The CDC intends to award a sole-source, firm-fixed-price purchase order to TSI Incorporated, Shoreview, MN, for repair and calibration of Government-owned Aerodynamic Particle Sizers. Only TSI can perform this work because proprietary models, internal flow regulators, and components behind embedded electronics require manufacturer access and proprietary knowledge. TSI will repair and calibrate three Aerodynamic Particle Sizers, with an expected turnaround of 21 days. Calibration must achieve and maintain a flow rate of 5 L/min (1 L/min sample + 4 L/min sheath), even with inlet pressure drops up to 50 inH2O, using microprocessor-controlled volumetric flow control, barometric pressure correction, and separate sheath/total-flow pumps. TSI must also calibrate for precise number-size distributions from 0.37 to 20 µm aerodynamic diameter using time-of-flight and a patented double-crest optical system, with the Government providing units S/N 1169, 1203, and 72321006.