Designs for a deposition gauge and a flux gauge for monitoring ambient dust

Pefgamon

Afmsphrrrr

Entrironmenr Vol. 28, No. IS. pp. 2963-2979. 1994 Elsevicr Science Ltd

135%2310(94)E0102-P

DESIGNS FOR A DEPOSITION GAUGE FOR MONITORING

GAUGE AND A FLUX AMBIENT DUST

D. J. HALL,* S. L. UPToNt and G. W. MARSLAND Warren Spring Laboratory, Stevenage, Herts, SGl ZBX, U.K. (First

recriued

28 June

1993 and in final form

5 April

1994)

Abstract-The paper describes designs for a deposit gauge and for a flux gauge intended to monitor windborne dust. The designs are the culmination of a three-year research programme into dust and precipitation gauges. They are not yet fully characterised, but show both fundamental and practical characteristics which are considerably closer to the ideal than has been achieved hitherto. The deposition gauge resembles shallow gauges which have been described previously and are in practical use, but with the addition of a deflector ring around the gauge to improve the aerodynamic behaviour and of a foam insert to improve particle retention. The flux gauge is novel, having a through flow driven by its shape past a particle trap, the gauge shape itself also being a natural trap. The design also has directionally sensitive sampling propertics, which can be usefully used for directional sampling, and is resistant to collection in reversed wind flows when used in this mode. Key

word

index:

Dust, dust gauge, dust deposition, flux, dust monitoring, ambient dust, British standard.

NOMENCLATURE A d D F u u “3 X .z

piers with fractionate the sample and pull a known volume of particle-laden air through a filter paper (see,

area of dust gauge sampling inlet gauge opening diameter rate of vertical dust deposition horizontal flux of dust time wind speed undisturbed wind speed particle falling speed ambient particle concentration height.

for example, Garland and Nicholson, 1991; Mark and Hall, 1994 for a review of these samplers and techniques). For nuisance dusts and those concerned with secondary pathways it is usually either deposition to the ground or the flux of particles past a point that is of interest. Deposition is defined as (1) and flux as

1. INTRODUCTION

(2)

Pollution problems due to wind-borne dust from human activities are well known as a source of nuisance. In number terms they are one of the major sources of complaint, alongside odours. Wind-borne dust can also be important for health reasons, due to

where A is the area of the gauge opening, x the concentration of particles in the atmosphere, u, the particle falling speed, U the wind speed and t is time. It can be seen that the two measurements can only be related by way of a detailed time-dependent relaentry into the respiratory tract or as a secondary tionship between the particle size distribution (and pathway for the ingestion of toxic materials. Dust thus the particle falling speeds)being collected and the monitoring is thus an important practical activity for wind speed. Generally this is not available. pollution control purposes. The directly inhaled parThe vertical deposition and horizontal flux of dust ticle fraction is normally monitored using active sammay be determined by measuring the ambient particle concentration and scaling by an estimated deposition velocity (for deposition) or the mean wind speed (for *To whom correspondence should be. addressed. Now at flux). Sometimes deposition is measured directly using the Building Research Establishment, Garston, Watford, surrogate surfaces. for example Carey (1959), Steen Herts WD2 7JR, U.K. (1986),Wu et al. (1992) and poll et al. (1988). However, tNow at ABA Technology, Harwell, Oxfordshire, U.K. 0 Crown copyright(1994).

2963

there is a dtficulty in measuring deposition directly at the ground in that it is not possible IO distinguish between genuinely deposited material and that salta ted along the ground. Wu L’I tr/.‘s. procedure of using a surface on an aerofoil set above the ground avoids this, as does that of Noll er 111.. but both techniques probably suffer from some of the same deliciencies in collection as the dust gauges described later in thts paper. Deposition and flux are often monitored with passive gauges of the type described here. Besides attempting to measure the relevant parameters directly. they are relatively cheap compared with pump-driven aerosol samplers, and siting is not limited by the need for a power supply. If used correctly these simple passive gauges can be effective monitors. There are in any case difficulties with sampling the larger end of the atmospheric particle size distribution range. which is of the greatest interest for dust nuisance. with pumped samplers. Apart from the Wide Range Aerosol Classifier (Burton and Lungren. 1987). whose performance is presently unquantified and of which only three models currently exist. there are no fully effective. commercial designs presently available which measure the total atmospheric suspended particulate up to and beyond lOO/tm in size. Measurements of the larger size fractions normally associated with nuisance are often made using passive gauges set above the ground. Deposit gauges have a horizontal opening and flux gauges a vertical opening. Deposit gauges are normally in the form of a cylindrical container or funnel of some sort and a fair variety of them are in use, worldwide. The U.K. standard design (British Standard Institution, 1969). which is described in more detail below. is a funnel with parallel sides 300 mm in diameter about 200 mm deep. The German standard (VDI. 1990) is a cylindrical glass jar about 100 mm in diameter and about 200 mm deep. The US standard (ASTM, 1990) is a cylindrical container I50 mm in diameter with a minimum depth of ?OOmm, surrounded by a wind deflector set at an angle of 45 (this design and its performance is considered in more detail in Hall et (I/.. 1993b). The Irish deposit gauges are plastic funnels of 200 and 250 mm diameter (they are also considered by Hall or ul.. 1993b). The proposed IS0 standard (International Standards Organisation, 1991) is a cylindrical container 200 mm diameter and 400 mm deep, derived from the Norwegian NILU gauge which is of the same size but with a different lip to the opening. The performance of these latter two designs is discussed in more detail by Ralph and Barrett (1984). There is also a comparative review of the performance of some of these devices by Kohler and Fleck ( 1966). Flux gauges are less common, the only standard design known to the authors is the British Standard directional gauge (British Standards Institution, 1972). to be described later. However, related devices are used as traps for wind-blown

sand

and

soil,

for example

Fryear

(1986).

