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REMOTE INTELLIGENT SENSOR - AREA MONITOR
Page 3-2 Instruction 2772-0803
This method eliminates the ‘zero drift’ that
could otherwise occur with slight variation in the
reflective value of the tape. It also ensures that
system accuracy is not compromised when reusing
the same tape spot. System accuracy is not compro-
mised when reusing the same tape spot.
3.2 DENSITY & TIME OPERATING
MODES
Actual sampling takes place under control of
one of two operating modes. The microprocessor
automatically selects the appropriate mode in
relation to the dynamic situation being monitored.
The two modes are:
3.2.1 DENSITY MODE. This operating mode is
used when the sampled concentration is below a
predetermined threshold. Under this condition, the
gas sample period is a fixed four minutes.
During the four minutes, a sample is drawn
into the system at a specified Flow Rate. The
product of time and flow provides the measured
volume used in the subsequent calculation of
concentration.
The term ‘Density Mode’ is used because the
sample volume is fixed and the variable used in the
concentration calculation is stain density.
3.2.2 TIME MODE. When the concentration level of
the sampled gas exceeds the preset threshold value,
the stain on the tape develops quicker. By measuring
the time it takes to reach the threshold density, it is
possible to calculate the sample concentration.
Using the ‘time mode’ provides real benefits; a
reading and an attendant alarm are provided rapidly
when concentrations are high, and the tape is never
allowed to become saturated. Because the tape doesn’t
saturate, the reaction is always in the linear region
where the monitor’s accuracy is maximized.
An additional benefit is that the tape continues to
act as a filter and prevents the sample getting past
the optics block and contaminating the system
beyond.
3.3 CHANGE-OVER FROM DENSITY
TO TIME MODE
With a 1,000 ppb instrument, the change-over
from density to time mode occurs at 20 ppb. Other
range instruments have change-over points in
proportion. This dynamic control of sampling mode is
automatic.
3.4 MINIMUM SAMPLE TIME
The minimum sample time in the time mode is
approximately 16 seconds. This reponse occurs when
the monitored concentration has reached, or ex-
ceeded, the maximum range of the instrument.
3.5 TAPE CASSETTE LIFE
The cassette has a nominal life of 30 days.
However the rate at which the system uses tape,
and the consequent cassette life is dependenty
mainly on the gas concentration monitored.
If the level is zero, or very low, a tape step will
occur every 16 minutes (4 x four minutes). This is
because the two tracks are each used for two
consecutive samples of four minutes as explained in
Section 3.1.2.
If the monitored concentration is higher, the
tape steps every two sample periods; also the
sample period itself can reduce below 4 minutes
(dependent upon the concentration reached). A tape
step will therefore occur at least every 8 minutes (2
x 4 minute sample periods) and more quickly if
levels are high.
Infrequent high gas concentrations will not have
much effect on cassette life, but if monitored levels
are continuously significant, cassette life will be
noticeably reduced.
In most applications, since monitoring is being
done to confirm an expected zero, or very low gas
concentration, cassette life will be close to the
30-day period specified.
3.6 DATA POINT STORAGE
At the end of each sample interval the calcu-
lated data point is stored, as are the calculated
TWA’s. The 15 minute TWA , and the 8 hour TWA
are both available for immediate display at a touch
of the appropriate front panel key switch. The
stored data points can be printed out as a record
when the optional Printer option is fitted.
3.7 SYSTEM DISPLAY
The displayed information, including symbols,
or icons, depends upon the operating conditions.
These and related information are as described below.
3.7.1 SYSTEM NORMAL, GAS CONCENTRA-
TION ZERO OR LOW. This is the usual operating
condition and three pieces of information are dis-
played.