pairwise evaluation of pdm3700 and traditional gravimetric...

THE AUSTRALIAN MINE VENTILATION CONFERENCE / BRISBANE, QLD, 28–30 AUGUST 2017 1

settle deep within the lungs that are not ejected by exhaling, coughing, or expulsion by mucus. Since these particles are not collected with 100 per cent effi ciency by the lungs, respirable dust is defi ned in terms of size-selective sampling effi ciency curves. This had led to internationally recognised respirable size-selective sampling (Orenstein, 1960) widely known as the British Medical Research Council (BMRC) defi nition of the respirable dust fraction or Johannesburg curve with a median aerodynamic diameter of 5 μm collected with a 50 per cent effi ciency (D50). In reality these size-selective curves represent lung penetration of dust particles that dust sampling instruments attempt to replicate. The International standards organisation (ISO) in 1995 recommended that the defi nition of respirable dust follow the convention described by Soderholm (1989, 1991) with a D50 of 4 μm. An international collaboration (ACGIH, 1985, ACGIH 1999, ISO 1995, CEN, 1993) for sampling harmonisation has led to the agreement

Pairwise evaluation of PDM3700 and

traditional gravimetric sampler for

personal dust exposure assessmentB Belle1

ABSTRACT

Dust sampling is pivotal in estimating the ‘dose’ of dust exposure and in deriving dose-response curves in epidemiological studies. Over the last half century, gravimetric sampling has been the fundamental means for dust exposure monitoring using recognised respirable size-selective standards. In Australia, a gravimetric sampling technique has been followed since 1983 using the Higgins-Dewell (HD) type cyclones (AS2985). The re-emergence of ‘Black Lung’ or Coal Workers Pneumoconiosis (CWP) in Queensland, Australia has re-kindled the need for better understanding of personal dust monitoring, compliance determination, accuracy, timeliness of sampling results and dust control systems. Reporting dust levels in real time empowers miners and operators to take immediate action to avoid being exposed to excessive airborne dust. Two decades of intensive research in the USA has led to the introduction of gravimetric based continuous personal dust monitors (CPDMs) called PDM3700 as compliance tools. The new real-time mass based sensor is a continuous mass based dust monitor using the Tapered Element Oscillating Microbalance (TEOM) principle and is regarded as signifi cantly superior to the current gravimetric sampling methodology.

Recently the overall respirable dust standard in US coalmines has been reduced from the historic 2.0 to 1.5 mg/m3. This paper provides the results of a comparative pairwise study of an existing gravimetric sampler and the PDM3700 carried out in 3 different Australian underground mines. The fi eld results consistently suggest that there is a signifi cant ‘measurement bias’ between the current gravimetric HD sampler and the PDM3700 monitor operated using a similar HD type cyclone. The results show that the currently used cyclone measurements are approximately 59 per cent higher than the ‘auditable’ PDM3700 monitor data at the current compliance limit of 3 mg/m3. Based on a review of the extensive data collected, the differences can in part be attributed to the aerosol size selectivity of the samplers with the current gravimetric sampler used in mines, collecting more larger sized particles (D90 of 15 μm). This fi nding has signifi cant consequences for historic exposure results and their use in medical surveillance programs, as well as the current approach to non-compliance determination.

INTRODUCTION

Respirable dust sampling is pivotal in estimating the ‘dose’ of individual coal worker exposure to dust and in deriving quantitative respiratory disease risks in epidemiological studies. Dose is the actual amount of dust penetrating and depositing in a workers lung; it can be estimated by personal dust sampling but they are not the same metric. This indicates that the dose received by different groups of miners may not be completely characterised by their personal exposures. This can be attributed fi rstly to a diverse mine workforce in terms of race, gender, body size, and secondly, miner lung dose depends on breathing rate, particle size, solubility, mouth versus nose breathing, and actual assignment of ‘dose’ using current defi nition of ‘breathing zone’ lapel samples.

Based on the past epidemiological knowledge (Orenstein, 1960), it has been established that the respirable dust particle size distribution is critical due to its potential health effects and quantifying the risks. Respirable dust refers to particles that

1. MAusIMM(CP), Group Ventilation and Gas Manager (Coal Global), Anglo American Coal, Brisbane Qld 4001. Email: [email protected]

THE AUSTRALIAN MINE VENTILATION CONFERENCE / BRISBANE, QLD, 28–30 AUGUST 2017

B BELLE

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on the defi nitions of health-related aerosol fractions in the workplace, defi ned as the inhalable, thoracic and respirable curve (Figure 1). The new respirable size-selective curve is different from previous defi nitions used in the United States, South Africa, Australia and Europe and truly represents an international harmonisation of the defi nition of respirable dust. This international harmonisation in personal dust sampling should eliminate confusions related to differences in dust exposure levels, exposure assessment and compliance determination methodologies. Table 1 summarises the BMRC and ISO size-selective curves for dust sampling in mines (NIOSH, 1995; ISO, 1995).

Therefore, for any personal sampling, the chosen respirable dust sampling device should achieve the theoretical sampling defi nition criterion as closely as possible to minimise bias using the D50 performance criteria at the recommended sampling rates. Due to the complex nature of sampler performance evaluations and their differences, regulatory bodies have dealt this aspect by decreeing one specifi c sampling device, (ie MRE, Dorr-Oliver, HD), as the reference sampler of choice. What is important herein is whatever sampler is used for exposure measurement, they are to be referenced to epidemiological health effects data to derive any meaningful benefi ts. Over the years, various types of dust sampling instruments have been evolved in line with the various size-selective sampling curves and the dust occupational exposure limits (OELs). OELs provide the necessary guidance for planning, engineering, monitoring and controlling the hazard and work practices for effective control of exposure to substances. There are wide variations in the OELs as defi ned by regulatory and research authorities or

scientifi c associations. The OELs set by regulatory authorities of countries worldwide need not be the same and must not be compared directly with each other because of the differences in each country’s exposure measurement method, instrument used and compliance assessment strategies. Moving from one size-selective sampling method to the other have some basic implications such as using OELs for compliance monitoring and dose-response estimation.

