 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates generally to mixing chambers and more specifically
to a dynamic mixing chamber for introducing known levels of a specific
foreign material to a flow of fluid having an established purity level to
provide a standard of testing of particle counters and other devices used
to monitor contaminant levels in flowing fluids.
FIELD OF THE INVENTION
Proper testing and characterization of particle counters for use in liquids
requires repeatable presentation of a standardized particle suspension or
a specific foreign material to the instrument being tested. The standard
suspension, of known particle size and concentration, must be measurable
by other certified methods and free of incidental contamination of unknown
type and/or concentration (artifacts) which would elicit spurious
responses from the instruments during testing.
Prior art liquid particle sensors have sensitivities that permit detection
of particles as small as 50 nanometers (nm) present at concentrations less
than one hundred particles per milliliter. Described as the ratio of
particle volume to liquid volume this equates to less than 1 part per ten
trillion (10.sup.-12). Preparation of test standards for these instruments
requires consummate precision to assure concentration accuracy and freedom
from incident artifacts. Correct scientific practice requires a
statistically significant number of data points be collected for test
validity. The test apparatus and technique chosen is obliged to be able to
consistently deliver a standard containing known concentrations of
detectable particles or foreign material to the instrument for the
duration of the test and must be repeatable at will to assure the accuracy
of future recalibrations and allow characterization studies of instrument
reliability, repeatability, drift, and useful life.
invariably it is necessary to test instrument sensitivity at different
ranges, consequently, the concentration of the standard solution
containing the detectable particles or foreign material should be easily
and predictably alterable to higher or lower levels without loss of
accuracy. Testing an instrument's freedom of concentration from spurious
induced responses in a dynamic state necessitates a test apparatus that
can rapidly return the test fluid to a particle free or foreign material
free condition without interruption of flow. Finally testing must be able
to be carried out in a cost effective manner that does not require complex
secondary measurement to certify the accuracy of each test segment.
There are three contemporary methods used for testing liquid particle
counters. All utilize a finite volume of particle suspensions mixed to a
fixed concentration and vary only in the manner in which this solution is
introduced to the particle counter: gravity feed, pumped injection, and
vacuum aspiration. These bulk volume methods have major problem areas.
Since they depend on individual batches of test solution each batch of test
solution of particles or foreign material must be precisely mixed using
multiple serial dilutions. The uncertainties introduced when diluting the
stock solution from approximately 10.sup.13 particles per milliliter to a
large volume of test solution containing less than 2.times.10.sup.3
particles per milliliter results in each batch of the test solution being
unique and non-reproducible. Also, the concentration of each batch must be
certified, an expensive proposition. Second, the components of the
delivery system contribute to the measurement uncertainty. The large test
solution vessel is difficult to clean but easily contaminated; and the
actions of the components of the delivery system can cause considerable
variations in the numbers of particle events detected by the particle
counter.
For example, if polystyrene latex (PSL) spheres are the particles to be
used as the injected foreign material and the fluid is deionized water
reducing the concentration of the bulk solution of foreign material can
and usually does increase the frequency of counts from incident artifacts
with respect to the total counts.
The use of a prior art gravity fed bulk test solution is dependent upon
being able to mount the vessel high enough to overcome the flow resistance
of the piping system and the particle counter being tested. Obviously, the
larger the vessel, the longer the testing period but physical restraints
and the sampling rate of the particle counter combine to limit the testing
period. Baseline restoration (e.g. returning to a foreign material free
carrier stream) is possible only if flow of the carrier stream is stopped
and the piping switched to a clean carrier stream source.
Installing a pump between the test vessel and the particle counter does not
eliminate all the problems of the gravity feed method and adds further
problems in that the pump has been added as another source of artifacts.
Uncertainty is also added due to the pressure and flow fluctuations caused
by the pump.
Locating the pump downstream of the particle counter, eliminates the
introduction of artifacts from a pump, but causes the solution to enter
the particle counter at a reduced pressure which can lead to the formation
of bubbles in the test solution resulting in specious particle counts.
SUMMARY OF THE INVENTION
The present invention which uses dynamic serial dilution, is a vast
improvement over the prior art.
