U.S. patent application number 09/757221 was filed with the patent office on 2001-06-28 for method and apparatus for calibrating personal air samplers.
Invention is credited to Hall, Peter M., Krajewski, Charles A..
Application Number | 20010004842 09/757221 |
Document ID | / |
Family ID | 23267451 |
Filed Date | 2001-06-28 |
United States Patent
Application |
20010004842 |
Kind Code |
A1 |
Krajewski, Charles A. ; et
al. |
June 28, 2001 |
Method and apparatus for calibrating personal air samplers
Abstract
A method for calibrating a personal air sampler. A desired flow
rate is set, the actual rate measured and recorded through a
plurality of runs, the results are automatically averaged and
compared to the desired flow and the pump is automatically adjusted
accordingly. The process may be repeated for a plurality of
selected flow rates, and a best-fit curve computed to cover the
entire range. Stability is tested by a series of sets of runs to
determine erratic behavior, and an overall calibration is conducted
by performing such a series of runs at each of several set point
flows.
Inventors: |
Krajewski, Charles A.;
(Bridgeville, PA) ; Hall, Peter M.; (McMurray,
PA) |
Correspondence
Address: |
William L. Krayer
Attorney at Law
1771 Helen Drive
Pittsburgh
PA
15216
US
|
Family ID: |
23267451 |
Appl. No.: |
09/757221 |
Filed: |
January 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09757221 |
Jan 9, 2001 |
|
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09325333 |
Jun 3, 1999 |
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Current U.S.
Class: |
73/1.16 |
Current CPC
Class: |
G01N 2001/2276 20130101;
G01N 1/24 20130101; G01N 1/2273 20130101; G01F 25/15 20220101 |
Class at
Publication: |
73/1.16 |
International
Class: |
G01P 021/00 |
Claims
1. Method of calibrating air flow in a personal air sampler having
a variable air flow pump, said sampler including means for
selecting a desired air flow, comprising (a) comparing a measured
air flow to a selected air flow, (b) generating a signal as a
function of the difference between said selected and said measured
air flows, and, (c) in response to said signal, controlling said
pump to reduce said difference.
2. Method of claim 1 wherein said signal is used to record an input
value as a function thereof, which value is used to control said
pump whenever said pump is set at said selected air flow.
3. Method of testing the air processing of a personal air sampler
including an electrically driven pump and a means for setting a
desired air flow therefrom to said air sampler, comprising (a)
setting a desired air flow in said air sampler, (b) generating a
signal representing a reading of air flow through said air sampler
set at said desired air flow rate, (c) recording said reading in a
data recorder (d) repeating steps (b) and (c) through at least one
iteration, (e) calculating an average of at least two of said
recordings, and (f) comparing said average to said desired
flow.
4. Method of claim 3 wherein if said average deviates from said
desired flow rate by more than a predetermined difference, then (g)
generating a signal as a function of said deviation for adjusting
the voltage to said pump to reduce said deviation to less than said
predetermined difference, whereby said sampler is calibrated at
said desired air flow.
5. Method of claim 4 wherein said adjusting of said voltage is
accomplished by said signal by recording a record value
representing voltage necessary for said pump to reduce said
deviation to less than said predetermined difference, whereby said
record value is used to control said pump whenever said pump is set
at said desired air flow rate.
6. Method of claim 3 which is repeated for a plurality of selected
air flows.
7. Method of claim 4 which is repeated for a plurality of selected
air flow rates in said air sampler.
8. Method of claim 4 followed by calculation of a curve
representing adjustment of said pump at any selected air flow
setting within a range, recording said curve, and employing said
curve to control said pump at any desired air flow setting within
said range.
9. Method of claim 8 wherein said pump is controlled by adjusting
the voltage supplied to said pump.
