Note: Descriptions are shown in the official language in which they were submitted.
7 ~ S S
BACKGROUND OF THE INVENTION
This invention is concerned with a device for
continuously monitoring the exposure of a carrier of the device
to an airborne contaminant. The need to insure that workers in
industrial environments are not subject to excessive dosages of
dangerous gases and vapours is well known. To achieve this end
in the past it has been the practice to provide a container of
activated charcoal or other absorbent to be worn by a worker
along with a pump which directs air through the container. At
the end of the day, the charcoal would be removed and the
contaminant which had been adsorbed upon it would be removed for
subsequent and rather costly analysis, as for example by
chromatography. This equipment was bulky and of course the
analysis was not completed until, at best, days end and usually
not until several days later, so that there was no immediate
indication of the dosage to which a worker had been exposed.
In recent years a so-called passive monitor has been
devised and is currently used which comprises a container of
activated charcoal having one side closed by a membrane (e.g. of
silicone rubber) through which contaminants diffuse to be
adsorbed on the charcoal. This device was small and could
conveniently be worn close to a worker's breathing zone.
Unlike classical dosimeters which use a pump to bring
air sample into contact with a collection element or a detector,
passive dosimeters rely on diffusion for this. Since the
"diffusion" rate of a gas or vapour is very sensitive to the
-- 2 --
~A`
~,~"j"",. . . .. .. . .
.. , ~ . . . .
.; . . . . ..... .
,. ~ . . .. . ..
~, ~ . . " ....
.. . . . . . .. . .... ;
,. . -, ; .. . , ~ .. ~
. ~ , . .
.. i ., ~ `. .. .
- 1 1 5 1 3 5 5
flow of air over and around the dosimeter, a membrane or other
limiting resistance must be located between the bulk air and the
collection element. The rate of mass transfer to the collection
element is now controlled by the rate of diffusion or permeation
across the membrane and the effect of face velocity past the
dosimeter if effectively eliminated.
Passive dosimeters of gas or vapour exposure have clear
advantages over the more classical methods of personal
monitoring in the workplace. Cumbersome pumps and tubing are
avoided, monitoring is less expensive and more reliable since
neither calibration of a pump nor charging of a battery pack are
needed. However, such dosimeters have the disadvantage of a
need for follow-up analysis to obtain quantitative results.
This is particularly true for activated charcoal based passive
monitors for which the adsorbed vapour must be desorbed in
carbon disulphide for quantification by gas chromatography. Not
only is there a significant delay (1 day to 1 week or more) in
obtaining the result thereby, but also the cost of analysis is
very high.
Widespread use of these devices is clearly hindered by
both this cost and the inability to obtain the result
immediately at the end of the worker shift, or even during the
shift. The ability to inform the worker that he has exceeded
his daily allowable dosage before his shift is actually
finished, offers an interesting potential to control his
exposure and therefore his health to a much finer degree.
- 3 -
.
:,
-- ,
. ~ . . .
-. . . . ..
-`: 115735S
Gas chromatography is not the only method used to
quantify the amount of gas or vapour collected by the passive
dosimeters. A dedicated spectrophotolneter has been proposed for
detecting NO, S02 or NH3, while spectrophotometers, specific
ion electrodes or wet chemistry methods are necessary for other
devices and the changing resistance of gold foil is the basis of
a device for ~g detection. While the analytical methods may be
simpler and quicker than gas chromatography, they still provide
an additional cost and delay in quantifying worker exposure. A
recently developed CO monitor changes colour when the TLV
exposure is exceeded and is an initial attempt to address these
limitations.
It is an object of the present invention to provide a
device effective continuously to monitor the exposure of a
carrier of the device to an airborne contaminant so that one may
have at any time an immediate indication of the dosage to which
the carrier has been exposed. The present invention also seeks
to provide a method for continuously monitoring the exposure of
a worker to an airborne contaminant.
