Measuring and testing – Gas analysis – Solid content of gas
Patent
1994-10-31
1997-01-07
Williams, Hezron E.
Measuring and testing
Gas analysis
Solid content of gas
324454, G01N 2500
Patent
active
055918956
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an apparatus and method for detecting particles in a gas flow. The invention is particularly concerned with an arrangement in which particles are detected as a result of those particles triboelectrically charging a probe in a duct along which a gas flows.
WO 86/02454 describes an apparatus which uses triboelectric effects. A metal probe projects into a duct and particles carried past the probe in the gas flow impart triboelectric charges to the probe. Those charges are conducted to ground through an electric circuit, resulting in an electric current. The current is converted to a unipolar voltage and amplified to provide an indication of the mass of the particles.
In WO 86/02454 an auto-zero circuit is preferably provided to rezero the circuit periodically (about once per minute) in order to prevent the output of the circuit varying with ambient temperature as a result of changing electrical characteristics of the current to voltage conversion circuit. As an alternative to the auto-zero circuit a system which uses only the a.c. component of the signal can be used apparently for the same purpose. In commercial versions of the apparatus described in WO 86/02454, the auto-zero circuit is employed.
We have found that apparatus of the kind described in WO 86/02454 is unable to give a reliable indication of the mass flow rate of particles in the gas flow and that the magnitude of the current generated by the charges is influenced by factors other than the mass of the particles.
It is an object of the invention to provide a method and apparatus for detecting particles in a gas flow which relies upon particles in the flow triboelectrically charging a probe and which is less affected by changes in other variables relating to the gas flow.
According to the invention there is provided a method for detecting particles in a gas flow in which a probe is charged triboelectrically by particles in the flow and the quantities of electrical charges transferred to the probe are evaluated to provide an indication of the particle flow in the gas flow, wherein, in order to reduce the effect of variations in "gas flow related variables" other than those relating to particle flow, an alternating component in the signal caused by the triboelectrical charging of the probe is monitored. The term "gas flow related variables" refers to variables that are related to the environment in the region of the gas flow and at the probe, to the gas flow itself and/or to the particles in the gas flow. Examples of gas flow related variables other than those relating to particle flow are therefore: humidity in the region of the gas flow, temperature of the gas flow, thickness of a particulate layer deposited on the probe, and the electrical charge of particles in the gas flow. Examples of gas flow related variables that relate to particle flow are the mass flow rate of the particles, and the velocity and size of the particles.
We have discovered that by looking at the alternating component in the signal from the triboelectrically charged probe rather than looking at the absolute value of that signal, it is possible to obtain an output signal that is much less affected by gas flow related variables, other than those relating to particle flow, than in the case of, for example, the apparatus described in WO 86/02454. Although it might appear to be disadvantageous to evaluate the much smaller amplitude alternating signal component involved, we have found that such disadvantages are more than compensated for by the much improved correlation between, for example, the mass flow rate of particles in the flow and the alternating component in the signal from the probe. More particularly, we have found that whereas variations in gas flow related variables other than those relating to particle flow are liable to affect the absolute value of a signal from a probe charged triboelectrically, they are not likely to affect the magnitude of the alternating component of the signal to the same extent. This hitherto undiscovered phenomenon is
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Politzer Jay I.
Pollution Control & Measurement (Europe) Ltd.
Williams Hezron E.
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