DMT UHSAS-0.055 User manual

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Ultra High Sensitivity Aerosol
Spectrometer
DMT Model UHSAS-0.055
0.055 – 1.0 microns
Manual
Droplet Measurement Technologies
5710 Flatiron Parkway, Suite B
Boulder, CO 80301
USA
Telephone [phone]

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Laser Safety Warnings
STRICT OBSERVANCE OF THE FOLLOWING WARNING LABELS IS ADVISED
This instrument is a Class I laser product. CAUTION: Use of control or adjustments or
performance of procedures other than specified in this manual may result in hazardous
radiation exposure.
This label is displayed on the top cover (front and back) of the instrument.
This label is located on the support structure underneath the instrument cover near the
laser interlocks to serve as a warning if the instrument cover is removed.
This label is located on the optical block and the arrow points to the cleaning port.

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Chapter One - Unpacking & Cabling
The UHSAS instrument is delivered in a shipping case which also contains the necessary
cables etc to make the instrument functional. Remove the instrument from the case by
putting your hands in the small cutouts on each of the long sides of the instrument. The
handles for the instrument are located in these cutouts. (The 2 small handle-like
protectors on the top lid of the instrument are protectors for the inlet. Do not use these as
handles.) Put your fingers into the handles located about ½way down each side of the
case lifting the instrument from the case and placing it on a stable working area. The
front legs of the instrument are hinged so that the front of the instrument case can be
elevated. The power cable, keyboard and mouse for the instrument are located in a pouch
in the lid of the case.
The instrument will be shipped with a cap on the inlet. Remove this cap before the initial
startup. Install sample line (1/8” OD Tygon tubing) and zero filter.
Connect the keyboard/trackpad to a USB port on the front panel. The USB ports are
located just below the floppy/CD combo drive.
Before connecting the power cable to the wall power outlet, make sure the instrument
on/off switch is in the off position. The on/off switch is a rocker-type, toggle located on
the back of the instrument near the power plug. Input power to the instrument can range
from 100-240VAC 50/60Hz.
If you are connecting the UHSAS instrument to an Ethernet network, the connector (RJ-
45 female) for that cable is located on the back of the instrument.
There is also a serial port connection (9 pin D-connector) located on the back of the
instrument that may be used to connect to an external data system.

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Chapter Two - Specifications & QC checklist
Laser Type solid state pumped Nd3+:YLF
Sample flow 10-100 SCCM (this unit)
1-10 SCCM optional
Sheath airflow setting 700 SCCM
Maximum count rate 3000 per second
Counting efficiency at least 50% at min. size
increasing to 100%
Number of channels 100
Minimum detectable size 0.055 microns
Size range 0.055 – 1.0 micron
Environmental 10 to 30C
Sea level to 4 km altitude
RH (non-condensing)
Dimensions 56 x 43 x 24 cm
Weight 31kg
Power requirements 100-240VAC 47-63Hz
BUSS fuse, GMA-2A

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Insert unit specific checklist here
UHSAS 55nm-1000nm QC Checklist

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Chapter Three - Getting Started & Shutdown/Power off procedure
The UHSAS instrument contains a computer which runs the Windows2000 operating
system. It is assumed that the instrument user is familiar with the normal operation of
this operating system on a computer.
Before starting the instrument, ensure that the keyboard/trackpad are connected to the
USB port on the front panel of the instrument, that the zero filter is in place on the inlet,
that the power switch is in the off position, and the power cable is connected.
Turn on the instrument with the power switch located on the back panel. When power is
applied to the instrument you will see the Windows2000 startup windows on the flat
panel display on the front of the instrument. The computer is configured to auto-boot
into Windows2000 without the need to supply a login and password at startup. The
default user called “droplet” is configured as a user with administrator privileges in
Windows2000.
When Windows2000 is booted, the desktop will contain a shortcut labeled UHSAS.vi
which will run the LabView executable program which controls the instrument. Select &
Run this shortcut. This will bring up the UHSAS window on the desktop.
LabView 7.1 is a software program from National Instruments which provides a user-
friendly virtual instrument (commonly called a vi) panel for the control, data display, and
data logging of the UHSAS instrument.
The UHSAS vi controls the instrument through a series of vi displays for various
instrument functions. Examples of this display are shown below. Actions on the vi are
initiated with the normal “mouse” and “select” buttons on the computer. The UHSAS vi
has different views which are selected with the appropriate tab near the top of the display.
The UHSAS.vi starts with the Controls screen as the default screen. The Controls screen
allows the user to monitor the various air flows in the instrument, the sample block
temperature & pressure, and has monitors for the laser & cavity characteristics. The air
flows are measured with electronic flow controllers or meters. The sample flow is
controllable by adjusting a flow controller with either the “slider” control or by manually
inputting a number in the set voltage box at the bottom of the flow indicator. At startup,
check that the Sheath flow is about 700 SCCM and the sample flow should be between
10 and 99 SCCM. Refer to the UHSAS 55nm-1000nm QC Checklist on a previous page
for nominal values for Reference, Scatter, Laser Current and Laser Temp.

