SASS^® 2300
Wetted-Wall Air Sampler

 

INTRODUCTION

The SASS 2300 is one of our most exciting portable aerosol particle collection systems. It employs a highly successful wetted wall aerosol collection method that has received U.S. Department of Homeland Security Certification under the U.S. Safety Act of 2002. Extremely long sampling protocols of up to several days are supported. Sample fluid volume is maintained constant over the entire collection period, independent of ambient air temperature or relative humidity, providing unsurpassed monitoring capabilities in environments ranging from farmyards to hospitals to battlefields. In addition, the units can be controlled wirelessly and users can download their own customized sampling protocols into the units.

This is the only wet-type air sampler technology that has been shown able, in concert with real-time PCR, to efficiently collect and identify airborne viruses. It has, for example, been successfully used to detect the airborne viral pathogens that cause exotic Newcastle disease and hoof-and-mouth disease, as well as strains of avian flu virus*.

* Sharon K. Hietala, et al., J. Vet. Diagn. Invest., 17, 198 (2005). Printed with permission of AAVLD, May 6, 2009.

• Download the SASS 2300 Data Sheet

OVERVIEW

The SASS 2300 collects both particulates and water-soluble chemical vapors from the air. These materials are extracted from sampled air and trapped in a small volume of liquid that can be removed, in part or in total, at any time for analysis. Distilled water is typically the sample liquid of choice- no additives or surfactants are required for high efficiency. Trace aerosol concentrations can be amplified by extending the sampling time to hours or more.

The SASS 2300 draws air through a convenient threaded adapter on the unit's exterior. This adapter allows the mounting of useful accessories such as flexible intake tubes, filters and nozzles. A built-in peristaltic pump can be used to transfer liquid samples to an external analyzer for immediate analysis, or to a sample vial filling station integral to the unit. The filling station is convenient for dispensing all or a part of the liquid sample into a dropper bottle of the type used with lateral flow bioassay tickets.

The air sampler is microcontroller-based and can function as a stand-alone unit or connected to other sampling, detection or communication systems via RS-232 or wireless link. Purpose-designed software allows for streamlined integration with Research International's RAPTOR and BioHawk biodetectors. The extensive use of microcontroller-based circuits allows overall system operating characteristics to be easily tailored to customer requirements. Reprogramming of sampler operation may be performed at any time over the RS-232 link without having to disassemble the unit.

Electric power consumption is minimized by operating the units blower at peak electric-to-pneumatic efficiency conditions and by using natural airflow through the cyclone structure to drive water re-circulation. Weight and power consumption figures are far below other aerosol collection systems of comparable sophistication and collection efficiency, and may be further reduced for specialized applications such as Unmanned Air Vehicle (UAV) applications.

OPERATING PRINCIPLES

A schematic representation of the SASS 2300 is shown in Figure 1. The cyclone has four main sections: a cyclonic cup, stripping column, cistern, and water feedback loop. A high-efficiency, brushless centrifugal blower at the cyclone exit pulls air into the unit. When the SASS is turned on, the blower is activated and a water charge injected into the cyclonic cup from an on-board fresh water reservoir. Incoming air enters at the 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 air must pass.

Figure 1: SASS 2300 fluid management system.

Concurrently, a centrally located nozzle projecting from the cup base injects additional water. This location is subject to high air shear and fluid discharged from the nozzle into the cup will be in the form of a fine spray. The cup plays a major role in collection of sub-micron particles and molecular species due to the intimate two-phase contact provided, whereas larger respirable particles are captured in both the cup and a stripping column to be described.

The air stream then flows from the cup into a stripping column connected to the cups 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 also wet by fluid outflow from the cup. The airflow rate and column inside diameter have been designed so that adequate shear force is produced to create a co-current upward flow of water on the column wall. The stripping column is operated beyond the so-called "flooding limit," meaning that liquid introduced at the base of the column cannot flow opposite to the upwelling air, and, in fact, must flow up the stripping column.

