from these microphotographs, the conventional tracer 
particles have the following drawbacks, 1) through 5), which amplify the 
measurement error. 
1) Because the tracer particles are morphologically not uniform, the 
sectional area of scattered light to be detected varies according to the 
real-time orientation of each particle. 
2) Because the particle size distribution is broad and the sectional area 
of light scattering varies with different individual particles, the 
comparatively large particles scatter light in two or more fringe at a 
time. 
3) Because the apparent specific gravity of the particulate tracer differs 
markedly from that of the fluid to be measured, the particles do not 
faithfully follow the on-going flow of the fluid. 
4) Because the particle size distribution is broad and the apparent 
specific gravity also has a distribution, the particles follow the fluid 
flow with varying efficiencies to prevent accurate quantitation of the 
flow measurement. 
5) Because the surface of the particle is irregular, the individual 
particles tend to be concatenated with each other to increase the 
effective particle size. 
The technique used generally for launching tracer particles into a fluid 
comprises either extruding tracer particles from a screw feeder and 
driving them into the body of the fluid with the aid of an air current or 
suspending tracer particles in a solvent and ejecting the suspension in a 
mist form using an ultrasonic humidifier. In any of the above methods, the 
rate of feed of the tracer particles is not constant so that the accuracy 
of flow measurement is inevitably sacrificed. 
OBJECTS OF THE INVENTION 
It is the object of the present invention to overcome the above-mentioned 
drawbacks and provide a method of flow measurement with improved accuracy. 
SUMMARY OF THE INVENTION 
The method of flow measurement according to the invention comprises 
measuring the flow of a fluid using an optical instrument and a porous 
particulate ceramic tracer, the diameter of which is 0.5 to 150 .mu.m. 
In another aspect, the method of flow measurement according to the 
invention comprises feeding a non-agglomerating particulate tracer to an 
optical instrument, such as a laser device, from a measuring wheel 
particle feeder. 
The method of flow measurement according to the invention comprises 
measuring the flow of a fluid using an optical instrument and a porous 
particulate ceramic tracer, said porous particulate ceramic tracer 
consisting of spherical particles having a diameter of 0.5 to 150 .mu.m. 
Particularly in the method of measuring the flow velocity using a laser 
instrument such as a laser Doppler velocimeter, spherical ceramic 
particles having a diameter of 0.5 to 10 .mu.m are preferred from the 
viewpoint of relation with fringe. A more satisfactory spherical particle 
diameter range is 1.5 to 2.5 .mu.m. In flow measurement which involves 
photographing, the use of spherical particles having a diameter of 5 to 
150 .mu.m is preferred from the viewpoint of detecting light and flowing 
the fluid flow. A more satisfactory particle diameter range is 30 to 100 
.mu.m. 
When the tracer particles for use in flow measurement with an optical 
instrument are spherical as in the invention, the sectional area of 
scattered light to be detected by a photosensor or the like is constant 
regardless of the orientation of particles at the moment of detection. 
Moreover, because such particles have no surface irregularities that may 
cause concatenation, it does not happen that two or more tracer particles 
flow as concatenated through the body of the fluid. Therefore, the 
accuracy of flow measurement is improved. 
Where the fluid to be measured is a gas, said tracer particles are 
preferably of hollow structure. 
When the tracer particles are hollow, the specific gravity of the particles 
is so low that even if the particle size is not critically uniform, they 
may readily follow the gas flow. Therefore, the accuracy of gas flow 
measurement is improved. The improved accuracy of measurement afforded by 
such hollow spherical particles over that attainable with solid spherical 
particles is more remarkable when the flow rate of the fluid is high. 
The shell thickness of such hollow spherical particles is not so critical 
but is preferably in the range of one-third to one-tenth of the diameter 
of the particle. If the shell thickness is less than one-tenth of the 
particle diameter, the particles tend to be collapsed in use. Conversely 
when the shell is thicker than one-third of the particle diameter, the 
advantage of the hollow structure will not be fully realized. 