There is also a fan-driven “tunnel sampler” designed by Regtuit er trl. (1988). which is a cross between a flux gauge and a size selective aerosol sampler. It will also be appreciated that there is a similarity to precipitation sampling. though the average size of precipitation is much greater than that ofdust particles. which signilicantly reduces the problems described collection problems below. Precipitation gauges are very similar to dust deposit gauges (there is an interesting review by Sevruk. 1993). though they are usually set closer to the ground than dust gauges. There are also flux gauges. or semi-flux gauges for fogwater (Hering L’I ul.. 1987. review the performance ofsome of these), though they tend to be active rather than passive devices which do not much resemble dust flux gauges or soil traps. It is not the purpose of this paper to describe the practical use of dust deposit and flux gauges, but they should be used in combination to assess different aspects of wind-blown dust problems. Deposit gauges give information on local rates of deposition to the ground, whereas flux gauges indicate the passage of material past the sampling potnt. Flux gauges can also possess natural directional properties, which can be used to identify the source direction of wind-blown material. In practice. dust gauges do not collect as indicated by equations (II and (2). above. usually showing serious deficiencies in performance. These problems are described in more detail in Section 2. but mainly arise from wind effects. As remarked earlier. gauges have to be set well above the ground, typically between I and 2 m height. in order to avoid collecting locally wind-raised material, so are well exposed to the wind. The end result is a collection performance which is strongly wind speed and particle size dependent, giving rise to unrepresentative samples. The general trend is for collection performance to reduce as wind speed increases, which is doubly unfortunate as the amount of windblown material also tends to increase at higher wind speeds. compounding collection problems. At Warren Spring Laboratory we have been investigating the performance of dust gauges and the means of improving their performance intermittently for the last 20 years. Most recently. we have completed a three-year programme looking at the behaviour of both dust and precipitation gauges (which suffer similar, but less severe, problems than dust gauges). The bulk of this work is described in Hall rt ul. (1993a, b). It does not. however. cover the final stages in the programme, during which improved designs for both a deposit gauge and a flux gauge were produced. AIthough it was not possible to fully characterise these designs, both showed marked improvements in their behaviour over the existing British Standard gauges. They are described here as prototype designs which deserve serious consideration as potential replacements for the present standards.

2. EXISTING

RRITISH

STAVDARD

(AL’GES

AND

form of a cylindrical frame holding a plastic mesh which closely surrounds the bowl. The basic design is similar to some early rain gauges and has altered only a little from its first appearance around the turn of the century, when it was used for making some of the first systematic measurements of air pollution (Brimblecombe. 1987). Its performance was first investigated by Ralph and Barrett (1984). who measured the collection efficiency over a range of windspeeds and single particle sizes (of glass spheres) using a one-third scale model of the gauge in a suitably scaled wind tunnel experiment. At

THEIR

PERFORMAN(‘E

2. I.

Dcpo,sn

qLlu(/c’

Figure I shows a diagram of the British Standard deposit gauge (British Standards Institutton. 19691. The collecting device is a plastic howl of 3OOmm diameter with a funnel-shaped bottom. It is fitted into a metal stand which sets the gauge opening about I.2 m above the ground. It is also fitted with a collecting bottle. for any matertal washed out of the bowl by rainfall. and a substantial “bird guard” in the (h)

ibrd-guard

Collecllng

bowl

,th adhesive

g bottle 12m

FIN. I. The

British

Standard

dust

deposit

gaug

ifrom

BSl737 PI I). (a) The collecring the bird guard.

howl.

1h1 The complete

gauge.

1.0

0.6 Collection Efficiency

0

2

4

6 Wind

Fig. 7. Collection

performance

of the British

Standard

B

Speed

(ms ‘)

deposit

gauge

10

(from

Ralphand

12

Barrett.

1984).

Including

D. J. H.\LI

796h the

same

guge depth). (still

time a similar

gauge

and discussed

also

be which

they

referred

to same

Upton

(1958).

is the

sequent

work

described

gauge disperse tunnel

model glass and

samplers.

speed particles

of the

gauge

the

actual

described

In

briefly

as been

of the

determined. catch

an cloud,

be calculated The (found

ratio by

and

f’ormancc

the

sub-

nind

of

mono-

of

isokinetic

concentration the

the

of this weighing)

rate

idea

catch

ideal

catch

gave

speed

\vas

gcncrally

speeds for

20”,) that

wind the

for

wind

the bird

the

fore

they

collected action Removing gauge

bj

2

s

III

BS gauge and

the

gauge.

Also

fairI!

rcadill

due

wind-driven

the bird

and

markedly

and

litting

performance

,

2

per-

lightest

reduced

rapidly

It was

less than

‘. It

was particles

be-

gauge

lost

the to

found

inhibited

the

inside a battle significantly.

Fig. 3. (a) Sketch (based on flow visualisation) of llow pattern around British Standard deposit gauge collecting bowl, showing the rising streamline over the opening;(b) vertical velocity profile through the centre of the collecting bowl of the British Standard dust deposit gauge (from Hall and Waters, 1986).

the

the

dctlccting

circulation

guard its

collection

seriously

trapping

particles

improved

beyond the

The varied

Bcqond lX)~nn.

in Fig.

12 m s- ’ and

size

ctlicicncy

bclov~

on

reached

of the

particle poor.

speeds guard

Inn.

gauges

and

sizes

-1 and

100

and

collection

particle

performance and

deposition

and

wind

and.

X7

eftici-

is shown

between

the other

and

collection

gauge

speeds

between

of this both

mrasurcd

Standard

of \vind

sixs

etficicnq

Hall

in the

British

a range

with

the

Their

el?ic~cnc!.

of the

and

in

array

enq particle

of

to expose

distributed

ambient

particle

could

used

concentration with

the

direc-

detail

collection for

method by

was

this

standard

Standard The

This

dust

twice

IS0

later).

(ballotini)

monitored

falling

British

more

a uniform

NILU

and

as an

the

here.