The dynamic serial dilution apparatus of the invention, consists of an
injection/mixing chamber and a precision injector from which foreign
material may be impelled into a flowing fluid leading to the device to be
calibrated. This arrangement allows the controlled injection of a
concentrate of foreign material into pressurized piping systems with a
negligible level of incidental artifacts. The injection/mixing chamber
assures even dilution of the concentrated stock solution and thorough
mixing with the carrier fluid without adding artifacts. As a result, the
particle counter being tested sees only a dilute, homogeneous sample of
specific, known concentration. Altering the delivery rate of the stock
contaminant solution or the flow volume of the carrier fluid permits the
particle concentration in the sample to be easily varied. The carrier
fluid, typically deionized water, can revert instantly to a condition free
of foreign material upon cessation of the foreign material injection.
An object of the invention is to provide a dynamic mixing chamber for
introducing foreign material into an ultra-filtered, ultra-pure fluid
delivery system having an output which can be coupled to test instruments
or liquid particle sensors either in parallel or in series to provide
continuous calibration of and accurate comparison between the instruments.
A further object of the present invention permits the introduction of a
standard concentrate of foreign material from a certified source which is
metered in and mixed with a pure filtered carrier liquid or fluid while
avoiding pressure fluctuations and other disturbances which can influence
the test results.
Still another object of the present invention permits the dilution ratio of
the introduced standard to be readily and predictably altered to match the
calibration requirements of the instrument at hand.
The injection chamber of the present invention assures that dilution and
mixing of the foreign material with the carrier fluid is without adverse
effects and can be operable for a fixed period of time without
interruption. Moreover, the invention can be easily and predictably varied
so that the fluid flow will revert instantly to a state, free of foreign
material, as soon as the injection of the foreign material into the fluid
flow ceases.
The present invention avoids all the known problems of the prior art and
utilizes a mechanism that permits injection of a preselected certified
concentration of foreign material into a high pressure system with exact
delivery of the foreign material to provide constant and repeatable
testing and calibration under the same fluid and pressure conditions
encountered in an industrial field application. In addition, any
contaminant which is soluble or suspendable in a carrier fluid can be
prepared or certified as a standard for calibration of fluid monitoring
instrumentation by dynamic serial dilution as taught by the present
invention.
DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic of the preferred embodiment of the invention.
FIG. 2 shows the mixing nozzle used in the apparatus of FIG. 1.
FIG. 3 shows actual test results obtained using the apparatus of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows the preferred embodiment of the mixing chamber 10 of the
present invention. Chamber 10 is generally comprised of a body 11 provided
with an inlet orifice 12, through which a clean undiluted carrier fluid,
such as deionized water, is transmitted into the mixing chamber from a
suitable source (not shown), an entrance aperture 13, through which an
impurity can be introduced into the fluid stream and at least one outlet
orifice or tube 15 through which the fluid exits the mixing chamber.
Preferably, the mixing chamber 11 is made of a plastic such as
perfluoroalkoxy (PFA). This PFA material is desirable because it is
chemically stable and does not introduce artifacts into the fluids passing
there through. Affixed to the entrance aperture 13 is a hollow guide 16
covered by a sealing septum 17, generally comprised of a soft resilient
material, such as rubber, which will seal the hollow guide 16 held in
place by a cap 18 which has a centrally located aperture therein that
lines up with the guide 16. An injector 21, which may be in the form of a
syringe and containing a selected concentrate of foreign material to be
introduced in the carrier fluid, is positioned over and coupled to the cap
18 so that a hollow injection tube 19 of selected size can pass from the
reservoir 21 through the sealing septum 17 and into and through the guide
16. The tip 19a of the tube 19 must emerge from and extend below the end
16a of the guide 16. In the present invention it has been found that the
tip 16a of the guide 16 must be positioned a sufficient distance below the
center line of the inlet 12 to assure that no cross currents or turbulent
flow is produced around the tip 19a of the tube 19 through which the
foreign material is introduced into the pure fluid stream. Preferably,
this position of the tip 16a of the tube 16 is positioned below the center
line of inlet 12 a distance equal to approximately four times the inner
diameter of the inlet orifice and can be positioned at a distance equal to
as much as twelve times the diameter of the orifice. Similarly, the tip
19a of the tube 19 through which the foreign material is actually
introduced into the flowing stream is also positioned below the center
line of the inlet 12 a distance approximately equal to seven to fifteen
times the inner diameter of the inlet 12. By so positioning the tip 19a of
the tube 19 a sufficient distance from the center line of the inlet 12 it
is assured that no turbulence or other non-laminar flow is created in the
flow of the introduced carrier fluid when the carrier fluid is coming in
in a laminar flow with a Reynolds number of 500 or less. Positioned beyond
the tip 19a, of the tube 19, a minimum length of one inlet inner diameter
from the tip of the tube 19a, is a mixing nozzle 23. This nozzle is
particularly shown in FIG. 2 and is tapered with an inlet diameter 24
substantially equal to the diameter of the tube in which it is inserted
and an exit diameter 25 approximately equal to 14% of the nozzle inlet 24.