Where the fluid to be measured is a liquid, said tracer is preferably a 
porous particulate ceramic tracer having closed pores with a porosity of 
not less than 0.1 cm.sup.3 /g. 
When the tracer particles have closed pores with a porosity of not less 
than 0.1 cm.sup.3 /g, the specific gravity of the tracer particles can be 
changed so as to minimize the differential from the specific gravity of 
the fluid to be measured, thereby making it easier for the particles to 
follow the dynamics of the fluid. In this manner, the accuracy of flow 
measurement can be further improved. 
Where the fluid to be measured is a liquid, tracer particles coated with a 
metal are used with advantage. 
When such metal-clad porous spherical particles are used for the flow 
measurement of a liquid, the intensity of reflected light is greater than 
it is the case when bare particles are employed so that the accuracy of 
flow measurement is improved. However, since such metal-clad particles are 
higher in specific gravity and expensive, they are preferably used where 
the conditions of measurement specifically call for the use of such 
particles. 
Particularly preferred are metal-clad porous ceramic tracer particles 
having closed pores with a porosity of not less than 0.1 cm.sup.3 /g. 
Application of a metal cladding increases the specific gravity of 
particles as mentioned above but the adverse effect of increased specific 
gravity can be minimized by using porous ceramic particles having closed 
pores with a porosity of not less than 0.1 cm.sup.3 /g. 
For application of a metal cladding, any of the electroless plating, 
electrolytic plating, CVD, vapor deposition and other techniques can be 
utilized but the electroless plating process is preferred in that a 
uniform cladding can be easily obtained (Claim 7). 
The cladding metal includes, among others, Ni, Pt, Co, Cr, etc. but nickel 
is preferred in that a quality cladding can be easily obtained by 
electroless plating and that the resultant cladding is comparatively high 
in chemical resistance. 
The thickness of the metal cladding is not critical but is preferably 
within the range of 0.05 to 5 .mu.m. If the cladding thickness is less 
than 0.05 .mu.m, the effect of increased reflectance is hardly obtained. 
If the cladding is over 5 .mu.m in thickness, the proportion of the metal 
in the whole particle is too large so that the bulk specific gravity of 
the tracer is increased. 
The starting material for said particulate tracer or for the ceramic part 
of said metal-clad particulate tracer is not limited in variety only if it 
is chemically stable. Thus, the starting material can be selected from 
among, for example, alkaline earth metal carbonates such as calcium 
carbonate, barium carbonate, etc., alkaline earth metal silicates such as 
calcium silicate, magnesium silicate, etc.; and metal oxides such as 
silica (SiO.sub.2), iron oxide, alumina, copper oxide and so on. Among 
these materials, SiO.sub.2 is particularly desirable in that it is 
commercially available at a low price and resistant to heat. When the heat 
resistance of the ceramic material is high, particles prepared therefrom 
can be effectively used without the risk of breakdown even in 
high-temperature fluids. 
The size distribution of tracer particles is preferably as narrow as 
possible but when not less than 70% of the particles have diameters within 
the range of .+-.50% of the mean particle diameter, there is obtained a 
substantially uniform sectional area of scattered light. Moreover, the 
kinetics of tracer particles in the fluid body, that is to say the pattern 
of following the fluid flow, are then rendered substantially uniform. 
The tracer particles of the invention can be applied to the measurement of 
fluids flowing at high speeds. Thus, in the conventional flow measurement 
using a laser Doppler device, an attempt to increase the sample data rate 
(the number of data generated per unit time) by increasing the flow rate 
of the fluid and, hence, the number of tracer particles passing through 
the fringe per unit time resulted in a decrease in the mean effective data 
rate, which is a representative indicator of measurement accuracy, thus 
making it difficult to achieve an accurate measurement of a high-velocity 
fluid. In accordance with the present invention, the mean effective data 
rate is high even at a high sample data rate so that the method can be 
effectively applied to the measurement of fluids flowing at high speeds. 
Furthermore, in the conventional flow measurement, the concentration of 
tracer particles cannot be increased over a certain limit because an 
increased feed of tracer particles for generating a larger number of data 
per unit time should adversely affect the mean effective data rate. 
