beta attenuation. Beta attenuation is a radiometric technique that
exhibits an exponential attenuation characteristic as a function of the
mass per unit area interposed between a beta emitting isotope and a
In an ambient monitoring application, beta attenuation can often provide the desired level of accuracy. However, it must be understood
that the measured mass can be affected by chemical composition (i.e.,
weight ratio). Therefore, certain precautions must be
taken to interpret the results
from discrete pollution
sources accurately. The atomic number-to-atomic weight
ratio has a direct impact on
the mass absorption coefficient used to correct the attenuation results to a mass value.
Research previously completed in 1981 by J.M. Jaklevic,
et al. demonstrated how particulate composition could
influence mass absorption
coefficient dependency. This
influence is shown in Table 1.
This information led to the
conclusion that accuracy
could not be reliably maintained across all particulate source categories and would likely not
meet acceptable performance standards.
Description of PM CEMS
One of the most critical parts of the design process of any CEMS is
protecting the system against failure. Moisture content, as either
vapor or condensed droplets, poses the greatest risk to the successful
operation of a PM CEMS. Another risk is overall sample transport to
the point of measurement. In the PM CEMS design, the sample is
extracted and immediately mixed with ultra-dry, clean gas to minimize moisture content and allow sample transport to occur under
controlled conditions. The final concentration can be calculated by
carefully monitoring the dilution ratio.
As seen in Figure 1, the sample gas is drawn from the stack through
an in-stack dilution probe. A portion of the diluted sample gas is
transported to the nephelometer stage where it is analyzed.
The particulate sample may subsequently be collected by an in-line
filter during normal monitoring operation or delivered to a filter
attached to the TEOM monitor for a mass calibration period. In each
case, the remaining gas flow is metered and returned to the stack.
The Thermo Scientific PM CEMS uses a ratio (PM Factor) of the
TEOM monitor and nephelometer readings in conjunction with the
dilution ratio to correct the real-time nephelometer output to generate
the final concentration output of the PM CEMS. This final concentration is referred to as “PMwet” since it is derived from a dilution
Discussion of Field Results
The Thermo Scientific PM CEMS has been installed at several coal-fired power plants. These power plants utilize an Electro Static
Precipitator (ESP) for primary particulate control, followed by a Wet
Flue Gas Desulfurization System (FGD) for SO2 scrubbing. During
this test, reference testing was done in accordance with the performance specifications PS- 11. The installation involved a heated probe, a
combined probe controller and mass monitoring enclosure, and a
clean air panel.
The main objective of these tests was to evaluate the performance of
the critical sensing functions of the PM CEMS during PS- 11 and
Procedure 2 evaluations. Further collocated data was gathered to
evaluate the field precision of the PM CEMS. The system was set up
to maintain a heated sample stream with an approximate 12:1 dilution
ratio and 2 lpm extraction flow with scheduled blowbacks and system
checks. The nephelometer was continuously operated. During PS- 11
testing the TEOM was operated continuously, and during normal
daily monitoring the TEOM was activated once per day for approximately 2 hours.
The variation of the PM factor was apparent during a de-tuned PS- 11
test. It varied from day to day and even within a given day of testing.
This variation supports the need for an internal mass reference device
(i.e., TEOM). The performance of the Thermo Scientific PM CEMS
showed that it meets the US EPA criteria for acceptability. The PM
CEMS also meets internal development goals of +/- 20% of the reference method.
Beta testing of the Thermo Scientific PM CEMS has been completed
and full production has begun. Field results show an accurate and
reliable instrument that can accommodate any changes in wet FGD
plant parameters or conditions that affect particulate characteristics.
Continued testing at multiple locations, including at cement kilns and
industrial boiler stacks, will further verify the instrument’s performance.
Jaklevic, J.M., R.C. Gatti, F.S. Goulding, and B. W. Loo. 1981. A beta gauge
method applied to aerosol samples. Environ. Sci. Tech. 15:680-686
Kevin Goohs, Thermo Scientific Environmental & Process
Thermo Fisher Scientific, 27 Forge Parkway, Franklin, MA 02038
Compound Z/A µ
Z/A: Atomic number to weight ratio
µ: Mass absorption coefficient for beta attenuation
Circle 127 on Card
Table 1: Effect of Atomic Number
Dependence on the Measured Mass of
Simplifying MACT and PS 11 Compliance
for Continuous Emission Measurement
Figure 1: Functional overview of the PM CEMS.