10. Method of determining instability in a personal air sampler
comprising (i) averaging a series of at least four flows a, b, c,
and d through said air sampler set at a desired flow rate and
determining the total deviation of said four flows from the average
thereof (ii) if said total deviation is higher than a predetermined
maximum, causing an additional flow e through said air sampler set
at said desired flow rate and determining the total deviation of
said flows b, c, d, and e from the average thereof (iii) if said
total deviation of said flows b, c, d, and e is higher than a
predetermined maximum, causing an additional flow f through said
air sampler set at said desired flow rate and determining the total
deviation of said flows c, d, e, and f from the average thereof
(iv) if said total deviation of said flows c, d, e, and f is higher
than a predetermined maximum, causing an additional flow g through
said air sampler set at said desired flow rate and determining the
total deviation of said flows d, e, f, and g from the average
thereof, whereby if said total deviation is greater than a desired
maximum, said personal air sampler is determined to be
unstable.
11. Method of claim 10 which is conducted after discarding at least
one flow.
12. Method of claim 10 wherein said flows are represented by
electrical signals which are functions of flow rates of air.
13. Method of claim 10 wherein said flows are represented by
electrical signals which are functions of volumes of air.
14. Method of claim 10 wherein at least one additional flow is
measured and included in the average in at least one of steps (i),
(ii), (iii) or (v).
15. Method of claim 10 wherein said predetermined maximum in each
of steps (i), (ii), (iii) and (iv) is computed as a percentage of
said average.
16. Method of claim 9 wherein said flows are measured at time
intervals of 1-60 seconds.
17. Method of calibrating a personal air sampler comprising (a)
determining actual flow within said sampler at least four times at
the same set point for selected flow at the same setting, averaging
said determinations of actual flow, subtracting the difference of
said determinations from their average, and comparing the total of
said differences to a predetermined standard of variance from said
set point, and (b) determining stability of the sampler at said set
point by performing step (a) at least four times with different
combinations of actual flows.
18. Method of claim 17 wherein the result of step (a) is used as a
feedback signal to adjust flow to said sampler.
19. Method of claim 17 wherein the determination of stability in
step (b) is used to determine whether to place said sampler out of
service.
20. Apparatus for calibrating an air sampler comprising (a) an air
sampler including a pump (b) a calibrator including a flow
transducer, said calibrator connected to said air sampler to
measure air flow therein, said transducer including means for
generating a signal as a function of air flow in said calibrator,
and (c) a controller connected to and interfacing with both said
air sampler and said calibrator, said controller including means
for (i) initiating a plurality of flow measurements by said
transducer at a desired flow rate (ii) means for measuring the flow
rates in said flows, averaging said flow measurements and comparing
the average to the desired flow, means for comparing to said
average the total of the differences of said flows from said
average, means for generating a control signal as a function of the
difference found in said comparison, and means for receiving said
control signal and adjusting pump action as a function thereof.
Description
RELATED APPLICATION
[0001] This application is a division of our copending application
Ser. No. 09/325,333 filed Jun. 3, 1999 and claims the benefit of
its filing date.
TECHNICAL FIELD
[0002] This invention relates to small personal air samplers and
particularly to their calibration for accurate readings of air or
gas flow and sample volume.
BACKGROUND OF THE INVENTION
[0003] Prior to the present invention, the calibration of personal
air samplers involved in large part empirically obtained readings
and sometimes unreliable manipulation by the user. Many of the
calibrators employed bubble flowmeters.
[0004] An early patent to Gussman et al, U.S. Pat. No. 3,994,153,
was directed to the calibration of the rotameter used to measure
flow.
[0005] Conkle et al, in U.S. Pat. No. 4,569,235, maintain
substantially constant air flow in an air sampler by monitoring
flow rate change and using a signal representative of the flow rate
change to manipulate the pump.
[0006] Padden et al, in U.S. Pat. Nos. 5,456,107 and 5,440,925,
describe generating electrical signals representing flow rates and
as a function of known volumes in one or more enclosures traversed
by a piston. It is suggested that these signals may be used in data
logging and report generation, but the device is not tied directly
to the sampler pump for calibration in the manner of the present
applicants.
[0007] Flow rates are calculated from the velocity of air in a
sampler, as described by Buchan in U.S. Pat. No. 4,375,667. The
flow rates are then integrated within the instrument over a sample
period to provide an indication of the volume sampled, which the
authors say enables them to obviate a calibration step. This
disclosure employs a microprocessor and converters for processing
the data obtained, but still may be said to calibrate only for a
current sample and not as a standard for use over an extended
period of time.
