PFT Sampling and release technology

BNL can release PFT compounds at rates that can be adjusted to match the spatial scale of any experiment. Releases have been scaled to study transport and dispersion in single room, in buildings, over small fields and urban canyons, and regional and continental scales. Source release mechanisms include permeation sources, syringe-pump tracer release system developed at BNL, and pressurized cylinders.

Permeation sources are small vials containing liquid PFT with permeable membranes. The membranes allow PFT vapor to diffuse into the environment at rates of micrograms per minute. The lower molecular weight, more volatile PFTs have the fastest permeation rates. Release rates depend on the temperature and the thickness of the membrane. The temperature at the release site is recorded so the actual release rates can be calculated. A sample of the deployed sources is weighed before and after each test to determine the average release rate. These sources are passive. They require no power. Multiple sources at single point can be used to increase the release rate. Permeation sources can be placed at intervals along lines or arcs to create effective line sources. They can be distributed in an area to create a uniform flux. These sources have been used in indoor building transport and infiltration studies, and atmospheric dispersion experiments on meters to kilometer scales.

Syringe-pump Tracer Release System

BNL has developed a precision PFT delivery system using a syringe pump. This system uses a syringe and a precision stepping motor to dispense PFTs onto filter paper where they are vaporized by the aistream from a small fan. Release rates can be varied from 1 μg/hr to 3 Kg/hr. These units are battery-operated. They can be modified for releases from airplanes or towers. Each unit can be run manually, from a computer, or programmed to execute as many as 41 changes. The units can be networked and monitored/controlled from a single computer. Dispensing accuracy is 1%. Seven of these systems were successfully used in a recent experiment to study urban dispersion over a 4 square kilometer area.

Pressurized Cylinders

PFT mixtures in nitrogen gas can be made in concentrations below their dew point in and stored in compressed gas cylinders. These mixtures can be used to release PFT vapors at rates up to ~100 mg/min. The release rate is controlled by the use of restrictor devices or mass flow controllers.

Tracer Release Unit (atomizer)

BNL has developed a novel release mechanism that can release perfluorocarbon tracers at rates up to 500 grams per minute. The Tracer Release Unit (TRU) creates a fine aerosol of PFT with a porous cup spinning at 30,000 revolutions per minute. The aerosol is vaporized in the atmosphere. The liquid is introduced into the cup through a 3mm tube. The release rate is controlled by the rate that the PFT flows into the cup. Centrifugal force pushes the liquid through the porous walls. When the liquid reaches the outer surface it forms a thin layer that is shears creating fine droplets. The droplets evaporate immediately. Power consumption is less than 50 watts. The unit is more reliable than ultrasonic atomizers or nebulizers because it uses a rugged, high-speed continuous duty motor with precision bearings. It is battery powered and compact. The prototype of this device was used in an atmospheric dispersion study that covered distances of 500 to 750 kilometers; it was operated at less than 10% of its maximum release capacity.

PFT Sampling Technology

The Brookhaven Atmospheric Tracer Sampler (BATS) is typically used to collect the PFT samples. The BATS consists of a base unit containing a pump, timer, and control electronics, and a lid containing Twenty-three, ⅛inch o.d. stainless-steel tubes packed with Ambersorb®. The lid also contains a multi-port valve that places the active sampling tube in the sample stream.
Analysis of the BATS Lids and CATS tubes is performed using gas chromatography with an electron capture detector (ECD). The quantity of PFT available for analysis is determined by the volume of air that is sampled. The ECD has the sensitivity to quantify background levels of PFT if the material in 1.5 liters of ambient air is collected. It takes approximately 15 minutes to analyze each sample.

BNL has developed a method of taking vertical samples using actively pumped sample lines from tether sondes . There have been several versions of this system. A system has been tested using a 500-foot long, 1/16-in OD plastic sampling tube. There are 15 ports along the length of the cable. An adsorbent tube is mounted in each port. The samples are collected by actively pumping the sampling line. This system was light enough to be carried by a single 5.25 m3 balloon. The spacing of the ports can be adjusted to get the desired vertical resolution.

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Last Modified: January 31, 2008
Please forward all questions about this site to: Linda Satalino