All the fluid exiting the system passes through the nozzle before reaching
the outlet tube 15. The taper of this nozzle is preferably such that the
length of the nozzle is 5-6 times the diameter of the outlet tube 15. This
nozzle provides the fluid with a Reynolds number of between 5,000 and
6,000. This high Reynolds number, at the mouth of the orifice, causes
adequate mixing of the foreign material in the carrier fluid to occur
downstream from the nozzle in the exit tube 15 and a well mixed stream is
created. A Reynolds number lower than 5,000 results in poor or no mixing
and a Reynolds number higher than 6,000 produces so much pressure drop
within the mixing chamber as to create undesirable or non-interpretive
results.
These specific dimensions of the actual unit used was the entrance orifice
0.25 inches ID (inner diameter), the exit orifice was 0.25 inches ID, the
tube 19 was a 28 gauge ID and the guide 16 had a 100 gauge ID. The
distance at the tip of 19a of the tube 19, was from the center line of the
entrance orifice 12, 1.78 inches while the tip of the exit of the mixing
nozzle was 21/4 inches from the central line of the entrance orifice. The
mouth of the nozzle pad had an internal entrance diameter of approximately
0.160 while its wall thickness was approximately 0.004 inches, its length
was 0.875 inches and its inner exit diameter was 0.024 inches. The nozzle
is preferably made from any relatively inert material that can be
precisely formed or machined such as polypropylene.
It should be noted that the present design can be modified provided the
turbulence around the guide tube 16 is minimal after the ultra-pure fluid
is passed into the mixing chamber, and that the tip 19a of the tube 19 is
sufficiently far enough downstream from the center point of the
introduction of the carrier fluid to prevent significant concentrations of
the injected foreign material to be retained within the chamber after the
flow of foreign material into the carrier stream is stopped, thus
permitting the mixing chamber to immediately clean itself.
Thus, the present invention comprises a dynamic injection mixing chamber
apparatus mounted on an ultra-filtered, ultra-pure chemical delivery
system so that test instruments can be connected singly or in multiples to
the outputs of the mixing chamber. Pure filtered liquid, such as deionized
water, can be introduced through the mixing chamber to become the carrier
stream and diluent of the injected concentrate of foreign material. The
metered concentrate of foreign material and the carrier stream pass
through the mixing nozzle which causes turbulence to thoroughly mix the
foreign material in the carrier stream.
In the case of particle counter calibrations and testing, ultra-filtered
deionized water was used as the carrier stream and ultra-clean precision
syringe and syringe drive a metered certified foreign material stock
solution through the injection mixing chamber into the contaminant free
deionized water stream. The dilution ratio of the standard to the carrier
was set to be nominally one part to a thousand parts. Any incident
artifacts in the standard stock solution, and or any generated by the
syringe and piping constitute such an insignificant contribution that they
can be statistically ignored.
Even the best quality deionized water contains artifacts. For this reason,
a 6.4 nanometer dual membrane ultra-filter is used to ensure the deionized
water carrier stream contains no artifacts of a size detectable by the
particle counters to be calibrated using the present invention. With this
type of filtration, it is possible to achieve consistent zero counts even
when using a particle counter sensitive to 50 nm particles. Also, the
materials used to assemble the injection device are selected to reduce or
exclude the creation or injection of artifacts such as may be caused by
corrosion, inclusions, and particle shedding. To further guard against
background artifact contamination error from the test apparatus, the
apparatus is continually flushed with ultra-filtered deionized water as
described above.
In one test purchased solutions of PSL spheres were diluted to a
concentration of nominally 1.times.10.sup.7 spheres per milliliter and
stored in a polysulfone bottle. A biocide added to this stock solution
prevented bacterial growth and a surface active agent was used to maintain
the dispersion of spheres in suspension. Samples were then characterized
using a scanning electron microscope (SEM) to determine the actual
concentration of spheres. SEM tests of these solutions show them to be
remarkably stable over time, with particle concentrations being within the
margin of error for the SEM even after 18 months in the case of spheres
less than 1000 nm (nanometer) in size. Also, the polysulfone bottles have
excellent characteristics for this storage duty as they neither shed
particles nor do they pull the PSL spheres out of solution and plate them
on the walls of the bottle.
