3.8
Normally, and if left unattended in air after power is applied, timing circuits within the hub
assembly will permit the system to remain on for approximately 5 minutes and then will
automatically turn the power off. However, when lowered into a gaseous environment containing
less than 18% oxygen, an internal voltage comparator senses this condition and prevents the unit
from turning off until it is once again exposed to a normal air environment, Thereafter,
approximated one minute of additional on power is permitted before the unity automatically turns
off during a measurement cycle.
3.9
The alarm circuit within the hub consists of 18% oxygen level comparator described above, an
8% comparator, a red lamp (LED) and horn as visual and audible alarm indicators, with other
ancillary electronic circuits.
3.10 When the sensor is powered in a normal air environment, the oxygen level is well above the 8%
level and the red LED lamp on the faceplate will flash slowly. The horn is silent. If the sensor is
now lowered into a tank which contains an oxygen level below 18% but above 8%, the horn will
now emit a pulsating audible tone and the red lamp will then begin to flash at a rapid rate. Below
8% the horn will be silent, and the red LED will extinguish.
To Summarize:
Above 18%
Lamp slowly Flashes, Horn Silent (Power on indicator)
From 8% to 18%
Lamp Rapidly Flashes, Horn Sounds
Below 8%
Lamp off, Horn Silent
3.11 As can be seen from the above table, during an above acceptable level of oxygen contents, both
red LED and horn are off. In addition, the action of the red LED is explained as follows:
3.12 The red LED lamp served two purposes.
3.13 The first purpose is to act as a simple power on indicator in normal air during day and nighttime
use. This feature is important at night since an operator may not be aware that automatic shut-off
has occurred and attempt a gauging procedure.
3.14 The second purpose is to provide an additional indication that an unsafe in-tank oxygen level
exists. In the event that an operator has difficulty in hearing the alarm horn, the rapidly flashing
red LED can be clearly seen.
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