Water flows from the stripping column into a larger diameter cistern section located above the tube. Due to parasitic shear forces created by the rotating airflow, water transitioning from the tube to the cistern is flung outward to a water trap zone where a rapidly rotating water ring is formed. Water in the ring flows back to the cyclonic cup by way of a liquid feedback tube. This water is re-injected into the cup via the spray nozzle, where it is once again available to collect additional particulates. Fluid re-circulation rates have been measured to be in the range of 30 to 100 cc/minute. This means a typical 5 cc water inventory is re-circulated through the unit from 6 to 20 times per minute.

Aerosol collection characteristics are similar to those of the SASS 2000, but with significant improvements in particle retention over long collection periods. Figure 2 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 2: The effect of water inventory on the total number of particles collected and particle concentration in the water sample. Polystyrene microbeads, 1 micron diameter.

The similar 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 2000 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 3. 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 2000 series.

Figure 3: Collection efficiency for various particle types at low aerosol concentration.

Fluid Control Subsystem

The particle extraction process involves intimate mixing of incoming air with re-circulating sample water. This water would be lost in a short time if no attempt were made to compensate for evaporation. To prevent dry out, liquid volume is monitored with a proprietary sensor attached to the water feedback tube. When the sample water inventory falls below a preset level, a microprocessor-controlled peristaltic pump meters a small amount of clean water into the re-circulation loop from an onboard water reservoir to bring the level back to set point. Water inventories can be maintained within a range of about 4 cc to 5 cc with an accuracy of a few tenths of a cc for periods of hours to days (U.S. Patent 6,532,835). Since the re-circulation loop does not use a mechanical pump, power consumption is minimized, delicate organisms are not damaged, and cleaning is simplified.

Samples may be removed at any time using an onboard peristaltic pump. When the unit is operated manually, air flow is stopped during the sample transfer process to allow fluid films to collect as a pool in the bottom of the unit. If the unit is being operated remotely via the serial digital link, air sampling may continue during the sample transfer process. The user can in either case elect to discharge the sample to an onboard sample vial or to a discharge spigot at the rear of the instrument.

The distilled water used by the SASS for sample collection is not generally compatible with analytical procedures used to identify any captured biologicals; most require some buffer such as phosphate-buffered saline. Sample vials for the SASS are provided with a lyophilized buffer that reconstitutes when the sample is pumped in to provide a liquid ready for direct application to lateral flow immunoassays such as the DoD Hand-held Assays (HHAs) or equivalent. Please see our Application Note, “Suitability of SASS 2300 Sample Vials and SASS 3010 Sample Vials for use with Hand-Held Assays,” for description of our test procedure.

Electronics

The microcontroller-based SASS 2300 has been designed to communicate with and be controlled by various triggers, bioanalyzers and data collection networks via its RS-232 serial data link. System firmware is located in flash memory and can be modified, customized and upgraded electronically at any time. There is no need to disassemble the unit. A program that operates under Windows^® is supplied with the instrument that allows the SASS 2300 to be operated from a PC. This program also allows users to change a large number of default settings resident in flash memory, such as fan speed and liquid inventory- essentially customizing operation to their purposes.

Sampling Protocols

Customizeable Firmware

Users can modify to their liking how the SASS 2300 collects and processes samples using the PC-based software supplied with the system. User-defined protocols can be stored in a PC and the sampler operated from that PC or the protocols can be downloaded to the 2300’s memory. The 2300 will then follow the downloaded instruction set automatically each time it is turned on, even if not connected to a PC.

Parameters are set on the Automatic Mode screen with radio buttons and text boxes as shown in Figure 4. For example, selecting Fan off while pump on will cause the fan to turn off while liquid is being pumped out of the cyclone. If you instead choose Fan on while pump on, the fan will continue to run while the cyclone is being pumped out.

Figure 4: PC-based window for customizing the collection protocol.

