40 CFR 90.414 - Raw gaseous exhaust sampling and analytical system description.
(a) Schematic drawing. An example of a sampling and analytical system which may be used for testing under this subpart is shown in Figure 2 in appendix B of subpart D. All components or parts of components that are wetted by the sample or corrosive calibration gases must be either chemically cleaned stainless steel or inert material (e.g., polytetrafluoroethylene resin). The use of “gauge savers” or “protectors” with nonreactive diaphragms to reduce dead volumes is permitted.
(b) Sample probe.
(1) The sample probe must be a straight, closed end, stainless steel, multi-hole probe. The inside diameter may not be greater than the inside diameter of the sample line 0.03 cm. The wall thickness of the probe may not be greater than 0.10 cm. The fitting that attaches the probe to the exhaust pipe must be as small as practical in order to minimize heat loss from the probe.
(2) The probe must have a minimum of three holes. The spacing of the radial planes for each hole in the probe must be such that they cover approximately equal cross-sectional areas of the exhaust duct. See Figure 2 in appendix B of subpart D. The angular spacing of the holes must be approximately equal. The angular spacing of any two holes in one plane may not be 180° ±20° (i.e., section view C-C of Figure 2 in appendix B of subpart D). The holes should be sized such that each has approximately the same flow. If only three holes are used, they may not all be in the same radial plane.
(3) The exhaust gas probe must be located in a position which yields a well mixed, homogenous sample of the engine exhaust. The probe must extend radially across the exhaust gas stream. The probe must pass through the approximate center and must extend across at least 80 percent of the exhaust gas stream. The exact position of the probe may vary from engine family to engine family.
(c) Mixing chamber. The exhaust mixing chamber is located in the exhaust system between the muffler and the sample probe. The mixing chamber is an optional component of the raw gas sampling equipment.
(1) The internal volume of the mixing chamber may not be less than ten times the cylinder displacement of the engine under test. The shape of the mixing chamber must be such that it provides a well mixed, homogenous sample at the sample probe location.
(2) Couple the mixing chamber as closely as possible to the engine muffler.
(3) Maintain the inner surface of the mixing chamber at a minimum temperature of 179 °C.
(4) Thermocouple temperature monitoring of the mixing chamber inner surface is required to assure wall temperatures specified in paragraph (c)(3) of this section. The temperature measurement must be accurate to within ±5 °C.
(5) The sample probe must extend radially across the exit of the mixing chamber. The probe must pass through the approximate center and must extend across at least 80 percent of the diameter of the exit. The exact position of the probe may vary from engine family to engine family. The probe must be located in a position which yields a well mixed, homogenous sample of the exhaust.
(d) Sample transfer line.
(1) The maximum inside diameter of the sample line may not exceed 1.32 cm.
(2) If valve V2 in Figure 1 of appendix B of this subpart is used, the sample probe must connect directly to valve V2. The location of optional valve V2 in Figure 1 of appendix B of subpart D may not be greater than 1.22 m from the exhaust duct.
(3) The location of optional valve V16, Figure 1 of appendix B of this subpart, may not be greater than 61 cm from the sample pump. The leakage rate for this section on the pressure side of the sample pump may not exceed the leakage rate specification for the vacuum side of the pump.
(e) Venting. All vents, including analyzer vents, bypass flow, and pressure relief vents, of regulators should be vented in such a manner as to avoid endangering personnel in the immediate area.
(f) Any variation from the specifications in this subpart, including performance specifications and emission detection methods, may be used only with prior approval by the Administrator.
(g) Additional components, such as instruments, valves, solenoids, pumps, switches, and so forth, may be employed to provide additional information and coordinate the functions of the component systems.
(h) The following requirements must be incorporated in each system used for raw testing under this subpart.
(1) Take the sample for all components with one sample probe and split it internally to the different analyzers.
(2) Heat the sample transport system from the engine exhaust pipe to the HC analyzer for the raw gas sampling method as indicated in Figure 1 in appendix B of this subpart. The NOX analyzer for the raw gas sampling method may be heated as indicated in Figure 1 in appendix B of this subpart. The HC analyzer and the NOX analyzer for the dilute sampling method may be heated as indicated in Figure 1 in appendix B of this subpart.