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Equipment that discharges its exhaust into intake air that is coursed directly into a ' return air course. Usually all such equipment would be excluded from the ' calculation. 3. Equipment that discharges its exhaust directly into a return air course. Usually all ' such equipment would be excluded from the calculation. 4. Other equipment having duty cycles such that the emissions would not significantly , affect the exposure of miners. The length, duty cycle and type of operation of the equipment must be evaluated in making this determination. In some cases sampling maybe needed to assess the effect of the equipment operation on the ' exposure of miners. Apnrova] of Reduced Minimum Ventilating Air Quantities under §75.325(1)_ ' The minimum ventilating air quantity required under paragraph (g) is based upon the nameplate air quantities for the equipment engines. These nameplate quantities are determined by laboratory testing using MSHA test procedures, which are designed to approximate the duty cycles of the , engines. The ventilation rates are based upon the exhaust contaminants measured at different engine speeds and loading factors. Because in-mine operation of multiple engines can vary depending on equipment loads and speeds, the regulations allow mine operators to request , reductions in the required nilnimum ventilating air quantity for multiple units of equipment. It is unportant to note that the minimum ventilating air quantity for an individual unit of diesel- ' powered equipment cannot be reduced. It is the mine operators responsibility to provide MSHA with data, such as results of on-shift, , environmental, and personal sampling, to support any request for a reduced minimum ventilating air quantity. Such data may include a continuous and complete record of carbon monoxide, nitrogen dioxide, and the air quantities measured on the section. Data should be collected for all locations where the minimum air quantity is required to be maintained. Data logging instrumentation generally provides the most usable results. Computer-based mine wide monitoring systems could also provide valuable data. Data should indicate the time-weighted ' averages for the contaminants measured, peak contaminant concentrations, the associated measured air quantities, section production records, and the reduced minimum air quantity the company is requesting. The sampling period should be of a sufficient time to provide MSHA with , enough data to make a valid determination. To minimize possible concerns about the validity of the sampling data, MSHA should consider ' observing some portion of the mine operator's data gathering and sampling to assist in evaluating the request for reduced air quantities. MSHA should review data to determine the potential effect of a reduction in ventilating air quantity by calculating the projected time-weighted average (TWA) concentrations for the contaminants. The following relationship can be used as a tool for making this determination: September 199'7 (Release 2) '