Teledyne 651 operation manual Flow System, Critical Flow

Technical Description Teledyne API Ultrafine Particle Monitor - Model 651

Changes in optical alignment, laser power, operating temperatures, flow rates, presence of water, or optical cleanliness can all reduce pulse amplitude, therefore the pulse amplitude indicates the “health” of the WCPC. A peak-sense and hold circuit within the Model 651 measures the pulse amplitude of 50 particles/sec. The average pulse amplitude is displayed both numerically and in a bar graph on the Status screen, and is also included in the data retrieved from the digital interfaces. The limitation of the Pulse Height indication is that is requires the presence of at least 50 particle pulses/sec to provide information. The Pulse Height fault status indication is displayed for particle concentrations over 1000 #/cm3 with a pulse height of less than 350 mV. When measuring very low concentrations (<10 #/cm3) the Pulse Height fault may be displayed even though the correct particle concentration is provided.

Flow System

Refer to Figure 1-2 while reviewing the instrument flow information.

The Model 651 relies on an external vacuum supply to maintain constant flows through three critical orifices. These independent flows can be verified by toggling into the different flow modes.

Orifice	Description
0.12 L/min aerosol	Carries the aerosol to be sampled. This is not
sample flow mode	user-selectable.
3.0 L/min auxiliary	Provides a higher flow rate for use with sampling
flow mode	systems.
	Note: 2.4 L/min auxiliary flow is removed to
	leave a transport flow of 0.6 L/min.
0.6 L/min transport	Reduces particle losses.
flow mode	Note: 0.48 L/min transport flow is removed to
	leave a sample flow of 0.12 L/min.

The flow rate through the sensor is always 0.12 L/min, independent of the inlet flow rate setting. Problems with the aerosol flow can be detected by monitoring the pressure drop across the nozzle and verifying that the critical orifice pressure is maintained.

Critical Flow

To achieve the 0.12 L/min sample flow through the sensor, an orifice is used (operated at the critical pressure ratio) to provide a critical flow. Critical flow is very stable and is a constant volumetric flow, ensuring accurate concentration measurements despite varied inlet pressure.

56 07506C DCN6727