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Next, select the Histogram tab to display counts versus channel number. You need to
select/push the Run button near the upper left corner of this window to show the particle
accumulations. This should have very few counts if the zero filter is in place.
Remove the zero filter for room air sampling and many counts will appear.

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The size range displayed on the histogram is controlled through the Map tab. Select the
Map tab to change/control the size range displayed. The Map tab shows the boundaries
of the bin width map that is used for the display. The map can be modified in multiple
ways depending on user preference. See Chapter 6 of the manual for details.

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Shutdown / Power off procedure
-Put zero filter on inlet line.
-If file recording is activated, go to Histogram tab and press the Record button. The
Record button will change color from a darker grayish appearance to the same color grey
as the background.
-Press the rectangular STOP button located towards the upper right portion of the
UHSAS screen. This stops the execution of the UHSAS.vi.
-Next the computer must be shut down using normal Windows2000 controls and the
on/off switch located on the rear panel of the instrument.

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Chapter 4 - Theory of operations
Instrument subsystems
The UHSAS is an optical-scattering, laser-based aerosol particle spectrometer system for
accurately and precisely sizing particles in the range from 0.055 µm to 1.0 µm diameter.
It uses fully user-specified size binning of up to 100 channels anywhere within its size
range.
The spectrometer instrument consists of 5 general subsystems, described in this chapter.
1) the main optical subsystem responsible for generating the laser light, detecting the
scattering from the particles and providing a mechanical enclosure for the optical
system and for delivery of the sample aerosol
2) the flow system for bringing the sample aerosol through the optical interaction
region, including flow control and measurement
3) the analog electronics system for amplifying and processing the particle signals
4) the digital electronics system for analyzing particle signals, binning signals
according to user-specified bin mappings and generating a histogram of number
of particles in the specified bins, and for communicating with the PC and system
monitor/control functions
5) an onboard PC running Windows and a specialized application GUI for
instrument control, setup and data reporting and collection
Figure: Block diagram of optical particle spectrometer
1) Optical system
Digital electronics (4)
PC & Display (5)
Monitor &
control
Histogramming
functions
Analog signal
processing (3)
Optical
system (1)
(2) Flow
system
Flow
control &
monitor
laser
control &
monitor
Flow
Analog monitor
Peak-
held
analog
particle
signal
Particle
photo-
signal
ADC

