There are two main competing technologies for the determination of levels of dust emissions. These are optical systems and tribo-electric systems. Within these two groups are a number of different types of instruments, all of which show similar operating characteristics.
Optical measurement
Historically, the earliest form of dust sensor was the simple optical smoke monitor, which effectively measured the opacity of the stack. Since then, more sophisticated forms of opacity monitor have been introduced along with a variety of light scattering sensors. All these instruments, however, share a number of common features.
1. All optical sensors respond to the mass concentration of dust within the duct.
Light beams are influenced by the scattering characteristics of large numbers of small particles and the measured parameter is a function of the number of particles per unit volume of gas, which in turn is proportional to the mass concentration.
2. All optical sensors are crucially dependent on the scattering characteristics of the particles in the gas stream. In particular, the size and material of the particles will have a major influence on the way in which light is scattered. One of the main criticisms of traditional opacity monitors is that the resultant mass concentration calibration varies considerably with particulate type.
Optical backscatter and forward scatter devices, introduced to enable lower density levels of emission to be measured, suffer especially from particle size and material variations.
Tribo-electric sensors
Tribo-electric sensors fall into two categories - DC sensors and AC sensors. Both types are fundamental sensors of mass flow and can be calibrated directly in units of gm/sec or kg/hr. DC sensors have been shown to have a linear response to mass flow over a wide range of gas velocities. These devices provide a simple low-cost mass flow measurement that, unlike mass concentration measurement, requires no further normalisation to deliver an absolute measurement of environmental impact.
However, AC devices, because of their additional AC coupling, are also dependent for their signal on the degree of inhomogeneity of the dust within the gas flow. This imbues them with unpredictable velocity sensitivity, making them unsuitable for any quantitative analysis, unless the process conditions and exhaust gas velocities are absolutely stable. Further measurement of flow is of no help since the flow dependency will vary from plant to plant and from location to location within even the same duct, because of the effects of gas turbulence patterns on the inhomogeneity of the dust flow.
Salah satu contoh dust emission monitoring dengan Tribo-electric Sensor teknik, Mark : Auburn
sumber : Instrumentation.co.za
