Thermo Scientific 5028i Instruction Manual page 32

Introduction
Principle of Operation
1-6 Model 5028i Instruction Manual
While the instrument allows the RH-threshold to be set from 30-95%,
Thermo recommends a value from 30-60%. %RH threshold setting
should be properly set with consideration of the local meteorological
conditions and particulate matter composition. This setting should be
compatible with local regulatory agency considerations and should be
set based on comparison with gravimetric sampling method results at
the same location as the instruments being operated. Always refer to
local regulations prior to setting this value.
At the location where the sampling tube attaches to the Model 5028i
instrument case, a radial insulated tube joins the sampling tube to the
collection chamber on each channel. The particulate is drawn onto the
glass fiber filter tape between the proportional detector and the C-14
beta source. Here, the beta beam passes upwards through the filter tape
and accumulated particulate layer. The intensity of the beta beam is
attenuated with the increasing mass load resulting in a reduced beta
intensity measured by the proportional detector. From a continuously
integrated count rate the mass on the filter tape is calculated. During
sampling, the mass on the filter spot is always known and the rate of
particulate accumulation is observed.
A continuous measurement of the flow rate determines the sample
volume. Two proportional valves use to maintain the sample flow rate
at its nominal value. From both integrated sample volume and mass
detection, the particulate concentration is calculated for Channel
A and Channel B. At any given time the output signal delivers the
concentration of particulate in actual or standard conditions.
The Model 5028i outputs particulate matter concentrations to the front
panel display as shown of PM and PMb, the analog outputs, and also
makes the data available over the serial or Ethernet connection. A visible
window of the Model 5028i can make an easy maintenance and service
for engineers and users. Two bigger filter tapes with 42 meters can use
for more spots.
Additionally, the Model 5028i proportional detector is sensitive to
natural alpha activity of the aerosol passing through the sampling
system. The additional alpha count rate is used to measure the
presence of Radon gas and infer the daughter nuclides of Radon gas.
These daughter nuclides create a small beta emission. This externally
sampled beta emission is perceived by the detector to be a higher count
rate and the higher the count rate, the less apparent mass detected.
Therefore, by measuring the natural alpha activity due to Radon gas, the
proportional beta interference from the daughter nuclide emissions are
subtracted from the count rate to provide a refined mass measurement.
This potential error from Radon gas interference may be (in lower
strength radioactive devices; e.g.; <100 µCi) as high as actual particle
concentrations, especially after filter change or when the natural activity
Thermo Fisher Scientific