Det-Tronics Eagle Quantum 95-8470-05, Инструкция по эксплуатации

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V-4
3. Ventilation characteristics of the immediate area
must also be considered. Movement of air will
cause gas to accumulate more heavily in one area
than another. The sensor should be placed in the
area where the most concentrated accumulation of
gas is anticipated. Also consider the fact that many
ventilation systems do not operate continuously.
4. The sensor should be pointed down to prevent the
buildup of moisture or contaminants on the filter and
to ensure proper operation. For PointWatch, refer to
instruction manual 95-8440.
5. The sensor must be accessible for testing and
calibration. The use of the Sensor Separation Kit
will be required in some installations.
6. Catalytic sensors should be located where they are
safe from potential sources of contamination that
can poison the sensing element.
7. Exposure to excessive heat or vibration can result in
premature failure of any electronic device and
should be avoided if possible. Shielding the device
from intense sunlight will reduce solar heating and
can increase the life of the unit.
For additional information on determining quantity and
placement for sensors in a specific application, refer to
Instrument Society of America (ISA) Transaction Volume
20, Number 2, titled “The Use of Combustible Detectors
in Protecting Facilities from Flammable Hazards.”
CATALYTIC GAS SENSORS
CAUTION
Exposure to a high level of gas can have an
adverse effect on the sensitivity of catalytic sensing
elements. If the level of gas at the sensor should
reach 100% LFL, it is important that it be tested
and recalibrated if required. In some cases, it may
be necessary to replace the sensor.
If an over-range condition should occur, the user
must exercise caution, since a highly explosive
condition could exist. The hazardous area should
be checked with a portable detection instrument to
determine the actual level of combustible gas
present.
For best calibration results, allow a new sensor to
operate for several hours to ensure a stable output
before performing calibration. For the highest degree of
accuracy, perform a second calibration after 24 hours.
When a sensor is exposed to a different or new
environment, calibration should be checked frequently
to determine the proper interval between periodic
calibrations.
Before performing calibration, the operator should
examine the sensor filter (flame arrester) to be sure that
it is not missing or damaged. If the filter is defective or
missing, it must be replaced. In some cases, an
exposed sensing element can act as an ignition source.
It should also be noted that a dirty cover can
significantly reduce the sensitivity of the sensor.
CATALYTIC SENSOR OPERATION
The sensing element used in the combustible gas
sensor consists of a pair of elements. One is an active
catalytic sensing element, and the other is inactive and
acts as a temperature compensating reference element.
Both elements are composed of a wire coil encased in
ceramic. The active element has a catalytic coating
applied to its surface, however, the reference element is
glazed so that it does not react to the presence of a
combustible gas/air mixture. The pair is computer
matched to have the same electrical resistance.
In the presence of a combustible gas/air mixture, the
resistance of the active sensing element increases in
proportion to the concentration of gas at the sensor.
The change in resistance of the active sensing element,
relative to that of the reference element, is used to
determine the actual level of combustible gas present at
the sensor.
Both elements operate at a high temperature and are
enclosed by a porous stainless steel cup (flame
arrester). See Figure V-2. This cup allows the diffusion
of gas to and from the sensing element, but prevents
the ignition of the atmosphere outside the sensor should
the combustible gas concentration exceed its LFL.
A barrier is placed between the element pair to prevent
thermal interaction and to prevent the transfer of
catalytic material from the active element to the surface
of the reference element.
All catalytic type sensors require oxygen to detect
combustible gases. The sensor response will decrease
if enough combustible gas displaces the normal oxygen
present in air. The sensor should not be used where the
oxygen level is less than 10%. See Figure V-3.
Figure V-4 shows the response of a typical sensor to
various levels of combustible gas. Note that a reading of
40% LFL will be given at 2.0% methane (40% LFL) and