BASICS OF PERSONAL DUST SAMPLERS

The primary purpose of personal respirable dust sampling is to characterise (with regard to mass and size) the quality of the ambient air to evaluate a miner’s dust exposure. The mass of respirable dust inhaled can be estimated by sampling. The measurement of dust in mines is usually carried out using various gravimetric sampling instruments. For personal coalmine dust sampling, the dust sampler or cyclone is normally mounted on the upper chest, close to the collarbone within his or her breathing zone (HSE, 2000). The breathing zone is the space around the worker’s face from where the breath is taken, and is generally accepted to extend no more than 30 cm from the mouth. Gravimetric dust monitoring involves sampling a known volume of ambient air through a fi lter. The fi lters are weighed before and after exposure to determine the mass of particles. The collected dust sample is expressed as mass of dust (mg) per cubic metre (m3) of air.

With the acceptance of a defi ned gravimetric based size-selective sampling, various types of dust samplers called ‘cyclones’ were developed and used in mines worldwide since the 1960s (NIOSH, 1995). Cyclones are named for the rotation of air within its chamber that separates and selects dust particles of interest from ambient airborne dust. The cyclone functions on the basis of a centrifugal force principle, ie the rapid circulation of sampled air separates particles according to their equivalent aerodynamic diameter.

In a cyclone, non-respirable particles are forced to the periphery of the airstream and collected in a grit pot, while the specifi ed particles remain in the centre of the air stream and deposited onto a preweighted fi lter medium. The size fraction sampled are very sensitive to type of cyclone used and variations infl ow rate. Various commercially available cyclones can approximate specifi ed size-selective curves when operated at certain fl ow rate. Any minor deviation from the recommended fl ow rate would lead to differences in measured dust results. For example, a mere change in fl ow rate of HD type cyclone from 1.9 Lpm to 2.2 Lpm can result in differences of up to 20 per cent in measured dust values (Kenny, Bristow and Ogden, 1996, Belle, 2004). This of course is dependent on the size distribution of the aerosol being measured. Lower fl ow rates collect less mass sample, but of a larger size fraction; higher fl ow rates collect more mass sample but of a smaller size fraction (Volkwein, 2016).

With the advent of the internationally accepted respirable size-selective curves, research studies have compared various dust samplers available for use in mines. What is obvious from the various studies (Liden and Kenny, 1991, Kenny and Gussman, 1997, Gudmundsson and Liden, 1998, Görner et al 2001) is that there are signifi cant differences in measured dust levels from different samplers measuring the same aerosol. The reasons affecting the performance of these different dust samplers can be attributed to inherent cyclone design, air velocity, and direction of airfl ow, humidity, sampler inlet size, geometry, orientation, aerosol particle size, aerosol density differences, electrical charge, particle bounce properties and conductive properties of cyclones.

FIG 1 – Respirable dust size-selective sampling curves (Soderholm, 1989).

BMRC Curve ISO Curvea

Particle size,

μm

% > stated size Particle size,

μm

% > stated size

0 100 0 100

1 98 1 97

2 92 2 91

3 82 3 74

4 68 4 50

5 50 5 30

6 28 6 17

7 0 7 9

8 5

10 1

a. NIOSH criteria document (page 72); ISO 7708 (1995).

TABLE 1

Collection effi ciencies for size-selective sampling.


PAIRWISE EVALUATION OF PDM3700 AND TRADITIONAL GRAVIMETRIC SAMPLER FOR PERSONAL DUST EXPOSURE ASSESSMENT

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Globally, over the last 6 decades, various size-selective conventions have been used, as well as various types of personal gravimetric samplers being used by mines. Until recently (Feb 2016), in the USA, the Dorr-Oliver 10 mm nylon cyclone (Jacobsen and Lamonica, 1969; Lippman and Harris, 1962; Caplan et al, 1977) was the widely used sampler operated at 2.0 Lpm across the entire US coal mining industry. On the other hand, most of the European countries use the HD type cyclone (Higgins and Dewell, 1967; Harris and Maguire, 1968; Maguire et al, 1973; Gwatkin and Ogden, 1979; Ogden et al, 1983; Blackford et al, 1985; Gudmundsson and Liden, 1998). In South Africa, mines use a locally manufactured HD type cyclone as confi rmed by HSE tests and are operated at a recommended fl ow rate of 2.2 Lpm (Kenny, Baldwin and Maynand, 1998). It is also important to note that, in the USA, the latest real-time continuous personal dust monitor (CPDM), PDM3700 uses a HD cyclone manufactured by MESA Laboratories (Lakewood, CO, USA) and is operated at 2.2 Lpm. The cyclone chosen for use in the PDM3700 followed the HD design previously shown through testing to have low bias relative to the ISO convention (Maynard and Kenny 1995). Bartley et al (1994) noted that the sampling effi ciency of cyclones drops towards zero much more sharply (Figure 2) than the theoretical curve with a D50 of 4 microns (Figure 1). In order to compensate for this effect, optimal cyclone operation was determined by a displacement of approximately 0.5 microns (Bartley et al, 1994) relative to the ISO (1995) respirable curve resulting in 4.5 microns as the cut-size of choice, and referred in research re-ports with a D50 value of 4 μm (Bartley et al, 1994).