However, in the method of the invention, increasing the rate of feed of 
tracer particles for increasing the sample data rate does not sacrifice 
the mean effective data rate, with the result that the desired measurement 
can be performed with an increased tracer concentration. 
The particulate tracer or the ceramic core of the metal-clad particulate 
tracer can be easily manufactured at low cost by the reversed micelle 
technology which provides spherical or hollow spherical porous tracer 
particles. 
In this connection, when an aqueous solution of the precursor for the 
tracer material is extruded from a porous glass or polymer membrane having 
substantially uniform pores in an organic solvent, there can be obtained 
uniform particles with a narrow size distribution, and such particles are 
well suited for use as the tracer particles or the core of metal-clad 
tracer particles. 
The above-mentioned porous glass or polymer membrane may be any of the 
known membranes such as the membrane obtainable by subjecting borosilicate 
glass to phase separation and washing the product with a pickling acid 
solution, the membrane obtainable by mixing a silica sol with a 
water-soluble organic polymeric material, subjecting the mixture to phase 
separation at polymerization and rinsing the product, and the membrane 
obtainable by a technology involving irradiation with laser light to give 
perforations of substantially uniform diameter. 
The tracer particles can be advantageously fed to the laser instrument by 
means of a measuring wheel particle feeder. 
When the tracer particles are fed from the measuring wheel particle feeder, 
the particles can be delivered quantitatively so that the accuracy of 
velocity measurement or photographic distribution measurement is further 
improved. Moreover, in the conventional method, for obtaining of the high 
measurement accuracy, it is essential to recalibrate the instrument after 
each measurement cycle for minimizing the measurement error. This 
operation is eliminated by use of the measuring wheel particle feeder so 
that as many more measurements can be performed within a given time 
period. 
The construction of the measuring wheel particle feeder and the mechanism 
of feed are described below, referring to FIGS. 15 and 16. As illustrated, 
a feeder body 101 is internally provided with a disk 102 which is driven 
by a motor not shown. The top surface of this disk 102 is provided with a 
circumferential groove 103. 
The reference numeral 104 indicates a hopper which is filled with a 
particulate tracer F. The hopper 104 has a lower portion 104a which is 
tapered towards the discharge end of the hopper and the lowest part 104b 
thereof is open in the form of an orifice 104c immediately over the groove 
103, so that the particulate tracer F in the hopper 104 may flow through 
the orifice 104c into the circumferential groove 103. 
The reference numeral 107 indicates a blow nozzle made of plate material. 
This blow nozzle 107 is configured as a sector in plan view and has a 
recess 109 having a tapered lateral surface 108 in a substantial center 
thereof. This recess 109 is centrally provided with an orifice extending 
in the direction of the thickness for passage of tracer particles (FIG. 
16). 
The reference numeral 105 indicates a particle duct which runs through a 
casing 106 of the feeder body 101 and through which the inside of the 
feeder body 101 is made communicable with the outside thereof. This 
particle duct 105 is attached to the top of the blow nozzle 107 in such a 
manner that its inward end 105a covers said recess 109 to establish 
communication with said particle duct 110. 
The atmospheric pressure within the feeder body 101 is maintained at a 
level higher than the external atmospheric pressure. Because of this 
pressure gradient, the air flows into the circumferential groove 103 
adjacent said blow nozzle 107 at point X beneath the blow nozzle 107. The 
air then flows out through a particle passageway 110, said recess 109 and 
said particle duct 105. The arrowmarks in FIG. 16 indicate the flow of 
air. 
As the particles F are carried by such an air flow, they are successfully 
metered out from the feeder body 101 into the body of the fluid to be 
measured. 
In a second aspect, the invention provides a method of flow measurement 
using an optical instrument and a particulate tracer material, wherein a 
non-agglomerating particulate tracer is fed to the laser or other optical 
instrument with such a measuring wheel particle feeder. 
