(a) Overview. This section describes how to calibrate flow meters for diluted exhaust constant-volume sampling (CVS) systems.

(b) Scope and frequency. Perform this calibration while the flow meter is installed in its permanent position, except as allowed in paragraph (c) of this section. Perform this calibration after you change any part of the flow configuration upstream or downstream of the flow meter that may affect the flow-meter calibration. Perform this calibration upon initial CVS installation and whenever corrective action does not resolve a failure to meet the diluted exhaust flow verification (i.e., propane check) in §1065.341.

(c) Ex-situ CFV and SSV calibration. You may remove a CFV or SSV from its permanent position for calibration as long as it meets the following requirements when installed in the CVS:

(1) Upon installation of the CFV or SSV into the CVS, use good engineering judgment to verify that you have not introduced any leaks between the CVS inlet and the venturi.

(2) After ex-situ venturi calibration, you must verify all venturi flow combinations for CFVs or at minimum of 10 flow points for an SSV using the propane check as described in §1065.341. Your propane check result for each venturi flow point may not exceed the tolerance in §1065.341(f)(5).

(3) To verify your ex-situ calibration for a CVS with more than a single CFV, perform the following check to verify that there are no flow meter entrance effects that can prevent you from passing this verification.

(i) Use a constant flow device like a CFO kit to deliver a constant flow of propane to the dilution tunnel.

(ii) Measure hydrocarbon concentrations at a minimum of 10 separate flow rates for an SSV flow meter, or at all possible flow combinations for a CFV flow meter, while keeping the flow of propane constant. We recommend selecting CVS flow rates in a random order.

(iii) Measure the concentration of hydrocarbon background in the dilution air at the beginning and end of this test. Subtract the average background concentration from each measurement at each flow point before performing the regression analysis in paragraph (c)(3)(iv) of this section.

(iv) Perform a power regression using all the paired values of flow rate and corrected concentration to obtain a relationship in the form of y = a · xb. Use concentration as the independent variable and flow rate as the dependent variable. For each data point, calculate the difference between the measured flow rate and the value represented by the curve fit. The difference at each point must be less than ±1% of the appropriate regression value. The value of b must be between −1.005 and −0.995. If your results do not meet these limits, take corrective action consistent with §1065.341(a).

(d) Reference flow meter. Calibrate a CVS flow meter using a reference flow meter such as a subsonic venturi flow meter, a long-radius ASME/NIST flow nozzle, a smooth approach orifice, a laminar flow element, a set of critical flow venturis, or an ultrasonic flow meter. Use a reference flow meter that reports quantities that are NIST-traceable within ±1% uncertainty. Use this reference flow meter's response to flow as the reference value for CVS flow-meter calibration.

(e) Configuration. Calibrate the system with any upstream screens or other restrictions that will be used during testing and that could affect the flow ahead of the CVS flow meter, using good engineering judgment to minimize the effect on the flow distribution. You may not use any upstream screen or other restriction that could affect the flow ahead of the reference flow meter, unless the flow meter has been calibrated with such a restriction. In the case of a free standing SSV reference flow meter, you may not have any upstream screens.

(f) PDP calibration. Calibrate a positive-displacement pump (PDP) to determine a flow-versus-PDP speed equation that accounts for flow leakage across sealing surfaces in the PDP as a function of PDP inlet pressure. Determine unique equation coefficients for each speed at which you operate the PDP. Calibrate a PDP flow meter as follows:

(1) Connect the system as shown in Figure 1 of this section.

(2) Leaks between the calibration flow meter and the PDP must be less than 0.3% of the total flow at the lowest calibrated flow point; for example, at the highest restriction and lowest PDP-speed point.

(3) While the PDP operates, maintain a constant temperature at the PDP inlet within ±2% of the mean absolute inlet temperature, T̅in.

(4) Set the PDP speed to the first speed point at which you intend to calibrate.

(5) Set the variable restrictor to its wide-open position.

(6) Operate the PDP for at least 3 min to stabilize the system. Continue operating the PDP and record the mean values of at least 30 seconds of sampled data of each of the following quantities:

(i) The mean flow rate of the reference flow meter, ṅ̅ref. This may include several measurements of different quantities, such as reference meter pressures and temperatures, for calculating ṅ̅ref.

(ii) The mean temperature at the PDP inlet, T̅in.

(iii) The mean static absolute pressure at the PDP inlet, p̅in.

(iv) The mean static absolute pressure at the PDP outlet, p̅out.

(v) The mean PDP speed, f̅nPDP.

(7) Incrementally close the restrictor valve to decrease the absolute pressure at the inlet to the PDP, p̅in.

(8) Repeat the steps in paragraphs (e)(6) and (7) of this section to record data at a minimum of six restrictor positions ranging from the wide open restrictor position to the minimum expected pressure at the PDP inlet or the maximum expected differential (outlet minus inlet) pressure across the PDP during testing.

(9) Calibrate the PDP by using the collected data and the equations in §1065.640.

(10) Repeat the steps in paragraphs (e)(6) through (9) of this section for each speed at which you operate the PDP.

(11) Use the equations in §1065.642 to determine the PDP flow equation for emission testing.

(12) Verify the calibration by performing a CVS verification (i.e., propane check) as described in §1065.341.