According to one aspect of the present invention there
is provided a device for continuously monitoring the exposure of
a wearer of the device to an airborne contaminant, said device
including an elongate transparent length-of-stain indicator
means for providing a visual indication of the amount of
airborne contaminant to which said device has been exposed, said
length-of-stain indicator means comprising:
. ., j :, ,
"`~ 115735~
(a) an air-impermeable elongate container comprising a
transparent elongate tubular portion closed at one end
thereof and having an end portion at an opposite end of the
tubular portion, said end portion widening towards said
opposite end to define an opening having a cross-sectional
area greater than the cross-sectional area of the tubular
portion, said elongate container being air-tight except at
said opening;
(b) reagent means accommodated in said transparent elongate
tubular portion for producing a colour change in response to
exposure of said reagent means to said airborne contaminant,
said colour change producing a stain in said reagent means
observable as a length-of-colour stain extending along said
elongate tubular portion as an indication of the amount of
said airborne contaminant to which said device has been
exposed; and
(c) membrane means extending across and covering said
opening for controlling diffusion from the atmosphere into
said elongate container, said membrane means being permeable
to said airborne contaminant to permit penetration of said
airborne contaminant into said transparent elongate tubular
portion and into contact with said reagent means, the
membrane means, the end portion and the opposite end of the
tubular portion defining an air space, said membrane means
being separated from said transparent elongate tubular
portion and from the reagent member by the air space.
- 4a -
Jll/ i,'
.
`,' ` , ~'~ `'.; ' : '
` : ,., :,,
: ~ , ` : '`
l 157355
Preferably the reagent is carried on a gel or other
carrier or packing and, by changing colour upon exposure to the
contaminant, indicates by the length of tube over which the
reagent has changed colour the dosage to which a wearer of the
device has been subject.
The material of the membrane and its size is selected
so that the quantity of air passing through it and reaching the
reagent during a normal working period is that which will, if
the air is contaminated, produce a change in colour of the
reagent over a length which is easily measured.
Further according to this invention there is provided a
method of continuously monitoring the exposure of an individual
to an airborne contaminant, said method comprising:
(i) attaching a device to a wearer, said device including
an elongate transparent length-of-stain indicator means for
providing a visual indication of the amount of airborne
contaminant to which the device has been exposed, said
indicator means comprising (a) an air-impermeable elongate
container comprising a transparent elongate tubular portion
closed at one end thereof and having an end portion at an
opposite end of the tubular portion, said end portion
widening from said opposite end to define an opening having
a cross-sectional area greater than the cross-sectional area
of the tubular portion, said elongate container being
air-tight except at said opening, (b) reagent means
'
'
``` l 157~S
accommodated in said transparent elongate tubular portion
for producing a colour change in response to exposure of
said reagent means to said airborne contaminant, said colour
change producing a stain in said reagent means observable as
a length-of-colour stain extending along said elongate
tubular portion as an indication of the amount of said
airborne contaminant to which said device has been exposed,
and (c) membrane means extending across and covering said
opening for controlling diffusion from the atmosphere into
said elongate container, said membrane means being permeable
to said airborne contaminant to permit penetration of said
airborne contaminant into said transparent elongate tubular
portion and into contact with said reagent means, the
membrane means, the end portion and the opposite end of the
tubular portion defining an air space, said membrane means
being separated from said transparent elongate tubular
portion and from the reagent means by the air space; and
(ii) checking the quantity of contaminant by observing the
length-of-colour stain in said reagent means to derive an
indication of the time weighted average concentration of the
contaminant to which the individual has been exposed.
An embodiment of the invention as illustrated,
schematically, in the accompanying drawing in which:
.
. - . . . .
. ::
.
,
- , . , . :.
.
- 1 1 ~j r~ 3 5 ~j
Figure 1 is a cross-section of a device embodying to
present invention;
Figure 2 is a cross-section view of an alternative
embodiment of this invention; and
Figure 3 comprises side and end views of a further
embodiment of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The device in the drawings comprise a transparent tube
10 closed at one end 12 and open at opposite end 14 and having
graduation or calibration markings 15. Within the tube there is
inert packing 16 carrying a reagent responsive to a contaminant
to be monitored by the device. Secured to the open end of the
tube 10 is a bell-like structure 18 the larger end 20 of which
is closed by a silicone rubber membrane or diaphragm 22.
