diff --git a/README.md b/README.md
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@@ -46,14 +46,25 @@ performance difference between pleated hydrophobic filters and electrostatic fil
 - ASTM standards uses latex spheres. could potentially use gold particles 
 - charge neutralizer recommended - but Lorena said viruses are typically polarized so maybe charge neutralization could be ignored initially 
 
+- optical particle counter [^opc]- fancy once can be used to measure particle size with lorentz mie scattering, but it shouldn't take too much work to make our own that just counts. 
+    -   visible laser : wavelength and power determine particle size resolution, higher power --> smaller particles, limited to >100nnm (intracavity systems for smaller than that)
+    -   photodetector placed where particles scatter light : placement could be difficult if the particles have a large area to explore, but photomulitplier tube could fix
+    - isokinetic sampling probes[^isokineticProbes] pull representative particle samples into the OPCs - so you don't care about area for PMT and can put the OPCs in a light box instead of the entire appartus 
 
-## shopping list 
-- nebulizer for aerosolization [^nebulizer_selection]
+## proposed testing device
+measure filtration efficiency and pressure difference with non-biological colloidal particles:
+![](proposed_apparatus.jpg)
+
+could also get away with only one OPC by adding a bypass around the test filter and a switch
+
+
+## equipment needed
+- nebulizer for aerosolization [^nebulizer]
     -   three-jet Collison nebulizer? 
-- impinger for collection 
 - wet and dry thermometer / hygrometer for humidity 
+- OPC : photodiodes or PMTs 
 - andersen sampler?? could be nice to bin different size particles - six-stage viable particle cascade impactor
-
+- OPC would be better than andersen sampler
 
 ### Resources
 [^DIYmask]: University of Cambridge [DIY mask efficacy testing](https://www.researchgate.net/publication/258525804_Testing_the_Efficacy_of_Homemade_Masks_Would_They_Protect_in_an_Influenza_Pandemic)
@@ -64,4 +75,6 @@ performance difference between pleated hydrophobic filters and electrostatic fil
 [^aerodynamics]: [Aerodynamic Characteristics of RNA concetration of SARS-CoV-2](https://www.biorxiv.org/content/10.1101/2020.03.08.982637v1.abstract)
 [^VFEreport]: [Nelson Laboratories: Viral Filtration Efficiency (VFE) at increased challenge level](https://gitlab.cba.mit.edu/pub/coronavirus/tracking/uploads/164d119b5a7a5751c685a6677378668a/ER_2235600_Attachment_5_Viral_Filtration_Efficiency.pdf)
 [^nelson]: [Nelson lab filtration test](https://www.nelsonlabs.com/testing/bacterial-viral-filtration-efficiency-bfe-vfe/?industry=medical-devices&category=protective-barriers-material-performance)
-[^nebulizer_selection]: [Nebulizer selection from CH tech](http://chtechusa.com/Manuals/Selecting%20the%20Model%20and%20Configuration%20Instructions.pdf)
+[^nebulizer]: [Nebulizer selection from CH tech](http://chtechusa.com/Manuals/Selecting%20the%20Model%20and%20Configuration%20Instructions.pdf)
+[^opc]: [optical particle counter](http://www.cas.manchester.ac.uk/restools/instruments/aerosol/opc/)
+[^isokineticProbes]: [Isokinetic particle Sampling](http://www.climet.com/accessories/isoprobes.html)
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