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No. 19
Using the Digital TDA-2G Aerosol Photometer
This bulletin is intended to familiarize users of "old
fashioned" analog photometers with knobs and switches
with the digital design by comparing the TDA-2D or TDA-2E
photometers with the digital TDA-2G model.
To Perform 100%
Analog Unit - To set the 100% baseline in the analog unit,
the range switch would first be set to the 100% range, the
selector valve would be turned to upstream and a sample of
the aerosol air mixture challenge upstream of the filtration
system under test would be drawn into the photometer. Then
the gain control would be adjusted for a 100% reading.
TDA-2G - To accomplish the same thing on the
digital TDA-2G unit, the <ENTER> key is pressed, then
100 and <ENTER> again. The selector valve is
turned to the upstream position to input the aerosol sample.
Since the unit is auto ranging there is no range switch to
place in the 100% position; it is done automatically. The
red LED over the 100 key is illuminated while the unit is
setting the 100%, and extinguishes when the process is completed.
The red LED over the 0 key illuminates and flashes prompting
the user to straylight or 0% the unit by pressing the <ENTER>
key.
Establishing 0.000% (Straylight)
Analog Unit - On the analog unit , after 100% was set, the
selector valve was switched back to the clear position to
allow particle free air to be drawn through the detector from
the internal HEPA filter. The range switch was then placed
in the 0.1% or 0.01% range and the straylight control adjusted
for a 0 indication on the analog meter.
TDA-2G - In the digital 2G unit, after the 100% has been
established the red LED on the 0 key flashes to prompt the
user to 0 or straylight the unit. This is accomplished by
placing the selector valve in the clear position and then
pressing the <ENTER> key. The digital unit will automatically
zero (straylight) itself, beep, and extinguish the red LED
on the 0 key to indicate straylight is completed. The display
reads either 0.000 or 0.0000, depending on whether three or
four decimal places were selected. Now the 100% and 0.000%
baselines have been established which means the photometer
is ready to test the system to which it was just calibrated.
Testing
Analog Unit - For testing on the analog unit,
the selector valve was placed in the downstream position which
then drew a sample, usually through the scanning probe, and
the user began to scan the filter system. If the reading went
up, the range switch would need to be switched to a higher
range so that the needle on the analog indicator wouldn't
be off-scale. The user would again scan and move the probe
about trying to pinpoint the maximum reading indicating where
the leak was emanating from the filter.
TDA-2G - To do this with a digital unit is
essentially the same; that is, the selector valve is turned
to the downstream position and the filter system scanned.
The main advantage with the new digital unit is that it is
not necessary to switch sensitivity ranges because of the
auto ranging feature. The display is digital and there is
a bar-graph below the numeric display for an analog indication.
The user can either observe the bar-graph and concentrate
on looking for a peak reading or can observe the digital display.
Concentration Measurement
Analog Unit - To measure concentration with
the analog unit, the manufacturer instructions were followed
to activate the Internal Reference signal with the push button
switch and set the gain control for a reading of (typically
in the ATI units) 10%. This established a gain level or sensitivity
level such that if 100 micrograms per liter of DOP (or PAO,
if the unit was calibrated for PAO) was drawn into the unit,
the gain control would read 100%. This was a nice feature
since, if the user wanted to know what the challenge concentration
was, the INT REF could be used to establish the 100 microgram
per liter sensitivity, the selector valve could be switched
to upstream and, if the reading obtained on the system under
test was 65%, it followed that the system had a concentration
of approximately 65 micrograms per liter.
TDA-2G - This is an occasion where the new digital unit is
a much better performer than the analog unit. First of all,
there are three Internal Reference settings: P1 for DOP, P2
for PAO, and P3 for any other alternative liquid, and all
that is necessary for P3 is entering a factor. For instance,
if mineral oil is to be used and the systems under test are
normally tested with DOP, the factor for mineral oil (which
is 0.90) would be used. To set the sensitivity to 100 micrograms
per liter on the digital TDA-2G, the <ENTER> key is pressed,
followed by Ref then <ENTER>. P1 flashes on the display if the
unit was programmed for DOP, P2 if programmed for PAO, and
P3 if programmed for an alternate liquid. A number will be
displayed, normally 100 (this number may be reduced if desired)
and the unit automatically sets the sensitivity level to 100
micrograms per liter of either PAO or DOP, whichever has been
selected. Once this has been completed, a red LED flashes
on the 0 key prompting the user to 0 or straylight the unit.
This is accomplished by placing the selector valve in the
clear position and then pressing the <ENTER> key. The digital
unit will automatically zero (straylight) itself, and when
it is complete beep and extinguish the red LED on the 0
key to indicate straylight is completed. The display reads
either 0.000 or 0.0000, depending on whether three or four
place display was selected. Now the valve selector is switched
to upstream just like the analog unit and the display reads
concentration in micrograms per liter. If an upstream reading
was taken on the system described for the analog unit, the
TDA-2G display would read 65.0 representing a concentration
of 65 ug/l.