Knowing

of the to

to

beads

the

diameter

proposed

(to

measurement. is

rxamined

700 mm

discussion)

gauge

which

also

of

under

tional

the1

(a cylinder

1’1 ,I/

scouring the gauge. inside

the but

it

remained rclati\el) poor. showing low collection eHiciencxs with ;L strong wind speed and particle size dependence. Both the NILU and the proposed IS0 gauges showed broadly similar performance characteristtcs. There are two principal reasons for the gauge collection performance described. The lirst is that the aerodynamic blockage of the gauge produces a rising and accelerating separation streamline over the gauge opening. As a result. particle trajectories are displaced away from the gauge opening and its collection etliciency IS reduced. A sketch (based on flow visualisation) of the flow pattern around the collecting bowl of the British Standard gauge is shown in Fig. 3a. together, in Fig. 3b with a velocity profile vertically through the centre of the gauge opening, taken from Hall and Waters (1986). The height and velocity scales in the plot have been nondimensionalised with respect to the gauge opening diameter, and the undisturbed wind speed, respectively. The profile shows the rapid rise in velocity through the free shear layer over the gauge opening to a maximum about 30% higher than the free stream at a height about 20% of the opening diameter over the opening. This phenomenon is common to all conventional collectors of precipitating material. which are usually in the form of cylinders or

funnels. It has been recopnised as a problem with rain or snow collection for over IOOyr. though due to the higher falling speeds of rain and snow the etTects are not so severe. Sevruk (1993) discusses this and also remarks on the limited attention that the problem has received with elevated precipitation gauges. It had. equally. received no attention with respect to dust gauges until the work of Ralph and Barrett (1984). The second reason is that there is a wind-driven circulation inside the gauge which tends to remove material already collected. There is a discussion and some experimental work on this subject described in Hall et (11. (1993b). The main point that will be made here is that the strength of the circulation depends on the ratio of depth to diameter of the gauge (which we have called its “aspect ratio”) and is at a maximum when the aspect ratio is approximately unity or a little less. The British Standard deposit gauge is close to this shape.

A diagram of the British Standard directional gauge (British Standards Institution. 1972) is shown in Fig. 4, it was designed by Lucas and Moore (1964) and was adopted as a British Standard shortly afterwards. It has four vertically aligned cylindrical collectors. ranged each pierced by a vertical collecting slot,

r

Fig. 4. The

British

Standard

directional

dust

gauge (from

BS 1747.Pt

IV)

D

2968

J. H.\II

at 90 intervals around a central support. There is a collecting bottle at the base of each cylinder to retain any collected rainwater. together with any dust it has washed from the gauge. It is essentially an arrangement of four flux gauges. as defined by equation (21. but it is erroneously described in the British Standard as a “directional deposit gauge”. Its collection characteristics (taken from Ralph and Hall. 1989) are shobvn in Fig. 5. for the collecting cylinder facing directly into the wind. The measurements were carried out on a one-third scale model at the same time as the work of Ralph and Barrett (1987) on deposit gauges. described above. As with the deposit gauge, these show a strong dependence on wind speed and particle size, but the pattern is different. For a given particle size. the collection efficiency rises to a maximum with increasing wind speed. then falls away, the collection efliciency at the maximum depending upon the particle size. Overall. the collection efficiency is low. Marc recent measurements using a half scale single collecting element, described in Hall et ~1. (1993a) showed even lower collection efficiencies than those in Fig. 5. This characteristic collection behaviour. passing thrcugh a maximum at some particle size in a given wind speed. is due to two features of the gauge: these are described briefly here. a more detailed consideration can be found in Ralph and Hall (19891 and in Hall er al. (1993a). The first IS that the gauge IS essentially a virtual impaction sampler. the flow at the collector entrance stagnates and particles are carried into the gauge by inertia. This behaviour can be and has been calculated (Bush et (I/.. 1976) and predicts. qualitatively but not quantitatively. the rising collection elficiency with increasing wind speed on th left-hand side ol

(‘I ,I/

the maximum. The %ccond feature of the gauge is the strong internal \vind-driven secondary circulation Inside the collecting cylinder. M hich ejects particles alread! collected. Thus is responsible for the falling collection characteristic on the right-hand side of the collection maximum and also for ;I reduced collectInn ellicicnc~. below the theoretlcal estimates. over the whole wind speed range. This internal circulation was the subject of ;I long investigation by Hall (‘I 111. (1993al. who examined a variety <~,fcollector shapes of this type. A shetch of the Internal circulation inside one of these gaupcs, taken from this wwk. is shown in Fig. 6. No design of this type of impaction collector with a simple internal cross section could be found which did not exhibit an internal circulation and no effective means could be found of preventing it short of loosely packing the inside ofthe gauge with glass wool. which was deemed to be impractical in use It was eventually concluded that this type of gauge was fundamentally unsuited to Hux measurement. The most effective virtual impactor type of design was found to be ;I short length of tube. with :I blanked-of rear end. facing Into the wind. Although still showing a sampling dependence on wind speed and particle size. this was both ;I Gmpler and a more etlicicnt collector than the British Standard design. Despite the problems described above. the gauge does have quite good directional propertics. Figure 7. taken from Ralph and Hall (19X9). shows thr collection etficiency as a function of the angle to the wind of the collecting slot. Although it shows some particle size dependence. there is a clear trend of CollectIon efficiency with wind direction. though it is below the ideal cosine characteristic.

-A

87um

---o0-

+ -0

285um bOOurn

No Partde Bounce With Bounce

Collection Efticiency

Windspeed

Fig. 5. Collection

performance

(ms ‘)

of the British Standard directional sizes (from Ralph and Hall. 1989).

dust gauge

with

four

partlclc

eflP0 Designs

for a deposition

Strong

and a Hux gauge

Internal

Trailing

Fig. 6. Sketch

of the wind-driven

circulation

2969

inside the directional al., 1993a).

dust

Vortex

gauge

type

Pair

ofcollector

(from

Hall

et

The latter approach is not practicable for dust monitoring or for precipitation measurement, where sam3.1. The deposit gauge ples are to be chemically analysed, as the surrounding The design described here is the end product of a disc is a source of additional material of uncertain number ofexperimental programmes on the collection provenance which can be blown into the gauge. of dust, snow and rain. These showed that it is only Our own efforts were concentrated on the altempossible to avoid the sharply rising and accelerating ative approach of greatly reducing the depth of the streamline over a conventional gauge set above the gauge. This reduces the aerodynamic blockage and ground by either greatly reducing the depth of the thus the rise and acceleration of the separation streamgauge or by fitting a plate or other isolating device line over the gauge opening. Hall and Waters (1986) around the rim (a form of displaced ground surface, as proposed a shallow collector-in the form of an inverted in the precipitation gauge design of Lindroth, 1991). frisbee (used simply because it was of the desired 3. IMPROVED

GAUGE

DESIGN.5

D. J. HALL

2970

-A

87um

-0150um ----+ -0

285um 4OOum

et al.