[0008] Ogden et al, in U.S. Pat. No. 5,551,311, uses a personal
computer and describes a calibrating apparatus connected to it from
the sampler. The system, however, does not utilize the data in the
manner of applicants. See also Ogden et al U.S. Pat. No.
5,646,357.
[0009] Peck et al, in U.S. Pat. No. 5,107,713, describes a
procedure which is manually repeated for several flow measurement
readings; the user is prompted to enter the readings on a CPU,
which further manipulates them based on an empirical compilation to
establish a relationship between air flow, pulse width modulation,
and pump motor RPM, using also a flow calibration meter. As
described, the device is essentially self-calibrating, but does not
tie the data generation to the calibrating device as applicants
do.
[0010] The prior art approaches to calibration are not conducive to
the calibration of a large number of personal air samplers within a
short period of time. One may, for example, manually change the
speed of the pump and then read a calibrator such as that described
by Ogden et al above, marking down the data as it is collected.
This process is susceptible to errors in setting the pump,
recording the setting of the pump, and recording the reading of the
calibrator. Over a period of time and a series of calibrations,
errors are statistically likely.
SUMMARY OF THE INVENTION
[0011] Our invention includes a method of calibrating an air
sampler at one desired flow rate or several wherein, for each
desired flow rate, a plurality of air flow rates are averaged and
analyzed, more or less automatically, for accuracy and stability.
Where a range of flow rates is calibrated, a best-fit curve and/or
a function representing it is generated. In either case--a single
point or where a range of flow rates is calibrated--pump action is
varied to deliver the desired flow.
[0012] Our technique permits the user to calibrate a large number
of air samplers within a short period of time while minimizing the
possibility of errors in entering data such as may happen with the
Peck et al system mentioned above. In addition, our system is
readily performed frequently and, perhaps more importantly,
minimizes the possibility of introducing human error. Records may
be maintained of the data obtained in and used for calibration.
[0013] We calibrate the air processing of a personal air sampler by
(a) setting a desired air flow rate in the air sampler, (b)
generating a signal representing a measured air flow rate through
the air sampler set at the desired air flow rate, (c) recording the
measured air flow rate in a data recorder (d) repeating steps (b)
and (c) through at least one iteration (preferably a total of at
least three repetitions), and (d) averaging at least two
(preferably at least four) of the recordings so obtained. The
average is compared to the setting of the desired air flow rate to
determine the degree of difference, which is converted to a signal
to be used as feedback for adjustment of voltage or power to the
pump, or otherwise to adjust its output. The feedback signal may
also be recorded and thereafter used to power the pump at the new
level each time the tested air flow rate is desired. The pump and
sampler are therefore calibrated for that particular desired air
flow rate. Note that the difference of the measured flow from the
desired flow, and the average difference from the setting, are both
treated as positive numbers whether they are above or below the
compared point.
[0014] Thus a straightforward calibration of a single desired air
flow will result in the power supply to the pump being controlled
to energize the pump to the degree necessary to achieve the desired
air flow rate. As the typical sampler is battery-powered, the
adjustment is preferably made to the voltage supplied to the pump.
The new voltage will thereafter be used for that desired air flow
rate until the sampler is calibrated again at that point.
[0015] But our procedure also guards against the possibility that
an average of two or more measurements may be very close to the
desired rate, while the individual measurements which make up the
average are quite removed from the desired rate. A high total
difference from the average indicates instability. Therefore we
conduct the test for stability described below. Unlike the results
of the averaging test, the results of the stability test are not
used to adjust the power input to the pump. They are used to
determine whether to discontinue use of the sampler.
[0016] To determine stability of the sampler, our procedure
continues beyond the above recited sequence of steps with the steps
of (e) determining and recording the absolute difference between
each of the recordings and the average so obtained, (f) totaling
the differences obtained, (g) expressing the total of differences
in terms of a percentage of the average, and (h) comparing the
percentage to a predetermined acceptable percentage. This
percentage value may be used to decide whether to continue or
discontinue the sampler in service, or the more extended procedure
described further below may be followed.