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The Optical system consists of
a) the laser and associated components and optics,
b) the detection system, including collection optics, photodetectors, and
reference monitoring
c) mechanical housing for above
a) The laser is a semiconductor-diode-pumped Nd3+:YLF solid-state laser. It operates in
the fundamental (TEM00) spatial mode on a 1054 nm laser line with an intracavity power
of ~1 kW. The pump laser is a 1 W single-stripe diode at 800 nm, driven by a stable
current source and is temperature-controlled. The laser is a high-quality factor optical
resonator built around an Nd3+:YLF active laser crystal, pumped end-on by the diode
laser. The laser mirrors have reflectivities near 0.99999 at the lasing wavelength. The
laser mode has a 1/e2intensity diameter of 600 µm. The standing wave laser mode is
perpendicular to the flow of particles; the light is linearly polarized with the electric field
vector parallel to the flow of particles. Particle scatter is collected in a direction
perpendicular to both the particle flow and the laser standing-wave. As particles traverse
the laser mode, they scatter light into the detection system. The amount of light scattered
is a strong function of the particle size.
b) The detection system consists of two pairs of mangin collection optics capable of
collecting light over a large solid angle. The mangins image the volume of space at
which the flow intersects the laser mode onto a photodiode. There are two pairs of
collecting optics: one pair images onto an APD for detecting the smallest particles (the
primary scattering detection system). The other pair (located on the opposite side of the
block) images onto a low-gain PIN photodiode for detection of the upper size range of
the instrument (the secondary scattering detection system). Each detector is amplified in
a current-to-voltage stage which feeds the analog electronics system. The imaging optics
have an acceptance aperture of 23 mm diameter at a distance of 8 mm from the
interaction region. The Mangin reflectors are dielectric coated with reflectivity of 0.9.
The system can detect particles as small as 55 nm (50% efficiency, 1-10 counts/minute
dark count rate). At this particle size, the peak scatter rate corresponds to 100 pW of
detected light power at the detector. The system size sensitivity is limited by several
noise sources: a fundamental noise process from the photon shot noise on the detected
molecular scatter from background gas, a fundamental noise process from the Johnson
noise in the photodiode transimpedence feedback resistor, and from technical noise of
various sources.
c) The laser and detection optics are built into a sealed mechanical enclosure (the optical
block). (See Figures)

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side view
Figure: Side view of optical block
Figure: Top view of optical block
block
Laser
crystal
Laser mode
High
reflector
Sheath inlet
Sample
inlet
Imaging
optics
photodiode
pump
Jet
assembly
Exhaust jet
Pump laser
Imaging
optics
Secondary
scattering
(PIN
photodiode)
block
Laser
crystal
Laser mode
High
reflector
Imaging
optics
Jet
assembly
Pump laser
High
reflector
Primary
scattering
(Avalanche
photodiode)
Laser
output
reference
photodiode

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2) flow system
The mechanical laser mount forms a sealed block around the laser and the inlet/exhaust
jets. A pump draws on an exhaust jet pulling flow through the inlet jet and across the
laser mode. The inlet jet is an aerodynamically focused assembly with a sample nozzle of
500 µm diameter and a sheath nozzle of 760 µm diameter. The tip of the sheath jet sits
within 1 mm of the edge of the laser mode. Sample flows are between 1 and 100 sccm
and the sheath flow is typically 700 sccm. Particle velocity depends on sheath flow rate,
but is on the order of 50 to 100 m/s. The particles are confined to a region of space whose
extent is approximately 10 % to 20 % of the laser beam diameter which is 0.5 mm (e-2
intensity diameter). This yields a sizing resolution of approximately 2% to 5% of the
particle size. (See Figure)
PUMP
Mass flow controller for
sample flow
control/measure
electronic
flowmeter
for sheath
flow
measure
mechanical
needle valve
for sheath
flow control
filter
sample
inlet
sheath flow
confined sample stream
laser mode
optical
block
Figure: Schematic diagram of flow system

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3) Analog electronics.
The analog chain converts the photocurrent of the detector photodiodes to a voltage and
processes that signal (called the particle signal). The chain is repeated for the primary
and secondary detection systems.
After the photodiode transimpedence amplifier, the photo signal is mixed with a signal
derived from the reference detection system for noise cancellation. This cancels noise
fluctuations of the laser, lowering the noise floor and improving sensitivity.
The particle signal is fed into two different AC gain stages, differing in gain as specified
below. In total there are four gain stages: high and low for each of the primary and
secondary detection systems.
Gain stage labeling convention
High gain
Low gain
Primary detector
G3
G2
Secondary detector
G1
G0
Gain ratios:
G3/G2 = 50
G2/G1 = 20
G1/G0 = 20
Note that the gain ratios G3:G2 and G1:G0 are pure electrical amplification gain ratios.
The G2:G1 ratio is more complicated since it involves two independent photodetectors
with independent electronics and on opposite sides of the optical block. See the
discussion in the Calibration section.
The gain stages also provide low-pass filtering to the signal. Each gain stage then feeds
it’s own baseline restoration circuit, which restores the 0 Volt baseline which is disturbed
by frequent particle signals after AC coupling. The particle signal is then passed to a peak
hold circuit which tracks the rise of the photo-signal as a particle crosses the laser and
holds the peak value. The digital system then processes the signal and issues a reset. (See
Figure)
The noise-cancellation is not used on the secondary side. The reference detector is used
both for noise cancellation and for monitoring of the laser output power (which is directly
proportional to the laser cavity power.) At present there is no gain control implemented:
if the laser power drifts, the instrument must be calibrated. However, only large drifts in
power > 25% really need to be corrected, since the particle sizing sensitivity is a sixth-
root of the laser power.