Collection Elflcienq

02-

20

40 Wind Directon

60

80

90

-0 -.25

(Degrees)

Fig. 7. Directional collection properties of the British Standdirectional dust gauge with four particle sizes (from Ralph and Hall, 1989).

.25

.s

1.25

-.5

Fig. 8. Vertical velocity profile through the centre of the opening of an inverted frisbee-type of deposit gauge (from Hall and Waters, 1986).

characteristics of the gauge are shown in Fig. 9 (from Hall and Upton, 1988). for a gauge with a sticky internal coating which improved the retention. Comparison with the same measurements for the British Standard deposit gauge, in Fig. 5, show the improvement in the collection performance that result from the shallower shape. Besides an improved catch, this shallow collector also showed better resistance to blowout than the British Standard gauge. Later experiments (Hall et al., 1993a, b) showed that, apart from very deep collectors, shallow collectors with about this ratio of internal depth to diameter were close to an optimum for resistance to blowout. A

Collectior EftiCif3fl~ (%)

z

1

UN,

ard

shape), which had a depth of about 17% of its opening diameter. This proved to have a much lower displacement and acceleration of the separation streamline than the British Standard gauge, it also showed greatly improved (though not perfect) collection characteristics (Hal) and Upton, 1988). A velocity profile through the centre of the opening of an inverted frisbeetype of gauge (from Hall and Waters, 1986) is shown in Fig 8. The reduced perturbation over the gauge opening, compared with that of the British Standard gauge in Fig. 3, can be clearly seen. The maximum increase in mean wind speed over the centre of the gauge was about 10% of the undisturbed wind speed and the rise in the stramline over the opening was about 10% of the gauge diameter compared with values for the British Standard deposit gauge, quoted previously, of 30 and 20%, respectively. The collection

.?5

4

6 Windspeed

8

10

(ms.‘)

Fig. 9. Collection performance of the inverted frisbee-type of deposit gauge with a sticky collecting surface (from Hall and Upton, 1988).

recent numerical Investigatmn hv Domhrowski c’t 111. (1993) of the internal Hews in t(vo-dimensional containers has come IO a similar conclusion. However. practical experience also showccl that rctcntion of collected particles was ;I problem and that effective drainage of rainwater and the avoidance of splashing during rainfall was needed. The Rat bottom of the frisbee-type of gauge dud not achieve this. In addition. some means was required of retaining particulate deposited into the gauge. The gauge shape was modified to include a curved bottom (Hall 1’1 (I/.. 1993a) which improved drainage. A cross section through this gauge design is shown in Fig. 10. Despite the slight increase in the depth of the gauge. the velocity vuriation over the opening and the collection performance remained the same as for the original frisbee design. shown in Figs 8 and 9. A sticky layer on the gauge inner surface was effective in improving retention (the collection measurements 111 Fig. 9 are for the gauge with a sticky coating). but caused some practical problems in recovering the sample. Also. none of these modifications reduced splashing during heavy rainfall. Later experiments with layers of open-celled foam in the base of the gauge showed some promise. Drainage was unaffected and splashing eliminated. whilst particles falling into the foam were resistant to blowout. However, a difliculty with the foam inserts (of about

All Dimensions

Fig.

10. Cross

section

IO mm thickness) in these existing gauge designs was that the Internal depth of the gauges was cxcessikel! reduced. However. llmlted field experiments (Vallack. 1993) have shown the foam inserts to be eflectivc In spite of this. During this period there was also an investigation of the design of a snow collector for precipitation studies (Hall (‘I (I/., 1987. 1989) which required a deeper collector to aid drainage of collected precipitation. To achieve the desired aerodynamic characteristics with a deeper collector, efforts were made to control the airflow around the gauge by a careful choice of shape. The two most important principles applied were, lirstly. of using a turned-in, horizontal lip to the gauge opening. This is a greater encouragement to obtaining a horizontal flow over the gauge opening than the usual vertical lip. The turned-in lip also assists the retention of collected particles by controllin_e particle paths inside the gauge. Secondly, attempts were made to shape the gauge so as to generate a downforce. which has the property of redistributing the flow around the gauge in the desired way. flattening the Row over the opening. This effect is not very powerful. but is nonetheless helpful. These design principles proved at least partially effective and it proved possible to produce a gauge with about three times the depth of the frisbee-type designs but with similar Roa

in mm

through

the modified

inverted frisbee-type of deposit (from Hall er al.. 1993a).

gauge

with

a curved

bottom

surface

2972

D. J. H\LI

characteristics over the gauge opening. For snow collectton. the gauge. through aerodynamically a good design. was too shallow to allow the accumulation of collected snow and utiliscd a low tcmperaturc heater to melt and drawn-offcollcctcd samples. It proved to be a quite good rain gauge. It was also tested as a dust collector by Hall and Upton (19XSl and proved to have a similar collection performance to the inverted frisbee-type of design. A cross section through this gauge design is shown in Fig. I I. The design also incorporated a boundary layer control device on the lip of the gauge (a transition trip wire) and an internal fence to control the internal circulatton and reduce blowout. The fence became desirable due to the tncreased depth of the collector reducing the blowout wind speed. At the same time that this work was in progress. Folland (198X) published a design for a precipitation gauge also based on a streamlined shape with greatly reduced depth. One of the main differences to the present work was that Folland used a sharp edged. lip to his gauge. compared with the inward-turned lips of the present designs. We believe this difference to be important as with the inward turned lip it is easier to obtain a horizontal flow over the gauge opening. it is less sensitive to the incidence of the airflow onto the gauge and it keeps a better control of the airflow (and retention of collected material) inside the gauge. In the more recent work on precipttation gauges, we

Boundary

Layer

Trip

1’1 111

have investigated the performance of flow dctlcctors around the paugc. which are litted to a number of prectpttation gauges. Thcsc arc intended to dellect downwards the How around the gauge. llattcning the Ilow over gauge opening and reductng the acceleration in wind speed. In practice dellectors proved useful if set at much lower tncidenccs (around IO&l5 ) than that usually employed (typically IS as in the ASTM dust gauge). Thctr main elfect was to reduce the acceleration in the separation streamline over the gauge. However. they reduced the vertical displacement of the streamline rather less. this was more effecttvely modified by reducing the depth of the gauge and shaping it more carefully. A combination of these two features. a shallow. carefully shaped gauge with a turned-in-lip and littcd with a flow deflector. is capable of producing the ideal aerodynamic gauge performance. a Hat unaccelerated tlon over the gauge opening. This was achieved vvith a precipitatton gauge (Hall (II rrl.. l993b). though the design was not entirely satisfactory. The Rat-plate defiector that was used in the design generated a leading edge separation which reattached itself to the leadtng edge of the collector. Besides not being very stable, this tlow pattern also left the possibility of material setting on the defector being carried into the gauge. A cross section through the improved deposit gauge design is shown in Fig. 12. The collecting bowl is a shallow streamlined shape, based on elliptic sections.

