With an increasing emphasis placed on protecting the environment, industries and governments are committing greater resources to monitoring and regulating existing stationary and non-stationary effluent sources as well as to developing new environmentally friendly stationary and non-stationary effluent sources. 
For instance, exhaust gases or emissions from motorized vehicles are regulated by the U.S. Federal Government so as not to exceed certain maximum contaminant levels.  Because of these regulations, increasingly more sophisticated testing equipment has been developed to test and analyze engines for conformance with such standards.  As an example, regulations set by the U.S. Environmental Protection Agency (EPA) involve particulate limit standards for various types of engines such as diesel truck engines.  The regulated particles are matter in the exhaust gas stream, other than condensed water, that can be collected after dilution.  These particulates can include agglomerated carbon particles, absorbed hydrocarbons, and sulfates. 
In order to comply with such regulations, industries involved in the manufacturing or use of effluent sources and government agencies responsible for enforcing such regulations have relied on systems that attempt to simulate the diluting process of the exhaust gases.  Known methods include adding diluting air to the exhaust gas through a controlled sampling system that has a dilution tunnel.  A significant challenge with these methods is the elimination of errors in measurements taken of the diluted exhaust gas and the diluting air streams and the need to precisely control their respective flow rates. 
When the size of the effluent source, and more particularly, the mass flow of exhaust gas from the effluent source, permits, full sampling dilution systems may be used in which the total exhaust gas flow from the effluent source is mixed with a quantity of diluting air.  However, when the size of the effluent source is so large that testing with a full sampling dilution system would not be practical due to the large size required for the corresponding dilution tunnel, proportional sampling dilution systems may be used in which only a portion of the exhaust gas flow is sampled, requiring a smaller dilution tunnel. 
Investigations into the performance of dilution systems used today continue to indicate excessive variability between governmental agencies, testing laboratories, and effluent source manufacturers.  This variability can have negative consequences.  On the one hand, the discrepancies between the testing laboratories may translate into competitive advantages for the low-result testing laboratories.  On the other hand, the observed test-to-test variability translates into increased test expenditure because a large number of tests are required to obtain statistically significant results.  Although there are several particle mechanisms that influence test-to-test variability, those most significant are particle deposition on the dilution tunnel and tailpipe walls by thermophoresis, by mechanical processes such as diffusion, gravitational sedimentation and turbulence, and by reentrainment of deposited particles and hydrocarbon gas phase exchange of the soluble portion of the exhaust particles with the deposited wall bound particles.  Therefore, elimination of the deposition mechanism is highly desirable. 
U.S. Pat.  No.  5,058,440 discloses a dilution tunnel aimed at reducing the variability of test results in part through the elimination of thermophoretic deposition of particles on the walls of the sampling device and corresponding hydrocarbon gaseous phase component exchange with these wall-bound particles; down-sizing the dilution tunnel to produce a fully portable sampling system that can yield results equivalent to those of large testing laboratories; and a sampling system that can monitor variable engine operating parameters, automatically control the rate of exhaust gas withdrawal and vary the air dilution rate within preselected guidelines within a normal range of operating temperatures and pressures. 
In particular, U.S. Pat.  No.  5,058,440 discloses a gas sampling system that uses a dilution tunnel, including a sampling probe disposed in an exhaust gas stream of an engine, a source of clean diluting air, and a filter assembly.  The dilution tunnel includes an air distribution tube or diffuser tube having a plurality of distribution holes therethrough, a porous center tube having a plurality of micron-sized pores and defining a first chamber within the air distribution tube, and a housing forming a second chamber about the air distribution tube.  The second chamber is connected to a diluting air source, and the center tube is connected between a sampling probe in the exhaust gas flow and a filter assembly. 
The known gas sampling systems, however, lack the ability to change the rate of dilution of a sample flow of gas, so that these systems do not allow accurate simulations to be made of a variety of diluting processes.  To appreciate the extent of this shortcoming, it is important to understand that accurate attainment of particle analysis, such as representative particle size measurement results, depends strongly on simulations of atmospheric diluting processes as qualified for a given application. 
For example, exhaust gas from the exhaust pipe of a large diesel truck hauling a trailer may experience the following particular diluting process characterized by a series of different diluting rates at various distances from the exhaust pipe: (1) a fast diluting rate near the exhaust pipe where the gas is initially introduced into the atmosphere, (2) a slow diluting rate over the trailer portion where the air flow is relatively stable or laminar, and (3) an intermediate diluting rate behind the trailer where the air flow is turbulent.  On the other hand, in a stationary effluent source, such as a power plant, the exhaust gas may experience a different diluting process determined primarily as a function of time out of the exhaust pipe or stack and distance from the exhaust stack. 
Therefore, there is a need for introducing additional degrees of freedom into gas sampling systems that use dilution tunnels, so that the sample gas can be diluted in a controlled manner that better simulates the actual diluting process for a given application. 
The present invention is directed to overcoming one or more of the problems as set forth above.