Table 2 provides the summary of respirable dust samplers used in mines. It is noted that the HD cyclone is the most commonly used sampler and is recommended by the Health and Safety Executive (HSE, 2006). Currently, these HD cyclones are referred to by commercial names such as Casella, SKC, BGI, MESA or also called SIMPEDS (Safety In Mines Personal Dust Sampler), which was developed for applications in the UK coal mining.

What is important in considering the adoption of a size-selective curve is that, in reality, it is not possible to emulate the laboratory test cyclones with experimental data. Because no device precisely follows the theoretical size-selective convention, individual cyclones used in practice at mines do exhibit inherent bias. It is expected that the suppliers of these cyclones demonstrate that they do meet a respective size-selective curve by test results from reputed and accepted independent laboratories. In addition, ISO7708 (1995), clearly cautions that it may not be possible to construct dust sampling

instruments or cyclones whose size-selective performance exactly match the recommended respirable curve, and it is only possible to make a statement of instrument’s ability to select particles within certain range. In summary, the D50 parameter is the most commonly used criteria in the selection of the gravimetric sampler for compliance determination in relation to the given OEL.

Australian gravimetric samplerSince the adoption of gravimetric sampling in Australia in 1983, it is noted that the plastic and aluminium HD cyclones have been used and operated at 1.9 Lpm. As per AS2985 (1987), Australian dust sampling followed the BMRC (Orenstein, 1960) with a zero effi ciency for particles of 7 microns. The AS2985 recommended dust sampling devices included the BCIRA (British Cast Iron Research Association), HD cyclones and SIMPED miniature cyclones. However, AS2985 (2004, 2009) defi ned respirable dust by using the inhalable convention with a ‘56 per cent’ effi ciency at 4 μmicrons (Table 1 of AS2985–2009, page 7) using the cyclones at 2.2 L/min fl ow rate. However, ISO7708 document states that the D50 of the size-selective curve to be 4 μm as the ‘total airborne particles’ convention. What is noteworthy here is that AS2985 uses the ‘inhalable convention [Table B.1-ISO7708]’ against the globally accepted ‘total airborne particles convention [Table B.2-ISO7708]’ in defi ning a respirable dust convention. Conforming to the international defi nition consistently would therefore, remove the ambiguity of D50 value of 4.25 μm against the NIOSH/ACGIH standard of 4.0 μm when deciding which cyclones should be used. In the context of confusion between respirable dust and PM2.5, it is worthy to note the difference between the two parameters. For the protection of the target population of children, or the sick or infi rm (the ‘high risk’ group), the corresponding D50 value is 2.41 μm which is commonly termed as PM2.5, while, D50 value for respirable dust is 4.0 μm.

Currently, further investigations have indicated that almost all of the sampling in some mining regions is carried out using a specifi c manufacturer, ‘plastic type HD’ cyclone without any sound knowledge of its size-selective performance. The reason for the selection of this particular supplier of cyclones could not be established other than ease of its availability. In addition, conformity of the currently available SKC cyclone or Casella cyclone test evaluation reports that are used in Australia are not readily available. The amendments to the FIG 2 – HD cyclone collection effi ciency data (source: Bartley et al, 1994).

Cyclone Name Flow Rate,

LPM

Comments

Dorr-Oliver 1.7a 10 mm nylon cyclone used by MSHA (USA)

HD Type (Casella) 2.2 Conductive plastic (UK, Australia)

HD Type (SKC) 2.2 Conductive plastic (Australia)

HD Type (SKC) 2.5 Aluminium cyclone (37 mm fi lter)

Casella SIMPEDS 1.9 Metal(UK)

HD Type (BGI-USA) 2.2 Nickel platted aluminium

HD Type (South Africa) 2.2b Conductive plastic (25 or 37 mm fi lter)

HD Type (BGI-MESA) 2.2c Conductive plastic used in PDM3700

a. 1.7 is the fl ow rate for metal and non-metal mines, the fl ow rate is 2 Lpm for coal with the

application of an empirically derived factor 1.38 multiplier to achieve MRE equivalency.; b. Another

European study had recommended a fl ow rate of 2.5 Lpm for the South African HD type cyclone

(Gorner et al, 2001); c. With an empirically derived factor of 1.05 to achieve equivalency to the

CMDPSU US sampling device up-on which health eff ects are predicated.

TABLE 2

Summary of commonly used respirable dust samplers in mines.


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OEL by switching over to the new size-selective curve has been already been incorporated into the NSW dust standards except in Qld dust limits. In the absence of original fi eld evaluation data on switch-over to the newly adopted curve has resulted in confusions over the dust limits between the two states. Consequently there is a need to conduct an empirical comparison between the currently used sampler and any proposed new personal sampler for compliance exposure monitoring.

PDM3700 samplerThe PDM3700 is a real-time instrument (Figure 3) utilising a tapered element oscillating microbalance (TEOM) that continuously weighs a fi lter as dust is deposited on it. The TEOMs have historically found their applications in ambient particulate monitoring (Patashnick and Rupprecht, 1991). The PDM3700 dust monitor is the scaled down version and adapted for mine use as a result of a two-decade long research and development program by the National Institute for Occupational Safety and Health (NIOSH) and the Mine Safety and Health Administration (MSHA) (Cantrell et al, 1997). The fi rst such TEOM-based device used in a coalmine was the Machine Mounted Continuous Respirable Dust Monitor (Kissell, Volkwein and Kohler, 2002). The technical details, operating principles and performance accuracy when compared to HD gravimetric samplers are documented extensively elsewhere on the NIOSH and MSHA websites and are not repeated here. The key aspect of PDM3700 is that it directly measures the mass of dust on a fi lter according to fi rst principles of physics regardless of dust composition, size, or physical characteristics.