Wire Blowout

Prevention

Fence

\

Fig. I I. Cross

Deflector

Fig.

Ring

12. Cross

section

through

Collecting

section

;I snow

collector

(diameter

of opening

200 mm) (from

Hall 1’1 al.. 1987).

Bowl

through

the collecting

head of the improved

design

of deposition

gauge.

with a turned-in-lip havtng a horizontal edge. It is surrounded hy a deflector ring. set a littlc heloa the gauge openrng. The gauge opening is 71Omm rn diameter. The bottom of the collecting hovvl contatns a layer of foam. This acts as a trap for collected dust. mrnimising the loss of collected material. and also prevents loss by splashing in heavy rainfall. There is a central drain for collected rainwater. which is best collected in a ground-based container so as to avoid any unnecessary aerodynamic disturbance to the gauge. The gauge is intended to he placed with its opening between I.5 and .! m above the ground. in order to avoid collectrng locally wind-rarsed dust. From the previous discussion. the significance of the features in the new deposit gauge design in Fig. I2 should be apparent. The combination of a shallow. carefully shaped gauge titted with a deflecting ring provides a Rat, unaccelerated flow over the gauge opening. The turned-in-lip of the gauge assists in providing this flow as well as encouraging the retention of particles. The layer of foam inside the gauge eliminates splashing and helps particle retention. The gauge is also deep enough to accommodate this foam layer whilst having a ratio of internal depth (above the foam) to diameter of 0.21, a little greater than the optimum of 0.17 noted previously. This latter figure should include in the gauge depth the height of the rising separation streamline over the gauge openmg, which occurred in the experiments from which this value was derived. In the present design the rising streamline has been eliminated. so that the effective internal depth of the gauge is reduced. The gauge has been made proportionately deeper to allow for this. Also. the deflecting ring has been given an aerofoil-like

lO(

form. This results in an attached How over most of its upper surface. unlike the Oat plate deflectors usually used. which inevnitably have a separated flow at the leading edge. Apart from improving its performance. this also means that the flow around the deflecting ring passes under the gauge opening. so that any particles accretmg on the ring and then blowing ofi pass under the gauge rather than entering it. Although etTective for its purpose. the design of the deflecting ring has not been optimtsed and a smaller. better designed deflector may be practicable. However, the low chord Reynolds number (CCL 3 x IO’) at which the deflecting ring operates makes a good design difficult to achieve and there was not time to investigate this in the present programme. Figure I3 shows a series of vertical velocity profiles along a windward line through the centre of the gauge opening (at a wind speed of 2.8 m s - ‘). which show the very small perturbations in wind speed over the opening in this design. They are within 2% of the undisturbed wind speed. which is of the order of accuracy of the measurement. The wind speed directly above the gauge opening appears to be below the undisturbed value. but this is due to the velocity profle in the free shear layer spreading over the gauge opening. If the flow over the opening is truly flat and the free shear layer spreads equally above and below the opening. then on the line of the gauge opening the wind speed should be half of the undisturbed value, ).4ms-‘. It is close to this for all but the forward measuring station, 35 mm from the upwind edge of the opening, that is over about 80% of the centreline of the opening. The combination of the foam insert and the optimum internal depth of the gauge appeared to give

‘r

Height Above Gauge Opening (mm)

-50

Distance

Fig.

from Upwind Edge of Opening

13. Vertical

AE 28:18-F

velocity

35mm

profiles

over

the opening

of the new deposit gauge design, the centre of the opening.

measured

along

the windward

axis through

II very

effective

to

blowout

the

glass

retention.

spheres

as if they

(hallotlm),

had

been

to the maximum Earlier

had

of the

also

new

design.

due

with

the

teristics been

and

The

reststancc

bolw,

and

the of

not

up

foam

layer

water

possible

to

efficiency of

the

is

a

of the of

model

is aerofoil-llkc

make

of the

chord

this

characso far

shape lower

are

part

half-ellipse

of

half-depth.

I7 mm the

opening spun

is also

Dimensions

The of the

and pauge

nest

10mm

was

radiused

efTective

overall is B

IS quite

angles

(around

radiused

was

Hat

45

The I mm. be

highest

part

the

opening

and

the

gauge

of about

1 mm

tilters:

of construction

for

Fairly

the

but

per

the

in gauge.

this easy

porosity

proved

to remove

set of

commonly most

the

Diameter 250

er 1 Oppi 1 Omm Thickness 23 e Text for Details

Bowl for Deposit

1 16 Radius

)

1

Gauge

Inner

I Outer

Dia 300 ?-7

I

Dia 500 G

(b) Deflector Fig.

14. DimensIonal

details

of the new

Ring for Deposit design of deposit deflector ring.

Gauge gauge.

(a) The

collecting

bowl.

for

effective.

from

43

Collecting

inside foam

12

(a)

below

foam used

Depth

La

also The

20 mm

polyester

-T Overall

Foam

a thickness

in mm

Overall

are

aluminium.

dust

tb)

.

large

it could

layer

the

an 6.5

very

had

spun

collected

gives

precipitation

critical: was

sort

the

70mm

previously. on

The

IO mm

about

the

noted

open-celled

of the

This of

with

deflector

the

inch.

I5 mm.

IS not

a fairly

inncr

detlector

this

to outer

remaining

;I>

shape,

by 7.5 mm.

