Teledyne 651 operation manual Pulse Height

Teledyne API Ultrafine Particle Monitor - Model 651 Technical Description

The sample flow is cooled with a thermoelectric device in the conditioner. The vapor passes into the growth tube where it becomes supersaturated and condenses onto the aerosol particles (acting as condensation nuclei) to form larger droplets. The droplets pass through a nozzle into the optical detector.

The sensor’s optical detector is comprised of a laser diode, collimating lens, cylindrical lens, elliptical mirror, and photodiode detector. The laser and collimating lens form a horizontal ribbon of laser light above the aerosol exit nozzle. The collection mirror focuses the light scattered by the droplets at a 90° angle (side scatter) onto a low-noise photodiode. The main beam is blocked by a light-stop in the back of the sensing chamber. A reference photodiode is used to maintain constant laser power output. The surface temperature of the optics housing is maintained at a higher level than the growth tube to avoid condensation on the optical surfaces.

The Model 651 operates in single particle count mode up to 106 particles/cm3. Rather than simply counting individual electrical pulses generated by light scattered from individual droplets, the Model 651 uses a continuous, live-time correction to improve counting accuracy at high particle concentrations. Live-time correction occurs when the presence of one particle obscures the presence of another particle creating an undercounting error that results in dead time.

Pulse Height

The Model 651 contains an electronic sub-system for monitoring the amplitude (voltage height) of the particle pulse generated by the optical detector. The actual amplitude of the pulse does not affect the particle counting performance as long as it is large enough to intercept the preset discriminator threshold. Typical pulse amplitudes (1 to 2 volts) are 10 to 40 times higher than the discriminator level which is typically 20 times higher than the RMS noise level of the photo-detector electronics. This large magnitude of ‘signal-to-noise’ margin provides robustness in performance in the optical detection of droplets.

Under normal operating conditions, the pulse amplitude decreases with increasing particle concentration. As particle concentration increases, depletion effects within the growth tube cause the nucleated droplets to grow to smaller sizes than they would at lower particle concentrations.

Note: The droplet size has been reduced in this instrument compared to those of previous generations - reducing the variation in pulse amplitude with respect to particle concentration to about 2:1 over the concentration range of the instrument.

07506C DCN6727 55