GASP Test Procedure and Instruments

The heating procedure that has been adopted for most GASP testing is designated the ‘fast ramp.’ Fuel samples are placed into a 9 cm alumina boat (at room temperature) which is

weighed before and after heating. Starting with a furnace temperature of 200 °C and 4.4 lpm

airflow through the tube furnace, the sample boat is inserted into the middle of the furnace tube.

The furnace controller is set to 640 °C for rapid heating to the set point at a rate of approximately

136 °C/min which typically takes 360 to 390 seconds.

Aging of the smoke in the smoke chamber cannot not be avoided, so GASP aerosol

measurements are of mixed fresh and aged smoke. This is a realistic fire scenario, particularly

on the ISS, where the absence of buoyant flow will cause smoke to concentrate at the source and

undergo aging, while fresh smoke is continually emitted. In some tests, the effect of humidity on

the gaseous products was explored and consequently there were some runs with the inline

bubbler providing humidity to the air flow through the furnace. It is documented, and past

GASP tests have shown that moisture promotes the production of some acid gases (Babushok et

al. 2015, Meyer et al. 2013).

3.8.1 DustTrak DRX Monitor

Aerosol photometers measure mass concentration by sensing the combined laser light

scattered from many particles at once. The DustTrak™ DRX Monitor (Model 8533, TSI Inc., Shoreview, MN) combines ensemble scattering with single particle detection to provide real-time

size-segregated mass fraction concentrations up to 150 mg/m3. In GASP testing, smoke aerosols are predominantly below 1 μm in diameter; however, it is of interest in fire characterization to determine which fuels generate particles above this threshold. The DRX can detect particles

between 100 nm and 15 μm and, thus is not sensitive to ultrafine particles. Although photometers can be used at high mass concentration, smoke aerosols in these tests required significant

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dilution with HEPA filtered air. The DRX was calibrated with lamp wick smoke to better

correlate its output to aerosol mass concentrations of typical GASP fuels.

3.8.2 Water Condensation Particle Counter

The Water Condensation Particle Counter (WCPC, Model 3787, TSI Inc., Shoreview, MN)

measures aerosol number concentration. This device operates by initially cooling the aerosol

sample and then passing the sample stream through a region of supersaturated water vapor where

the particles grow as water condenses on them. The particle-containing droplets pass through a

laser and scatter the light, creating a pulse, which is detected and counted. This device has an

extended single particle counting range with continuous, live-time coincidence correction

allowing measurement from 0 to 250,000 particles/cm3 at sizes from 5 nm to 3 μm in diameter. Substantial dilution with HEPA filtered air is required, since the smoke concentration is higher

than the upper counting limit. The response time of this instrument to a step change in aerosol

concentration is on the order of 0.75 second. Placement of the WCPC and DRX outside the

smoke chamber and dilution equipment adds to the path length the aerosol must travel before

being sampled. The delay in the instrument response attributed to tubing length between the

smoke chamber and the instrument inlet is 0.02 seconds and 0.06 seconds for the DRX and the

WCPC, respectively.

3.8.3 Sharp Dust Sensor

A Compact Optical Dust Sensor (model GP2Y1010AU0F, Sharp Corporation) was tested for

the purpose of comparing time to detection of smoke from common spacecraft materials. This

commercial-off-the-shelf (COTS) device uses an infrared emitting diode and a phototransistor to

determine the presence of aerosols passing through a hole in the center of the 4.6 x 3 cm device,

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calibrated to aerosol mass concentration (mg/m3) with a sensitivity to 0.1 mg/m3. The Sharp sensor is shown in Figure 3.14.

Figure 3.14: The Sharp Compact Optical Dust Sensor which weighs 16 grams.

The calibration used for the GASP data is from Yang et al. 2015, which reported that the

sensor output is strongly material dependent, and provided a correlation to mass concentration of

incense smoke. For the purpose of this preliminary study, this calibration is assumed to be

adequate, however, if the device is selected for further studies, it will be calibrated to one or

more representative smoke aerosols outlined in this work. Three of the sensors were placed

inside the smoke chamber, attached to the wall cantilever-style with double-sided tape. This

configuration oriented the central smoke path through the sensor in the vertical direction. Figure

3.15 shows the three sensors mounted on the interior wall of the smoke chamber. The

electronics accompanying the sensors were placed in the ante-chamber of the glove box to

protect them from exposure to the smoke and acid gases. There was no forced convection

provided for smoke through the device other than what was produced by the mixing fan in the

smoke chamber. The Sharp sensor has a much lower measurement threshold than other aerosol

instruments, however, the main interest for this study is the early response of a third moment

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Figure 3.15: Positions of the three Sharp sensors mounted on the interior wall of the GASP smoke chamber.

While the GASP laboratory has an SMPS to measure particle size distributions, it is not

suitable for tests performed with the fast ramp furnace profile, as smoke concentration is

typically too high, and 2 min scan is too long for the dynamic conditions of aging smoke in the

chamber. The 55-gallon drums used in the SAME ground-based experiments are necessary for

smoke particle size distribution measurements, in order to dilute sufficiently to stop coagulation

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