IncNectually

manufactured of

kept

deflector

I which.

length

of the

The

compared

is a co;lrse

10 pores

the

experimental

of

been

b>

for

rather

of about

3 chord

downwards

and

small

collectors.

halfesperi-

was

used

readily

has

The

diameter

details

of munufacture.

downwards

which

20 mm The

but

case

incidence

is 55 mm

is ZlOmm. form

collecting

XOmm

in alumlnium a good

in cross

the

turned-in-lip

half-depth

depth

shown of

for

the

in the

well.

csternal

giving

shows

used

survived

mm

dlamcrcr. I4b

form

of sh;lpe and

IS of 500

internal Figure

typically

total was

This

300 mm

of IO0 mm. which

research

it has

and

frisbee-type

f-‘ig.

simple

to a gauge

that

deflector

drops for

approach

blowout

gauge

43 mm

diameter

thickness.

IOb.

of aerodynamic

14. The

14a.

Thus

mental

that

closest to

of the Fig.

quarter-ellipse width.

ring.

luyer

to achxve.

Fig.

diameter

aIuminium

the foam

termination

combination

details in

the

anodlsed

tunnel

collection

it is the

desired

possible

section

to

but

the modified

was

of IO m S- I.

It MS of

use.

field

occurred

the

measurements

lield

Using

wind

shown

gauge.

design

high.

no blowout

splashing

programme.

the

collected.

of this very

over

eliminated into

direct

rexlstance proved

sprinkled

speed

tests

completely poured

The

of particles

the

of dust It

foam

is by

.Ihout

50

(I mm) opcn~ng.

There

is prchcntl! for

sland

the

bct\tccn

gauge.

I.5

and

of 2

15 dchirahlc

that

aerod\

namic

abo\c

III

for

111

dr\ign

~h~~trld

It 2

use ;I height

I;)rmal

no

With

bc 5~1 \\ ith

the ground.

modlliecl

there

frishcc-t!

should

hlochapc

of

not

collectins

hottlc

around

mndtlied

frlabcc-thpc

gaugcb

mount

which

v,ill Thih

stand.

the

lit

it

could pauge.

due

The

outside best

stringing

gaupe

i>

opcninp)

strike

pre\cnter

slightly

slack)

national 19X6).

This

is the

\\ater bclo\r

simple

the

chemistry metal

the and

arran@cment used

and

ring

for

of

opening the

U.K.

network outside

the

and

-

P

passiic

sampler

inp

tube n\< ind

end.

\\ith

so that all

hut

isokinctic

sampler

substitute

snow

for

applied particles.

tubular

led box.

additionally

helpful

tial drvelopmcnt Hall ment

(lY93a).

design

this

stages paper

box.

wedge-shaped towards

angle

The and

flat,

of 24.5

the

whole

slot

of 80 mm

box)

horizontal

200 mm width

entry at the

height

set at the top

The

of the

top

is a slot front

(also

over

The

of the box

and a baffle

is 360 mm

upwards

of 36 mm

vertical

flux

elevation

box whole face

is

is a simple

holding

slopes

in

design

improved

design in

the

ini-

develop-

final

the

of the

Rat

the

rear

bottom

wide.

The

the

a of

is described

16. The

sides

extended

a

number

;I

of the gauge

of

in Fig.

high of

features.

and

in elevation

is shown

and

principle

with long

gauge

after

below. section

design

inherent later

dewith

development,

possessed

the

original

demands

diHerent

design

a flux carried

produced

the

parallel-sided plate.

for deposl[ 1987).

but

a

of

isokinetic

which

of this

followed

described A cross gauge

to

this

lirst

Steen’s

However.

in

work

were were

approaching

of a tunnel

reported

development

gauges

eticiencies. gauge

also

present

following

flux

conditions

the

success-

of a passive particles

Experiments

shapes.

etTective

collection

to

(unfor-

in wind

was

the

were

not

used

N;IS

design

(this

aerosols

sampler has

It

to was

principle

small

lo\+-denstt)

In

a1so

Ho\+

design

pol\st>rene)

gauges

\vas

wedge-shaped

Arrangement

large.

1990).

dust

design Inlet

a duct

the

a-

particle

proportional

for

use.

tube.

tube ;I

passive

The

in

(espanded

on snow

and

entr)

further

Upton.

of the

past

principle

or ~1. (IYXY)

as its

the

tube

sampler and

in a

a coIIectthe Hare.

rear

sampler

ingenious

much

Hall

for

the

speeds.

sampler

this

had

on

wind

They

(1977)

across

a near-[sokinetic

pasG\e

;I

gauge.

trap.

out\vards

the

at

diKcrcncc into

the

around

1115s. The

ambient

and

Hared

prcssurc

was

IO de-

used

accclcratcd

tlou

10

tunatell)

wind,

the

31

dcsigncd).

This

through

applied an

by Steen

preshurc

maintained

sign.

the M Ind

(Hall.

in~lcad

particle

used

virtual

gauge

through

tlom

tlow

speed.

by

Into

of

turncd

aerosols.

The

the carlier

t!pc

small

;I

the wind

thih

:I

small

rcduccd

\rith

an internal

pressure

:I

produced

(both

uas

isoklnetic

a11

practical

IS. Sketch of bird guard and stringing (from Hall. 1986, after Asman.

ring

a ct>n\rni-

dcscrihcd

originally

facinp

;I

lowed

out

Metal ring wit&equispaced ins 25mm long for stringing Ii ing is sat 50mm above gauge opening

Fig.

struts

producing

\\a

past

for

resulting

gauge

\

attcnt~on

principle

dou

ws

of

there

;I

principle

/

Stringing

,A

in Fig.

supportIng

ax.\ociatcd

Jc\elopmcnt

h) the wind,

Hall

II \

pcrfcct.

of the rlcllector

dianictcr).

gauge.

e\perimcnts Stringing A

rhc

rim

Ilu\

and

utilised

trap

of

in which

driven

fully

-5OOmm

not

tint

gauge

I\ \ho\rn

ring.

outer

the dlllicultlc\ t!‘pe

IYc)!a).

The

British

aboLe

gauge

hut

;I

the

qrrrrqc

f-~~lla\\ing impactor

producing

bird

;I

cll?cttvenrss.

the

precipitation ;I

rain well

and

(set

o\‘er

for

blockage. on

of the

\rlth o\c’r

dc+n.