In the upper right hand corner of the Automatic Mode screen is the Delay Before Starting First Cycle text box. A value placed in this box will cause the air sampler to pause before starting the first sampling cycle. This is useful for example if you have stirred up dust during set-up and want the dust to settle before sampling starts. Other text boxes allow you to set how long the fan is on; the amount of time the sample discharge pump is on; the amount of pause time there is between samples; and the number of sampling cycles. Alternatively, setting a desired number of total cubic meters of sampled air will define the fan time per collection cycle.

The protocol defined by the Automatic mode setting is started by pressing the RUN button. This also downloads and stores the settings to the 2300. It will now be the default procedure executed each time the 2300 is used as a stand-alone system in the field or when connected to the PC.

For example, the selected Automatic Mode settings in Figure 4 cause a 1 hour delay before the fan does an initial 10 minute aerosol sampling. The fan then shuts off and the onboard peristaltic sample transfer pump runs for 40 seconds, transferring sampling fluid to an onboard storage vial or to an external collection device (user selectable). A cleaning cycle 240 seconds in length is then executed, as explained in the next paragraph. The fan does not run for another 2720 seconds, after which the sampling pattern is repeated four more times. At the end of the fifth sampling cycle the system will stop and no further action will occur: A total of 16.25 cubic meters of air will have been processed.

The Cleaning group of controls shown in the bottom of Figure 4 allows the user to run an automatic cleaning protocol at the end of each fan/pump cycle. This is a procedure wherein the sampler is repeatedly flooded with excess fluid and the fan used to circulate this fluid before it is pumped out, thereby flushing contaminants collected during the previous sampling cycle. If no cleaning cycle is needed, simply enter ‘0’ in the Number of cleaning cycles box.

The Cleaning Routine Settings group allows you to set the parameters for this routine. The Cleaning initial fill time allows the user to set the amount of time that the cyclone is filled with fluid. This fill time can be set in tenths of seconds and occurs immediately upon the fan starting. The Cleaning fan run time and the Cleaning pump run time allow the user to set the fan and pump time for the cleaning routine. The Cleaning number of cycles allows the user to repeat the fan/pump cycle a number of times. Typical values for a cleaning protocol are shown in the Figure.

Once all parameters have been set as desired, there are several options:

• The Run button allows you to run the protocol directly from the PC. In this mode, the remaining time to complete the sample protocol is shown in the Count Down Timer window. The PC can be connected to the SASS via an RS-232 port; by a USB port using a USB-to-RS232 adapter; or by a set of Research International’s wireless BioLinks. They are particularly convenient for remotely controlling one or several 2300s, and eliminate the need for any wires between the PC and the 2300s. These mount directly to a USB port. Antennas are available with ranges from 100 to 1000 meters.
• The Save to PC button transfers the selected settings to a file on the PC. This and other stored protocols can be reloaded at a later time using the Load From PC button. This allows several different collection protocols to be saved and used as needed.
• The Load from SASS button allows you to see what protocol is currently loaded into the SASS.
  

Expert Users

For users who need even more control over the SASS 2300 or need to integrate it into a proprietary system, the Operating Manual provides the user with a set of over 60 ASCII commands that can be sent over the serial data link to either control or interrogate the sampler.

Main Window for Remote PC Operation

If the SASS 2300 is being operated remotely from a PC by wire or wireless connection, the main program window in Figure 5 continuously shows key operating parameters associated with the sampler. A large graph displays the liquid sample volume circulating in the cyclone. The vertical labels on the left side of the graph represent the raw counts from a liquid level detector inside the SASS 2300 while the vertical labels on the right side represent the corresponding milliliters of liquid circulating in the cyclone. The signal from the liquid level detector is displayed in red and normally oscillates about the set point that is represented by a blue horizontal line.

Figure 5: Main SASS 2300 Windows software screen showing the unit operating in Collection Mode.

The upper left of the main window shows the Set Point information. The set point is the desired operating volume for the cyclone that the microcontroller-driven electronics in the SASS 2300 maintains while the fan is running, by adding make up fluid as it evaporates or is pumped out.

