Direct reading aerosol monitors

Transcription

1 Health and Safety Executive Direct reading aerosol monitors 27 th June 2018 Andrew Thorpe

2 Introduction Direct reading Aerosol Monitors (DRAMs) are often used as a complementary approach to conventional filter-based gravimetric methods for measuring airborne particles. Immediate knowledge of the result without going through lengthy and expensive laboratory analysis, allowing for rapid intervention. They can be used to evaluate instantaneous concentrations produced by various work activities and to monitor changes in concentration with time. Can be set to alarm above a pre-set airborne dust concentration. DRAMs are becoming smaller and lighter and of relatively low price

3 Uses of direct-reading aerosol monitors Walk-through surveys. Background sampling to assess concentration variations and mean concentration during a working shift period. Assessment of the effectiveness of dust control systems. As part of exposure video visualization systems.

4 Principles of measurement Optical Optical Particle Counters Hand-held and fixed instruments available Particles are passed one by one through a small illuminated volume. Scattered light intensity from each particle is detected and converted into an electrical signal. Signal analysis allows particles to be counted and their size to be estimated.

5 Principles of measurement Optical Optical Particle Counters Coincidence occurs when two or more particles pass simultaneously through the optical sensing volume. Maximum measurable concentration is of the order of a few hundred particles per cubic centimetre. Require calibration of both the particle count response and the particle diameter response.

6 Principles of measurement Optical Optical Particle Counters Advantages and disadvantages of OPCs

7 Principles of measurement Optical light scattering photometers Based on the principle of light scattering by aerosol particles. Light scattered by particles contained within a finite volume is used to determine their concentration.

8 Principles of measurement Optical light scattering photometers A wide range of hand-held instruments available that use this principle. Passive or pumped o Passive run for long periods and are quiet, but can underestimate concentration in still air and optics easily contaminated. o Active use a pump to draw aerosol into instrument sensing volume. Can use a sheath of clean air to protect optics, and can be used with size-selective inlets. Can measure a wide range of particle concentrations (from micrograms to hundreds of milligrams per cubic metre.

9 Principles of measurement Optical light scattering photometers Ideally suited to the respirable fraction and become successively inaccurate for coarser particles within the thoracic and inhalable aerosol conventions (scattered light intensity is at a maximum for particles from 1 μm to 2 μm, and drops to zero for particles greater than 10 μm) Reading is only valid for a specific dust - factory calibrated with Arizona Road Dust. This is because of its consistency in mineral content and particle size. It also has a similar refractive index to many workplace aerosols.

10 Principles of measurement Optical light scattering photometers Need to be recalibrated to measure an aerosol of particles with different refractive indices or particle shape or a different particle size distribution. On-site calibration should be performed by comparison with a reference gravimetric sampler for accurate measurements. Some active aerosol monitors include an integral back up filter onto which the aerosol passing through the sensing zone is captured, and can be used to determine the reference gravimetric concentration. Photometers are becoming smaller and less expensive (< 500 as opposed to 2,000-6,000 traditionally).

11 Principles of measurement Optical light scattering photometers Advantages and disadvantages of aerosol photometers

12 Some optical DRAMS currently on the market (photometers)

13 Some optical DRAMS currently on the market (photometers)

14 Some optical DRAMS currently on the market (Particle Counters)

15 Other technologies Piezoelectric mass monitors Particles drawn into the instrument are collected on the surface of a vibrating piezoelectric crystal. The mass of deposited particles causes a reduction in the oscillation frequency which is directly proportional to the particle mass. TEOM Tapered Element Oscillating Microbalance Oscillating frequency is applied to a tapered glass tube equipped with sampling filter at its narrow end.

16 Other technologies Mass of particles collected on the filter based on the oscillation frequency difference before and after sampling. Beta attenuation mass monitors Mass of dust particles sampled on a filter (or collection substrate) is estimated by attenuation of beta rays crossing the filter. Response is affected by the uniformity of particle deposition on the filter and so the method is normally limited to the thoracic (PM10) or respirable (PM4) particle fractions.

17 Other technologies Piezo balance TEOM Beta attenuation monitor

18 Recent developments in cheap DRAMs Alphasense NP1 OPC (~ 300) 16 size channels ( µm). Very small and lightweight. Measures PM1, PM2.5, PM 10, PM4. Logs data onto micro SD card. Can run for long durations (external battery). No display of concentration (requires computer or tablet). Largely untested.

19 Recent developments in cheap DRAMs Airbeam 2 (~ 150) Air Casting Small and lightweight (personal). Long run times with external battery. PM 1, PM 2.5 & PM 10 concentrations temperature and relative humidity. Mobile session - measurements sent to a Smart phone once a second to the AirCasting Android app via Bluetooth.

20 Recent developments in cheap DRAMs Airbeam 2 (~ 150) Air Casting Fixed session - measurements sent once a minute to the AirCasting website via WiFi or cellular. Generates concentration hot spots. Uses cheap sensors, questionable response and accuracy. Largely untested especially at elevated concentrations.

21 Recent developments in cheap DRAMs PATS+ photometer (~ 390) Berkeley Air Small and extremely lightweight. Logs PM 2.5, temperature, and RH. Long battery life, can run for weeks on an external battery. Doubts over accuracy, uses cheap sensors. Designed for stove fume (PM 2.5 ) - largely untested against anything else. No real time display.

22 Recent developments in cheap DRAMs Dylos DC1700 OPC (~ 286) Dylos Simple to use. Measures >0.5 and >2.5 µm particles. Logs data in internal memory. OPC and so limited concentration range until coincidence errors occur. Gives readout in particles/cc rather than mg m -3.

23 Choosing a DRAM Instrument Requirements o Instrument price. o Ease of operation. o Portability (size/weight). o Accuracy o Ease of maintenance. o Reliability.

24 Instrument Requirements o Concentration range. o Is particle size information required? o Length of operation on full charge. o Is a concurrent filter sample required? o Is a size selective inlet required? o Data logging capacity. o Calibration costs.

25 Thank you for listening - any questions? Health & Safety Laboratory andrew.thorpe@hsl.gsi.gov.uk