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In the recent years the small particles became more and more interesting
not only in the scientific research but also in the development of new
commercial techniques. But measuring nano particles is limited by the
instrumentation. The condensation particle counter (CPC) technique can't
reach a smaller cut off d50 as 2 to 5 nm. The CPCs also have the
disadvantages of a relative slow response time and their handling may be
a little complicated, because of the condensation liquid and supply. An
alternative to the condensation of particles is the direct counting of
charged particles (or ions) by electrometers. Faraday Cup Electrometer (FCE)
don't have to magnify the particles and therefore require no
condensation agent and temperature control, no optical and laser parts
and are theoretical not limited in the particle size. The only
limitation is given by the number of charged particles (or ions) and the
necessity to combine it with a Differential Mobility Analyser (DMA) to
cut out adequate charged particles. Solution - SMPS+C
GRIMM has developed a highly
sensitive FCE which is able to count charged airborne particles in the
size range of 0.8 nm - 700 nm. The instrument is especially adapted to
measure very small particles up to very high concentrations. The
airflows and the voltages are controlled by a DMA Controller. This
instrument also manages the voltage settings of the DMA and the
electrical adjustments of the FCE. It acquires all the data of the DMA,
FCE and attached sensors.
The charged particles are measured in
a Faraday Cup Filter isolated and connected to a highly sensitive
electrometer and amplifier with a total resistor of 2.5 TW (figure
below). Electrometers have much shorter response times in
comparison with classical CPC solutions and are able to give highest
time resolutions. The original electrical mobility spectrometer (EMS)
scanning method was originally developed (Flagan/Reischl/ et al. 1991)
for Nano DMAs in combination with electrometers in the nucleation and
Aitken mode (only single charge correction, shortest response time and
therefore no smearing)
.
Sensitivity
The GRIMM FCE has a sensitivity down to 0.1 fA (1 Hz, ~600 charged
particles), an automatic zero point adjustment, changeable
concentration ranges and air-flows.
Resolution
The new GRIMM FCE #5.700 (see image below), developed together with the
university of Vienna is highly sensitive to low concentration but on the
other side robust enough to mechanical shocks and pressure fluctuations.
The FCE in combination with the GRIMM Nano SDMA (a Vienna type DMA),
which is able to measure below 1 nm, is the proper solution for
nucleation measurements and can be used as a SMPS system. With this
system it is possible to measure even larger ion clusters around 0.8 nm
and up to about 50 nm.
Software
By using different ratios of sample airflow to sheath airflow, the
software enables the feature of four different measurement modes. There
is a high-resolution mode with 176 size channels, a medium mode with 88
size channels and a mode for lower resolution with 44 size channels (the
fourth mode has its resolution between low and medium mode and can be
operated with 44 or 88 channels). The software also calculates the
multiple charge correction up to five charges per particle.
Benefit
The SMPS +E system can be operated in a wide range of atmospheric
pressure (400 to 1100 hPa) and has a very great concentration range (55
to 100,000,000 P/ccm). Due to its very fast response time, it allows
ultra fast scanning (16 channels).The option to use the GRIMM FCE with
all three different GRIMM DMA (Short, Medium and Long Vienna U-Type DMA)
offers the ability to choose three different size ranges according to
the respective application between 0.8 and 700 nm.
Technical Specifications:
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Voltage* steps
(DMA): |
254 (also with
changeable 16 Hz) |
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Pressure range:
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400 to 1,100
hPa |
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Signal filter:
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with/without
low pass filter |
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Output
(selectable): |
[fA] or [Q/cm3]/DMA
voltage/date |
|
+ time location
|
Nr./error
code/user comments |
|
Particle size:
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< 0.8 to 700 nm
|
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Sample flow:
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1 to 5
liters/min** |
|
Sheath air
flow: |
3 to 20
liters/min** |
|
Output:
|
3x ± 0..10V (±4
fA/V; ±40 fA/V; ±400 fA/V) |
|
Noise:
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(=0.25s 90%)
0.35fA (65 elementary charges/cm³ at 2 liters/min) |
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Noise:
|
(=0.95s 90%)
0.25fA (19 elementary charges/cm³ at 5 liters/min) |
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Offset
correction: |
Electronically
|
|
Rinse air
flow*** |
0.3 to 0.6
liters/min |
*Also
selectable directly as particle size Dp for use as a monodisperse
generator
**Changeable: volume constant or mass constant.
***If the aerosol contains particle with high mobility or the sample is
very humid, the offset could drift. An additional flow with
dry air could be added
(supplied by the DMA-controller) to keep the isolation chamber around
the filter free of electric charges.
The Calibrator
The GRIMM CPC Calibrator consists of a nano DMA + FCE (= SMPS+E) + Air
Conditioner (dryer + adsorber) + WOx Generator (picture below).
CPC and SMPS Measurement Systems are
normally calibrated or referenced against DMA/FCE-combinations. A DMA/FCE
is the established reference method for validation of any other aerosol
measurement system in the nanometer range. A calibration of this system
is not needed. Any other measurement principle will be calibrated
and validated with this unit. The very high resolution in the nano range
is depicted below: 
The TR-DMPS
GRIMM has introduced a unique particle counting
system. Based on our proven SMPS +C system, this technology has been
incorporated into a battery of 10 differential mobility analyzers
(Vienna type DMA cluster), combined with high performance Faraday Cup
Electrometers (FCE), assuring extremely short response times with
highest time resolution.
This high quality technology is
virtually insensitive to internal contamination and is also extremely
robust. This unique concept is not matched in sensitivity and
flexibility by any other commercial instrument. This system is an
absolute and a reference system for any other particle seizer and
counter system. Airflow changes or air pressure changes cannot result in
errors or wrong measurements and the results can be compared to any
existing SMPS system. In addition this system does not have any problems
of sensitivity due to dirt in the system. The consistency, high time
resolution and accuracy of the Model #5.600 makes this system a very
powerful measurement technique in the field of motor efficiency testing,
monitoring fast atmospheric changes, filter efficiency tests and various
other research topics. The primary benefits of the TR-DMPS are:
-
Changing aerosol concentrations
and sizes:
- transient driving cycles for automotive emission
measurements
- environmental monitoring of fast changing aerosols like in
nucleation events or onroad/drive by tests
-
Changing sample air pressure. The instrument is also
operating under different pressure conditions even for measurements
in front of filters/traps
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Fast scans in any field (the TR-DMPS spectrometer determines a
complete particle-size distribution every 200 milliseconds)
-
Flexible setting and combination of the DMAs: User specific choice
of particle size to be measured.
Results and Software
The data are collected from all Faraday Cup Electrometers (FCE) at a
rate of 100 Hz (100 times per second, the response time of these
extremely sensitive FCEs is 200 ms). This leads with a data reduction
algorithm to complete particle size distribution measurements with 5 Hz.
Real-time data display are shown 5 x per second and can be averaged
during data collection. The TR-DMPS software is full features and user
friendly. It includes a complete data inversion algorithm and allows
data to be displayed and exported in many different ways to meet a
variety of needs. A three dimensional representation of size
distribution over time is included.
The software collects data with 5x
per second and can display averaged data with 0.5 to 60 second averages.
Pull down menus and dialog boxes simplify setup and operation. Tests may be started by a few simple
mouse clicks, at a specific time, or by an external trigger |