To the right of the Set Point group in Figure 5 is the Detected Value group of controls. These controls display the reading from the internal liquid level sensor that is used to maintain the liquid volume in the cyclone. This is the value plotted by a red line on the graph.

The Status group in the upper right of Figure 5 shows several system operating parameters. The real-time internal electronics temperature is displayed in the Temp field, in degrees Celsius. The temperature sensor that generates this reading is located on the SASS 2300’s internal PC board. The operating voltage is displayed in the Volts field. The Filling field displays the status of the pump that is used to fill the cyclone with makeup fluid when the fan is running. This pump turns on momentarily for a few seconds at a time to assure that the proper liquid level is maintained in the cyclone. When this pump is off, the Filling field will display the word OFF with a white background. When the pump is on it will display the word ON with a bright green background. The Batt field displays the percentage of battery life remaining. The Liters field displays the number of liters of air that have been sampled since the fan was last turned on. If you are running in Automatic Mode (Figure 4) this field will display the cumulative liters of air sampled since these operations were started.

Long Battery Operating Times

The SASS 2300 can be used with three different lithium ion-based batteries. Two are primary types, the BA-5590/U and the BA-5390/U extended life, while the third, the UBI 2590, is a rechargeable battery. Their capacities are respectively 210, 308, and 168 watt-hours at 20C. These capacities translate into continuous run times of approximately 13, 19, and 10 hours, respectively. If the SASS 2300 is on standby, that is, the fan is not running, these batteries can power the units for about 8, 13, and 7 days, respectively.

For customers who will use the sampler frequently, we suggest purchasing the rechargeable battery and its electronic recharger. These two items are about the same price as six primary batteries, so the rechargeable approach may pay for itself in a short time if the SASS 3100 will see heavy usage and the shorter continuous run time with the rechargeable battery is acceptable.

New international air transport rules make shipping batteries with these high energy contents difficult, if not impossible: Batteries must generally be shipped by other methods. If this will make use of the battery-powered SASS 2300 difficult for you, please contact RI for other battery options.

Battery life is monitored in two ways: by ampere-hours of electrical power consumed, and by battery voltage. In the field, a low battery condition is signaled by blinking of the yellow “Pump” LED. When the Sampler is connected to RI’s Windows Operating Software by the serial data link, two indicators of battery life are displayed at the PC- a “Percent Capacity” number based on ampere-hours, and raw battery voltage.
For situations where mains power is available, a universal (IEC 320) wall-plug power supply accepts 82-265 VAC at 47-63 Hz.

Accessories

Air inlet and outlet ports feature an industry-standard screw thread that allows many different types of input and output hoses, nozzles and filters to be mounted to the unit. System electronics and firmware are designed for ease of customization. Please contact us if you are interested in GPS, weather, long-range RF, visual, thermal imaging, or any other specific add-on feature.

Application Areas:

• Medical facilities
• Public health
• Academic research
• Military
• Food safety
• Power plants
• Agriculture
• Indoor air quality
• Environmental
• Homeland security
• Mailrooms

Features:

• Collection periods may be many hours in length
• Liquid sample volume unaffected by ambient conditions
• User-specified automated protocols
• Onboard automated sample vial filling
• Wireless operation and control option
• Convenient threaded inlet and outlet ports
• Long-life primary and rechargeable battery options
• Portable: Impact- and water-resistant case
  

Product Information :

• SASS 2300 Frequently Asked Questions
• SASS 2300 Parts and Accessories Page
• View Product bibliography
• Download SASS 2300 Data Sheet
• Power Measurements and Battery Life Estimates (89 KB, PDF)
• SASS Air Sampler Comparison Chart
• SASS Series Collection Efficiency chart
• Candidate Disinfectants for Research International Products (978 KB)
• SASS 2300 technology used by Lawrence Livermore National Laboratory in their Autonomous Pathogen Detection System (APDS) This is a 7.80MB PDF
• Bluetooth Biolink™ Wireless Units provide plug-and-play wireless communication between any Research International product and a remote PC or laptop.
• Using SASS 2300 Presentation (PDF)

 

 

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