IOtech products used in this application:

• WaveBook Portable High-Speed Waveform Acquisition System • WaveView Out-of-the-Box Software

The WaveBook/561E is ideal for measuring dynamic signals in portable and laboratory applications.

WaveView software is a setup, acquisition, and real-time display program included with all WaveBooks.

Most pyrotechnic formulas, especially for military and industrial uses, are intended for a specific application and primarily produce heat or pressure to accomplish their mission. When used in fireworks displays, however, a pyrotechnic comprises a fuel, an oxidizer, a binder, plus other chemicals that generate the sparks and flames of various colors that we enjoy.

To ensure that the formulas used for mixing the ingredients consistently produce devices that are accurate, safe, and repeatable, manufacturers depend upon special data acquisition systems to acquire critical information during various tests. These data acquisition systems are extremely flexible and typically accommodate a wide variety of sensors for both impulse and steady-state test modes.

For example, one test procedure called “bomb calorimetry” determines the enthalpy of combustion. A companion test called “closed-bomb testing” measures pressures generated within a certain time period. And the test can be done in a number of different ways. For example, technicians can establish the burn times of pellets. Although the pellets have various odd shapes, including holes, the size and density of the pellets largely determine the burn time. For some applications, a quantity of pellets becomes a propellant when they are enclosed in a chamber and ignited. Technicians then record the closed bomb curves and compare them to an equation that the government supplies for evaluating burn time based on the length of the pellet. When ignited in a container, they typically burn from end to end, or from both ends to the center. The test team can determine the instant when the output pressure reaches its peak value. Here, the fuel has been completely consumed, and the descending slope of the wave represents the slowly burning remnants.

Another application for pyrotechnics is in oil-well fields where the wells have stopped producing petroleum at the normal rate because of plugged rock strata. Since the wells are expensive to drill and maintain, the investors cannot afford to wait for them to rejuvenate on their own, so they are primed with what’s called perforator charges. An array of charges are deposited in the wells to a depth of about three feet and ignited. The charge perforates the rock strata in different directions and releases the trapped oil.

There are several US companies that manufacture a wide variety of pyrotechnical products. They have a number of different facilities for assembling and testing the products, from buildings that contain several diverse setups for testing relatively small devices to outdoor ranges for testing parachute flares, M2 and M16 grenade launchers, and larger military devices.

One such company makes or assembles products that an agency of the US government has designed. For example, it makes the component of a bullet that has a controlled burn rate and ensures that it meets the specs exactly. Precision is needed because out-of-spec parts have been known to self-destruct before hitting the target and causing damage where it was not wanted.

“Our company tests many different kinds of devices, including practice diversionary charges,” says the technical director. “Here, we place a photodiode near the test specimen and four transducers around it, spaced 90°, to measure the acoustic blast. When the firing pin comes around the test specimen, it hits a device that triggers our data acquisition system, an IOtech WaveBook, which begins collecting the data.”

First, the primer fires, and a few milliseconds later, it kicks off the fuse. It does not become a projectile, however, it just separates into several parts about 20 feet away and emits light, which the photodiode picks up. “We monitor the four pressure transducers on individual channels,” continues the director, “so we can see the output amplitude, and the time that the output peaks. We record the time from the initial trigger to the peak and measure the amplitude of the peak, which we translate into dB.”

A telephoto lens is aimed at the light output from the event, which is about 30 to 40 feet away. The photodiode is located at the focal point of the lens, a surface where the plane of the film would be on an attached camera. The photodiode senses the light and feeds the signal back to the WaveBook. The speed of sound measured is based on several factors: The relationship between the flash of light at the photodiode, the pressure created at the pressure sensors from the sound wave, the distance above the concrete pad, and the distance between the device and the overpressure detector. The WaveBook measures the time from the flash of light to the primary sound wave and to the reflected wave.

“We use 4 channels for measuring the overpressure, plus one channel for the light output, which also serves as the trigger — to conserve the number of channels,” says the director. The transducers are secured to low profile, steel brackets of narrow cross-section, about 2 to 2.5 feet long, located 3 to 4 feet away from the test specimen. The brackets have a pointed nose and hold a sensor about halfway back so the blast wave first sees the nose of the transducer body. The pressure transducers pick up pressures of 3 to 5 psi. This design lets the signal be monitored without interference.

The present test setup evolved after some trial and error work. “Initially, we set the test specimen about one foot above a concrete pad,” says the director. “We measured extremely high pressures and couldn’t figure out why. Someone proposed the theory that the wave reflected off the concrete pad and reinforced the main wave. Eventually, we proved that to be the case. We then elevated the test specimen 3.5 to 4 feet. The primary wave still travels downward, hits the ground, and reflects, but it’s delayed long enough that we can isolate the reflected wave. We see the initial peak when the blast wave reaches the transducers, then we see the blast wave go up and back down, and several milliseconds later, we see another smaller wave, the reflected wave.”

Before the company purchased the IOtech WaveBook, it used one of the first 4-channel digital oscilloscopes available. But the ‘scope had limited storage capabilities, and no computer interface; it saved everything on a 5.25 inch floppy diskette. “We tried a standard analog ‘scope,” says the director, “but it could not accomplish what we needed. We even tried to trigger multiple ‘scopes simultaneously, but to no avail. By comparison, the WaveBook is basically an 8-channel scope and can be expanded to many more channels than we could possibly use.”

The director developed an automated spreadsheet to present the data. He simply pulls up the ASCII file on the WaveBook and loads it into the spreadsheet. It displays a graph on the screen that shows the wave, reduces some of the data automatically, and lets him do a better job of giving the customer the information it requested.

Moreover, the WaveBook does the conversion from mV to psi. There is no need to calibrate the peak of the photodiode; it is strictly a time hack. “We use either pressure transducers to measure other pressures defined in psi/mV, or we let the WaveBook do it,” says the director. “We put the conversion into the WaveBook and use that extensively. But anything else that has to be calculated is accomplished with the spreadsheet.”

The director didn’t have nearly as tight a time per point available using the analog scope, but the bigger problem with it was that it didn’t have the number of points capability that the WaveBook has. And he is sampling every 10 ms over 4 to 5 channels, which records a lot of data. “When I was running the earlier system,” he says, “we never would have considered that such capability existed. I bought one of the first WaveBooks, and it had been in service until a lightning strike took it out. Then I replaced it with the newer WaveBook/516E. This is a big cost advantage; we are competitive and we stretch every dollar. We don’t do a lot of research, but one objective for engineering is to make the most of its resources. I take that to heart.”

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