Document ID: chunk:federal_register_of_legislation:F2021C01210:reg:200:p6
Version: federal_register_of_legislation:F2021C01210
Segment Type: reg
Provision Reference: reg 200 (pt 6/8)
Character Range: 98129–100765

ratio of the cross-sectional area of the probe to that of the exhaust pipe shall not be less than 0.05. The back pressure measured in the exhaust pipe at the opening of the probe shall not exceed 75 mm (water gauge).
        2.2.2 The probe shall be a tube with an open end facing forwards in the axis of the exhaust pipe, or of the extension pipe if one is required. It shall be situated in a section where the distribution of smoke is approximately uniform. To achieve this, the probe shall be placed as far downstream in the exhaust pipe as possible or, if necessary, in an extension pipe so that, if D is the diameter of the exhaust pipe at the opening, the end of the probe is situated in a straight portion at least 6 D in length upstream of the sampling point and 3 D in length downstream. If an extension pipe is used, no air shall be allowed to enter the joint.
        2.2.3 The sampling system shall be such that at all engine speeds the pressure of the sample at the opacimeter is within the limits specified in annex 8, paragraph 3.8.2. This may be checked by noting the sample pressure at engine idling and maximum no-load speeds. Depending on the characteristics of the opacimeter, control of sample pressure can be achieved by a fixed restriction or butterfly valve in the exhaust pipe or extension pipe.
        Whichever method is used, the back pressure measured in the exhaust pipe at the opening of the probe shall not exceed 75 mm (water gauge).
        2.2.4 The pipes connecting with the opacimeter shall also be as short as possible. The pipe shall be inclined upwards from the sampling point to the opacimeter, and sharp bends where soot might accumulate shall be avoided. A by-pass valve may be provided upstream of the opacimeter to isolate it from the exhaust-gas flow when no measurement is being made.

  3.  FULL-FLOW OPACIMETER
        The only general precautions to be observed in steady-speed and free-acceleration tests are the following:
        3.1   Joints in the connecting pipes between the exhaust pipe and the opacimeter shall not allow air to enter from outside.
        3.2  The pipes connecting with the opacimeter shall be as short as possible, as prescribed in the case of sampling opacimeters. The pipe system shall be inclined upwards from the exhaust pipe to the opacimeter, and sharp bends where soot might accumulate shall be avoided. A by-pass valve may be provided upstream of the opacimeter to isolate it from the exhaust-gas flow when no measurement is being made.
        3.3  A cooling system may also be required upstream of the opacimeter.
Annex 10
"ECE" METHOD OF MEASURING