//II\

7-/I,)

signs

practice

rcqulres

of Ilmited

L’I trl. (1YXZ) uses

The

unacccptahlc

I>

(left

secondary

drain 111

clT’cct~\c.

i\ of the \;inic

,thandoncd

a simple

pround.

gauges

stem 25mm

to

used

some

The

about iratcr

alho

or

short

;I

3.2.

x11

gat~pc.

acrod!namic

bird

h> Asman

li\cd

design.

gauge

011

gauge

proposed

(lf

bc

structure

used

the

laqc

the

deposit rins

(Hall.

its

massl\c

Standard simple The

to

ptpc

the

on

It

srcat

co~l~~‘n~~‘~it

Ned

set

the

and

arrangcnicnt

dianictcr

nicch;inIcaI

ciit

structure

end

quite

l’or the

too

pro\4

support

ttrung

around

NC

pc gauges. hc

on

mounting

is best

mounting.

guard.

can

pro\cd

ha\

hattlc

Besides

plastic

both

;15

he used

collecting the

;I

whtch

act5

gauge.

and

into

diamctcr.

opening

prcscnt

\upportlng

a11)

rain\rater

external

it\

at

gauge.

run

\triiiging

lhis

alho

(M hich

ol’ supportlnf

ha>

of the

could

the

lint

It

diagram IS.

aho\c

111111

plastic

height

and

the

at an over exit

width

of the

which

forms

a

Sides Extended lo Backilow Preventel Top and Bottom Open Backflow

Preventer

i Wind DIrection ‘: 150

1 36

t 90 Air In 1

T

the rear of the box. The top of the box overhangs the exit slot by 80mm. The sides of the box extend rearwards by 200 mm to carry the vertical baffle plate, which is of ISOmm depth with its bottom edge set 90 mm above the bottom of the box. The box contains a particle trap made from IO poresin- ’ open-celled foam, sprayed with a thin sticky coating to retain any impacting particulate. The layer of foam is 30mm deep and is set with its rear face 40 mm from the back face of the box. Because of the external shape of the gauge, there is an accelerating flow over its outer surfaces. This produces a low pressure in the base region where the outlet is situated, providing a pressure difference across the front and rear openings sufficient to drive a flow through the gauge and to additionally overcome the pressure drop of an internally fitted particle trap (the layer of porous foam). Because the design is passive, the flow through the gauge is, nominally, proportional to the wind speed, satisfying equation (2), for flux. With the present design, the inlet velocity is about 80% of the forward wind speed, Thus the sampling is slightly sub-isokinetic. However, since it is large particles with relatively high inertia which are mainly being sampled, the inlet efficiency remains high. The gauge shape is additionally a naturally good particle trap. Particles enter the gauge low down, so are encouraged to deposit on the floor of the box. The internal wedge-shape acts as a diffuser, reducing internal air speeds which further encourages deposition to the floor. It also reduces the air speed though the foam trap, reducing its pressure losses, so allowing a large flow rate through a relatively efficient trap. The pressure drop across the foam trap additionally improves

the elTectiveness of the diffuser. whtch otherwise has a too rapid rate of expansion to retain an attached flow. Besides the foam trap itself, the bottom corner at the rear of the box is also a natural particle trap. After passing through the foam. the airtlow is dtrected upwards towards the exit. so that this region acts as an impaction collector. It is important that if the gauge is fixed in direction (the modes of operation are described later). then when the wind is reversed over the gauge there should be minimal particle collection. The size and position of the baffle plate, in combination with the overhanging upper surface of the gauge. acts as a back-flow preventer, producing a stalled airflow in the gauge. When the wind direction is reversed, there is no flow through the gauge in either direction. In reversed flow. the baffle plate also produces a strongly rising and accelerating airflow over the exit opening, which is effective in reducing the particle collection performance just as it is with conventional deposit gauge designs. Also, there is only a very limited direct pathway into the exit opening for particles with high inertia. The overhang of the upper surface of the gauge beyond the exit opening also helps to prevent the ingress of rain. Direct measurements showed that the collection efficiency of the gauge in reversed flow was less than 10% of that when facing forwards into the wind. The baffle plate appeared to have no significant effect on the gauge’s performance in the forward wind direction, because it did not encroach on the path of the airflow out of the exit opening. Limited collection efficiency measurements for the flux gauge were made with a half-scale model using correctly dynamically scaled wind speeds and particle

Dcslgns

for a deposition and a flux gauge

sizes. The details of the procedures are described in Hall er 111.(1993a). following the principles outlined by Ralph and Barrett (1984). The results of these measurements are shown in the bar graph in Fig. 17. for two wind speeds and two particle sizes. The bar graph also shows measurements made at the same time with a half-scale model of a single-slot collector of the existing British Standard directional gauge. these are shown as the shaded lower parts of the bars. These latter measurements give lower collection efficiencies than the earlier measurements of Ralph and Hall (1989). shown in Fig. 5. The matter is discussed in more detail in Hall et ul. (1993a). It can be seen that the new gauge design gives high collection efficiencies, around 8OG90%, with very little effect of wind speed and only a limited variation due to particle size. These variations with wind speed and particle size follow the form that might be expected considering the effects of particle inertial on the collection. However, since the accuracy of measurement of collection efficiency was estimated to be only within 10%. they are also almost uniform within the order of accuracy of the experiment. By comparison. collection efficiencies of the present British Standard design are low, between 20 and 40%, and show a greater variation with both wind speed and particle size. There are two possible modes of operation of a flux gauge. The first is to collect the total flux of practicles past the sampling site, irrespective of wind direction. The second is to determine the flux of particles from some specific direction. In practical use, the most common need is to attribute the flux of particles from

0

New Design

q

British

Standard

Directional

Gauge

81-k’

r

-

80

60 Collection Efftclency (%I 40

Fig. 17. Bar graph ofcollection performance of the improved design ol flux gauge, compared with that of the British Standard directional gauge, for two wind speeds and two particle sizes.

Fig.

