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SASS 2000 Series Air Samplers

The SASS® 2000 Plus and NEW SASS® 2100 are highly efficient, multiple-effect, wetted-wall cyclone collectors that extract and transfer pathogens from sampled air to a small fixed water volume for subsequent user-defined wet analysis'. A unique and patented feature is their ability to perform this function continuously for an extended time period without fluid sample loss: Fluid sample volume is maintained constant regardless of air temperature or relative humidity using a proprietary liquid inventory sensor and onboard make-up water. This feature is particularly valuable for continuous long-term monitoring and longer sampling periods also provide useful amplification of low-level airborne concentrations.

The SASS 2000 model is ideal for fixed installations where monitoring may take place over an extended time period and the sampler will be integrated with other equipment. The new '2100 model provides '2000 series features as well as a washable inlet filter box. Both series allow grab-samples to be taken for use with lateral flow bioassay tickets- features of particular interest to first responders.

The standard air sampling rate for both units is 265 LPM, while a new brushless blower option (–HS) allows either unit to sample at 325 LPM or more. The standard blower is powered by a high efficiency DC brush-type motor with an estimated operating life of about 8000 hours, while the –HS unit is powered by a brushless motor with an estimated life of about 30,000 hours. Both blower modules may be easily replaced.

Principles of Operation

A schematic of the fluidic and pneumatic systems is shown in Figure 3. A miniature air pump and two microprocessor-controlled valves use pneumatic pressure to move collection liquid from an onboard air bladder-actuated reservoir to the sampler. One valve controls system air pressure while the second meters liquid into the sampler.

Figure 3: Schematic diagram of the pneumatic and fluidic system of the
SASS 2000 Series air samplers.

The samplers have four main sections: a cyclonic cup, stripping column, cistern, and water feedback loop. A high-efficiency DC centrifugal blower pulls air through the unit. On start-up, the blower is activated and a water charge is injected into the cistern section from the fresh water reservoir. Incoming air enters at the cyclonic cup perimeter, creating strong vortex action and a rapidly swirling film of water on exposed surfaces. The water film passes across the air inlet region, forming a water curtain through which the air passes. Concurrently, additional water is injected by a nozzle in the cup base. This water is subjected to high air shear at its central location, and discharges as a fine spray. Sub-micron particles and molecular species are primarily captured in the cup due to the intimate two-phase contact thereby provided.

Sampled air then flows from the cup into a stripping column connected to the cup's upper surface. As air enters the smaller diameter stripping column it increases in rotational velocity, enhancing particulate collection through centrifugal action. The inner surface of the stripping column is wet, and airflow and tube diameter have been selected so that adequate shear force is produced to create vertically upward cocurrent air and water flow. The stripping column is operated beyond the so-called 'flooding limit,' meaning that liquid introduced at the base of the column cannot flow counter to the upwelling air, and, in fact, must flow up the stripping column.

The upward moving film of agent-laden water is captured in a cistern at the top of the unit. From that point, a small diameter tube allows this water to flow back down into the cyclonic cup via the spray nozzle previously mentioned. This air-driven fluid pumping process typically causes the fluid inventory to recirculate from 6 to 20 times for each minute of operation.

The microprocessor continuously senses liquid level with a proprietary fluid level detector attached to the water feedback tube and causes liquid to be added as necessary to make up for evaporative losses (U.S. Patent 6,532,835). When the water inventory detector signals low water, the microcontroller opens the makeup water solenoid for a predetermined period to bring water inventory back to nominal.

Water inventories may be maintained within a range of about 4 cc to 8cc, with an accuracy of a few tenths of a cubic centimeter. Recent innovations allow the optically-based fluid level detector to operate with quite dirty collection solutions. It will function properly with fluids that attenuate 99% of the light impinging on a 1 cm sample, that is, with samples having an optical transmission of only 1% over a 1.0 centimeter path length.

Figure 4 shows the effect of water inventory on the concentration and total number of 1 micron particles in the water phase. From this Figure it can be seen that about 5 cc of water provides optimum performance. The effect of fluid charge is modest as long as very lean, low water charges are avoided.

Figure 4: The effect of water inventory on the total number of particles collected and particle concentration in the water sample. Polystyrene microbeads, 1 micron diameter.

Collection Efficiency

The base 265 LPM SASS 2000 system has been tested at a number of facilities including Dugway Proving Grounds, Aberdeen Proving Grounds, Lawrence Livermore National Laboratory, Battelle Columbus, and the U.S. Naval Research Laboratory (NRL). A test of nine 'personal' sampler designs was conducted at Dugway Proving grounds in April 1997 against airborne Bacillus globigii in a controlled atmosphere chamber. In these tests, the number median aerodynamic spore diameter was 0.9 microns while the mass median diameter was 2.9 microns. At that time, Research International's prototype system tied with two cyclone-based devices for highest concentration factor versus time.

A second set of tests was run at BIO911 in December 1997 in which 11 different samplers were examined. In the period between the two test series, Research International made several improvements to the design that reduced water inventory and made the interior less likely to trap particles. In these tests, the SASS 2000 came in second, performing slightly lower in overall performance than the front-runner, but both it and the first-place system were substantially better than other systems tested. Its favorable performance is particularly notable since the number one system weighed four times as much and consumed 42 times more electric power.

Detailed examinations of SASS capture efficiency have been performed by Lawrence Livermore Laboratory and Battelle Columbus over the past five years. A compilation of these test results, plotted as capture efficiency versus particle size, is provided in Figure 5. Finally, in portable air sampler tests performed by Battelle Columbus in 2004, no other portable unit was found to be more efficient that the SASS series.

Figure 5: Collection efficiency for various particle types at low aerosol concentration.

Samples may be removed from the collector at any time, independent of the air sampling process, using a low-power internal microcontroller-controlled peristaltic pump that provides on-demand delivery of liquid samples to external detectors at a flow rate of approximately 12 cc/minute and at a maximum delivery pressure of about 300 mmHg. Design qualification tests have demonstrated a pump life in excess of 1000 hours of continuous operation. Power consumption is a modest 180 mW at 12 volts DC.

The microcontroller also monitors interior temperature using a small solid-state sensor, controls operation of the centrifugal blower and pneumatic air supply, and supports bi-directional RS-232 communication and control. The basic system weighs only 2.7 kg (6.0 lb) (add 1 kg, or 2.3 lb, for the BA5590/U battery) and measures approximately 19 cm L x 20 cm W x 36 cm H. Specifications for the two models can be found in Tables 1 and 2.

Research International reserves the right to change specifications on any of the devices mentioned without prior notification.

Notes:
1. Consult with factory for other mesh sizes or for a blank screen element if the tube stub air inlet is preferred.

For more information, see:

• SASS 2000 Plus specifications (PDF, 411 KB)
• SASS 2000 Series Collection Efficiency chart (PDF, 31 KB)
• SASS 2000 Parts and Accessories page (photos and part numbers)
• SASS 2100 specifications (PDF, 247 KB)
• SASS 2100 Parts and Accessories page (photos and part numbers)

• Product bibliography
• Link to Federal Supply Schedule price list for SASS 2000 Plus
• A new addition to our product family is the Biolink™ RF serial data link. Biolink™ provides plug-and-play wireless communication between Research International's air samplers/detectors and a PC or laptop. View and print data sheet (PDF, 253 KB).

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