The PDM3700 uses the plastic BGI-4 HD type cyclone manufactured by BGI (now Mesa Labs, USA). The D50 cut point for the PDM3700 at 2.2 Lpm ± 0.022 L/min is 4.37 μm and commonly termed as 4 μm and this is previously shown to have low bias relative to the ISO convention (Maynard and Kenny 1995; Volkwein et al, 2004; Volkwein et al, 2006; NIOSH, 1995). Based on NIOSH studies, the unit incorporates an embedded equivalency or correction factor of 1.05 to account for the difference between the manual gravimetric sampler and the PDM3700 for its use as a MSHA compliance sampling device. This factor is expected to account for any internal losses (inlet design change from cap mounted straight entry to lapel mounted 90° entry) due to the difference in design between the PDM3700 and the manual gravimetric sampler (Volkwein et al, 2006).

Prior to MSHA regulatory approval, NIOSH had demonstrated that the CPDM is an accurate instrument that meets the NIOSH Accuracy Criteria and, therefore, can be used as a compliance instrument (Volkwein et al, 2006). The NIOSH study (Volkwein et al, 2002; Volkwein et al, 2006) established that the PDM3700 measurement outputs

would meet ±25 per cent standard analytical measurement accuracy criteria 95 per cent of the time and with a 95 per cent confi dence that the individual PDM3700 measurements were within ±25 per cent of reference measurements for mass loadings between 0.5 and 4.0 mg/m3. Also, the US coalmine data (Volkwein et al, 2006) showed a fi eld precision of 0.078 (RSD value) for the PDM and 0.052 (RSD value) for the gravimetric Dorr-Oliver cyclone sampler operated at 2.0Lpm. In addition, fi eld studies were also evaluated for the instrument durability, precision, and worker acceptability. In-mine testing determined the precision, durability, and miner acceptance. For example, the largest data set with 955 samples in US coalmines by having miners wear a CPDM and a CMDPSU (gravimetric sampler) concurrently was recorded by the MSHA. In order to determine the bias, NIOSH reviewed the data set and concluded that these results support those already published as a result of the NIOSH PDM studies (Kohler, 2010). The results showed that the average concentration measured by the traditional US gravimetric sampler (CMDPSU), 0.83 mg/m3, was virtually identical to the PDM3700 (CPDM) average value of 0.82 mg/m3. In addition, NIOSH further concluded after reviewing the 955 samples, that there was no statistically signifi cant difference between the data sets, and that the bias between the CPDM and the approved CMDPSU is zero (Kohler, 2010). In US coalmines, the technology proved durable enough to successfully measure 108 shifts of data out of 115 attempts in the mines. Under US specifi c test conditions, the PDM was shown to be convenient to wear and robust, easy to use and provided accurate and timely data that could be used to monitor and prevent overexposure. Table 3 shows the comparison of the traditional gravimetric and the state-of-the-art PDM3700 sampling methodology.

In Australia, a pairwise underground evaluation of a precommercial version of PDM3600 and Casella HD metal cyclone was made by members of the Standing Committee on Dust Research and Control in NSW that produced promising results (Mace, Goodwin and Shepherd, 2005).

FIG 3 – MSHA approved continuous personal dust monitor (CPDM)/PDM3700.

Description Cyclone

Sampler

PDM3700

Real-time

Size-selective curve ISO (1995); AS2985 ISO (1995); AS2985

Size selective criteria, D50

4 μm 4 μm

Type of Sampler HD cyclone HD cyclone

Pump fl ow rate, L/min 2.2 2.2

Sample mass determination Manual Real-time TEOM

technology

Flow rate variation, L/min ± 5 % ± 2.5%

Silica Determination XRD/IR XRD/IR

Integration of dust, ventilation, gas,

cooling data in real-time

Not possible/

manual

Automation

potential

Real-time exposure data Not possible Possible

Time lag for exposure results Up to 2 weeks Immediate

Workplace Barometric pressure data Not possible Immediate

Workplace real-time DBT and Humidity

data

Not Possible Immediate

Reporting and data integrity Manual handling Automated and

tamperproof

TABLE 3

Comparison of traditional gravimetric and PDM3700

dust sampling methodologies.



5

METHODOLOGY: PAIRWISE DATA

COLLECTION AND VALIDATION

IN AUSTRALIAN MINES

This paper quantifi es the measurement differences between the current gravimetric sampler and the PDM3700 monitor under fi eld conditions and its implications in dust exposure assessment. Figure 4 shows the personal gravimetric sample (a) and static (b and c) pairwise sample collection set-up. Despite the PDM3700 being an MSHA approved tool used in the USA, it could not be fully used in Australian mines due to it not meeting the Queensland intrinsically safety (IS) electrical approval. However, the PDM3700 unit was trialled in Queensland coalmines in underground areas where the gas levels were below 0.5 per cent.