[0017] While we speak throughout of personal air samplers, it
should be understood that the same procedures and techniques may be
used with respect to gases and gaseous media other than air, where
the samplers are used to collect and/or measure contaminants,
aerosols, microorganisms, and concentrations of various substances
and other gases.
[0018] When we speak of "signals", we include not only the usual
and typical electrical signals, but also pneumatic or other
signals. They may be continuous (analog) or discrete (digital).
Note also that the action of the pump may be varied by varying the
voltage or power to it, by braking, actuation of a clutch,
adjustment of the pump stroke, and/or any other practical
means.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a flow sheet of our process.
[0020] FIG. 2 is a plot of data points collected through a series
of tests on a personal air sampler conducted according to our
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0021] In a preferred, more elaborate, practice of our invention,
we average a series of at least four flow measurements a, b, c, and
d through an air sampler set at a desired flow rate and determine
the total deviation of the four measurements from the average
thereof. If the total deviation is higher than a predetermined
maximum, we again measure the flow through the air sampler set at
the desired flow rate; the new flow measurement may be designated
flow measurement e. We then determine the total deviation of flow
measurements b, c, d, and e from the average thereof. If the total
deviation of flows b, c, d, and e is higher than a predetermined
maximum, we take a new measurement of the flow, f, through the
sampler set at the desired flow rate and again determine the total
deviation, this time of the individual measured flows c, d, e, and
f from the average thereof; if the total deviation of the flows c,
d, e, and f is higher than a predetermined maximum, a further flow
g is measured through the air sampler set at the desired flow rate
and again the total deviation of flow measurements d, e, f, and g
is determined from the average thereof. Then, if the total
deviation of flows d, e, f, and g is higher than a predetermined
maximum, we again measure the flow, h, through the air sampler set
at the desired flow rate and determine the total deviation of flow
measurements e, f, g, and h from the average thereof, whereby if
the total deviation is greater than a desired maximum, the personal
air sampler is determined to be unstable. Normally and preferably,
the pump remains running throughout the above outlined procedure,
but we do not rule out a procedure utilizing discrete pumping
action. Note that this describes a series of five overlapping or
cascading routines, beginning with measurements a, b, c, d and b,
c, d, e. Four such routines are normally sufficient and four is our
preferred number, but any number of sequential routines in excess
of one may be used.
[0022] Persons skilled in the art will realize that the above
procedures may be varied and that the underlying principle is to
take at least two flow readings at the same setting, average the
readings, and determine the difference of those readings from their
average, for a comparison with a predetermined standard of overall
difference. Thus the test for that particular setting may be
satisfactory if the readings are generally near the target flow
rate, either over or below. If the difference between the average
of the measured flow rates and the desired flow rate is greater
than the predetermined standard of overall difference, an
electrical signal is generated to adjust the pump in a manner to
bring the air flow rate into acceptability. But even if the average
is very close to the target flow rate, our test for stability may
demonstrate that there are large differences above and below the
target flow, meaning that performance is erratic and imprecise. We
also recognize that it may take a certain period of time for the
sampler to stabilize to the conditions of the test, and accordingly
we pass it through several sequential and, preferably, overlapping
tests. Hence, in the above description, we average the first,
second, third and fourth result, then the second, third, fourth and
fifth result, then the third, fourth, fifth and sixth result, and
so forth to at least the fifth, sixth, seventh and eighth runs.
This sequence can be followed in any case, but is preferred if the
first result is unsatisfactory. In a preferred version of our
procedure, we discard the results of the first run regardless of
whether it is satisfactory, and proceed to a series of at least two
(or other desired number higher than two, preferably four)
runs.
[0023] After the above described process is employed for a
particular selected (desired) flow rate, a different flow rate may
be selected and the process repeated for the new flow rate. The
above described procedures can be performed automatically by
programming the pump and microprocessor to perform the various
steps. Thus, although a large number of steps and samples can be
processed, particularly when a quantity of personal samplers is to
be tested, the procedure can be accomplished conveniently and in
good time.
[0024] Our invention includes the programming of a microcomputer or
controller to perform automatically both the individual selected
flow calibration and calibration over a range of flow rates
followed by generation of a best-fit curve using well-known
algorithms.
