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Calibration -
The Answer to Reliable Detection
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The BS EN 50291 British and European Standard for CO Alarms requires a much lower trigger level than the previous BS 7860,which it
supersedes.This is designed to give added protection to the more vulnerable members of society: the very young and the elderly.
However, this can mean that nuisance alarm from very low levels of CO can occur if the sensor is not very accurately calibrated.To
minimise this risk:
• Prior to alarm assembly every Ei CO sensor is independently calibrated at the factory in CO gas
• After alarm assembly every Ei CO Alarm is further tested in CO gas at the factory
These facts, coupled with sustained levels of quality throughout the manufacturing process ensure the very highest standard of
reliability and immunity from nuisance alarm.
Comprehensive Precision Calibration and Testing at 5 stages of the
Manufacturing Process
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10
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50
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80
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50 100 150 200 250
CO Parts per million
Alarm Response
Time (minutes)
▲
▲
▲
▲
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0.5
1
1.5
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3.5
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4.5
% COHb*
▲
Ei CO Alarm Trigger Level
Every Ei Alarm calibrated at these critical points
Nominal COHb level at
Ei Alarm Trigger points
The sensors are analysed
and are grouped
according to their
response. Each group is
then installed into a
specifically calibrated
circuit within the alarm,
ensuring that all alarms
accurately respond
when subjected to CO
gas.
* When CO is inhaled,it combines with the
haemoglobin in the blood to form
Carboxyhaemoglobin (COHb).The CO displaces
the oxygen attached to the haemoglobin and as
such causes oxygen starvation throughout various
parts of the body.
BS EN 50291:2001 requires CO Alarms to trigger
before the %COHb in a typical user has risen
above approx 5 % COHb at low CO levels,such
as 50 ppm CO.At higher levels,such as 300 ppm
CO,the alarms must trigger before the level in a
typical user has risen above 2 %COHb.The effects
of COHb vary from one person to another,
people at most risk are the elderly,the very
young,pregnant women and those whose health
is already compromised.The adjacent chart
shows that the level of CO present over a period
of time increases the %COHb and that Ei CO
alarms respond well before a dangerous %COHb
level is reached.However,it should be noted that
people with heart disease and other associated
illnesses may suffer harmful effects even at low
COHb levels.
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Ei CO alarms made to exceed BS EN 50291: 2001 -
Precise and reliable calibration is now even more important
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Sensors mounted on
bespoke calibration
circuits are loaded into
the CO enclosure, where
the CO gas exposure
program can be run.
Once recorded,the
sensors are loaded into
a custom interface to
extract data on each
individual sensor’s
response.
As the CO gas exposure
program is run, the
sensors are subject to
precise CO levels for set
times with each sensor’s
individual response
being recorded by the
calibration circuitry.
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Following assembly, all
finished alarms are
functionally checked for
response and accuracy
in CO gas before
despatch.
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Nominal Time to Alarm & % COHb