Teledyne 4250, Installation And Operation Manual, Page: 42 / 204

4250 Flow Meter
Section 2 Programming
2-14
When the level of the flow stream falls below the Minimum
Depth, the flow meter approximates the velocity readings using
velocity readings taken earlier, when depths were greater than
the minimum. Under these conditions, the flow meter will not be
able to detect the direction of the flow in the channel.
Determine the correct selection for the Minimum Depth by
testing the flow meter for a period of time with the default
selection: 3 inches. In most installations this will be the optimum
selection. If, after examining the velocity data, you see a sharp
decrease in velocity as the level readings approach 3 inches,
increase the minimum depth to 4 inches. You can use the 2-inch
option when the channel produces flows of very low velocity.
Low Profile A-V Sensors – T h e s e s e n s o r s d o n o t h a v e a
program selection for minimum depths. Low profile sensors
attempt to measure velocity regardless of depth. Typically a low
profile sensor provides error-free velocity readings to depths as
low as 1 inch (25 mm).
Note
The MINIMUM DEPTH setting will only appear if you are using
a
standard 10 ft or 30 ft AV probe. This setting will not appear
if you are using a low profile AV probe. The standard probe
cannot sense velocities below two inches of depth. Measure-
ments below the minimum depth are approximations based on
the velocity characteristics of the stream and previous mea-
surements at other levels.
Figure 2-2 Measuring Minimum Depth
If you select ZERO LEVEL OFFSET, the following will appear:
This option lets you mount the probe somewhere other than at
the bottom of the channel. This is useful when there is a problem
with silt or sludge in the center of the channel.
Measure from bottom of sensor when
determining minimum depth setting.
Area Velocity
Sensor
Channel Bottom
ZERO LEVEL OFFSET
OFFSET = 0.00FT