Biomedical Engineering Reference
In-Depth Information
Once this hardware was transferred to measure velocity in the pneumatic
gun, data was collected electronically with the Tektronix digital oscilloscope.
For this application the magnitude of voltage change due to change in light
intensity was not of interest, only the voltage rise. Therefore, the calibration of
the sensor is not important. The rise time of the sensor, whose consistency
could be a factor if there is variation from one sensor to another, is stated as 43
ns. Variations in rise time are not stated in the manufacturer's literature for
these photodiodes. However, it can be shown that even with a rise time
variation of 50% from the stated value, the error in velocity measurement will
be less than 0.2% at the highest recorded velocities. For all velocity
measurements except the final actual shots in which metal particles were fired
at the substrate, plastic balls of 6mm diameter were used for the purposes of
consistency and to conserve the amount of metal particles, of which the
amount was limited. Pressure data recorded for the subsonic pneumatic gun
was recorded manually by reading the pressure gage mounted onto the air
tank; according to the manufacturer, the pressure gage has a resolution of 1 psi
and an accuracy of +/- 3% full span. To determine the precision of the
subsonic pneumatic gun, thirty shots were fired consecutively while the
pressure was held constant. The results can be seen below in Figure 53. Based
on this data, standard deviation was calculated to be 31 ft/s. If the two extreme
sample points were considered to be outliers, the standard deviation would
decrease to 10 ft/s. Pressure on the supersonic pneumatic gun was monitored
only visually by manually watching the regulator and may have resulted in
errors as large as +/- 10%.. Therefore, pressure data for the supersonic
pneumatic system is used only as an approximate reference for the burst discs.
Speed at 90 psi
750
700
650
600
550
500
0
5
10
15
20
25
30
35
sample number
Reproduced with permission. Copyright retained by Inderscience Publishers.
Figure 53. Velocity Distribution of Subsonic Pneumatic Gun at Constant Pressure.
