Analytical Instrumentation & Environmental Monitoring


Dust emission monitoring by laser

November 2000 Analytical Instrumentation & Environmental Monitoring

One of the main causes of atmospheric pollution is uncontrolled emission of dust. Classified industries are required to fit an approved dust emission measurement system such as that offered by Oldham.

The Oldham Observer EP 1000 is based on the principle of laser beam retrodiffusion by dust particles. Pointed towards the dust-bearing atmosphere, the laser beam is reflected by the suspended particles back towards the source, providing an indication of dust concentration. The amount of reflected light is used to determine dust concentration in the atmosphere. Measurement is instantaneous and permanently recorded.

Figure 1. The Oldham Observer EP 1000 dust monitor
Figure 1. The Oldham Observer EP 1000 dust monitor

The EP 1000 uses light reflection (instead of absorption as in other equipment) and has the ability to measure very low concentrations with no zero drift. It is relatively insensitive to water droplets, flowrate and particle temperature. The unit features excellent signal linearity, especially at very low concentrations.

The laser transmitter is modulated, only the portion of the detected light that exhibits modulation identical to transmitting laser's modulation is measured as reflected light. This makes the dust monitor essentially immune to interference from any spurious light sources. The detector is relatively insensitive to water droplets and it has a built-in protection from dust penetration. This is implemented by means of a positive air pressure (from an air blower) and an optional shutter. These features, together with its insensitivity to temperature variations make it a reliable instrument.

The detector is housed in an easy to install polyurethane box, only one side of which has to be mounted into the stack. The unreflected portion of the transmitted laser beam enters a light trap, which is mounted on the stack, opposing the sensor/transmitter installation. The sensor requires no special alignment.

Figure 2. Accessories: air blower, chart recorder, remote keyboard and light traps
Figure 2. Accessories: air blower, chart recorder, remote keyboard and light traps

Several selectable ranges of operation are available, the device, which has a linear response characteristic, may be run continuously or be used for instantaneous measurements. Outputs available are 2 to 20 mA, RS485, RS422 and Modbus. The Oldham Observer EP1000 has a setpoint overshoot alarm and is specified to operate between -25 and +55°C.



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