Dynasonics DDFXD Doppler Ultrasonic Flow Meter - Manual (Page 47)

06-DPP-UM-00147 02/12 47
When the unit is powered up, there is a delay before the unit begins transmitting sound into the pipe.
During this time, the signal strength is measured and a base signal level is obtained. Typically this is a
value of about 20. The unit measures flow by measuring the Doppler frequency shift. The frequency shift
is approximately 70 Hz per foot per second. For every foot per second increase in velocity, the signal
strength should increase by 1. The unit automatically adjusts the gain and selects the proper hardware
filter for the measured velocity. The control can be observed when the DISPLAY mode is set to DIAG. See
Figure 4.4.
RUN
Frequency
Gain
Signal
Strength
FIGURE 4.4 - DIAGNOSTIC DISPLAY
MANUAL OPERATIONS
When NORMAL is selected, the unit will automatically control the gain and front end hardware filter for
optimum measurement of the Doppler signal.
Select HIGH for applications where the unit reads flow rates consistently, but much lower than the actual
flow rate. This may be required when sound is not getting through the pipe as well. Selecting HIGH will
cause the unit to look for the signal strength to increase by 2 for every foot per second increase in flow
rate. Basically, the gain is doubled, but still automatically controlled.
For applications where the flow is constant, but you may need to tune the unit to filter out extraneous
noise, select the MANUAL mode. Typically, this would only be required at very low flow rates. When MAN-
UAL mode is selected, the GAIN POT and FILTER settings are manually set. Automatic control is disabled.