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Figure. Block diagram of analog electronics
4) Digital electronics system
a) ADCs and peak height analysis
For each of the four gain stages (2 primary, 2 secondary) there is an associated Analog-
to-digital converter. The ADCs run a 16-bit conversion at 100 kHz sample rate. The
chain of events is begun as a particle traverses the laser mode and begins scattering light.
The particle signal from the highest gain stage on the primary detector (G3) feeds an
analog comparator. If the signal exceeds a preset (user-settable) threshold it generates a
particle trigger. The threshold value is independent of the particular active bin map: it
should be set to register the smallest detectable particle (55 nm diameter) under typical
operating parameters. After a trigger is generated and after a small delay to allow the
particle signal to reach its maximum, the 4 ADCs sample the 4 peak-held particle signals
from the four gain stages. Starting from highest gain (G3) and working down in gain, the
first ADC that is not in saturation is the valid particle ADC. The value of this ADC is
read and compared to a look-up table of bin boundaries previously loaded into memory.
Depending on where in the look-up table the particle signal belongs, a counter for the
appropriate channel is incremented (There are certain conditions which will post-
invalidate a particle event, for example, if the event falls outside certain timing
requirements.) After the particle signal is sampled, a reset is sent to the peak hold circuit
and the cycle repeats for the next particle.
The look-up table is the heart of the peak-height analysis in this instrument. It can be
reset by the user at any time to generate an arbitrary bin mapping. The user specifies the
boundaries of the channels and this is automatically converted, via the calibration curve,

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relative gains and calibration points into a mapping of voltages at each of the gain stages.
The mapping process is transparent to the user and occurs every time a bin map is
committed to the instrument. (See the calibration section below.)
b) monitoring/control
The digital system also provides monitoring and control of onboard systems. It reads and
sets the mass flow controller for monitoring and control of the sample flow. It reads the
electronic flowmeters for the sheath and purge flows (flows are controlled by
mechanically-actuated needle valves). It controls and monitors the pump laser diode by
enable/disable lines and current and temperature setpoints. The laser reference from the
reference photodiode and the molecular scattering level are sampled on an ADC and read
in the digital electronics module. Additional housekeeping parameters such as case
temperature and ambient barometric pressure are also monitored. These could be used
for correction to flow meters and noise-cancellation circuitry but at present are not. All
parameters which are read by the digital system are logged with the sample data. All
parameters which are set are stored in configuration files.
5) On-board PC
The onboard PC provides a user interface to the instrument. It is a Pentium-III, 845MHz,
single board computer running the Windows2000 Professional OS. The computer has
256Mb of physical memory and a 30Gb hard drive. The monitor is a standard LCD
display built into the front panel of the instrument. All normal OS operations are handled
by Windows, e.g., networking, file management, printing, etc.. The user interface is a
virtual instrument written in LabView 7.1(See VI section). Communication with the
digital electronics system is via internal RS232 (115,200 baud, 8N1). The update rate of
the PC I/O is controlled from a user settable variable .
Calibration
Calibration is an important process for any particle spectrometer instrument. The
UHSAS with it’s high resolution and large number of arbitrarily settable bins poses
unique challenges in this area. Several features have been added to this instrument to
make the calibration process as easy and accurate as possible.
There are 4 separate gain stages which must be “stitched” together for accurate, seamless
sizing across the full dynamic range of the instrument. The gain stages are labeled in the
table in a preceding section. There are two types of gains that need calibrating: absolute
and relative gains. Relative gains are used to calibrate gain stages to one another.
Absolute gain is used to fix the overall scale to a known particle size.
The relative gain calibration is somewhat automated, though the results can always be
altered if the user needs to make slight adjustments. The relative gain calibration works
by sampling an ambient air distribution which contains particles of all sizes measured by
the UHSAS. The instrument detects a particle on adjacent gain stages (for example G3
and G2), noting the signal size on both gain stages (in volts). (For example a 100 nm