In this study, gravimetric and real-time PDM3700 side-by-side dust samples were collected underground during the production shifts. All personal side-by-side samplers were collected in the breathing zone area, where the samplers were worn in random on the left and right side of the lapel. For static side-by-side sampling, in mine A, four of each the PDM3700 samplers and four of the gravimetric samplers were placed in a box and positioned on the maingate drive in underground longwall face. As shown in Figure 4 b, c the location of the sample pairs PDM2-HD2 and PDM4-HD4 were on the left side of the sample box and PDM3-HD3 and PDM1-HD1 were on the right side of the sample box. For each underground evaluation, different cyclones of the same HD type were used. In Mine B, the pairwise static samplers were collected at the maingate transfer point area. The samplers were operated at 2.2 Lpm fl ow rate using personal or area dust samples from 3 different coalmines. For all pairwise test data, the widely used HD plastic cyclone (supplier A) was used side-by-side with the PDM3700 monitor. The study involved multiple shifts of dust measurements representing actual underground production conditions. Individual pairwise HD and PDM3700 monitors both selectively collect the fraction of airborne respirable dust nominally less than 10 m particles on a pre-weighed fi lter disc. The procedures for determining the particulate mass and pump calibrations were followed as per the AS2298 (2004) for the gravimetric sampler, while the PDM3700 provides the respirable mass in real-time. The fl ow rates of the pumps were measured before and after the shift for traditional samplers, while the PDM3700 provides the data in real-time.

The following data validation criteria were applied on the pairwise fi eld data prior to data evaluation:

• each of the pairwise samples were collected for the same sampling time

• pairwise data was rejected if one of the sample pair failed, ie failure of gravimetric pump or PDM3700

• if Gravimetric sampler time was less than the PDM3700 sampling time, only data from the corresponding time period of the PDM3700 was used

• if the PDM3700 sample time was less than the gravimetric sample time, the data was rejected

• the actual PDM3700 instrument number for the sampling pair was also recorded

• for this Australian pairwise study, there was no separate correction factor applied other than the embedded HD cyclone and PDM sampler equivalency or correction factor (1.05) based on the NIOSH study

• for comparison purposes, the BGI plastic cyclone used in the PDM3700 is considered as a ‘true reference sampler.’ This is based on the previously well-established research study by Maynard and Kenny (1995) that the Casella

plastic cyclone and the BGI cyclones have been proved to be identical.

RESULTS AND DISCUSSIONS

Figure 5 shows an example of the benefi ts of having real-time PDM3700 monitoring results, together with ventilation and gas monitoring data for an operating Australian longwall mine. It can be seen that at the end of shift the dust concentration for PDM3700 was 1.7 mg/m3, while the measured gravimetric concentration result was 2.42 mg/m3. It is to be noted that this example was not a pairwise PDM3700 and gravimetric sampler data. What can be inferred from the real-time PDM3700 data is that 61 per cent of the shift dust exposure came from only three exposure spikes as a result of outbye chock yielding, chock advancement, face slabbing and wrong operator position, ie 1.05 mg/m3 of the 1.7 mg/m3 shift average (664 minutes of sampling time).

This information is very useful considering the current focus on dust exposure management and application of suitable engineering controls on longwall dust sources. The plot also demonstrates the usefulness of having additional features such as real-time longwall ventilation fl ow, ventilation and goaf gas drainage effectiveness during the

FIG 4 – (A) Pairwise personal and (B) and (C) static

gravimetric and PDM3700 sampling.

A

B

C


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entire shift to manage dust and methane during a production shift in modern longwall panels. However, there is a potential for the PDM3700 monitor to provide thermal stress data such as the dry bulb temperature (DBT), relative humidity and barometric pressure data that is already inherent to the PDM3700 monitor.

From the underground measurements, a total of 34 pairwise valid sample data were collected from three different underground operations. The data contained eight personal sampling results and 26 area sampling data from three different underground coalmines. The minimum average sampling period for the pair wise sample data was 300 minutes. The measured dust levels using the gravimetric sampler and PDM3700 samplers is summarised in Table 4 and plotted in Figure 6.

From the results it was noted that despite the gravimetric sampler and the PDM3700 being operated at the same sample fl ow rates, the average measured dust levels for the sampling period were 1.62 mg/m3 and 0.81 mg/m3 respectively. Overall, from the underground measurements, it was noted that the current HD gravimetric sampler consistently showed an approximately 50 per cent increase in measured dust values when operated as per AS2985. From the linear regression plot of the data (Table 5), it can be inferred that

there is a ‘measurement bias’ in respirable dust levels by approximately 59 per cent at the current coal dust compliance limit of 3 mg/m3 limit. The implications of this fi nding is signifi cant for compliance determination process as well as on the development of ‘correction factor’ for light-scatter based real-time monitoring tools. Correction factor in this discussion is the ratio of the pairwise gravimetric reference sampler and the average measured data by the light-scatter based real-time monitor, such as PDR1000. For example, if the PDR1000 measured an average dust value of 0.81 mg/m3, the correction factors would be either one (0.81 mg/m3/0.81 mg/m3) or two (1.62 mg/m3/ 0.81 mg/m3) depending on the gravimetric sampler used in collecting the pairwise gravimetric and light scatter based dust data.

Sampler fl ow rate analysesIn order to understand the changes in sample fl ow rates between the PDM3700 and the gravimetric samplers, the measured fl ow rates before and after sampling was evaluated. The fl ow rate data for the analyses was obtained from a record of 22 PDM3700 underground evaluations. However, the corresponding side-by-side gravimetric sampler data recorded in the sampling sheet before and after always showed 2.2 Lpm, which is highly unlikely. The potential reasons

FIG 5 – Example of real-time personal TEOM PDM3700 dust monitor, methane, and ventilation data in an operating longwall.

