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How to Conduct Static Tests of Aerial Retardant Delivery Systems
Flow data are collected from a series of drops that include all possible system drop configurations. Float output ranges from 0 to 10 volts. Voltage is highest when the ball is closest to the sensor. Voltage decreases as the ball travels to the lower end of the rod. In other words, voltage is high when the tank is full and falls as the tank is evacuated. While the tank is being evacuated, the data logger software records the voltage output of all the instruments at a selectable scan rate. Typically, a scan rate of 100 times per second is used. Immediately after each release is complete, the acquisition program displays a graph of voltage output over time. This display provides an initial interpretation of tank performance. The voltage curve on the display should drop off evenly as the tank is evacuated.
Effects of tank geometry may cause the curve to be nonlinear. For example, if the tank is wide at the top and narrows at the bottom, the voltage curve will not be as steep at the top and will be steeper at the bottom. Because the fill file will contain the same nonlinearity, the calculated volume discharged will be accurate.
If the curve is irregular or contains a sine wave pattern (figure 8), causes other than the tank geometry may be generating the irregularities. In such cases, the problem must be investigated. A visual inspection, including a video record of the fluid surface during the evacuation, often will reveal the problem. The solution may be to move the float to a different location where the change in fluid height more accurately represents the flow from the tank.
Figure 8—This raw voltage trace indicates uneven
float movement due to
surface turbulence
during evacuation.
After the test team collects and analyzes data from the first series of drops, the team can determine whether adjustments or modifications are needed to flow rates, door movement, or internal pressures. Often, the operator can make immediate adjustments and testing can continue. As adjustments are made, data are collected and analyzed to determine the effect of the adjustment. Additional adjustments may be needed depending on results. It is not unusual to repeat a drop configuration several times while adjusting the system to achieve the desired effect.
Once all the necessary adjustments have been made, another drop series is collected for all configurations. This drop series quantifies tank performance in its final design configuration. If no problems are encountered during this drop series, a replicate drop series is collected to establish reproducibility. If no problems are encountered after the second series has been completed, and the two series meet IAB reproducibility requirements, data collection is complete.
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