Carbon Monoxide Alarm User Manual

Page 39
ACTUAL LEL CONCENTRATION
RELATIVE LEL
METER RESPONSE
CALIBRATION STANDARD
OTHER GASES
OTHER
GASES
Note that the response to the gas to which the instrument
was calibrated, the "calibration standard," is still precisely
accurate. For the other gases the responses are a little
off.
In the case of some gases the readings are a little high.
This results in the instrument going into alarm a little bit
early. This type of error is not dangerous, since it results
in workers exiting the affected area sooner than they
absolutely have to.
Gases which produce lower relative readings than the
calibration standard can result in a more potentially
dangerous sort of error. In the chart example above the
"worst case" gas only produces a meter reading of 50
percent LEL even when the actual concentration is 100
percent explosive. If the alarm were set to go off when the
display reads 50 percent LEL, the alarm would sound
simultaneously with the explosion!
If on the other hand the alarm is set to go off when the
display reads 20 percent LEL, a 50 percent concentration
of the same "worst case" gas is enough to cause an
alarm.
It may be seen from the graph that the amount of relative
error decreases the lower the alarm point is set. If the
alarm point is set at 10 percent LEL, the differences due
to relative response of the combustible sensor are
minimal.
The best results are obtained when calibration is done
using the same gas that is expected to be encountered
while actually using the instrument. When it is not
possible to calibrate directly to the gas to be measured, or
when the combustible gas is an unknown, an alarm set
point of 10 percent LEL or less should be selected.
In the new standard for "Permit Required Confined Space
Entry" (29 CFR 1910.146) OSHA has determined that a
combustible hazard exists whenever the concentration of
combustible gas or vapor exceeds 10 percent LEL. Per
this standard confined spaces with concentrations which
exceed 10 percent LEL may not be entered. Likewise,
workers are required to immediately leave anytime
readings exceed 10 percent LEL.
The standard combustible alarm set-point for the
PhD
2
is 10 percent LEL.
When not sure what combustible gases might be
encountered, use a mixture of 1.1% propane in air (50%
LEL) combustible gas to calibrate.
Propane provides a sensor response which is more typical
of the wide range of combustible gases and vapors than
any other calibration mixture.
2.1. Calculating relative responses
There are theoretical ways to estimate the relative
response of a sensor calibrated on one combustible gas
to exposure to another gas. This is done by taking the
actual instrument reading, and multiplying it by a
correction factor.
It is very important to treat gas concentration
calculations based on theoretical relative response
ratios cautiously.
In actual practice, the relative response varies somewhat
from sensor to sensor. The response ratios may also shift
over the life of a particular sensor.
It is also very important to understand that if an error is
made in determining the specific kind of gas present, and
the wrong correction factor is used, the accuracy of the
calculation may be significantly affected.
The best results are obtained when calibration is done
using the same gas that is expected to be
encountered while actually using the instrument.
3. Correction factors for PhD
2
combustible gas
sensors:
Combustible
Gas / Vapor
Correction factor
when instrument is
calibrated on Propane
Correction factor
when instrument
is calibrated on
Methane
Hydrogen 0.54 0.83
Methane 0.65 1.0
Propane 1.0 1.5
n-Butane 1.0 1.5
n-Pentane 1.1 1.7
n-Hexane 1.2 1.8
n-Heptane 1.3 2.0
n-Octane 1.6 2.5
Methanol 0.65 1.0
Ethanol 0.76 1.2
Isopropyl
Alcohol
1.0 1.5
Acetone 0.93 1.4
Ammonia 0.46 0.71
Toluene 1.6 2.5
Methyl Ethyl
Ketone
1.2 1.8
Ethyl Acetate 1.2 1.8
Gasoline
(Unleaded)
1.1 1.7