Casella CEL Airborne particulate monitoring system User Manual
Page 4
HB3256-03
Page 2
P
RINCIPLE
O
F
O
PERATION
Please refer to the APM950 schematic drawing.
Ambient dust is drawn into the Total Suspended Particulate (T.S.P.) or size selective PM10
sampling head.
Sample flow is precisely regulated by means of a built in diaphragm pump and electronic control
circuitry. A state-of-the-art mass flow transducer is used to provide flow feedback information,
this ensures a highly reliable and stable flow rate of 16.7L/min is achieved. (Temperature
compensated volumetric flow is available as an option). A ‘flow smoother’ unit reduces the
amplitude of pump pulsations within the air flow.
The sample air passes through a heater assembly where it is warmed to eliminate errors resulting
from suspended moisture droplets and high humidity levels. The sample is raised to typically 20-
25°C above ambient.
The high sensitivity optical sampling head is based upon the forward scattering effect of an near
infra red beam. The response is optimsed for the measurement of respirable particulates below
10
µ
m in diameter. Signal processing electronics are used to control the infra red emitter and
provide conditioning of the return signal.
After passing the optical sensing stage, all particulates within the sampled air are collected and
retained on a 47mm filter. Gravimetric results may be correlated to the real time data to provide
optimised calibration accuracy. Additional analysis of filters may be used to establish the
chemical composition of contaminates.
A purge pump maintains a supply of clean dry air to the optically sensitive components within
the probe. The protective air shield protects against particle deposition and ensures reliable long
term operation. It is also used to establish a zero dust level during automatic calibration checks.
A programmable logger and micro controller are used to sequence the overall system operation,
monitor signals and record data. The data saved within its internal memory may be extracted
using a laptop P.C. memory card or remotely via a telephone modem.