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particle might be 3 V on G3 and 0.060 V on G2.) By noting many such events, a
relationship between the signal size of a particle on the two gain stages can be
determined---the relative gain. A linear fit to the data for many events produces a relative
gain and an offset between adjacent gain stages. By running this procedure on all
adjacent gain stage pairs (G3 and G2; G2 and G1; G1 and G0) a complete specification of
the relative gains can be developed, linking the optical and electronic signals across the
range of the instrument (which spans 6 decades of signal size in volts).
In addition to the relative gains, there is an absolute calibration curve, that is, the shape of
the particle signal size (in volts) versus the particle size (in nm) from calibration
standards. Once the relative gains are known, the corrected response for the entire
instrument can be formed. Since the wavelength of the instrument is 1054 nm, it is
expected that all particles below approximately 200-300 nm will lie on a sixth-power
curve, that is, the particle signal is a sixth-power of the particle size. This has been hard-
coded into the instrument by forcing the signals from G3, G2 and G1 to fall on a sixth-
power curve. The final gain stage, G0, used for particles from 300 to 1000 nm has a Mie
curve appropriate for the scattering response of the instrument (see Figure). It is a
complicated function which is calculated and confirmed by test particle measurement.
It’s accuracy is questionable. In the event that the user has a preferred curve (empirical
or theoretical) with, for example, a different index of refraction), this curve can be
entered instead.
Figure: Example calibration curve

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In principle, if all the relative gains are known accurately, and the calibration curve is
known, the instrument need only be calibrated in an absolute sense at one point---any
point in fact. In practice it is best to use a trusted particle or a few trusted particles. For
factory calibration, the NIST SRM 1963 100 nm standard reference material is used to fix
the calibration at one point--- and thereby with the relative gains and calibration curve,
calibrate the instrument over its range. Other points in the instruments range are checked
for accuracy with for example the NIST SRM 1691 269 nm reference particle.
In some cases, the user may have preferred particles to use for calibration. In this case as
many particles as needed may be used. If the particles do not all fall on the preset
instrument calibration curve, the calibration curve is altered slightly to ensure that the
calibration particles will return a result which is the stated size of the particle. The data
representing signal size for a given particle size is entered in the VI and is referred to as a
calibration point. Note that alteration of the calibration curve from the preset may be
required in order to accommodate several possible inconsistencies: for example, particles
that have been inconsistently sized with other methods; nonlinearities in the instrument’s
detection electronics; or improved empirical data on the non-power law portion of the
curve.
One comment on the relative gains is needed. In the particle-size regions where detection
passes from one gain stage to another, there can be discontinuities in the histograms
produced. The histograms are very sensitive to the relative gain parameters, and the
relative gain parameters are experimental quantities, subject to statistical and systematic
error. The stitching region between G2 and G1 is particularly prominent in this regard,
since detection technique changes between these gain stages (they are physically different
photo-detectors). The ability to zoom in on these transition regions can overemphasize
the stitching errors. The user can optimize the stitching parameters to accommodate
unusual requirements in this area, however, the semi-auto-calibration provided should be
adequate in most cases.
Whenever changes are made to the relative gain parameters, the calibration curve or the
calibration points, the new parameters will be used in the generation of the next bin map
as it is committed to the instrument.

