* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download 12Sept_Synergist Solutions article
Survey
Document related concepts
Atmospheric optics wikipedia , lookup
Magnetic circular dichroism wikipedia , lookup
Cross section (physics) wikipedia , lookup
Thomas Young (scientist) wikipedia , lookup
Fluorescence correlation spectroscopy wikipedia , lookup
Rutherford backscattering spectrometry wikipedia , lookup
Nonlinear optics wikipedia , lookup
Retroreflector wikipedia , lookup
Dispersion staining wikipedia , lookup
Optical tweezers wikipedia , lookup
Nonimaging optics wikipedia , lookup
Ultrafast laser spectroscopy wikipedia , lookup
Transcript
Synergist® Solutions: Real-time Dust Monitoring Advantages of Real-time Measurements in Industrial Hygiene By Sreenath Avula and Greg Olson Real-time monitoring in industrial hygiene has revolutionized the measurement of workplace hazards. Direct-reading instruments have enabled industrial hygienists to be proactive, allowing them to measure physical and chemical hazards as they are being generated. The time-consuming alternative is to wait for an 8-hour reference method sample to be completed at the end of the work shift and then send it to a laboratory for subsequent analysis. Real-time monitoring using direct-reading instruments also enables source identification and is a very useful tool for source apportionment and modeling, evaluation and validation of engineering controls, and corrective actions during personal and ambient work area monitoring activities. Immediate access to information supports real-time decision making and is the industrial hygienist’s greatest advantage over reference sampling methods. This article will focus on advancements in real-time monitoring for dust, also known as particulates or aerosols. There are plenty of instruments with which industrial hygienists can monitor dust, and these instruments are primarily based on two distinct technologies: photometers and optical particle counters. Both instruments rely strongly on the optical properties of the particulates to indirectly estimate the mass concentration. The light scattered by particles can be converted to mass concentration through calibration. Other technologies used to measure mass include beta attenuation gauge and vibrating mass monitor, both of which are more suited for outdoor ambient monitoring and will not be discussed in this article. Photometers Photometers are ideal for industrial hygiene applications and are highly desirable for applications such as personal exposure monitoring, ambient workplace monitoring, point source monitoring, emissions monitoring, validation of engineering controls and trend analysis and screening. They can be used to sample solid and liquid aerosol-like dusts, mists, fumes, smoke, diesel exhaust, fog and condensates. They are capable of handling a wide range of concentrations and have a linear response to particulate concentration from very low (as low as 0.001 mg/m3) to extremely high (>100 mg/m3). Photometers are calibrated to Arizona Road Dust, or ISO 12103-1 A1 test dust. Figure 1. Schematic of a typical photometer. A photometer’s principle of operation is based on measuring scattered light from a cloud of particles. The light source is typically a laser diode that produces a coherent beam of light, which is then focused onto the The Synergist | September 2012 1 particles using focusing optics. The higher the concentration of particles, the higher the amount of light scattered from the particles. The resulting scattered light from the particles is collected by the collecting optics—an assembly of lenses or a mirror. The collecting optics transfer the collected light onto the detector, which produces a current that is converted to a measurable voltage. The change in the voltage is directly proportional to the mass concentration. Figure 1 details the schematic of a typical photometer. Photometers cannot discriminate between different particle sizes. Therefore, if the user is interested in sampling a specific size fraction (for example, respirable fraction PM 2.5 or PM 10 ), a size-selective inlet must be used on the front end of the photometer. Size-selective inlets are designed to operate at a specific flow rate. Photometers with size selective inlets also come with flow control, where the desired flow rate is maintained under all operating conditions. The DustTrak™ DRX Aerosol Monitor from TSI Incorporated is an exception and does not need any size-selective inlets. The DustTrak DRX uses a patented technology that utilizes both the photometric and single particle-counting technologies to enhance the performance of the real-time, direct-reading photometer. The DRX technology counts only particles greater than 1.0 µm, unlike an optical particle counter (OPC), which counts all particles in the sensing chamber. The OPC detection circuit in the DRX appends to the photometer measurement range that tends to drop off for particles larger than around 4.0 µm. The optics and electronics are more complicated than those of a simple photometer (see Figure 2), but the measurement is based on the speed of the signal processing and size-discriminating algorithms. Figure 2. Schematics of the optics and electronics of the DustTrak DRX Aerosol Monitor. Aerosol Measurement Signal Acquisition and Processing Aerosol Inlet HEPA Filter Optics Light Trap Chamber Orifice Photometric DC Voltage Offset Analyzer Sheath Air Size Segregated Mass Concentration (eg. PM1, PM2.5, PM4 & PM10) Mirror Photo Detector Beam Shaping Optics Viewing Volume Gravimetric Filter Protection Filter Pump Flowmeter Pulse Height Pulse Height Offset