DigitalFlow™ XGM868i Startup Guide 69
Appendix C. Measuring P and L Dimensions
Appendix C. Measuring P and L Dimensions
C.1 Introduction
When programming the PIPE menu in the Model XGM868i’s User Program, the path length (P) and the axial
dimension (L) must be entered. These parameters are determined by measurements on the actual transducer
installation, with P equal to the face-to-face distance between the transducers and L equal to the axial distance between
the centers of the transducer faces.
The accuracy of the programmed P and L values is critical to precise flow rate measurements. If GE supplies the
flowcell for the system, the correct values will be included in the documentation supplied with the system. For
transducers installed on an existing pipe (see Figure 22 on page 70), P and L must be measured at the site. This
appendix provides instructions for properly determining these dimensions.
C.2 Measuring P and L
Whenever possible, physically measure the face-to-face distance (P) and the axial distance (L) between the centers of
the flat faces of the transducers. Refer to Figure 22 on page 70 for an illustration of the proper distances to measure, in
a typical installation.
In some situations only one of the required distances can be directly measured. When this happens, a knowledge of the
installation angle () of the transducers, permits the second distance to be calculated from Equation C-1
(C-1)
As an example, assume that the transducer installation angle is known to be 45° and the L distance is measured to be
10.00 inches. Then, the P distance is calculated to be P = 10.00/0.707 = 14.14 inches.
With a Bias 90° transducer installation, it sometimes happens that the only known parameters are the transducer angle
() and the centerline distance between the transducer bodies (CL). In these cases, it is still possible to calculate P and
L by combining Equation C-1 with the additional Equation C-2 (see Figure 22 on page 70):
(C-2)
Standard GE 90° transducers have the face offset from the centerline of the body by 0.6 inches. Thus, a pair of
transducers has a total offset of 1.2 inches, as indicated in Equation C-2. For example, suppose that the transducer
installation angle is 30° and CL is measured to be 12.00 inches. Then, P = 12.00 – 1.2 = 10.80 inches and L = 10.80 x
0.866 = 9.35 inches.
cos
L
P
---=
P CL 1.2–=
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