FIG 6 – Relationship between measured dust levels using current gravimetric HD sampler and PDM3700.



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could be due to rounding error of fl ow rate measurements or pumps inability to record the second digit after the decimal. Therefore, history of gravimetric sample records from the sample mine B was obtained to understand the variations. Based on the data analyses, it is noted that the fl ow rate data of the PDM3700 for the sampling period was more consistent before and after sampling. For example, a sample of 22 records of PDM3700 fl ow rates before and after the sampling period was found to be 2.198 and 2.199 L/min respectively or 0.013 per cent change. In the case of gravimetric sampling, historic sample fl ow rate database of 303 samples from mine B showed a variance of fl ow rate ranges from 0.0 per cent to 9 per cent with an average of 1 per cent. This clearly shows the benefi ts of PDM3700 in maintaining the fl ow rate and ensuring the required size-selective curve is followed during the entire sampling period.

Particle size analysesIn order to understand the reasons for such a large and signifi cant deviation from the traditional gravimetric sampler, particle size analyses on collected respirable dust samples were carried out. Respirable dust samples collected over the polyvinyl chloride (PVC) membrane fi lters were submitted to the CSIRO Particle Analysis Service in Perth (Australia) for laser particle size analysis using a Malvern Mastersizer 3000 (CSIRO, 2016). The fi lter loading of dust was found visibly loaded to be high with a thick layer and a small amount of dust was coming off the fi lters. To be able to analyse the samples the dust had to be removed from the fi lters and was done by cautious sonication of the fi lters. For standard particle size analyses process, the dust fi lters were washed with ethanol in an ultrasonic bath to remove nearly all fi lter dust deposit. The results obtained by a laser particle size instrument provides particle equivalent optical diameter using an estimated coal density factor of 1.4 g/cm3. No attempt was made at this stage to determine the size distribution of the material on the CPDM fi lters.

Table 6 and Figure 7 shows the summary of samples analysed and the D50 and D90 values. As can be seen from the analyses, the reported D50 values varied from 4.5 μ to 7.5 μ and 90th percentile values were between 11.5 to 23.4 μ. Based on 14 sample analyses, on an average, 20 per cent of the respirable sample exceeded the respirable dust size limit of 10 μ and collection of up to 36 per cent of sample dust with an average D90 of 15 μm. It is unknown if the effectiveness of the sonication process has any infl uence between smaller and larger particles on the fi lter. In addition to sampler ineffi ciency, the potential for ‘dumping,’ ie oversized particles from the grit-pot entering the respirable air stream in personal samples was considered as the potential sources of measurement and sample handling errors. In summary, it can be concluded that the current HD type gravimetric sampler do not conform to the AS2985 requirements and its continued use for compliance determination and exposure assessment should be considered invalid.

In order to ascertain the large particle sizes collected in the current gravimetric samplers and cyclone performance, additional study was carried out to determine the penetration characteristics of a total of 3 used plastic cyclones from each of the three mines and 3 new plastic samplers (HSE, 2016). The design of the sampler test system is based on that described by Kenny and Liden (1991) used for the measurement of aerosol penetration through cyclone samplers and is not discussed here. Figure 8 shows the average penetration curve for the 3 used and 3 new current gravimetric sampler with a D50 value of 5.9 microns. Similarly, the plot shows the average penetration curve for the 3 new HD cyclone used in

Statistic # HD type sampler PDM3700

No. of Samples 34 34

Min, mg/m3 0.13 0.09

Max, mg/m3 6.96 2.23

Avg, mg/m3 1.62 0.81

Stdev, mg/m3 1.68 0.64

TABLE 4

Summary statistics of measured dust levels using

gravimetric sampler and PDM3700.

Sample

#

Sample

Type

Mine PDM

#

Gravimetric

Sampler

PDM3700

Sampler

1 Personal A 37015101826 1.40 0.74

2 Personal A 37015101755 0.70 0.35

3 Personal A 37015101755 0.45 0.38

4 Static A 37015101755 1.50 0.51

5 Static A 37015101557 1.40 0.68

6 Static A 37015091442 1.60 0.64

7 Static A 37015101826 1.00 0.54

8 Static A 37015091442 5.70 2.23

9 Static A 37015101755 6.90 2.10

10 Static A 37015101826 3.60 1.49

11 Static A 37015101557 4.40 1.66

12 Personal A 37015101826 3.10 1.25

13 Static A 37015091442 1.22 0.83

14 Static A 37015101826 1.13 0.77

15 Static A 37015101755 0.22 0.26

16 Static A 37015091442 0.96 1.39

17 Static A 37015101755 0.97 1.35

18 Static A 37015091442 0.20 0.23

19 Static A 37015101755 0.19 0.21

20 Static A 37015091442 0.29 0.15

21 Static A 37015101755 0.26 0.18

22 Static A 37015091442 0.25 0.11

23 Static A 37015101755 0.18 0.17

24 Static A 37015101755 1.14 1.44

25 Static A 37015091442 0.13 0.09

26 Static A 37015101755 0.13 0.17

27 Static B 37015111999 0.40 0.41

28 Static B 37015101598 2.10 1.09

29 Static B 37015101598 1.30 0.43

30 Static B 37015111999 4.70 2.04

31 Personal C 37015080829 2.05 0.79

32 Personal C 37015080829 1.30 0.61

33 Personal C 37015080829 1.60 0.47

34 Personal C 37015080556 2.46 1.85

TABLE 5

Measured pairwise dust data using the gravimetric and PDM3700 samplers.