When a non-agglomerating particular tracer material is fed with the 
measuring wheel particle feeder for optical instrument, the feed rate can 
be critically controlled even when the tracer has a large particle size 
distribution and is morphologically divergent as it is the case with the 
conventional tracer particles. Thus, the conventional non-agglomerating 
tracer particles are generally large in particle size and high in bulk 
specific gravity so that they cannot faithfully follow the fluid flow but 
when this measuring wheel particle feeder is employed, a better tracking 
performance can be obtained for enhanced measuring efficiency under 
conditions of high flow rate and least turbulence.

PREFERRED EMBODIMENTS OF THE INVENTION 
The following examples are further illustrative but not limitative of the 
invention. 
EXAMPLE 1 
Using a hollow spherical particulate SiO.sub.2 tracer with 70% of 
individual particles having diameters within the range of mean particle 
diameter=1.5 .mu.m.+-.0.4 .mu.m, the shell thickness of which is one-fifth 
of the diameter of the particle, the velocity of air within a cylinder was 
measured using a laser velocimeter under the following conditions and the 
relationship between the sample data rate and the mean effective data rate 
was investigated. Thus, for increasing the number of data per unit time 
(sample data rate) stepwise, the flow rate was increased stepwise (with 
the concentration of tracer particles kept constant) to increase the 
quantity of particles passing through the inference figure at the 
flowmeter. Of the resulting data, the percentage of data useful for 
velocity assessment (effective data rate) was determined. (Mean flow rate 
=ca. 20 m/min.) 
1. Instrument: Fiber type laser Doppler velocimeter (FLDV) 
(cf. Ikeda, Y., Hikosaka, M., Ohira, T., and Nakajima, T., Scavenging Flow 
Measurements in a Fired Two-Stroke Engine by FLDV., 1991. SAE Paper No. 
910, p.670) 
(Specification) 
Laser: He--Ne laser 
Laser power: 8 mW.times.2 
Lens diameter: 55 mm 
2. Measuring conditions: 
Center frequency: 20 MHz 
Band width: .+-.16 MHz 
Effective sample number: 5,000 
Signal gain: 24 dB 
Photomultiplier voltage: 760 V 
The results are shown in Table 1. 
[The mean effective data rate was determined with Dantec's burst signal 
analyzer. When the symmetry of scatter signals is disturbed, the peak 
frequency value after Fourier transformation is depressed. Therefore, only 
the signals with a frequency peak/reference frequency peak ratio over a 
given value were regarded as valid data. In other words, the data lacking 
in signal symmetry were 
TABLE 1 
______________________________________ 
Sample data rate (Hz) 
Mean effective data rate (%) 
______________________________________ 
300 82 
600 80 
900 75 
1,200 70 
1,500 73 
1,800 75 
______________________________________ 
It will be apparent from Table 1 that increasing the sample data rate does 
not result in any appreciable decreases in the mean effective data rate 
which is a representative indicator of measurement accuracy, indicating 
that the tracer particles of the invention are fully effective for the 
measurement of high-velocity fluids. 
EXAMPLE 2 
The same measurement as Example 1 was performed using a hollow spherical 
particulate SiO.sub.2 tracer with 90% of individual particles having 
diameters within the range of 1 to 5 .mu.m (the shell thickness was 
one-fifth of the diameter of the particle). The results are shown in Table 
2. 
TABLE 2 
______________________________________ 
Sample data rate (Hz) 
Mean effective rdata rate (%) 
______________________________________ 
300 80 
600 79 
900 55 
1,200 60 
1,500 65 
1,800 57 
______________________________________ 
It will be seen from Table 2 that although the mean effective data rates 
are not as high as those obtained in Example 1 because of the broader 
tracer particle size distribution, there are obtained stable effective 
data rates even at high sample data rates. 
COMATIVE EXAMPLE 1 
The same experiment as Example 1 was performed using a wet-process white 
carbon, shown in FIGS. 3 and 4, which is a representative prior art tracer 
(mean primary particle diameter 0.2 .mu.m, mean agglomerated particle 
diameter (effective particle diameter) 6 .mu.m; NIPSIL SS-50F, 
manufactured by Nippon Silica Industry Co., Ltd.). The results are shown 
in Table 3. 