[0025] FIG. 1 is a flow sheet of our invention. Controller 1
performs several functions which may be separated into several
different pieces of equipment or combined into one, or may be built
into the air sampler 2 or the calibrator 6. It is programmed to
carry out the tests and calibrations in the sequences described
herein, on command of the operator, who initiates the process. Air
sampler 2 includes an air pump which draws air through intake 3,
passes it through its sampling system, and then through air
discharge 7. Air intake 3 is normally open to the air, but we
connect it through duct 4 to calibrator 6, which includes a flow
transducer. Calibrator 6 takes in air through air intake 5 and
measures the air flow to and through it as it passes through from
air intake 5 to exit duct 4. The flow transducer in calibrator 6
generates an electrical signal representative of the measured flow
and transmits it over electrical connection 9 to controller 1,
which includes a data recorder and microcomputer. The air flow is
recorded by the data recorder. After a preferably adjustable preset
interval, a second air flow is measured and recorded in the same
manner. The microcomputer may then average the recorded values or
wait for additional recordings, depending on the programmed
sequence. The intervals between the measurements, the programmed
sequence of the procedure, and all other controls and data
collection of the system are communicated either through connection
8 between the controller 1 and sampler 2 or connection 9 between
controller 1 and calibrator 6. The calibrator 6 is in this instance
located on the intake of the sampler 2 at least partly because the
operation of sampler 2 calls for discharge to atmosphere. For other
types of samplers and/or sampler pumps, the calibrator may
effectively measure flow from the discharge. Any of the values
measured, generated or recorded may be displayed for the operator,
preferably on the sampler if the controller 1 is built into the
sampler 2.
[0026] Following is a description of a Single Point Calibration
Process program in pseudo-code, where k is the flow reading count
and F is the volumetric flow measurement from a Bios DRYCAL
calibrator:
[0027] 1 k=0
[0028] 2 k=k+1
[0029] 3 if(k>8) Error "Too much variation among readings".
END
[0030] 4 F=Reading from DRYCAL
[0031] 5 Avg[k]=flow
[0032] 6 If(k<4) goto 2
[0033] 7 F.sub.avg=(Avg[k]+Avg[k-1]+Avg[k-2]+Avg[k-3])/4
[0034] 8 Calculate the variation between F.sub.avg and Avg[k] . . .
Avg[k-3]
[0035] 9 If(variation>acceptable limit) goto 2
[0036] 10 If(.vertline.FlowDesired-F.sub.avg.vertline.<10
ml/min) goto 13
[0037] 11 Adjust voltage in pump to make run faster or slower as
indicated
[0038] 12 Goto 1
[0039] 13 Successful single point calibration
[0040] The numbers 8 and 4 in lines 3 and 6 are typical but not
essential and may be varied considerably, as explained elsewhere
herein.
[0041] Following is a Full Calibration Process program in
pseudo-code where k and F are as above and E is the voltage that
the sampler provides internally to control motor speed for the pump
mechanism (i.e. more volts=higher flow), x[i] is the flow rate of
the i'th measurement and y[i] is the voltage for the i'th
measurement:
[0042] 1 E=initial value (a minimum voltage that makes the pump
operate very slowly or not at all)
[0043] 2 k=0
[0044] 3 k=k+1
[0045] 4 if(k>8) Error "Too much variation among readings".
END
[0046] 5 F=Reading from DRYCAL
[0047] 6 Avg[k]=F
[0048] 7 If(k<4) goto 3
[0049] 8 F.sub.avg=(Avg[k]+Avg[k-1]+Avg[k-2]+Avg[k-3])/4
[0050] 9 Calculate the variation between F.sub.avg and Avg[k] . . .
Avg[k-3]
[0051] 10 If(variation>acceptable limit) goto 3
[0052] 11 x[i]=F.sub.avg
[0053] 12 y[i]=E
[0054] 13 if (F>3000 ml/min) goto 17
[0055] 14 E=E+.DELTA.E (raise the voltage by a small amount)
[0056] 15 If (E=max voltage) Error "Cannot achieve 3000 mL/min".
