Teledyne 651 operation manual Technical Description, E o r y

C H A P T E R 6

Technical Description

The Model 651 is a continuous-flow, water-based, condensation particle counter that detects particles down to <7 nm at a sample flow rate of 0.12 L/min. This section describes the function of the particle counter, its subsystems and its components. A discussion of operation theory is given first.

T h e o r y

The Model 651 acts very much like an optical particle counter. However, the particles are first enlarged by a condensing vapor to form easily detectable droplets. Portions of the following discussion (focusing on how to condense the vapor onto the particles) are taken from a paper by Keady, et al. [1986].

In heterogeneous condensation, the vapor surrounding particles reaches a certain degree of supersaturation and begins to condense onto the particles. In homogeneous nucleation (self-nucleation),supersaturation is so high that condensation can take place even if no particles are present because molecules of the vapor form clusters (nucleation sites) due to the natural motion of the gas and attractive van der Waals forces. The Model 651 operates below the supersaturation ratio to avoid homogeneous nucleation.

The degree of supersaturation is measured as a saturation ratio

(P/Ps), which is defined as the actual vapor partial-pressure divided by the saturation vapor pressure for a given temperature:

P supersaturation =__ Ps

For a given saturation ratio, the vapor can condense onto particles only if they are large enough. The minimum particle size capable of acting as a condensation nucleus is called the Kelvin diameter and is evaluated from the following relationship:

P	(4γ M)
saturation ratio =__ = exp _____
Ps	ρRTd

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