Alignment with Flow and Depth
For the calibration to be valid, the transducer needs to be horizontally and vertically aligned with the flow. While Ultraflow QSD 6537 instruments are calibrated pointing into the flow, they can be pointed downstream with little loss of calibration accuracy. You may want to do this when fouling of the sensor face is a problem. Any angled flow in the horizontal plane will reduce the recorded velocity.
The Ultraflow QSD 6537 instrument must be mounted in the water parallel to the surface for depth readings to measure accurately (~ +/- 10 deg), if not the depth may read inaccurately and therefore the recorded depth can be recording incorrectly.
Instantaneous Versus “Averaged” Velocity
When you observe Ultraflow QSD 6537 velocities, they will be seen to vary by 10% or more from scan to scan at some sites. Because Ultraflow QSD 6537 is very sensitive to variations in velocities, you are able to see the natural velocity changes in the channel. Although the discharge in a channel may be reasonably constant for a period of time, the velocity distribution is always changing. Different velocity streams wander from side to side and bed to surface as they progress down the channel. Turbulent swirls and eddies are carried downstream for long distances while they slowly decay. Hydrographers will be used to having this action partly removed by the mechanical inertia of a current meter and the period over which a typical measurement is timed. However all will have noticed that the rate of revolutions of the current meter varies during the timing period. Continual velocity logging at one location with a Ultraflow QSD 6537 will show these cyclic velocity pulsations. The characteristics will be different for different sites and will vary with discharge. Cycles will typically include short period fluctuations (a few seconds) overlaid on longer cyclic fluctuations (up to many minutes). Longer term pulsations may also be seen particularly in larger streams when in flood.When comparing Ultraflow QSD 6537 velocity and mechanical current meter readings, the display should be observed long enough to estimate the mean of the readings. The Ultraflow QSD 6537 will do most of this processing internally but if an external logger is being used to record the readings averaging could also be done here this will help attenuate short frequency variations.
Conversion of Logged to Mean Velocity
The measured velocity data may have to be adjusted during post processing to reflect a mean velocity for the channel. The factors used will be site specific and have to be determined by the operator. This is done by obtaining a mean channel velocity by conventional techniques and comparing it with the average logged velocity. If necessary this process should be repeated at various discharges
Where the relationship is complex or unstable, the accuracy of this method is compromised.
In laminar flow conditions the channel mean velocity could be expected to be between 90% and 110% of the logged velocity.
In small channels (say a 500mm diameter pipe) the factor may be close to 100% as a representative area of flow will have been “seen” by Ultraflow QSD 6537 and contributed to the logged velocity.
In larger channels only the area adjacent to Ultraflow QSD 6537 will be “seen” and the relationship will depend on how this portion relates to the vertical and horizontal velocity distribution in the channel. An instrument located in the centre of the stream would normally be in a higher velocity area. However in a deep channel Ultraflow QSD 6537 may only see the slower portion of the velocity profile.
Post time: Dec-02-2022