TABLE 3 
______________________________________ 
Sample data rate (Hz) 
Mean effective data rate (%) 
______________________________________ 
300 53 
600 63 
900 20 
1,200 15 
1,500 12 
1,800 5 
______________________________________ 
It will be apparent from Table 3 that the mean effective data rates are 
invariably lower than the rates obtained in Examples 1 and 2, with 
extremely low rates found at high sample data rates. 
It is predictable that the use of the prior art tracer particles shown in 
FIGS. 5 through 14 will also yield results similar to those described 
above for white carbon. 
EXAMPLE 3 COMATIVE EXAMPLES 2 AND 3 
The velocity of a fluid flowing through an acrylic resin pipe with an 
internal diameter of 100 mm was determined using: a spherical particulate 
SiO.sub.2 tracer having the particle diameter distribution of FIG. 18 
(Example 3; FIGS. 1 and 2), a particulate TiO.sub.2 tracer having the 
particle diameter distribution of FIG. 19 (Comparative Example 2; FIGS. 5 
and 6) and a particulate SiO.sub.2 tracer having the particle diameter 
distribution of FIG. 20 (Comparative Example 3; FIGS. 3 and 4). For 
determinations, the same fiber type laser Doppler velocimeter (FLDV) as 
used in Example 1 was employed. A measuring wheel particle feeder (MSF-F, 
Liquid Gas Co., Ltd.) was used to supply said spherical particulate 
SiO.sub.2 particle and a fluidize bed feeder (Durst et al., 1976) was used 
to supply said conventional TiO.sub.2 and SiO.sub.2 particles. 
In each determination, the sample data rate was varied by changing the 
concentration of tracer particles. The same average measuring speed and 
root mean square velocity (r.m.s.v.), 122 m/s and 3.5 m/s, respectively, 
were used for the three tracers. 
The relationship between sample data rate and effective data rate is 
diagrammatically shown in FIG. 21. 
It will be apparent from FIG. 21 that, in accordance with the present 
invention, the effective data rate is not decreased even if the number of 
data per unit time is increased by increasing the feed rate of particles. 
Production Example 1 
The following example is intended to illustrate the production of tracer 
particles by the reversed micelle method. 
A 10 .mu.m-thick polyimide film was irradiated with a KrF excimer laser 
(wavelength 251 nm) to provide perforations sized 2.0 .mu.m. This 
perforated polymer film was mounted in an emulsification device 
illustrated in FIG. 17 and an aqueous solution of the tracer precursor 
substance was fed under pressure into an organic solution with a syringe 
pump. The feeding rate was 1 g/cm.sup.2 and the temperature was 25.degree. 
C. 
The construction of the device shown in FIG. 17 is summarized below. The 
reference numeral 10 indicates a volumetric syringe pump 10. The polymer 
membrane, indicated by 12, is mounted in the forward portion of the 
volumetric syringe pump. The reference numeral 14 indicates a screen for 
supporting said polymer membrane. Indicated by the numeral 16 is a 
cylindrical reactor which is communicating with said syringe pump 10. The 
reference numeral 20 indicates a feed pipe for feeding an organic solvent 
25 from a solvent beaker 24 to said reactor 16 through a metering pump 22. 
Now, an aqueous solution 11 of the tracer particle precusor substance is 
quantitatively injected into the organic solvent 25 within the reactor 16 
by said syringe pump 10. After formation of a large number of emulsion 
particles, the organic solvent is returned from the reactor 16 to the 
solvent beaker 24 via a withdrawal pipe 26. 
In the example, a hexane solution of polyoxyethylene (20)-sorbitan 
trioleate (20 g/l) was used as the organic solvent. 
As to the aqueous solution, a solution prepared by adding 1.0 mol of 
tetraethoxysilane, 2.2 mol of methanol, 1.0 mol of N,N-dimethylformamide 
and 4.times.10.sup.-4 mols of ammonia to 10 mols of water was employed. 
