0557-2812-MTDC:How to Conduct Static Tests of Aerial Retardant Delivery Systems, page07

he collection of raw voltage outputs from the instruments is saved in data files. These files contain columns of data where each column represents the output of each instrument. For example, raw data files from a static test that uses two floats, a pressure transducer, and a door potentiometer would contain four columns of data. The number of data points in each column depends on the scan rate and duration. For example, if output was being recorded 100 times per second for 10 seconds, each column would contain 1,000 data points.

Data are collected several seconds before the start and several seconds after the end of the evacuation. The first step in data reduction is to “trim” the raw data files so they contain only the data collected during the evacuation. Using the fill file discussed above, the float data are converted to gallons. The scan rate allows float data to be related to time. The change in volume over time is used to calculate flow rate. A second test team member usually reduces the data that have been collected while the first member collects additional data.

The two methods of flow rate calculation are known as instantaneous flow rate (IFR) and average flow rate (AFR). Table 1, derived from raw static test data, contains time, volume discharged, instantaneous flow rate, and average flow rate.

Both IFR and AFR are calculated by dividing the volume discharged by the time required for evacuation. The IFR is calculated by dividing individual increments of volume discharged by the corresponding time increments (as defined by the scan rate) during the evacuation. The IFR data reveal changes in flow rate while the evacuation is taking place, but the data are sensitive to signal noise and surface turbulence. During the 0.1-second increment between 0.2 and 0.3 seconds into the evacuation (table 1), the volume discharged goes from about 57 to 44 gallons (negative 12.6 gallons discharged) with a corresponding IFR of negative 126 gallons per second. Obviously, fluid did not flow back into the tank. Surface turbulence caused the float ball to rise during this time increment.

Time	Volume Discharged	Instantaneous Flow Rate	Average Flow Rate
0	0	0	NaN
0.1	13	126	126
0.2	57	440	283
0.3	44	-126	147
0.4	44	0	110
0.5	57	126	113
0.6	69	126	115
0.7	69	0	99
0.8	82	127	102
0.9	94	125	105
1	100	60	100
1.1	107	64	97
1.2	113	65	94
1.3	120	63	92
1.4	126	67	90
1.5	132	62	88
1.6	139	64	87
1.7	145	61	85
1.8	151	59	84
1.9	157	65	83
2	164	62	82
2.1	170	67	81

Using the beginning of the evacuation as a reference point, the AFR is calculated by dividing the cumulative volume discharged by the corresponding time. For example, after 1 second of the evacuation (table 1), 100 gallons had discharged with an AFR of 100 gallons per second. The complete evacuation took 2.1 seconds, discharging a total of 170 gallons for an AFR of 81 gallons per second. Typically, the AFR at or near the end of the evacuation is used to report the “flow rate” of a given release and also to qualify tank performance based on IAB criteria.

Figure 9 is a graph of instantaneous and average flow rate data. The IFR curve displays wide oscillations during the evacuation. These oscillations probably are caused by surface turbulence. Often, the IFR curve is smoothed to help interpret flow rate trends during the evacuation. The smoothed IFR curve indicates that the flow rate peaks early in the evacuation and decreases to zero at the end.

Image of a graph displaying the instantaneous and average flows.

Figure 9—Graphs of instantaneous and average flow data recorded from a
single tank release. The IFR curve often is smoothed to remove wide
variations caused by signal noise and surface turbulence. The AFR curve
provides an overall average of the flow rate.

The AFR curve follows the IFR curve, but is smoother because variations in flow rate are averaged. Figure 9 is typical of a low-flow drop from a conventional tank. Throughout the evacuation, the door opening at the bottom of the tank does not change. At the start of the release, the head height is the greatest, producing the highest static pressure at the bottom opening and the highest flow rate. As the tank evacuates, head static pressure and IFR decrease.

Constant flow systems open the doors wider as the tank evacuates, producing an even IFR throughout the evacuation. As the tank evacuates, head and static pressure decrease, but the door opening increases to compensate, keeping the flow rate constant. Significant variation in IFR will cause uneven drop deposition patterns.

The test team maintains a notebook during the test. The notebook contains the date, system tested, drop series and sequences, drop configurations, and comments. The comments document problems encountered with test equipment or the system being tested, observations, and adjustments made to the system during the test. A comprehensive notebook is valuable when writing the final test report.

The test team uses video equipment during the test to measure door operation. Video footage often will reveal anomalies unnoticed at the time of the test. These anomalies can be quantified using video analysis. Video footage is used to measure the door speed, timing of trail drop sequencing, side deflection, residual flows, and internal tank dynamics during releases. Video narration should be included as much as possible. Video images are used as illustrations in test reports.

Data Reduction