818 PARTICULATE EMISSIONS
taken in the prescribed method with the sun at the observer’s
back. Nevertheless, when properly made, observations of
Ringlemann numbers are reproducible among observers and
agree well with actual plume opacity.
Opacity regulations specify a maximum Ringlemann
number allowable on a long-term basis but often permit
this to be exceeded for short prescribed periods of time. For
instance, a typical requirement specifies that emissions shall
not exceed Ringlemann 1, except that for up to 3 min/hr emis-
sions up to Ringlemann 3 are permitted. This allowance is of
considerable importance to such operation as soot blowing
or rapping of electrostatic precipitator plates, which produce
puffs to smoke despite on overall very low emission level.
Federation regulations of the Environmental Protection
Agency^14 specify that opacity observations be made from a
point perpendicular to the plume, at a distance of between
two stack heights and one quarter of a mile, and with the sun
at the observer’s back. For official certification, an observer
under test must assign opacity readings in 5% increments
(1/4 Ringlemann number) to 25 plumes, with an error not
to exceed 15% on any single reading and an average error
(excluding algebraic sign of individual errors) not to exceed
7.5%. Annual testing is required for certification. In view of
previous studies, 11,13 this is a very high standard of perfor-
mance and probably represent the limits of visual quantifica-
tion of opacity.
Perhaps the greatest advantage of the Ringlemann Number
approach is that it requires no instrumentation and very little
time and manpower. Readings can usually be made by con-
trol authorities or other interested parties without entering the
premises of the subject source. Monitoring can be done very
frequently to insure continual, if not continuous, compliance
of the source. Finally, in terms of public awareness of par-
ticulate emissions, plume appearance is a logical candidate
for regulation. Air pollution is, to a great extent, an aesthetic
nuisance affecting the senses, and to the extend that plume
appearance can be regulated and improved, the visual impact
of pollution is reduced.
The Ringlemann Number concept has drawbacks reflecting
its simple, unsophisticated basis. Most serious is that, at pres-
ent, there is no really quantitative relationship between stack
appearance and the concentration of emissions. Additional
factors; such as particle size distribution, refractive index,
stack diameter, color of plume and sky, and the time of day,
all have a marked effect on appearance. On a constant weight
concentration basis, small particles and large smoke stacks will
produce a poor Ringlemann Number. Plumes that have a high
color contrast against the sky have a very strong visual impact
that does not correspond closely to the nature of the emissions.
For example, a white plume may be highly visible against a
deep blue sky, but the same emission can be practically invis-
ible against a cloudy background. As a result, it is often dif-
ficult to predict whether or not proposed control devices for a
yet unbuilt plant will produce satisfactory appearance. Certain
experience factors are presented in Table 1 for emissions, mea-
sured on a weight concentration basis, which the Industrial Gas
Cleaning Institute has estimated will give a Ringlemann 1 or
a clear stack.A second objection is that Ringlemann number is a
purely aesthetic measurement which has no direct bearing
on physiological effects, ambient dirt, atmospheric corro-
sion, or any of the other very real and costly effects of par-
ticulate air pollution. There is some concern that regulations
of very low Ringlemann numbers will impose very costly
control measures upon sources without producing a com-
mensurate improvement in the quality of the environment.
Thus a high concentration of steam will produce a visually
prominent plume, but produce virtually no other undesirable
effects. Opacity restrictions are usually waived if opacity is
due entirely to steam but not if any other particles are pres-
ent, even if steam may be the major offender.Instrumental OpacityMany factors affecting the visual appearance of a smoke
plume are external variables, independent of the nature of the
emissions. In addition, visual reading cannot be taken at all
at night; and manpower costs for continuous daytime moni-
toring would be prohibitive. For these reasons, instrumental
measurements of plume opacity are sometimes desirable.
A typical stack mounted opacity meter is shown in
Figure 2. It consists, basically, of a light source, an optical
path traversing the smoke stack, and a phototube receiver
which responds to the incident light intensity and, hence,
to the light attenuation caused by the presence of smoke.
Various techniques including beam splitting, chopper stabi-
lization, and filter comparison are used to maintain stable
baselines and calibrations. At present, however, there is no
way to distinguish between dust particles within the gas
stream and those which have been deposited on surfaces in
the optical path. Optical surfaces must be clean for mean-
ingful measurements, and cleanliness is difficult to insure
for long periods of time in dusty atmosphere. The tendency,
therefore, is for such meters to read high, indicating more
smoke than is actually present. For this reason, and because
of reluctance to have a continuous record of emissions, there
has not been a very strong push by industries to supplant
Ringlemann observations with opacity meters.
Stack mounted opacity meters, of course, will not detect
detached plumes, which may contribute to a visual Ringlemann
observation. Detached plumes are due to particles formed by
condensation or chemical reaction after gas leaves the stack
and are thus beyond detection of such a meter.
At present, Texas is the only state with emissions control
regulations based on use of opacity meters,^15 as described
by McKee.^11 The Texas regulations is written so that smoke
of greater optical density (light attenuation per unit length
of light path) is permitted from low velocity stacks or small
diameter ones. Basically, a minimum transmittance of 70%
is allowed across the entire (circular) stack diameter if the
stack has an exit velocity of 40 ft/sec, and adjustment equa-
tions are provided for transmittance and/or optical path
length if non-standard velocity or path length is used.
Perhaps the greatest dissatisfaction with emission regula-
tions based either on visual observation number or on instru-
mental opacity is due to the fact that there is presently noC016_001_r03.indd 818C016_001_r03.indd 818 11/18/2005 1:15:34 PM11/18/2005 1:15:34 PM