SMPS Size Calibration Considerations

1. SMPS Size Calibration Considerations Mike Q, Dec 2006

2. The Problem • Absolute pressure at the SMPS inlet is not the equivalent between when: • You have an atomiser on the front • You are pulling down a sampling line • The Cunningham Slip Factor is dependent on pressure... so what is the magnitude of the difference in sizing between these two situations?

3. = pump Scenarios vent to atm at this point... 1 bar here SMPS CPC very very nearly 1 bar here 1 bar here SMPS CPC long complicated inlet 1 bar less dp(inlet) here

4. Example Inlet Line Pressure Drop Real inlet from Riverside.... • URG 2.5 micron 2000-30EN Cyclone. 2 mbar at 10 lpm. • Large diameter tube from roof, about 0.5 mbar • ¼” tube to instrument from bypass, incl. fittings about 6 mbar • total pressure drop is about 8.5 mbar. !!! Including a Nafion dryer in the system will greatly increase this... at 10 lpm, our 24” dryers give dp = 46 mbar. • Without a drier, of order 1% pressure drop from atmospheric • With drier, of order 5%.

5. The effect at 100 nmSmaller dp = more effect... larger dp = less • 2% drop in pressure = 1% increase in diameter selected for equivalent V, flow. • c.f. your PSL cal, you could be OVERsizing by 2-3% with a Nafion inclusive inlet system. AMS users this is up to 9% for your volume comparisons.... (at 100nm)

6. And a footnote... • The influence of pressure on the slip factor is reviewed in http://nvl.nist.gov/pub/nistpubs/jres/110/1/j110-1kim.pdf • Kn = mean free path / radius. For constant T,dp, Kn is thus proportional to 1013/p [mbar]

7. Thus using that plot.... • for 100 nm PSLs, at Boulder altitude would expect change in slip factor of about 2%. Which you get.... 98 nm peak This is obviously much less than the “straight line” plot I measured in lab.... so “the point is”, try atomise your stuff into the SMPS at the pressure it’ll be at during ambient sampling