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Chapter 5 - Acceptance/Performance Checks
Particle size calibration confirmation
In order to confirm calibration of the UHSAS instrument, the following steps should be
taken.
1) The instrument’s primary calibration is to PSL standard reference materials from
NIST: the NIST 100 nm SRM and the NIST 269 nm SRM. Other manufacturer’s
particles may or may not agree with the sizes reported by NIST and therefore in
general cannot be used to confirm the factory calibration with high accuracy.
2) Confirm that the laser reference level and scattering level are at their original
values. If they are not, cleaning may be needed, see the maintenance chapter.
3) Run a monodisperse sample of PSL particles to the instrument sample inlet.
4) Configure a size bin map around the particle size used. For example a bin map
with 99 channels and with a start value of 0.5 times the nominal size and a stop
value of 1.5 times the nominal value is a good choice.
5) Start a histogram.
6) Accumulate a sufficient number of counts and confirm that the peak position is in
agreement with the known particle size. If it is not, recalibration may be needed,
see below.
Particle size recalibration
It may be necessary to recalibrate the size scale of the UHSAS. Some reasons for this
include:
1) the laser power has drooped and cannot be fully restored
2) internal relative calibration has degraded
3) other types of particles (not PSL) with a different index of refraction will be used
4) to assure agreement with previously used standard materials.
The “Calibration” tab in the VI is where all calibration procedures are performed. The VI
controls for calibration are discussed in the VI chapter and the calibration theory is
discussed in the theory section. A complete outline of the calibration process is described
here.
First, re-measure the relative gain parameters. To do this choose the relative gain tab
labeled “G3:G2 Gain” and press “clear.” Open the sample inlet to ambient air in a
normal lab (not cleanroom) environment. If this is difficult, arrange the sample inlet to
see a broad distribution of particles from 0.1 µm diameter to 1.0 µm diameter from some
other way. Press “Run.” As particles are sampled, they will begin to appear on the
relative gain plot. Those particles which can be measured on both G3 and G2 will be
used to measure the relative gain (and offset) between these stages. The gain parameters
will begin to appear in the boxes when the required number of data points has been

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reached. The process will stop when the preset number of points has been acquired. The
graph should show the data points scattered around a best-fit line. Repeat this procedure
for the other relative gain tabs (labeled “G2:G1 Gain” and “G1:G0 Gain”). Acquiring a
sufficient number of large particles (for G1:G0) may take a significant amount of time (1-
10 minutes), especially at flow rates below 60 sccm. This completes the relative gain
portion of the calibration.
Next, gather a collection of particles to be used as reference particles and click on the
“Calibration curve” tab. The present calibration curve should be shown. The x-axis of
the graph shows particle size in nm and the y-axis shows the instrument response in
millivolts as it would appear on the highest gain stage (G3). Note that the signal size of
large particles is on the order of 106mV on G3. Of course, these particles are measured
on other gain stages with significantly less overall gain.
The region covered by each gain stage is indicated by the red dots on the graph. The red
dots are the saturation levels of each gain stage, in other words, the largest particles
measureable on each gain stage are shown at the red dots. For example, the region below
the first red dot is covered by G3, the region between the first and second red dots is
covered by G2, etc..
The white dots on the graph show the calibration points, or those points where particles
have been run to fix the response of the instrument. These points provide the absolute
calibration of the instrument: they move the curve up and down appropriately to convert
voltage on various gain stages to particle size. There are two white points fixed by the
instrument: one at 10 nm and one at 300 nm. These are used only to define the region of
the instrument described by a power-law curve. They are not absolute reference points---
they are used only for forward and backward extrapolation of the curve from the real
calibration points. For example, if the user supplied one calibration point, say at 100 nm,
then the 10 nm point will be used to extrapolate a power-law curve downward from 100
nm and the 300 nm point will be used to extrapolate a power-law curve upward from 100
nm. Above 300 nm, the calibration curve is defined by the “cal curve” shown in the
lower right hand corner of the “calibration curve” tab. This is an array of points which
specifies the relative signal size of particles above 300 nm. It is used to specify the
SHAPE of the calibration curve outside of the power-law response region.
The calibration of the instrument is achieved by entering data into the “calibration points”
data array. This is an array of triplets of numbers. The triplets are : The gain stage
0,1,2,or3; the particle size in nm and the particle response in millivolts. If you are
recalibrating the instrument, clear all data from the calibration points array by right
clicking on a data point and choosing “delete point” EXCEPT for the 100 nm point on
gain stage 3. To calibrate follow the following procedure:
1) With your collection of particles, make note on which gain stage each particle will
be measured by looking at the size of your particle and the calibration curve.
2) Choose a particle and introduce it to the sample inlet of the UHSAS.
3) Go to the “Map” tab and set up a voltage bin map by :