In operation a worker carries the device in his pocket
or pinned to his clothing as close as possible to his face, so
that the device is in effect sampling or monitoring the
atmosphere in the region where it is being breathed by the
worker. AS the contaminant diffuses through the membrane 22 it
reaches the reagent on the packing 16 and the gel changes colour
progressively, as indicated by a stain front 17. ThuS, the
distance represented by the length of the tube over which the
reagent has changed colour is directly related to the quantity
of contaminant passing through the membrane 22 and reaching the
reagent.
Typical contaminants to be monitored and the reagents
used in that monitoring are listed below:
'AI
1 1$7~5~
Contaminant Reagent
carbon dioxide crystal violet/hydrazine
benzene iodine pentoxide/sulphuric
acid
hydrogen sulphide heavy metal salts
carbon monoxide* selenium dioxide/sulphuric
acid
vinyl chloride permanganate/o-toluidine
ammonia acid/bromophenal blue
etc.
*drying or precleanse layers are required to minimize
interferences.
It will be recognized that the device of the present
invention gives a continuous indication of the dosage of a
contaminant to which a worker has been exposed.
Thus, the present passive dosimeter provides a
continuous readout of the exposure. The accumulated dose is
read as the length of coloured stain on the tube 10, specific to
a particular gas or vapour.
The dosimeter is a combination of the gas indicator
tube 10 and the membrane 22. The gas or vapour diffuses through
the membrane to the colour change reagent dispersed on an
appropriate support in the gas indicator tube 10. Reaction
between the diffusing gas and these reagents causes the reagent
on the support to change colour. As more gas or vapour diffuses
into the indicator tube 10 more of the reagent changes colour,
-- 7 --
,
, ~ ,
5 $
with the volume of coloured reagent and hence the stain length
related to the amount of gas or vapour to which the dosimeter
has been exposed (i.e. the dose).
The membrane 22 should be large enough or have a
sufficiently high permeability to permit sufficient gas to move
through the membrane 22 to react with the colour change
reagents. Silicone rubber has been found appropriate for CO ,
benzene and NO . The support should be able to retain
sufficient colour change reagent yet not adsorb the diffusion
gas. This is particularly important for the low concentration
detectors (e.g. benzene) where adsorption of vapour without
reaction is significant and essentially slows down or stops the
movement of the stain down the gas indicator tube. Glass bead
or clay supports are preferable to silica gel in these cases.
At high stain lengths, the calibration curve begins to
level off. As the stain length increases, diffusion of the gas
in the indicator tube 10 becomes limiting with a consequent
reduction in the slope of the calibration curve. The indicator
tube 10 should be packed with support in such a way as to
maximize the diffusion coefficient in the tube 10. Alternative
methods of supporting the reagents to increase the useful
measurement range of the device are available.
Since the limitation to long stain lengths is the
p~ck~d
resistance to diffusion through a long tube ~a~e~ with solid
support, means to increase the diffusion coefficient in the
indicating layer will be greater as the stain length scale will
- 8 -
~ ;
5 ~35~
be expanded over a longer tube. Unfortunately, the obvious
variation of support mesh size has a rather limited effect on
the diffusion coefficient in the tube. Furthermore, very large
particles are capable of retaining only very small amounts of
colour change reagents and are therefore not practical.
Alternatively short lengths of capillary tubes (e.g. glass) can
be used as supports for the colour change reagents. Provided
s sufficient loading is obtained, the diffusion coefficient be
. identical to the relatively high diffusion coefficient in air.
The specificity of the dosimeter is limited by the
specificity of the colour change chemistry. For example, the
; benzene dosimeter is essentially an aromatic hydrocarbon
detector with interference expected from toluene and xylene.
The presence of the membrane with its own permselectivity
towards lower molecular weight compounds minimizes this problem.
Another limitation of the colour change chemistry is
its sensitivity to extraneous effects, most especially
humidity. ThiS is particularly so for NO . For conventional
gas indicator tubes used to measure ambient concentration, a
drying layer must be added to the tube to control the humidity
of the ambient air.