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8

the PDM3700 with a D50 value of 4.4 microns. The ISO (1995) respirable convention, defi ned in EN 481 (1993), is also shown in red for comparison.

The measured D50 (the particle size that will penetrate the cyclone with a 50 per cent effi ciency) of the gravimetric sampler (both new and used) was up to 59 per cent higher than the target value of 4 μm. The gravimetric samplers also exhibited a high sampling bias, giving a positive bias often greater than 30 per cent higher than the respirable convention (EN 481, 1993) for theoretical aerosols with mass median aerodynamic diameters 1–30 μm and geometric standard deviations 1.75–4. These independent laboratory results with higher D50 values and bias have reinforced the earlier conclusions that the current gravimetric sampler results signifi cantly overestimated the measured respirable airborne dust concentration levels.

Laboratory cyclone evaluations – a perspectiveThe following paragraph provides some background to the dust sampler acceptance process supplied by various dust manufacturers for personal monitoring. Typically, prior to their acceptance as a dust sampling device, their designs are evaluated fi rstly in a reputed laboratory such as HSE (UK). Typically, cyclones were exposed to aerosols of ultrafi ne and medium grades of Arizona road dust (ARD) generated in a

calm air chamber against the British Safety in Mines Personal Equipment for Dust Sampling (SIMPEDS) as a reference sampler operating at a fl ow rate of 2.2 Lpm which has been characterised previously by Maynard and Kenny (1995). It is possible that different laboratories recommend different fl ow rates for the same cyclone. For ex-ample, a HSE study recommended a fl ow rate of 2.2 Lpm, while the Gorner et al (2001) recommended a fl ow rate of 2.5 Lpm. Similarly, the study recommended no changes to the fl ow rate for Casella or SKC cyclones. The SIMPEDS or Casella cyclone sampler of the generic HD type is recommended for use in the UK for optimal agreement with the respirable convention. Other suitable cyclone types that also agree with the respirable convention include the GS-3 cyclone (2.75 Lpm) and the GK2.69 cyclone (4.2 Lpm). Bartley et al (1994), noted that a HD cyclone should be operated at 2.2 Lpm for best matching the ISO defi nition of respirable dust. In addition, Maynard (1993) carried out an independent experiment on the fl ow rate dependence of the HD type SIMPEDS sampler at 2.2 L/min, manufactured by Casella for use within UK.

Gudmundsson and Liden (1998) also investigated in laboratory studies the various cyclone models (Table 7) at a fl ow rate of 2.1 Lpm and observed that D50, increased with increasing inner diameter of the vortex tube or surface properties of cyclone material. For example, what this would mean is that Supplier D HD cyclone vortex tube with an inner diameter of 3.12 mm would result in higher D50 (a difference in D50 of 0.7 microns) than the Supplier A HD plastic cyclone vortex tube with an inner diameter of 3.02 mm. There are differences in laboratory results when the samplers are tested with the same controlled test spherical aerosols (eg silicone oils, glass beads, potassium sodium tartrate, polystyrene, polyvinyl acetate (PVA)). In contrast, outcome would be very discomforting when samplers are challenged with dynamic mine dust with varying characteristics with time. Therefore, absolute comparisons of samplers for their performance

Sample # Sample

Type

D50

(microns)

D90

(microns)

% > 10

(microns)

AS2985

Pass/Fail

1 Personal 7.5 22.82 36.38 Fail

2 Personal 5.26 14.32 20.10 Fail

3 Personal 5.05 12.71 16.87 Fail

4 Static 4.96 12.61 15.95 Fail

5 Personal 6.57 23.42 32.29 Fail

6 Personal 4.73 11.46 14.92 Fail

7 Personal 4.62 12.74 16.61 Fail

8 Personal 4.49 14.82 16.39 Fail

9 Personal 4.83 11.97 15.01 Fail

10 Personal 6.03 14.80 23.45 Fail

11 Personal 6.02 14.80 22.95 Fail

12 Personal 5.31 12.96 17.73 Fail

13 Personal 5.04 12.80 16.70 Fail

14 Personal 5.84 14.13 21.54 Fail

TABLE 6

Size distribution of personal and static respirable dust samples.

FIG 7 – Particle size distribution of respirable dust

from the current HD gravimetric sampler.

FIG 8 – (A) Fractional penetration average of particles through

six gravimetric samplers and (B) PDM3700 sampler as a

function of aerodynamic particle diameter (HSE, 2016).

A

B



9

would be impossible, let alone aspects related to compliance determination. The laboratory results and a study by Liden (1993) provide the explanation on the differences (of up to twice as large) increased measured dust concentrations by supplier D cyclones (HSE, 2016) when compared with the Supplier A metal cyclones. It is certain that manufacturer modifi cations such as blacking, tapering of the vortex tube inlet, and gasket type do infl uence the cyclone penetration curves.

While there may be differing expert views, based on static and controlled laboratory evaluations, on using D50 as a selection criteria for the performance of cyclones, it is the only widely used standard for dust sampler selection. It has been shown from various studies that the changes in fl ow rates can also result in a different respirable curve during the gravimetric sampling. However, PDM3700 is less susceptible to such changes in fl ow rates and ensuring the sampler consistently follows the size selective curve with minimum deviations. The impact of differences are signifi cant at the operations for dust compliance determination. Therefore, it is important to establish the conformance of the gravimetric samplers as prescribed in the AS2985/ISO1995 for traditional personal exposure assessment. When the respirable dust is collected using different size selective curves, it will result in different dust masses. While the exposure limit is set in

accordance with the specifi c size selective curve in mind, the comparison of measured dust mass collected using a different sampling curve or cyclone or fl ow rates must be understood and accepted before comparing it to the dust standard.

