Update README.md

README.md

 # COVID-19 Filter Testing
 
 ## testing instrument
-<img src="./cad_1.png" height="600">
+<img src="./cad_1.png" height="500">
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+This filter efficency testing device is a blend of ASTM standards F2299, F2101, and the henderson apparatus detailed by Wilkes[^filterPerformance]. 
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+compressed air enters at the center of the device and first passes through a HEPA filter (a).