Additional precleanse or drying layers, when necessary,
cannot be incorporated as additional lengths of the tube between -
the membrane and the indicator tube 10 because of the
relationship between tube length and the rate of diffusion in
the tube. Any additional resistance between the indicating
',
,
_ g _
i ~A:
~ ~ .
~.
~ . . . . .
.
: . , ` ~ . .
: ` . . .. , .
. . ~ . .
. . . ` ~
3 5 ~
layer and the membrane only serves to shorten the actual
coloured stain and make the device more difficult to read.
Alternatively, the configuration of Figure 2 can be used. In
Figure 2, parts which correspond to those of Figure 1 have been
indicated by the same reference numerals. In this case, a
drying or precleanse layer 24 is provided as a thin, wide layer
between the membrane and the indicating tube, the layer 24 being
retained by a glass wool layer 26. Provided sufficient
adsorptive or reaction capacity is present in the layer 24,
drying or precleaning of the diffusing vapour can be achieved
without significantly lengthening the tube 10 and compromising
the stain length relationship to accumulated dose. In addition,
these layers must be sufficiently specific, so that inadvertent
adsorption of the test gas does not occur.
15Assessment of each device is essentially achieved by
- the preparation of a calibration curve relating time weighted
average cocentration (e.g. 8 hr) to length. The dosimeters are
exposed to a constant concentration atmosphere in a suitable
chamber which is continually flushed with the gas-in-air mixture
of the desired concentration. Since these mixtures can be
problematic, the gas concentration in the chamber is
periodically or continuously measured.
T~
Silica gel, clay (Chromosorb/W) and glass beads (all
40-60 mesh) are appropriate supports for the indicating
; 25 precleanse, and/or drying layers. Particular attention must be
given to the minimization of adsorption of the test gas at low
-- 10 --
' ~ .
,~ ' .
7' 3 ~ 5
concentrations.
The reagents are dispersed on the support, generally by
loading small volumes of reagent solution on to relatively large
. amounts of solid, followed by drying in a rotary evaporator
Although the required amount of primary colour change reagent
~,
can normally be calculated, the amounts of the other reagents
must be determined by trial and error. Changes in the mechanism
of reaction from the homogeneous to the heterogeneous systems
~ typically complicates the chemistry of the reactions.
- 10 The presence of the large area membrane 22 at the end
of the small diameter tube 10 in Figure 1 is probably not the
most convenient configuration of this device. Folding of the
membrane into a "concertina" shape with the same outside
diameter as the tube would be one improvement. Alternatively,
as shown in Figure 3, a disk-shaped badge comprising an
impermeable container 27 with diffusion in a radial direction
through a membrane 28, an air space 29 and an annular body 30 of
the reagen~ of the packing might yield a more compact device,
although the "stain length" (now read as the radius of a
coloured ring defined by a stain front 31) would be even more
sensitive to the limitation of diffusion through the reagent.
The development of this passive monitor with instant
readout will facilitate the personal monitoring of gases and
vapours in the workplace. The device is much less expensive and
easier and more convenient to use than existing devices. It is
a device with which workers themselves can monitor their own
,,
~;A 11
1 157355
exposure. It will make them more aware of their environment and
encourage them to be more concerned with their health on the
job. It is a device which is sufficiently inexpensive to permit
,:
more widespread monitoring than is currently economically
feasible. Problem areas can be identified more readily and with
greater reliability. Government inspectors will be able to
assess the in-plant exposure essentially during a visit and the
need for follow up visits can be reduced.
In general, this device will place industrial hygiene
monitoring of gas and vapour exposure on a more routine and
widespread level. Consequently the incidence of occupational
desease will decrease as the danger of unknown exposure is
minimized.
- 12 -
JA
, . . :,
-
.~ . .