CONCLUSIONS

This paper discusses comparative results between a traditional gravimetric dust sampler (plastic HD type) and the PDM3700 continuous monitor which uses an approved plastic HD type cyclone operated in accordance with the AS2985 as a ‘true sampler.’ The following conclusions can be drawn from the pairwise fi eld evaluation in Australian underground coalmines:

• The study has demonstrated that there is a signifi cant ‘measurement bias’ between the current gravimetric sampler and the PDM3700 ‘true reference’ sampler. For the current Australian compliance limit of 3 mg/m3, the currently used gravimetric sampler overestimates the measured respirable coal dust levels by approximately 59 per cent. The reason for the ‘measurement bias’ may be attributed to the inherent design and non-compliance of the cyclone to AS2985/ISO1995 curve and potential impact of cyclone collection effi ciencies due to changing fl ow rates during sampling.

• The results have shown that the large difference between the two sampling methods in measured dust results will have signifi cant infl uence on the exposure assessment and the compliance determination. Continuing with the use of current plastic sampler will result in higher measured dust levels.

• Size analyses of the respirable dust collected using the current gravimetric sampler have shown to fail the cyclone performance criteria as prescribed in the AS2985/ISO 1995 with D50 values between 7 μm to 12 μm. In addition, dust samples had D90 values of between 11 μm and 23 μm thus impacting the validity of the single or 95 per cent upper confi dence limit (UCL) compliance determination methodology for homogeneous exposure group (SEG) data.

• An independent investigation of the gravimetric sampler performance indicated that the currently used cyclone D50 values were signifi cantly higher (5.9 μm) than the AS2985 specifi cation unlike the PDM3700 HD cyclone with a measured D50 of 4.37 μm when exposed to same test aerosol.

• The study recommends that due to its inherent technological advances in mass based monitoring with minimised sampling errors, the PDM3700 should be used as a ‘true reference’ sampler.

• The use of the current plastic HD sampler used in this evaluation study and used in Australian mines result in signifi cant exposure overestimation due to its non-conformance to the AS2985 requirement based on particle size analyses and must be discontinued from use in its current design.

• A secondary inference that can be made based on the pairwise comparison is its implications when developing ‘correction factor’ to adjust the light-scatter based real-time monitoring dust data. Correction factor in this discussion is the ratio of the pairwise gravimetric reference sampler and the average measured data by the light-scatter based real-time monitor, such as PDR1000. Therefore, the use of an unverifi ed gravimetric sampler for applying the correction factors for real-time light-scatter based instrument data can be misleading and the

Cyclone model D50

(μm)a D50

±SD (μm)a D50±SD (μm)b

HD Metal-Supplier A 4.78 4.78 (0.04)

4.74

4.82

HD Plastic – Supplier A 4.38 4.54 (0.11) 4.36 (0.05)

4.51 4.33 (0.06)

4.68 4.16 (0.03)

4.61

4.47

4.56

HD Blacked-Supplier C 4.67 4.66

4.66

HD Supplier C 4.91 4.92

4.92

HD Aluminium Supplier D 5.39 5.32 (0.09)

5.36

5.22

HD Plastic New Supplier D 5.64 (0.16)



HD Plastic Used Supplier D 5.68 (0.03)



PDM3700-HD Plastic 4.43 (0.05)



a. Source: Gudmundsson and Liden, 1998; b. Current data (HSE, 2016).

TABLE 7

Summary of historic and recent D50

test values of various

cyclone models including PDM3700 cyclone.


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10

conclusions reached can contribute to the ineffectiveness of engineering controls.

• Although it is not well known, due to inherent manufacturing differences between these cyclones and their geometry, there will certainly be consistent bias in the measured dust levels. Therefore, if there is a signifi cant measured bias rather than random variation due to dust gradient or air turbulence, this may lead to differences in interpretation and decision-making that ultimately impact the industry, dust control assessment and compliance determination of operations.

• The mass based PDM3700 operational experiences in US and Australian coalmines have been found to be of signifi cant benefi t to mine worker and to provide quick turnaround results for personal exposure assessment as they provide measurements in real-time thus addressing immediately any dust management defi ciency issues. In addition, if the operator decides to create an action trigger, for example if personal dust exposure reach 80 per cent of the limit, the PDM3700 instrument can give a warning and enable the worker to exit the workplace.

• The benefi ts of the PDM3700 are so substantial that having just a few such units in each mine can re-place the current suite of gravimetric-based sampling instrumentation for all types of mining conditions – where currently, results of exposure may take up to a week or two to yield results. Therefore, its approval for use in Australian mines as a compliance tool must be treated as a priority to benefi t coalmine workers.

Lastly, the paper also reminds the reader that careful handling and understanding of past dust exposure data for assessment is essential in deriving the ‘dose’ for dose-response studies. The paper also stresses the need for transparent compliance determination processes which include the measurement challenges which have been identifi ed.

ACKNOWLEDGEMENTS

The author would like to thank all the underground staff in the collation of data and reviewers (J Volkwein (USA), H Phillips (SA)) for their constructive comments and encouraging remarks. It is hoped that these comments will assist in the speedy implementation of the PDM3700. This paper and the work contained herein is an effort to improve the exposure monitoring and assessment to minimise worker exposure and improve engineering controls in workplaces. The opinions and/or conclusion expressed or recommendations made, do not necessarily refl ect policy or views of any operations but a professional opinion. The mention of any company name or product does not constitute endorsement by the author.

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