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 4 th 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 multiple measurements with the drycal: record avg and stdev of flow and laminar flow meter ΔP. For at least one valve setting use the bubble flow meter to repeat a drycal measurement (flow rate is change in volume swept out by bubble divided by change in time). Later you will make a graph of flow rate versus ΔP. Flow Meter ΔP Vacuum Pump Small GAST pump Laminar Flow Meter Note the direction of flow and position of press taps Valve Experiment #1b critical orifice: Now replace the laminar flow meter with the critical orifice and make sure you measure the pressure in the line after the orifice, but before the valve. (Note, this critical orifice is rated at 10 L/min). 1
Make a series of measurements at different P from P=0 to P greater than or equal to ½ atm (15 mmhg vacuum P) and by starting from valve completely closed to completely open. Make sure to record statistics of flow rate and record P at each valve setting. Make a table of flow rate and P. You will eventually plot flow rate versus P 2 /P 1, where the pressures are total pressures (note, the pressure measured in the lab is vacuum gage pressure, the pressure below ambient atmospheric pressure), P 1 is the ambient atmospheric pressure (you can use 1 atm), and P 2 equals P 1 -P Flow Meter P Vacuum Pump Small GAST pump Critical Orifice Valve 2
Experiment # 2. DMA Sizing Comparison to PSL Size Set up the expt as shown below. To begin, disconnect the CPC inlet so it is open to the room. Turn on the CPC and vacuum pump, wait until the CPC comes up to temperature (green lights stop flashing-10min or so). Put the laminar flow meter just calibrated on the CPC inlet and measure the flow rate, or use a flow meter (this will be Qmono). Put the CPC into the experimental setup shown below. NOTE: DON T PLUG THE CPC INLET WHILE RUNNING, IT SHOULD NOT BE UNDER A LARGE VACUUM AT ANY TIME. Valve A Q poly Q sh Diffusion Dryer Neutralizer Laminar Flow Meter (w/in DMA) Nebulizer with PSL Solution DMA Air Pump ~ 30 psi On/Off Valve Vacuum Pump Pressure Gauge Q exc Critical Orifice (w/in DMA) Q mono Pressure Gauge CPC Turn on the small Air Pump connected to nebulizer, pressure on pump should read ~ 30 psi (don t go larger than that!). Set up the DMA as shown, noting the following: Make sure the On/Off Valves by the Vacuum Pump are open. Make sure valve on DMA Qsh is all the way open (tight Counter Clock Wise) Qmono should be close to 1 L/min, use the value measured above for your calculations (CPC flow is set by an internal critical orifice) Qexc will be the critical orifice calibration value at choked flow (~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. 1. Check to make sure the nebulizer has at least a few cm/s deep of liquid. It looks milky due to PSL. 2. 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 3
dramatically. At zero voltage the CPC should read close to zero concentration (record the value). 3. Calculate the DMA voltage corresponding to Dp* equal to the PSL size 0.11µm). This will help deciding what voltages to run the expt at (hint, V should be around 1,600 Volts). 4. Choose various DMA voltages at finer voltage increments in the vicinity of the Voltage for Dp*. 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. A total of about 30 measurements would be appropriate. Record a table of DMA V and CPC Concentration. 5. 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, as discussed above, you need total P for the slip correction, not gage pressure: Ptotal = 1 atm Pgage, when Pgage is measuring vacuum pressure). 4
Experiment # 3. Measure the ambient dry particle size distribution Q poly Q sh Diffusion Dryer Neutralizer Laminar Flow Meter (w/in DMA) Outside Inside DMA On/Off Valve Vacuum Pump Pressure Gauge Q exc Critical Orifice (w/in DMA) Q mono Pressure Gauge CPC 1. Set up the experiment as shown in the schematic above. Note, that basically you will have only disconnected the nebulizer. When you re-apply power to the system, make sure the small pump on the nebulizer is off. Also, close the valve on the vacuum pump so suction from the nebulizer is also off. 2. Make sure you have connected the Qpoly leg to the ambient aerosol sample tube (ie, tube that runs to the outside) and that there are no openings in the line to room air. 3. Adjust large valve on DMA Qsh (closing it) while watching the laminar flow meter recording Qpoly, close Qsh valve until Qpoly is same as Qmono (ie, 1 L/min). 4. 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). 5. 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, DMA pressure, Vacuum pressure on the large vacuum pump so your sure the Qexc flow controlled by the critical orifice is always critical, along with the Voltages and CPC concentrations (dn). You will eventually need to make a table on the form: 5
V Dp* ddp dn* f 1 Ω dn dn/dlndp da/dlndp dv/dlndp dn# da dv dm.............................. 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 f 1 = 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 6. 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. 7. Turn everything off (power strip off). DMA dimensions: L = 43.6 cm r1 = 0.937 cm r2 = 1.958 cm Experiment # 4. Measure the ambient dry particle size distribution with an SMPS and OPC Connect the TSI SMPS and OPC to the inlet line that samples outside air. Now measure the ambient aerosol size distribution with both the SMPS and OPC (only run one SMPS voltage scan and a average OPC measurement over 5 or so minutes. Download all data to a memory stick. 6
Interpret Your Experimental Results: 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. Compare the flow rate measured by the bubble flow meter to that of the drycal. Is there a real difference (difference outside the error bars) between these two measurements? Provide details. Experiment #1b. Make a graph of Q in L/min vs P 2 /P 1 and discuss the shape (is it as expected, at roughly what pressure ratios is Q constant? Give the critical orifice flow rate and uncertainty in L/min that will be used in the DMA expt. P 2 /P 1 will be less than 0.5 during the measurements. 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/d p * {1.257 + 0.4 exp [-1.1 D p * /(2λ)]}, gas mean free path (λ) is 0.066 µm (at 1 atm) and viscosity (µ) 1.83x10-4 dyn s/cm 2. 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. 7
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 DMA.pdf). For each voltage and diameter you will also have to calculate the charging probability f 1, (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 f 1, 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 dv Zxcvbnmiscrepancies. Now plot the number distributions (dn/dlogdp vs Dp) from the SMPS and OPC on a single graph. Do they match up, if not any ideas why not? Does the SMPS size distribution agree with our manual measurement? Discuss. 8