END
[0057] 16 Goto 2
[0058] 17 //Final Calculations
[0059] 18 Find last i where x[i]<750. Call it x[first]
[0060] 19 Use last i (if no errors occurred x[i] will be>3000)
Call it x[last]
[0061] 20 Calculate a best-fit curve from (x[first], y[first] to
(x[last], y[last]).
[0062] 21 When done, we will have a curve y=f(x)
[0063] 22 We can then plug our desired flow (x) into the equation.
It will calculate `y`, the voltage that we need to most accurately
achieve that flow.
[0064] Note that the program literally does not input desired or
selected flow rates, but rather simply incrementally increases the
voltage and measures the resulting flow rate, to generate the
curve. After full calibration, the sampler is in effect a secondary
standard at any flow rate selected in its range.
EXAMPLE 1
[0065] An SKC AIRCHEK 2000 air sampler (obtained from SKC, Inc.,
Eighty Four Pa. and constructed as described in U.S. Pat. No.
5,892,160), was to be calibrated according to our technique. It was
accordingly set to a desired flow rate of 1000 milliliters per
minute and connected to a DRYCAL DC-Lite calibrator model 717-01,
obtained from BIOS, Inc., Pompton Plains, N.J., using our
interfacing controller, a CALCHEK communicator (source--SKC, Inc.,
Eighty Four Pa.), which takes the output from the calibrator to the
pump. The process was initiated by turning the system on,
permitting the pump to activate and initiating the single point
controlled procedure described above. The DRYCAL calibrator
includes a flowmeter or flow transducer; it is itself calibrated
and may be considered a primary standard. It reads the actual flow
and converts the flow to an electrical signal representative of the
flow. The flow may be converted to either a digital or analog
signal, or both. A representative series of data, which can
displayed or printed, is shown in Table 1:
1TABLE 1 Desired Flow Setting - 1000 mL/min Flow Reading Difference
from Average 1 discarded -- 2 994 10 3 1009 5 4 1002 2 5 1011 7
Average 1004 Sum 24
[0066] An immediate observation is that the average flow was above
the desired flow rate of 1000 mL/min. Although the sampler's
performance is acceptable at the setting of 1000 mL/min., the
difference of 4 mL/min. from the average may be used as automatic
feedback to adjust the voltage delivered to the pump for the
desired flow setting. After the same type of data collection is
made at several selected flow rates, the microcomputer may generate
a best-fit curve y=f(x) where y is the voltage to be applied and x
is the flow set point. This information is used thereafter for any
desired flow rate in the entire range of the sampler. Thus our
system can be used to control the pump, or to calibrate the air
sampler, over a range of air flow rates.
[0067] For the data in Table 1, the limit for the total difference
for the four points had been set at 6% of the average, considerably
above the variation here, which is slightly less than 2.4% of the
average, and accordingly the sampler successfully passed the
stability test at the input flow rate of 1000 mL/min
[0068] Although, as noted above, the BIOS calibrator is taken as
the primary standard, it is subject to possible error as well as
the sampler. A plurality of measurements is used to obtain at least
a minimum sampling base. These are averaged for comparison to the
selected flow, in order to minimize the effect of possible errors
originating in either the sampler or the calibrator or both.
[0069] An example of measurements over a full range is seen in FIG.
2, which is a plot of averages computed of four readings at each of
26 incrementally increased flow settings in liters per minute. The
X axis is in terms of volts applied to the pump, in this case to
measure the flow resulting at the voltage shown. It will be seen
that the plot is not linear. Our invention contemplates calculating
a curve from such data so that all possible settings on the sampler
can be handled by the feedback loop. At least three data points are
needed for such a calculation, and they are preferably obtained by
four iterations of the calibration procedure described above at
each set point. Any of numerous "best-fit" algorithms may be used
To achieve the optimum balance of accuracy and speed, we use from
six to eighteen desired flow set points for curve calculation, most
preferably about ten to about fourteen.
[0070] While we speak above of using the feedback signal to adjust
the voltage of the pump, it may adjust any operating factor which
will in turn adjust the pump's output in the desired manner, such
as power, a clutch, or the length of stroke. Any type of adjustment
which will respond to the feedback signal may be used; the
adjustment need not be electrical, nor does the feedback
necessarily need to be electrical--it may be pneumatic, for
example.
* * * * *