Historically, regulators were concerned with the visual impact of the discharge from a stack or chimney and therefore, emission limits were expressed in terms of colour or opacity. Modern methods for opacity measurement still use the darkness of the stack gases to measure the amount of smoke or dust emitted within the exhaust gases. However, with the advent of emission limits for a process being defined, continuous particulate (or dust) monitoring has become a regulatory requirement for many industries.
Our Opacity Monitors employ the light transmission technique to measure dust, smoke and particulate emissions, which uses a high intensity light source to project a beam of light across the stack, duct or flue. Some light is transmitted and some is lost due to scattering and / or absorption. The amount of this light lost in crossing the stack is the opacity, which correlates to the amount of dust present. The fraction of light transmitted is known as the Transmittance.
The mathematical relationship between the light transmitted by a medium and the quantity of pollutant present is known as the Beer-Lambert Law and the fundamental measurement made by all of opacity instruments is Transmission Ratio;
T = Ir / It
where It = Intensity of outgoing light beam and Ir = Intensity of returning light beam
This is normally expressed as Opacity, where percentage opacity is:
Opacity = 100 × (1− T)
Opacity monitors are typically either Single Pass like DynOptic Systems DSL-220 Opacity Monitor and DSL-230 Dust Monitor or Double (Dual) Pass, like DynOptic Systems DSL-320 Opacity Monitor and DSL-330 Dust Monitor.
Single Pass Opacity Monitors
Single Pass Opacity Monitors have two parts; a Transmitter (TX) and Receiver (RX), on either side of a stack or duct. The Transmitter contains a light source (Green LED) of known (measured) intensity and the amount of light transmitted across the stack is measured by a sensor in the Receiver. The initial intensity of the light is memorised and any reductions in that intensity, caused by particulate suspended in the gas stream, are measured as opacity.
Double (Dual) Pass Opacity Monitors
Double Pass opacity monitors consist of a Transceiver (TRX) and Reflector positioned on either side of the stack. A light beam generated in the TRX passes across the centre of the duct (through the gas stream) and falls directly onto the Reflector on the other side. The Reflector returns the light back through the gas stream to the TRX. The initial intensity of the light is memorised and any reductions in intensity, caused by smoke & particulate suspended in the gas stream, are measured as opacity.
The innovative Dynamic Detection Principle (DDP) measures the dynamic fluctuation in light transmission as the dust particulates move through a light beam, i.e. the fluctuation in the amount of transmitted light is measured. As the particulate density in the stack increases, the amplitude of the fluctuations also increase. This rapid fluctuation in the optical transmission is sometimes called scintillation. When calibrated against standard reference measurements, the amplitude of this signal relates directly to the particulate concentration in the stack and can be used to present a reading in mg/m³.
One key advantage of DDP over opacity is that by measuring the ratio of the noise variance to the mean transmission, the resulting signal is almost independent of the mean transmission value. Therefore unlike the standard transmission technique, DDP has immunity to gradual reductions in the absolute intensity of the light signal, which has the advantage of being significantly less susceptible to drift with time, temperature or dirtying optics, than traditional opacity monitors using the standard transmission technique. It is also less sensitive to optical misalignment.
DynOptic Systems DDP monitors can be either Single Pass, our DSL240 Dust Monitor or Double (Dual) Pass, our DSL340 Dust Monitor range.