GE DigitalFlow GS868, Startup Manual

Page: 73 / 81

Measuring P and L Dimensions D-1
March 2008
Introduction
When programming the
PIPE
menu of the Model GS868’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 the factory supplies the flowcell for the
system, the correct values will be included in the documentation
supplied with the unit. For transducers installed on an existing pipe
(see Figure D-1 on page D-2), P and L must be measured at the site.
This appendix provides instructions for properly determining these
dimensions.
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 D-1 on page D-2 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
(
θ
) of the transducers, permits the second distance to be calculated
from Equation D-1 below:
(3-1)
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 (
θ
) and the centerline
distance between the transducer bodies (CL). In these cases, it is still
possible to calculate P and L by combining Equation D-1 above with
the additional Equation D-2 below (see Figure D-1 on page D-2):
(3-2)
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 D-2 above. 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.
θ
cos
L
P
--- =
P CL 1.2 – =