Dynamic serial dilution has enabled a regime which can be used to fairly
evaluate any liquid particle counter presently produced. Direct
comparisons of up to four particle counters can be made simultaneously
with only one stipulation: all of the particle counters being compared
must operate at reasonable similar flow rates. This capability has been
extremely valuable in making one-to-one comparisons between liquid
particle counters.
Simultaneous testing of multiple particle counters is possible because the
design of the test apparatus produces fully developed turbulent flow.
Injected standard particle suspensions are evenly dispersed in the exiting
carrier fluid by the turbulence, created at the mouth of the nozzle to
achieve the final serial dilution seen by the particle counters being
tested. Since the concentration of particles in the standard suspension
has been predetermined with an SEM, all that remains to verify the test
concentration is regulation of the carrier flow and adjustment of the
injection speed. Precision syringes used as the injector, ranging from 10
to 1000 microliters in total volume, provide a third means of controlling
the particle concentration achieved in the final dilution.
Once the dynamic serial dilution apparatus has been installed and the
desired sizes of standard particle suspensions have been prepared and
certified, testing can progress quickly and smoothly. Precise, repeatable
dilutions can be produced without rigorous specialized training.
In one test of interferometric particle counters involving a 1200 hour
sustained operation test, thirty-two sample points were collected for each
of two particle sizes from three prototype particle counters running in
parallel. Each of the sample points consisted of ten runs of ten minutes
duration at a flow rate of 40 ml/minute through each particle counter. Two
of the particle counters 21 and 24 experienced hardware failures during
the test period. The relative standard deviation of the delivered particle
concentrations, as measured by Sensor 10, the surviving particle counter,
was 1.1% (FIG. 3).
Background counts, counter response to artifacts inherent in the test
system, which have always plagued contemporary testing apparatus and
methods can essentially be ignored when using the dynamic serial dilution
apparatus and method of the present invention. The dual membrane
ultra-filter is 99.9% effective at 50 nm and assures the carrier fluid is
free of artifacts within the size range of interest. Continual flushing of
the entire test apparatus and distribution tubing by the ultra-filtered
deionized water prevents artifacts due to shedding or biological
contamination from becoming a factor in the background counts.
The standard particle solutions of foreign material being used is tested
and verified during the concentration certification by SEM thus
eliminating it as a source of uncertainty. A precision syringe can be used
as a reservoir from which the standard particle solutions can be injected
into the stream, the reservoir or syringe can be cleaned and then their
cleanliness verified before use with the particle counters to be tested.
Verification of the syringe's condition is accomplished by inserting the
syringe needle through the septum of the injection/mixing assembly, and
backflushing with the ultra-filtered deionized water, described above,
from the carrier fluid stream. The fluid used to backflush the syringe is
then injected into the fluid stream at a high volume rate so the syringe
is emptied in 15 seconds time. This continued until a count of no more
than 50 artifacts is detected after injecting the entire volume of the
syringe in 15 seconds into the fluid stream. This assures that there will
be no significant background artifacts contributed by the syringe over the
course of the normal test period which is approximately two hours.
Acceptance tests conducted on prototype liquid particle counters obtained
results that were not only conclusive, but statistically sound.
An example of a typical calibration regime for a particle counter utilized
Standard Polystyrene Latex (PSL) spheres of monodisperse size diluted to a
certified concentration of 106 spheres per milliliter injected, at a rate
of 1 microliter per minute, into a carrier fluid stream flowing at 100
milliliters per minute.
With a minimal investment of money and training, anyone with a consistent
supply of clean deionized water can have a repeatable, statistically valid
means of calibrating, characterizing, and standardizing all of their
liquid particle counters by using the present invention and method
described above. Additionally, the dynamic serial dilution apparatus
provides a test facility for other instruments designed to measure
conditions in a flowing fluid stream, e.g. pH meters, resistivity meters,
specific ion monitors, total oxidizable carbon meters, turbidity meters,
etc.
The present invention avoids all the present difficulties known to be used
with the pump delivery of bulk sample solution at high concentrations. It
should now be obvious to one skilled in the art that by varying; the
injection rate of the concentrate into the carrier stream, the dilution
rate of the concentrate, the size of the input and exit orifices as well
as the position of the needle, within certain limits, from those set forth
in the above specification without departing from the present invention as
set forth in the appended claims.
* * * * *
|
|
 |
|
 |
Description |
|
