Methods Comments
The general information below includes comments on sampling techniques, compensation for
temperature and pressure, number of passes required, and accuracy and speed. To summarize, the
3-D probe is used to determine the velocity pressure and the yaw and pitch angles of the flow velocity
vector in a stack or duct. The method determines the yaw angle directly by rotating the probe to null
the pressure across a pair of symmetrically placed ports on the probe head. The pitch angle is
calculated using probe-specific calibration curves. From these values and a determination of the
stack gas density, the average axial velocity of the stack gas is calculated. The average gas
volumetric flow rate in the stack or duct is then determined from the average axial velocity. Other US
EPA Methods may be mentioned and can be found at www.epa.gov/ttn/emc/promgate.html . A copy of
the final rule that details the methodology for Methods 2F, G & H is available from the US EPA
website at http://www.epa.gov/airmarkets/monitoring/flow/rule.pdf
It is important to inspect the pitot head for any damage and all equipment for proper operation prior to
sampling. Auditing and calibrating the electronics is also required. Please see the
Calibration/Recalibration in this section for assistance. It is equally important to document these
steps. Forms are included in the appendix of this manual as an aid.
Method 1
The first step required is to perform US EPA Method 1 to determine the traverse point locations on the
probe. Mark the probe. See
Figure 10
Method 2
Determine the atmospheric pressure at the sampling elevation during each test run following the
procedure described in section 2.5 of Method 2.
Method 3
Determine the stack gas dry molecular weight. For combustion processes or processes that emit
essentially CO2, O2, CO, and N2, use Method 3 or 3A. For processes emitting essentially air, an
analysis need not be conducted; use a dry molecular weight of 29.0. Other methods may be used, if
approved by the Administrator.
Method 4
Determine the moisture content of the stack gas using Method 4 or equivalent.
Method 2F Axial Velocity Determination
Yaw angle measurement may be obtained in two alternative ways during the field test, either by using
a yaw angle-measuring device (e.g., digital inclinometer UINC-3D-##) affixed to the probe, or using a
protractor wheel and pointer assembly (optional). For horizontal traversing, either approach may be
used. For vertical traversing, i.e., when measuring from on top or into the bottom of a horizontal duct,
only the protractor wheel and pointer assembly may be used.
At each traverse point, yaw-null the probe. Rotate the probe until a null differential pressure reading
(the difference in pressures across the P2 and P3 pressure ports is zero, i.e., P2 = P3) is indicated by
the yaw angle Magnehelic® pressure gauge. The angle displayed on the angle-measuring device is
considered positive when the probe’s impact pressure port (as viewed from the ‘‘tail’’ end of the
probe) is oriented in a clockwise rotational position relative to the stack or duct axis and is considered
negative when the probe’s impact pressure port is oriented in a counterclockwise rotational position.
See
Figure 11
for a visual representation.
Revision:01 Page 22 of 23 9/26/2017