ISA-40 Oxygen Monitor
ENMET
Corporation
11
4.0 Operation
4.0 Operation
Install and power the unit as previously described in section 3.0 through 3.3.3.
The green lamp on each channel is an indication that power is on and the unit is not in alarm. The red lamp indicates
alarm, which is usually set at an oxygen content of 18.0%. The green lamp goes out when the red lamp goes on. If no
lamps are on, power has been interrupted.
The meter, one per channel, gives a visual indication of oxygen content at the location of the sensor for that channel.
The meter scale reads from 16% to 26% oxygen by volume and is linear. The percentage of oxygen for fresh air,
20.9%, is the center point on the scale. The alarm point is typically 18.0% oxygen. Do not work in a space where the
indicated oxygen content is less than 16%. Follow your organization's policy and procedures in an alarm situation.
The audio alarm mounted on the front of the control unit enclosure activates when any one or more channels is in
alarm. In order to meet stringent safety requirements, the horn cannot be turned off during maintenance.
4.1 Rough Test
An oxygen alarm can be triggered to verify the alarm point by exhaling over the cell. If the
ISA-40
includes a remote
sensor, two people are required to verify the alarm point, one at the sensor assembly and the other at the control unit.
1.
The individual at the sensor assembly inhales and holds his breath for 5 - 15 seconds.
2.
Then he exhales slowly through the sintered screen in the cell enclosure.
3.
The individual at the control unit observes that the alarm triggers at the alarm point, usually 18.0%.
4.
A pressurized cylinder containing a mixture of 17.0% oxygen in nitrogen can be used with an appropriate
calibration assembly to trigger the alarm. See Section 7.1.
4.2 Precautions Regarding the Cell
Do not expose the oxygen cell to temperatures below 32
°
F (0
°
C) or above 130
°
F (55
°
C).
Never adjust the oxygen cell in an area where an oxygen deficiency may exist. Always adjust the oxygen cell in fresh
air. Use cylinders of 20.9% by volume oxygen in nitrogen if necessary to guarantee oxygen content. See Section 7.1.
Avoid exposure to toxic gas environments.
4.3 Atmospheric Pressure Variations
The galvanic oxygen deficiency transducer cell current output is directly proportional to the partial pressure of the
oxygen in the atmosphere surrounding the cell. The partial pressure of oxygen is a function of the percent by volume
of oxygen in the air and the air pressure. Air pressure varies as natural weather systems move through the area,
causing changes in barometric pressure as well as oxygen content. The barometric pressure changes cause a relatively
small change in oxygen content indication; for example, if the gain is set to give an oxygen content meter indication of
20.9% oxygen when the barometric pressure is 29.9 inches of mercury, normal variations of barometric pressure cause
variations in meter readings of + or - 0.25%, from 20.65% oxygen by volume to 21.15% oxygen by volume. If the
gain is set during a normal high pressure weather cycle, the variation is 0.50% downscale; conversely, if the gain is set
during a normal low, the variation is upscale. This response to atmospheric pressure is not distressing when
understood. The variation of the alarm point by + or - 0.25% is not significant when the liberal safety factor between
the alarm point, usually 18.0%, and the point at which oxygen deficiency first emphatically affects human
performance, approximately 16.0%, is taken into consideration
4.4 Temperature Variations
The cell output is temperature dependent; consequently, the cell is temperature compensated by means of a thermistor
resistor network on the circuit board in the sensor assembly. Significant changes in temperature cause erratic initial
readings, as the cell and thermistor do not track due to the difference in their locations and thermal masses. This
phenomenon is significant in portable instruments, but usually does not affect the operation of permanent systems.