(13) During emission testing ensure that the PDP is not operated either below the lowest inlet pressure point or above the highest differential pressure point in the calibration data.

(g) SSV calibration. Calibrate a subsonic venturi (SSV) to determine its calibration coefficient, Cd, for the expected range of inlet pressures. Calibrate an SSV flow meter as follows:

(1) Connect the system as shown in Figure 1 of this section.

(2) Verify that any leaks between the calibration flow meter and the SSV are less than 0.3% of the total flow at the highest restriction.

(3) Start the blower downstream of the SSV.

(4) While the SSV operates, maintain a constant temperature at the SSV inlet within ±2% of the mean absolute inlet temperature, T̅in.

(5) Set the variable restrictor or variable-speed blower to a flow rate greater than the greatest flow rate expected during testing. You may not extrapolate flow rates beyond calibrated values, so we recommend that you make sure the Reynolds number, Re#, at the SSV throat at the greatest calibrated flow rate is greater than the maximum Re# expected during testing.

(6) Operate the SSV for at least 3 min to stabilize the system. Continue operating the SSV and record the mean of at least 30 seconds of sampled data of each of the following quantities:

(i) The mean flow rate of the reference flow meter ṅ̅ref. This may include several measurements of different quantities for calculating ṅ̅ref, such as reference meter pressures and temperatures.

(ii) Optionally, the mean dewpoint of the calibration air,T̅dew. See §1065.640 for permissible assumptions.

(iii) The mean temperature at the venturi inlet,T̅in.

(iv) The mean static absolute pressure at the venturi inlet, P̅in.

(v) The mean static differential pressure between the static pressure at the venturi inlet and the static pressure at the venturi throat, ΔP̅SSV.

(7) Incrementally close the restrictor valve or decrease the blower speed to decrease the flow rate.

(8) Repeat the steps in paragraphs (g)(6) and (7) of this section to record data at a minimum of ten flow rates.

(9) Determine an equation to quantify Cd as a function of Re# by using the collected data and the equations in §1065.640. Section 1065.640 also includes statistical criteria for validating the Cd versus Re# equation.

(10) Verify the calibration by performing a CVS verification (i.e., propane check) as described in §1065.341 using the new Cd versus Re# equation.

(11) Use the SSV only between the minimum and maximum calibrated Re#. If you want to use the SSV at a lower or higher Re#, you must recalibrate the SSV.

(12) Use the equations in §1065.642 to determine SSV flow during a test.

(h) CFV calibration. Calibrate a critical-flow venturi (CFV) to verify its discharge coefficient, Cd, up to the highest expected pressure ratio, r, according to §1065.640. Calibrate a CFV flow meter as follows:

(1) Connect the system as shown in Figure 1 of this section.

(2) Verify that any leaks between the calibration flow meter and the CFV are less than 0.3% of the total flow at the highest restriction.

(3) Start the blower downstream of the CFV.

(4) While the CFV operates, maintain a constant temperature at the CFV inlet within ±2% of the mean absolute inlet temperature, T̅in.

(5) Set the variable restrictor to its wide-open position. Instead of a variable restrictor, you may alternately vary the pressure downstream of the CFV by varying blower speed or by introducing a controlled leak. Note that some blowers have limitations on nonloaded conditions.

(6) Operate the CFV for at least 3 min to stabilize the system. Continue operating the CFV and record the mean values of at least 30 seconds of sampled data of each of the following quantities:

(i) The mean flow rate of the reference flow meter, ṅ̅ref. This may include several measurements of different quantities, such as reference meter pressures and temperatures, for calculating ṅ̅ref.

(ii) The mean dewpoint of the calibration air,T̅dew. See §1065.640 for permissible assumptions during emission measurements.

(iii) The mean temperature at the venturi inlet,T̅in.

(iv) The mean static absolute pressure at the venturi inlet, P̅in.

(v) The mean static differential pressure between the CFV inlet and the CFV outlet, ΔP̅CFV.

(7) Incrementally close the restrictor valve or decrease the downstream pressure to decrease the differential pressure across the CFV, ΔpCFV.

(8) Repeat the steps in paragraphs (f)(6) and (7) of this section to record mean data at a minimum of ten restrictor positions, such that you test the fullest practical range of ΔP̅CFV expected during testing. We do not require that you remove calibration components or CVS components to calibrate at the lowest possible restrictions.

(9) Determine Cd and the highest allowable pressure ratio, r, according to §1065.640.

(10) Use Cd to determine CFV flow during an emission test. Do not use the CFV above the highest allowed r, as determined in §1065.640.

(11) Verify the calibration by performing a CVS verification (i.e., propane check) as described in §1065.341.

(12) If your CVS is configured to operate more than one CFV at a time in parallel, calibrate your CVS by one of the following:

(i) Calibrate every combination of CFVs according to this section and §1065.640. Refer to §1065.642 for instructions on calculating flow rates for this option.

(ii) Calibrate each CFV according to this section and §1065.640. Refer to §1065.642 for instructions on calculating flow rates for this option.

(i) Ultrasonic flow meter calibration. [Reserved]

[70 FR 40516, July 13, 2005, as amended at 73 FR 37305, June 30, 2008; 75 FR 68463, Nov. 8, 2010; 76 FR 57445, Sept. 15, 2011; 81 FR 74165, Oct. 25, 2016]