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3.2.2.14.2 Mode
There are four alarm concentration-calculation modes available to calculate the gas
concentration average:
• INST - Instantaneous, generates alarm as soon as gas concentration
exceeds concentration alarm level.
• STEL - Short Term Exposure Limit, 15 minute time weighted average of
measured gas concentration.
• TWA - Time Weighted Average, 8 hour average of measured gas concentration
• CUSTOM - Similar to TWA but allows a customer to provide his own time-base
to perform a customized time-weighted-average.
CAUTION
When power is restored after a power failure, displayed averages will be
reset and valid data will not be available until 15 minutes or 8 hours have been
accumulated for the STEL and TWA modes, respectively.
CAUTION
Averaging data will be accumulated from the time the RUN CALIBRATION
key is pressed until the calibration process is actually begun by activating
the sensor’s magnetic switch. Once calibration has been activated, data
accumulation is suspended until calibration is completed. Data accumulation
is suspended during SENSOR CHECK, CALIBRATION (begins when the
sensor’s magnetic switch is activated), and SELF-TEST.
*CALIBRATION is defined as beginning when the sensor's magnetic switch is activated.
CAUTION
If the measured gas concentration exceeds the maximum range,
the data will be accumulated at the maximum range rate. The average data after
the over-range event will be in error for the next averaging period. The over-range event
is displayed and may also be retrieved from the data archive. If the instrument
is set up such that alarm level 3 is at the maximum range, remote notification of
this type of occurrence is possible.
Should a power failure occur during the concentration averaging in the above modes,
the averaging period will be reset and the averaging will start over. When power is
restored, the displays will indicate zero concentration until updated. Displays are
updated every hour on the hour for TWA and CUSTOM modes and every minute for
STEL mode.
3.2.2.14.3 Latch
The selection made in this menu item determines whether a detected alarm condition is
“latched” or self-resetting. Two selections are possible
• YES - Once an alarm condition occurs, the alarm status will be "latched” or non
self-resetting. The unit will hold the alarm state even if the gas concentration
goes back below the alarm threshold. The alarm indication may be cancelled,
after clearing, by pressing the ACK (acknowledge) key.
• NO - Alarm status self-resets. When the alarm condition clears, normal
operation will resume.
Self-Resetting Alarms: Figure 3-3 shows the behavior of a self-resetting (unlatched)
alarm. The gas concentration rises above the programmed alarm level at time t1. The
alarm activates and remains active until the average gas concentration falls below the alarm
threshold at time t2, at which time the alarm conduit deactivates automatically.
The display indicates current gas concentration level.
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