Laboratory Experiments No 1: Measuring the Number Distribution
Purpose: To test the operation of the DMA by comparing the calculated size to a
monodisperse aerosol particle, and to use the DMA to measure the ambient aerosol
particle size distribution. To combine two instruments to measure ambient size
distribution over a large diameter range.
Note: Record all results from the following experiment for later use in the lab write-up.
Experiment # 1. Calibration of Laminar Flow Meter and Critical Orifice
To operate the DMA we need to know 4 flow rates: Qpoly, Qmono, Qexc, and Qsh (see
DMA fig below). Note that if we know 3 of the 4, the 4th can be found by sum of flow in
equals sum of flow out. A laminar flow meter will be used to monitor Qpoly during the
expt, Qmono will be measured at the start of the expt and assumed constant throughout,
and Qexc will be controlled by a critical orifice, measured and also assumed constant (see
DMA fig below). To calibrate the laminar flow meter and critical orifice do the
following:
Experiment #1a laminar flow meter:
• Set up the system as shown below (make sure correct flow direction).
• Adjust the valve to change the flow rate. You should do a total of about 5 to 6
different flows spanning the full range of laminar flow meter’s ΔP range (include
Q =0, valve closed).
• For each valve (flow) setting make approximately 3 or so repeated measurements
with the bubble flow meter. For each, record the stop-watch elapsed time for a
bubble flow meter change in volume, and the laminar flow meter ΔP. Later you
will make a graph of flow rate versus ΔP.
Bubble Flow
Meter
ΔP
Vacuum Pump
Small GAST pump
Laminar Flow Meter
Valve
Note the direction of flow
and position of press taps
Experiment #1b critical orifice:
• Now replace the laminar flow meter with the critical orifice but keep the bubble
flow meter in line.
• Remove the valve (we will only calibrate for one flow rate).
• Make sure the vacuum pressure on the pump is greater than 15 mmHg (i.e.,
vacuum has to be less than ½ atm for the critical orifice flow to be critical).
• Using the stop-watch and bubble flow meter to determine the flow rate for the
critical orifice under critical flow conditions. Repeat the measurement about 5
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times so you can get a mean and uncertainty associated with the critical orifice
flow rate.
Experiment # 2. DMA Comparison to Monodisperse Aerosol Particles
To begin the experiment first turn on the CPC and vacuum pump, wait until the CPC
comes up to temperature (no green lights stop flashing). Put the laminar flow meter just
calibrated on the CPC inlet and measure the flow rate (this will be Qmono). Put the CPC
back into the experiment. NOTE: DON’T PLUG THE CPC INLET WHILE RUNNING,
IT SHOULD NOT BE UNDER A LARGE VACUUM AT ANY TIME.
Valve A
Filter
Qpoly
Neutralizer
Qsh
Laminar
Flow Meter
Diffusion
Dryer
Filter
(w/in DMA)
Nebulizer with
PSL Solution
DMA
Filter
Air Pump
~ 30 psi
On/Off
Valve
Pressure
Gauge
Vacuum
Pump
Pressure
Gauge
Qexc
Qmono
Critical
Orifice
CPC
Filter
(w/in DMA)
Set up the DMA as shown, noting the following:
Make sure the On/Off Valve by the Vacuum Pump is Open (handle parallel to flow).
• Make sure valve on DMA Qsh is all the way open (tight Counter Clock Wise)
• Qmono should be close to 1 L/min, measure it and use the measured value in your
calculations (CPC flow is set by an internal critical orifice)
• Qexc will be the critical orifice calibration value (~10 to 11 L/min)
• Qpoly: set Qploy to same value as Qmono using Valve A (vacuum pump and
CPC must be running) use Expt 1 laminar flow meter calibration results.
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3.
Check to make sure the nebulizer has at least a few cm/s deep of liquid. It looks
milky due to PSL.
To begin the experiment start the Air Pump and make sure pressure gauge on
pump is reading ~30 psi, meaning that there is a flow to the nebulizer. (You can
disconnect the black tube from the top of the nebulizer and see a mist exiting).
Set the DMA voltage to zero, wait about 20 seconds to make sure the system has
reached equilibrium. The CPC concentration should not be changing
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dramatically. At zero voltage the CPC should read close to zero concentration
(record the value).
Calculate the DMA voltage corresponding to Dp* equal to the PSL size. This
will help deciding what voltages to run the expt at (hint, V should be around
1,600Volts).
Choose various DMA voltages at finer voltage increments in the vicinity of the
Voltage for Dp* to prove to yourself that only this size is passing through the
DMA. Make measurements over coarser increments away from the peak. Once
you set a voltage, wait about 20 sec for the system to reach equilibrium. Then
watch the CPC and determine the particle concentration. In the vicinity of the
peak, make finer voltage adjustments. A total of about 30 measurements would
be appropriate.
Note the pressure gauge by the CPC – this is the vacuum gage pressure within
the DMA, which will influence the slip correction factor when doing the DMA
Voltage/Particle size calculations. (Note, you need total P for the slip correction,
not gauge pressure: Ptotal = 1 atm – Pgauge, when Pgauge is measuring vacuum
pressure).
When finished, shut off the Air Pump and Close the On/Off value on the vacuum
pump.
Note the size of PSL used for this expt. (see PSL bottle) (0.09 µm diameter).
