3 Description of the CNC
3–1
The Model 3760 CNC measures the number concentration of particles that
measure 0.014 micrometer in diameter and larger. The particles are detected by
condensing n=butyl alcohol vapor onto the particles, causing them to grow into
droplets. The droplets are easily counted by a simple optical particle detector.
The CNC is small, quiet and clean; it is designed to be operated continuously with
the Model 3701 Multisensor Cleanroom Monitoring System.
Figure 3-1
Schematic Diagram of the Cleanroom CNC
The sample flow is drawn into a saturator where n-butyl alcohol (butanol)
evaporates into the sample stream. Although the saturator portion is not heated, it
tends to be several degrees above room temperature. The sample then cools by
thermal diffusion in a cooled condenser tube. The butanol vapor becomes
supersaturated and condenses on the particles. A thermoelectric device, cools the
condenser, which is sandwiched between the condenser and a heatsink. The
temperature of the condenser is based on the temperature of the saturator. The
particles are detected by a simple single-particle-counting, laser-diode, optical
