Teledyne 651 operation manual S i g n o f t h e M o d e l 6 5, Sensor

Technical Description Teledyne API Ultrafine Particle Monitor - Model 651

where γ = surface tension of the condensing fluid

M= molecular weight of the condensing fluid ρ = density of the condensing fluid

R = universal gas constant T = absolute temperature d = Kelvin diameter

The higher the saturation ratio, the smaller the Kelvin diameter.

The saturation vapor pressure Ps is defined for a flat liquid surface. For a curved liquid surface, such as the surface of a droplet, the actual saturation vapor pressure is greater. The smaller the droplet, the easier it is for the vapor molecules to escape the liquid surface. The Kelvin diameter defines the critical equilibrium diameter at which a pure droplet is stable (there is neither condensation nor evaporation). Liquid particles with diameters smaller than the critical equilibrium diameter will evaporate and larger particles will grow even larger by condensation. The larger particle will grow until the vapor is depleted, causing the saturation ratio to fall until it is in equilibrium with the particle droplet. The lower size sensitivity of the counter is determined by the operating saturation ratio.

D e s i g n o f t h e M o d e l 6 5 1

Submicrometer particles are drawn into the particle counter and enlarged by condensation of a supersaturated vapor into droplets that measure several micrometers in diameter. The droplets pass through a lighted viewing volume where they scatter light. The scattered-light pulses are collected by a photodetector and converted into electrical pulses. The electrical pulses are then counted and their rate (live-time corrected) is a measure of particle concentration.

The basic instrument consists of three major subsystems: the sensor, the microprocessor-based signal-processing electronics, and the flow system. The sensor and the flow system are described below.

Sensor

The sensor contains a conditioner, a growth tube, and an optical detector (shown schematically in Figure 1-2). The sensor grows the sampled aerosol particles into larger droplets that are detected optically.

54 07506C DCN6727