100% Adjustment Without Upstream Sample
All certifiers must occasionally adjust the sensitivity of
the photometer to a calculated concentration level because
it is impossible to get an upstream sample to do this. An
excellent example of this is a contaminated biological safety
cabinet.
Analog Unit - With the TDA-2D or TDA-2E, concentration had
to be calculated and then the concentration calculation had
to be used to determine the Internal Reference setting for
a correct sensitivity level. This was one of the most complicated
operations using the photometer and it was difficult to learn.
Essentially the total flow of the cabinet would be measured;
for example, assume a total flow of 540 CFM. Since it is known
that one Laskin nozzle at 20 psig provides 100 micrograms
per liter into 135 CFM, that 135 can be divided by the new
CFM (540), assuming one nozzle, resulting in a calculation
of 25 micrograms per liter. Since the concentration is now
1/4 of 100 micrograms per liter the Internal Reference setting
would be increased from the normal 10% to 40% (4 times the
gain) to have the correct sensitivity for 25 micrograms per
liter.
TDA-2G - All that is necessary to do this on
the TDA-2G is to press the <ENTER> key, followed by
the REF key, then scroll up or scroll down until the calculated
concentration is displayed (which in this case is 25 micrograms
per liter) and press <ENTER> again. The unit automatically
adjusts the sensitivity level of the photometer to display
100% when exposed to 25 micrograms per liter of concentration,
and then prompts the user to 0 (stray light) again.
Alarms
Analog Unit - The TDA-2D unit didn't have an
alarm unless it was ordered as a special option. Generally
a contact meter was used and the red pointer was set to the
point of ALARM. In the later versions of the TDA-2E photometer
an electronic ALARM was developed that could be set at any
point on the unit or disabled. An audible ALARM sounded when
that set point. was exceeded.
TDA-2G - The digital TDA-2G has two types of
alarms. The first alarm is a visual alarm; when the alarm
set-point has been exceeded the display will flash on and
off visually indicating that the alarm set point has been
exceeded. The second alarm is an audible alarm. The visual
and audible alarms can be enabled or disabled.
To do this, the <ENTER> key is pressed, followed by the ALARM
key. Either three or four dashes will be displayed to indicate
that both the audible and visual alarms have been disabled
and the alarm light will flash red. If the alarm light was
already on, indicating that the alarm had been previously
set, a reading will be displayed which will be the actual
alarm set point. If the dashes are displayed indicating that
the alarm features were disabled, the alarm key must be pressed
once more before the set point can be adjusted.
To increase or decrease the set point, the up scroll or down
scroll keys are pressed to change it to the desired
set point , then <ENTER> is pressed again. Both the visual and
audible alarms are now going to activate when the set point
just entered is exceeded. When programming parameters, there
is an option (L1) to turn the audible ALARM on or off.
Programming
Programming the TDA-2G consists of selecting the photometer's
operating parameters. The unit is turned on and when self
diagnostics are completed the display contains a series of
8's and the bar-graph is illuminated. At this point, if the
<ENTER> key is pressed followed by the scroll up key, the L0
parameter is displayed. If the unit is up and running and
the <ENTER> key is pressed followed by the scroll up key, the
L0 parameter is also displayed. L0 sets the parameters selected
using the L1 through L10 parameters.
Pressing the scroll up and scroll down keys scrolls through
the parameter list. When L1, the parameter for the audible
ALARM is displayed, the <ENTER> key is pressed and scroll up
and scroll down keys are used to turn the audible ALARM on
or off; when the ALARM is enabled a 1 is displayed and when
it is disabled a minus sign is displayed. When the ALARM is
set to the desired operating mode, <ENTER> is pressed and the
display returns to the parameter list.
To change, another parameter use the scroll up and scroll
down keys to display the desired parameter, scroll through
the options for the displayed parameter, and press the <ENTER>
key when the desired option is displayed. L2 allows the display
rate to be changed and L3 allows the 100% sample time to be
adjusted. L4 selects the number of decimal places displayed
and L5 selects the Internal Reference. The Internal Reference
choices are P1 for DOP, P2 for PAO, and P3 which is a DOP
based correction factor for the desired substitute liquid.
The L6 setting displays the number of operating hours on the
unit and L7 allows the unit to be reset to the factory defaults
which are listed in the operating manual. L8 indicates the
software revision that currently is installed on the unit
and L9 allows the bar-graph to be enabled or disabled. Finally,
L10 allows the intensity or brightness of the display to be
increased or decreased. Consult the operating manual for more
details on these programmable features.
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