After emulsification at 5.degree. C., the slurry was refluxed for 30 hours 
and the resulting emulsion particles (sol) in the organic acid were 
precipitated by gelation. The precipitate was dried and heated at 
800.degree. C. to give a silica (SiO.sub.2) tracer uniform in particle 
diameter. The silica tracer particles thus obtained were spherical 
particles, 70% of which had diameters in the range of mean 
diameter=2.5.+-.0.7 .mu.m (FIGS. 1 and 2). 
EXAMPLE 4 
For comparing the measuring accuracy obtainable with spherical tracer 
particles with that obtainable with hollow spherical tracer particles, the 
same experiment as Example 1 was performed using the solid spherical 
particles prepared in Production Example 1, that is the particles with 70% 
having diameters within the range of mean=2.5.+-.0.7 .mu.m. The results 
are shown in Table 4. 
TABLE 4 
______________________________________ 
Sample data rate (Hz) 
Mean effective data rate (%) 
______________________________________ 
300 81 
600 80 
900 74 
1,200 73 
1,500 63 
1,800 65 
______________________________________ 
Comparison of Table 4 with Table 1 indicates that both at low velocity (low 
sample data rate) and high velocity (high sample data rate), high 
measurement accuracy values are obtained and that particularly at high 
velocity, the hollow spherical tracer particles yield a higher measurement 
accuracy than the solid spherical tracer particles, even when the minor 
difference in particle size is taken into consideration. 
EXAMPLE 5 AND COMATIVE EXAMPLE 4 
Using the conventional particulate TiO.sub.2 tracer for fluid visualization 
having a mean particle diameter of 5 .mu.m and a particle specific gravity 
of 6 g/cm.sup.3 (Comparative Example 4) and a porous spherical particulate 
SiO.sub.2 tracer having a mean particle diameter of 30 .mu.m and a 
particle specific gravity of about 1 g/cm.sup.3 which is substantially 
comparable to the first-mentioned tracer in average fluid tracking 
performance (Example 5, 72% within the range of mean particle diameter 
.+-.50%), a fluid visualization test was performed by the photographing 
method using a flash lamp as the light source. 
As a result, the mean reflection light quantity per particle was about 20 
times the value of the conventional tracer. 
In terms of the width of spread of particles in the laminar flow region, 
the porous spherical particles showed values about 0.8 to 0.5 times the 
values of the conventional particles. 
It is easy to see that, with the average fluid-tracking performance being 
fixed, the larger the reflection light quantity, i.e. the signal quantity, 
and the narrower the spread of tracer particles in the laminar flow 
region, the higher is the measurement accuracy. 
It is easily predictable that similar results will be obtained when the 
conventional tracer particles illustrated in FIGS. 5 through 14 are used 
in lieu of the above tracer particles of Comparative Example 4. 
EXAMPLE 6 AND COMATIVE EXAMPLE 5 
The same visualization test as above was performed using, instead of the 
porous spherical particulate SiO.sub.2 tracer with a mean particle 
diameter of 30 .mu.m, a porous spherical particulate SiO.sub.2 tracer with 
a mean particle diameter of 100 .mu.m (Example 6; 72% of particles within 
the range of mean.+-.50%) and a conventional particulate TiO.sub.2 tracer 
for fluid visualization which is comparable to the first-mentioned tracer 
in fluid tracking performance (Comparative Example 5). 
Like the tracer of Example 5, the porous spherical SiO.sub.2 tracer having 
a mean particle diameter of 100 .mu.m was superior to the conventional 
tracer in average reflection light quantity and in terms of the width of 
spread of particles in the laminar flow region. 