Laser Diode Exhaust Single Particle Pulse Height Analyzer Dampening Chamber No Particles (background) Low Concentration (Counting Pulses) Photometric Signal Laser Beam High Concentration (Pulses + Photometry) Single-particle counting allows particles to be classified into PM 1.0 , PM 2.5 , respirable and PM 10 fractions without the use of size-selective inlets. The advantage of DRX over a traditional OPC is DRX’s ability to discriminate particle size even at very high concentrations (up to 150 mg/m3). Photometers have their limitations like all other instruments. They are sensitive to humidity and will tend to overestimate actual concentration if the humidity exceeds 70 percent. When sampling under high humidity conditions, some The Synergist | September 2012 2 manufacturers provide the option to heat the inlet to remove humidity. Others provide humidity compensation by determining the humidity in the sensing chamber and making a correction to the measured concentration. Photometers’ responses are also sensitive to particle size distribution, density and refractive index of the sampled aerosol. To overcome these sensitivities, all photometer manufacturers allow the user to program a custom calibration factor. By performing a side-by-side comparison between a photometer and gravimetric analysis, the photometer response can be adjusted to match gravimetric concentration. This correction factor, or calibration factor, is valid as long as the dust being sampled does not change much in terms of size distribution or composition. Optical Particle Counters (OPCs) OPCs measure particle concentrations based on the optical properties of dust. However, unlike photometers, they are designed to count and size particles being sampled one at a time. The particles are focused into a thin beam and made to pass a thin sheath of laser light. As each particle passes though the laser light, it registers a pulse of light across the detector, which is then converted to a voltage pulse. The height of the pulse is proportional to the particle size. The pulse heights are related to particle size through calibration with spherical polystyrene latex particles (PSL). OPCs are also sensitive to changes in dust composition (density and refractive index), but are not affected by changes in size distribution. OPCs cannot handle high dust concentrations and are more difficult to calibrate to the particulates being measured. Their measurements are also significantly affected if the sampled flow rate changes. Therefore, flow control is very critical to accurate concentration measurements. The optics in an OPC are more complicated than in a photometer because the light scattered from individual particles has to be collected and focused onto the photo detector. Since the light scattered from individual particles has a lot less intensity than a cloud of particles, the collecting optics are designed to collect as much scattered light as possible using an elliptical mirror with a large aperture. Figure 3. Schematic of OPC optics. See Figure 3 for a schematic of OPC optics. Since OPCs count particles one at a time, they cannot handle high concentrations (> 1.0 mg/m3). The upper concentration limit depends on the particle size. At high concentrations, the probability of more than one particle in the sensing chamber at a given time increases, resulting in coincidence losses. OPCs are therefore ideal for applications such as semiconductor cleanroom monitoring, pharmaceutical cleanroom monitoring, indoor air quality studies and research studies. OPCs work well at low concentrations, but at high concentrations they tend to underestimate concentration due to coincidence errors. The conversion from The Synergist | September 2012 3 count concentration to mass concentration also depends on the assumptions made about the density and refractive index of the sampled particles. This plays a larger role in an OPC than in a photometer because OPCs can be calibrated only with PSL, which are perfectly spherical unit density particles. Choosing the Appropriate Instrument Industrial hygienists have plenty of direct-reading instruments to choose from in order to measure dust in real time. IHs must consider the application and choose the appropriate instrument and technology for their sampling needs. Not all instruments are ideal for all applications. Instrument choice is determined by units of measure needed (mass and/or size distribution), flow rate, expected concentration, frequency of sampling and application. See Figure 4 for general guidelines for practitioners for choosing the appropriate instrument for each specific application. Figure 4. Guidelines to aid IHs in choosing appropriate instruments. Photometer Indoor Air Quality Conventional Studies Indoor Air Quality UF Particle Studies Industrial Workplace Monitoring Outdoor Environmental Monitoring Emissions Monitoring Respirator Fit-testing Filter Testing Clean Room Monitoring Pharmaceutical & Semiconductor Clean Room Research and Development Cost Comparison Optical Particle Counter Single Channel Multichannel Portable Handheld Good Excellent Poor Good Poor Poor N/A N/A N/A N/A N/A Good N/A N/A Good N/A N/A Excellent Excellent Excellent Good Excellent Excellent Excellent Good Good Good Good Poor Poor Excellent Poor Poor Poor Excellent Poor Fair Good Good Fair $ $$ $$ $ Sreenath Avula, PhD, is the product management specialist for TSI, Inc. He can be reached at (651) 490-3867 or [email protected]. Greg Olson is senior industrial hygienist and global product specialist for TSI, Inc. He can be reached at (651) 490-4002 or [email protected]. The Synergist | September 2012 4