Experiment # 3. Measure the ambient dry aerosol particle size distribution
Qpoly
Outside
Diffusion Neutralizer
Dryer
Qsh
Laminar
Flow Meter
Filter
(w/in DMA)
Inside
DMA
On/Off
Valve
Pressure
Gauge
Vacuum
Pump
Pressure
Gauge
Qexc
Critical
Orifice
Qmono
CPC
Filter
(w/in DMA)
1. Set up the experiment as shown in the schematic above.
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2. Make sure the On/Off valve is closed so that the critical orifice operates correctly
(pump not just open to room).
3. Make sure you have connected the Qpoly leg to the ambient aerosol sample tube
(1/4 inch copper tube).
4. Adjust large valve on DMA Qsh (closing it) while watching the laminar flow
meter, close Qsh valve until Qpoly is same as Qmono.
5. Starting at 100V, step through a voltage range from 100V to ~ 10,000V.
Increment the voltage by Vi+1 = Vi * sqrt(2). (i.e., 100V, 141V, 200V … 9051,
10,000). End with a final voltage of about 10k V. This should produce about 15
measurements. In each case, after adjusting the voltage wait about 20 sec, then
read the CPC for a short period and record what you believe is the number
concentration (try to record some average value if fluctuating).
6. Keep an eye on all flow rates (ie, the laminar flow meter, eg Qpoly) during the
expt. You may wish to record the laminar flow rates ΔP’s in a table along with
the Voltages and CPC concentrations (dN). You will eventually need to make a
table on the form:
V Dp* dDp dN* f1 Ω dN dN/dlnDp dA/dlnDp dV/dlnDp dN# dA dV dM
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Sum
N
A V M
dDp = width of transfer function based on DMA theory
dN* = raw concentration from CPC (particles/cm3)
dN = calculated ambient concentration
f1 = fraction having 1 elementary charge
Ω = DMA transfer function
dN# = dN*/dDp delta Dp; where delta Dp distance between centers between Dp lower
and higher =[ Dp(i+1) – Dp(i-1)]/2
7. As a final test, remove the ambient air sample line and the CPC from the DMA.
Connect the CPC directly to the ambient air sample line and after waiting for it to
stabilize, record the ambient aerosol total number concentration.
8. Turn everything off (power strip off).
DMA dimensions:
L = 43.6 cm
r1 = 0.937 cm
r2 = 1.958 cm
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Connect the OPC to the inlet line and now measure the ambient aerosol size
distribution with the OPC. Download all data to a memory stick.
Interpret Your Experimental Results:
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Experiment #1a.
• Make a graph of Q in L/min versus ΔP. Include both x and y error bars for each
data point. (If the error bars are too small to plot, state what they are on the
figure). Make a reasonable plot; axis labels with units, graph title, minor ticks.
• Fit data with a line and give the slope and intercept (ie, give the equation to
predict Q from a measured ΔP).
• If the intercept is not zero, give a reason why.
Experiment #1b.
• Give the critical orifice flow rate and uncertainty in L/min.
Experiment # 2.
• Report all DMA flow rates used in the calculations.
• Summarize your results in both a table and plot of concentration versus Dp
(always assume number of charges, n=1). To do this use the following equations
and iterate. Where the particle diameter in µm is:
where the slip correction factor (ignoring pressure effects) is:
C = 1 + 2 λ/Dp*{1.257 + 0.4 exp [-1.1 Dp*/(2λ)]},
gas mean free path (λ) is 0.066 µm (at 1 atm) and viscosity (µ) 1.83x10-4 dyn s/cm2.
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Compare Dp* measured with PSL size (its best to fit the peak with a Gaussian
(normal) distribution to determine the mean size and 2 σ as the width, ΔDp).
Compare the peak width to that predicted from the transfer function. The easiest
way to do this is to note that ΔDp/Dp* in theory equals
2(Qpoly+Qmono)/(Qsh+Qex). Compare your observed ΔDp/Dp* to this theory
value.
Discuss the effect of DMA pressure on the calculated size (Can we ignore this
complication)?
Discuss the results, do they make sense, do they agree with your calculations –if
not why not? Consider uncertainties.
Experiment #3.
• Report all DMA flow rates used in the calculations (note Qmono is the CPC flow
and Qexc the critical orifice flow both determined in Expt 2. Use the laminar
flow calibration and measured ΔP to determine Qpoly, then use conservation of
flows (in=out) to fine Qsh.
• Complete the table shown above; use the equations above with measured Qsh and
Qexc to convert voltage to Dp*. Again always assume that n=1, and assume the
transfer function (Ω) is always equal to 1 (see last page of Class Notes The
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DMA.pdf). For each voltage and diameter you will also have to calculate the
charging probability f1, (see Class Notes Formulas.pdf for equation). Note that
sign of charge does not matter (the DMA center rod is negative so we are really
measuring only positive charged particles). Convert measured concentration
(from the CPC) to ambient by dividing concentration by Ω and f1, see last page of
Class Notes The DMA.pdf).
Plot the number, surface, and volume distributions in the form of dN/dlogDp, etc.
The Dp axis should be a log scale. Label (with units) both graph axis.
Calculate the number mean particle diameter and the total number, area, and
volume concentration for particles in the measured size range.
Consider uncertainties associated with the results (this can be difficult, eg, include
an estimate of ΔDp).
How well does the DMA integrated total number concentration compare with the
measured CPC total number concentration (the last part of Expt 3). Offer
explanations for any discrepancies.
Now plot the distributions from both the DMA and OPC on a single graph. Do
they “match up”, if not any ideas why not?
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