[Code of Federal Regulations]
[Title 40, Volume 31]
[Revised as of July 1, 2007]
From the U.S. Government Printing Office via GPO Access
[CITE: 40CFR1065.240]

[Page 707-708]
 
                   TITLE 40--PROTECTION OF ENVIRONMENT
 
         CHAPTER I--ENVIRONMENTAL PROTECTION AGENCY (CONTINUED)
 
PART 1065_ENGINE-TESTING PROCEDURES--Table of Contents
 
                    Subpart C_Measurement Instruments
 
Sec.  1065.240  Dilution air and diluted exhaust flow meters.

    (a) Application. Use a diluted exhaust flow meter to determine 
instantaneous diluted exhaust flow rates or total diluted exhaust flow 
over a test interval. You may use the difference between a diluted 
exhaust flow meter and a dilution air meter to calculate raw exhaust 
flow rates or total raw exhaust flow over a test interval.
    (b) Component requirements. We recommend that you use a diluted 
exhaust flow meter that meets the specifications in Table 1 of Sec.  
1065.205. Note that your overall system for measuring diluted exhaust 
flow must meet the linearity verification in Sec.  1065.307 and the 
calibration and verifications in Sec.  1065.340 and Sec.  1065.341. You 
may use the following meters:
    (1) For constant-volume sampling (CVS) of the total flow of diluted 
exhaust, you may use a critical-flow venturi (CFV) or multiple critical-
flow venturis arranged in parallel, a positive-displacement pump (PDP), 
a subsonic venturi (SSV), or an ultrasonic flow meter (UFM). Combined 
with an upstream heat exchanger, either a CFV or a PDP will also 
function as a passive flow controller in a CVS system. However, you may 
also combine any flow meter with any active flow control system to 
maintain proportional sampling of exhaust constituents. You may control 
the total flow of diluted exhaust, or one or more sample flows, or a 
combination of these flow controls to maintain proportional sampling.
    (2) For any other dilution system, you may use a laminar flow 
element, an ultrasonic flow meter, a subsonic venturi, a critical-flow 
venturi or multiple critical-flow venturis arranged in

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parallel, a positive-displacement meter, a thermal-mass meter, an 
averaging Pitot tube, or a hot-wire anemometer.
    (c) Flow conditioning. For any type of diluted exhaust flow meter, 
condition the flow as needed to prevent wakes, eddies, circulating 
flows, or flow pulsations from affecting the accuracy or repeatability 
of the meter. For some meters, you may accomplish this by using a 
sufficient length of straight tubing (such as a length equal to at least 
10 pipe diameters) or by using specially designed tubing bends, orifice 
plates or straightening fins to establish a predictable velocity profile 
upstream of the meter.
    (d) Exhaust cooling. You may cool diluted exhaust upstream of a raw-
exhaust flow meter, as long as you observe all the following provisions:
    (1) Do not sample PM downstream of the cooling.
    (2) If cooling causes exhaust temperatures above 202 [deg]C to 
decrease to below 180 [deg]C, do not sample NMHC downstream of the 
cooling for compression-ignition engines, 2-stroke spark-ignition 
engines, and 4-stroke spark-ignition engines below 19 kW.
    (3) If cooling causes aqueous condensation, do not sample 
NOX downstream of the cooling unless the cooler meets the 
performance verification in Sec.  1065.376.
    (4) If cooling causes aqueous condensation before the flow reaches a 
flow meter, measure dewpoint, Tdew and pressure, ptotal at the flow 
meter inlet. Use these values in emission calculations according to 
Sec.  1065.650.