EXAMPLE 7 
Using the spherical particles manufactured in Production Example 1, namely 
a spherical particulate SiO.sub.2 tracer with 70% of particles having 
diameters within the range of 2.5.+-.0.7 .mu.m (FIGS. 1 and 2) and the 
same laser Doppler velocimeter as used in Example 1, the velocity of water 
flowing in a turbulent flow within a pipe of circular section was 
determined and the relationship between sample data rate and mean 
effective data rate was investigated. Thus, the flow rate was increased 
stepwise to increase the number of data per unit time (sample data rate) 
and, hence, the quantity of particles passing through the fringe in the 
velocimeter, with the concentration of particles being kept constant. Of 
the data thus generated, the percentage of the data useful for velocity 
assessment (effective data rate) was determined. 
1. Instrument: A fiber type laser Doppler velocimeter (FLDV) 
(Ikeda, Y., Hikosaka, M., Ohira, T., and Nakajima, T., Scavenging Flow 
Measurements in a Fired Two-Stroke Engine by FLDV. 1991, SAE Paper No. 
910670.) 
(Specification) 
Laser: He--Ne laser 
Laser power: 8 mW.times.2 
Lens diameter: 55 mm 
2. Measuring conditions: 
Center frequency: 20 MHz 
Band width: .+-.16 MHz 
Effective sample number: 5,000 
Signal gain: 24 dB 
Photomultiplier voltage: 60 V 
The results are shown in Table 5. 
TABLE 5 
______________________________________ 
Sample data rate (Hz) 
Mean effective data rate (%) 
______________________________________ 
1,000 72 
2,000 69 
3,000 70 
______________________________________ 
COMATIVE EXAMPLE 6 
Using the conventional particulate TiO.sub.2 tracer with a mean particle 
diameter of 2 .mu.m (FIGS. 5 and 6), the velocimetric test was performed 
under the same conditions as used in Example 7. The results are shown in 
Table 6. 
TABLE 6 
______________________________________ 
Sample data rate (Hz) 
Mean effective data rate (%) 
______________________________________ 
1,000 35 
2,000 20 
3,000 10 
______________________________________ 
COMATIVE EXAMPLE 7 
Using the conventional particulate SiC tracer with a mean particle diameter 
of 3 .mu.m, the velocimetric test was performed under the same conditions 
as in Example 7. The results are shown in Table 7. 
TABLE 7 
______________________________________ 
Sample data rate (Hz) 
Mean effective data rate (%) 
______________________________________ 
1,000 50 
2,000 42 
3,000 37 
______________________________________ 
It will be apparent from Tables 5 through 7 that, compared with the tracers 
of Comparative Examples 6 and 7, the tracer of Example 7 yields high 
effective data rates which are substantially constant up to a very high 
data rate. 
EXAMPLES 8, 9, AND 10 AND COMATIVE EXAMPLES 8 AND 9 
The five particulate tracers shown below in Table 8 were respectively 
immersed in water for a predetermined time and the bulk specific gravity 
of each wet tracer was determined. The results are also shown in Table 8. 
TABLE 8 
______________________________________ 
Particulate tracer Bulk specific gravity 
______________________________________ 
[Example 8] 1.45 g/cm.sup.3 
Porous spherical SiO.sub.2 particles, closed pore 
0.05 cm.sup.3 /g, mean particle diameter 2.7 .mu.m 
[Example 9] 1.20 g/cm.sup.3 
Porous spherical SiO.sub.2 particles, closed pore 
0.21 cm.sup.3 /g, mean particle diameter 2.8 .mu.m 
[Example 10] 1.26 g/cm.sup.3 
Porous spherical SiO.sub.2 particles, closed pore 
0.15 cm.sup.3 /g, mean particle diameter 15 .mu.m 
[Comparative Example 8] 
2.3 g/cm.sup.3 
Conventional SiC particles, mean particle 
diameter 3 .mu.m 
[Comparative Example 9] 
3.1 g/cm.sup.3 
Conventional TiO.sub.2 particles, mean particle 
diameter 2 .mu.m 
______________________________________ 
It is apparent from Table 8 that compared with the tracers of Comparative 
Examples 8 and 9, the tracers of Examples 8, 9 and 10 are smaller in the 
bulk specific gravity differential from water, suggesting the greater ease 
with which they may follow a water flow and that the tracer of Example 9 
is particularly excellent. 