. . . ~ - :
1 1 57355
SUPPLEMENTARY DISCLOSURE
Referring again to Figure 1, for detecting H S, lead
chloride has been found to be a satisfactory reagent when used
in the proportion of 7.10 multiplied by 10 - 7 moles per gram of
inert packing 16, while for detecting benzene, 10 - 5 moles of
iodine pentoxide in 96% sulfuric acid added to each 2.5 grams of
the inert packing 16 has been found to give satisfactory results.
The inert packing 16, preferably an acid washed
diatomaceous earth sold under the trademark CHROMOSORB W, mixed
with the reagent until the packing is free flowing, is added to
the tube 10, which is a glass tube with an inside diameter of
6mm., and vibrated to obtain a packed density of 0,254 grams per
cubic centimeter, and is kept in place by a piece 15 of glass
fabric and an overlying piece 17 of wire mesh inserted into the
open end 14 of the glass tube 10.
The membrane or diaphragm 22 is of silicone rubber
having a thickness of 5 mils. and manufactured by Sci-Med Inc.,
of Minneapolis, Minnesota. The membrane 22 is glued to the
structure 18 by an RTV adhesive.
Figures 4 to 9 show diagrammatic views in cross-section
through six further dosimeters embodying the present invention.
Figure 4 shows a modification of the device of Figure
1, in which modification the bell-like structure has been
replaced by a glass adaptor, indicated generally by reference
numeral 44, comprising a cylindrical portion 42 and end portion
44, on which the membrane 22 is provided.
- 13 -
`: :
` ~ l 15735~
In addition, the glass tube lO is provided with a
spring metal clip 45 to enable the dosimeter to be clipped, e.g.
to the lapel of the wearer's shirt.
The contents of the tube 10 are the same as those
described hereinabove with reference to Figure l.
In the embodiments of the invention illustrated in
Figure 5 of the drawings, the packing and reagent are contained
in a glass tube lOa which is open at it's lower end only and
which is supported by means of annular plastic spacers 46 within
a protective houslng indicated generally by reference numeral
47, which may be made of glass but which is preferably made of a
breakage-resistant transparent plastics material. The housing
47 has a cylindrical wall 48 and a closed end 49, the opposite
end of the housing 47 defining an opening which is closed by
diaphragm 22a.
The material of the diaphragm 22a and the contents of
the tube lOa correspond to those of the diaphragm 22 and the
tube lO of Figure 1.
As indicated above, diffusion of the gas along the
indica~or tube becomes limiting as the stain length increases,
the limitation to long stain lengths being the resistance to
diffusion through the tube packed with the solid packing.
Hence, in order to increase the precision and ease of reading of
the above device, the cross-sectional area of the indicator tube
may be varied, so as to decrease in the direction of diffusion
of the contaminant.
- 14 -
r~..
`
,...
'` ' :' ~ ' ' '
.: : , . :
7355
For example, Figure 6 diagramatically illustrates a
dosimeter having a container, indicated generally by reference
numeral 49, which is made of glass and which has an
exponentially curved wall 50, the container 49 being closed by a
silicone rubber membrane 51 at an open end thereof and
containing a packing and reagent such as those described above.
The embodiment illustrated in Figure 7 is similar to
that of Figure 6 except that, in this case, the wall of the
container, which is indicated by reference numeral 53, is
conical.
Figure 8 shows another possible configuration for the
indicator tube and it's protective housing. In this case, the -~
tube, which is indicated by reference numeral 55, opens at it's
upper end into a frusto-conical adaptor 56, having an open upper
end coincident with the open upper end of the protective
housing, which is indicated by reference numeral 57 and
comprises a transparent plastics material, the open upper ends
of the adaptor 56 and the housing 57 being closed by a silicone
rubber membrane 58. 'r
Figure 9 shows an embodiment in which a glass indicator
tube 60, which is cylindrical and which contains the same
packing, reagent etc. as the tube 10 of Figure 1, is provided at
it's upper end with a cylindrical support structure 61, e.g.
wire mesh, which is provided within and supports a cylindrical
silicone rubber diaphragm 62.
- 15 -
. - ,, .
, , '~ ''' , ' ' " ' .
. . ~ - ~: . ~ , - :
- :
' ' '::. . - '