18. Photograph

2971

of a commercial gauge.

version

of the flux

some specific source. To do this a pair of flux gauges would usually be used, one omni-directional gauge recording total flux and one fixed gauge directed at the source of interest. The contribution of the source of interest to the total flux can then be determined. The fixed gauge must resist collection when the wind reverses, which is achieved by the baffle plate acting as a backflow preventer, as described above. The omnidirectional gauge must rotate into the wind. This can be done by pivoting the gauge and fitting it with a vane to drive it into the wind, though careful design is needed to achieve this at low wind speeds. A commercial design of this gauge, which has already been produced, is shown in the photograph in Fig. 18. The vane is set below the gauge and there is a forward counterbalance weight set below the inlet. It is manufactured of aluminium and is sufficiently light and well balanced to turn into the wind at all speeds above 1 m s- ‘. The top of the gauge is removable for access to the foam trap and to clean dust out of the rest of the interior. There are some other practical matters to be considered for field use. The height of the gauge above the ground needs to be between I.5 and 2 m, for the same reasons as for the deposit gauge. The British Standard directional gauge is set a little low in this respect. Some arrangement is needed for retaining and draining

r> .I

297x

H .\I I l’,
collected preciprtatian. Although the gauge is designed to avoid rain collection. it would bc a goad collector for line precipitation (fog or dri/rle) and this must be drained and collected, together with an! particulate run-or. The easiest way to do this would be to tilt the gauge slightly backwards and to one side (a few degrees should bc sulficlent), so that an> precipitation collects in a back corner and can be drained off. This would also prebent water running out of the front of thr gauge and carrying particulate with it. In the wind tunnel experiments a silicon grease spray was used for the sticky coating. This can be inconvenient in practice since an organic solvent is then needed 10 wash out the collected dust and this has caused some minor difficulties with the field USC of deposit gauges. However. since in this gauge the trap is enclosed. glycerin (which is water soluble) may bc a satisfactory alternative. One linal need may be for a coarse mesh of line threads over the inlet and outlet slots to prevent birds from nesting in the gauge. These features are Incorporated Into the commercial design. The dimensions given are for the gauge corresponding to the scaled experiments for which the collection efficiency measurements of Fig. I7 were obtained. However, the gauge is probably not very sire cr-sii;.v,e as long as the shape is maintained. In principle. a smaller gauge should have a higher collection efliciency in this sort of design. though the smaller gauge size results in a smaller sample. If the gauge size is altered. the depth of the foam trap. and its pore size. should remain the same as this produces the same pressure drop coeticient. which is a constant irrespective of the gauge size.

4. CON(‘I.l'SIO~s ( I ) The deficiencies in performance of the existing British Standard ambient dust monitoring devices. the depostt gauge and the directional gauge (a type of flus gauge), have been described and two improved designs offered. (2) The improved deposit gauge is a shallow bowl with a turned-in lip to IIS opening and a flow deflecting ring to improve its aerodynamic performance. It is lined with a layer offoam to improve particle retention and reduce splashing. Its aerodynamic performance is close to the ideal. having a nearly Hat. unaccelerated Row over its opening. It is also very resistant to the blowout ofalready collected material. Direct measurements of its collection efficiency have yet to be made. (3) The improved flux gauge is a passive. wedgeshaped device. designed to drive a flow through the gauge past an internal parttcle trap (a layer of foam). The shape of the gauge itself also has natural particle trapping qualities. The flow into the inlet is subisokinetic (about 80% of the wind speed) and remains proportional to the wind speed. Limited measurements have shown its collection efficiency to be high, cu. 80-YO%, and largely unaffected by wind speed or

partlclc ~17~‘. II i\ littcd wtth a baltlc plate at its rear \\hlch c!Tectlvel! prc\cnts any internal 0ow or particle collectiun when the ulnd dIrection IS re\,erscd.

Buah A. W., Cross M.. Glhson R. D. and Owbt A. P. (1976) The collcctlon cllic~cnc) oldlrcctional dust gaugea. .Irfn~j,\p/,lTl‘ f:rlr~lrlmnlc~rrr 10, 997 IOOO. Carey W. F. t 195’)). Atmospheric deposits In Britain. a study in dingincha. //jr ./. :tlr P0ll~rr. 2, I-26. Dombrowskl N.. Foumeny E. A.. lngham D. B. and QI Y. D. (lYY.1) Modclling or no\v charactcristicb within deposirion gauge.\ under ~IOH.IIUI cc~ndltlon~. :lrrn~~\phcri~~ Er~rirorlmwr 27:~. 1435 Ia? Folland C. K;. (19x8) NumerIcal models or the raingauge exposure pruhlcm. lleld espcrimenrs and an improved collector dosIgn. (! J K. .Ifcr. Ser. 114, 4X5- 1516. Fryrar D. W. (19X6) A field dust sampler. J. .SIJI/ Il’urrr C‘r~rl.\l~r~~urfrllI 41 Garland J. A. and Nicholson K. W. (1991) A review of methods l%r sampltng Iarge airhornc particles and associated radioactivity. J. .4rrr1sr1/ Sci. 22. 479-499 Hall D. J. (19X6) The precipitatmn colleclor t%r use in the secondary natIonal acid deposItIon networh. Report No. LR56l (AP)M. Warren Spring Laboratory. Hall D. J. and Waters R. A. 11986) An Improved. readily available dusr gauge. .4rfnr~sphcrlc ErlrrrrJ,lnlrnr 20. 219~221. Hall D. J. and Upton S. L. (198X) A wind tunnel study of the particle collectton eHiclency of an Inverted frisbee used as a dust collector. :lrn~o.sphrrir~ t‘nrir~~!tmrrlr 22, I383 - 1394. Hall D. J. and Upton S. L. (1990) A passive isokinetic sampler. In Prr~c. .4er0.s0/ S0cirry 4rh .4!1n. C0r!/.. ‘.4rro.so/s. -Ihrir Genrrurii~rt. Bdlariour ufld Applrccrrims’. University of Surrey. 9-l I April. Hall D. J.1 Cotlrill S. M.. Goldsmith A. L.. Upton S. L.. Waters R. A and Wright P. (1987) The develonment and ticld use of a snow collector f’or acid precipitatihn studies. Report No. LRSRS(AP). Warren Spring Laboratory. Hall D. J.. Upton S. L.. Campbell G. W.. Waters R. A. and Irwin J G. ( 1989) Further development ora snow collector For use in acid precipitation studies. Report No. LR752 (PA). Warren Spring Laboratory. Hall D. J.. Upton S. L. and Marsland G. W. (1993a) Improvements in dust gauge design. In Meusurrmrnrs 0.l