Since the fluid-tracking performance is inversely proportional to the 
specific gravity differential from the fluid, the tracer of Example 10 is 
considered to be substantially equivalent to the tracer of Comparative 
Example 8 in tracking efficiency. However, because the sectional area of 
the tracer particle of Example 10 is approximately 25-fold greater, it is 
easy to anticipate that, in the photographing method, it produces a 
greater intensity of scattered light. The greater the intensity of 
scattered light, the higher is the measurement accuracy. In other words, 
the smaller the specific gravity differential from the fluid to be 
measured, the larger is the tracer particle that can be employed. 
Therefore, the fact that the tracer particle has closed pores and the 
specific gravity of the particle can be controlled by taking advantage of 
such closed pores has a great significance in a measuring system where the 
distribution of tracer particles is photographed using an instantaneous 
powerful light source such as a flashlight or a pulse laser. 
EXAMPLE 11 
A velocimetric test was performed using a metal-clad spherical particulate 
tracer prepared by depositing a nickel plate about 0.05 .mu.m thick on the 
particles manufactured in Production Example 1 by the electroless plating 
technique. The test conditions were otherwise identical to those used in 
Example 7. The results are shown in Table 9. 
TABLE 9 
______________________________________ 
Sample data rate (Hz) 
Mean effective data rate (%) 
______________________________________ 
1,000 80 
2,000 75 
3,000 74 
______________________________________ 
Comparison with Tables 5 through 7 and 9 indicates that the effective data 
rates in Example 11 are higher than those obtained in Example 7 and 
Comparative Examples 6 and 7. 
EXAMPLE 12 
Using a porous hollow spherical particulate SiO.sub.2 tracer with a mean 
particle diameter of 1.5 .mu.m.+-.S.D. 0.3 .mu.m, the shell thickness of 
which was one-fifth of the diameter of the particle, a comparative feeding 
test was performed with the measuring wheel particle feeder (MSF-F, Liquid 
Gas Co., Ltd.) and the screw feeder. In both cases, the feed rate was set 
at 0.3 g per minute. 
The feeding accuracy was high for both the measuring wheel feeder and the 
screw feeder but with the measuring wheel feeder the tracer could be 
introduced with an accuracy of 0.3.+-.0.01 g/min. This accuracy is about 5 
times as high as the accuracy with the screw feeder. 
In the measurement of fluid velocity with a laser instrument, it is easy to 
see that the higher the accuracy with which the tracer can be fed to the 
instrument and, hence, to the fluid to be measured, the higher is the 
accuracy of flow measurement by the instrument. 
COMATIVE EXAMPLE 10 
Using the conventional non-agglomerating particulate SiO.sub.2 tracer with 
a mean particle diameter of 1.5 .mu.m, a feeding test was performed with 
the measuring wheel particle feeder and the screw feeder. In both cases, 
the feed cate was controlled at 0.3 g per minute. 
With the measuring wheel feeder, the tracer particles could not be 
successfully delivered due to agglomeration. 
The feed accuracy with the screw feeder was 0.3.+-.0.14 g/min and it was 
found that, compared with Example 12, the use of spherical tracer 
particles insures a comparatively higher accuracy of feeding to the laser 
instrument. 
In the measurement of fluid flow with a laser instrument, it is easy to see 
that the higher the accuracy of feed to the fluid, the higher is the 
accuracy of measurement by the instrument. 
EXAMPLE 13 AND COMATIVE EXAMPLE 11 
Using the conventional non-agglomerating particulate TiO.sub.2 tracer with 
a mean particle diameter of 5 .mu.m, a feeding test was performed with the 
same measuring wheel particle feeder as used in Example 12 (Example 13) 
and the screw feeder (Comparative Example 11). In both cases, the feed 
rate was set at 0.3 g per minute. 
The feeding accuracies were 0.3.+-.0.02 g/min and 0.3.+-.0.08 g/min, 
respectively, indicating that the measuring wheel particle feeder is 
conductive to a higher measurement accuracy.