Carbon Monoxide Alarm User Manual

Page 40
As an illustration, consider a PhD
2
calibrated on methane,
which is then used to monitor ethanol. When calibrated to
methane, the instrument is actually less responsive to
ethanol than to methane, so the readings will be low.
Multiplying the instrument reading by the correction factor
for ethanol will produce the true concentration.
Given that the correction factor for ethanol is 1.2, if the
instrument reading is 40 percent LEL, then the true
concentration is seen to be 48 percent LEL.
(40% LEL) X (1.2) = (48% LEL)
INSTRUMENT CORRECTION TRUE
READING FACTOR CONCENTRATION
It is important to note that the correction factor for ethanol
is different when the instrument is calibrated on propane.
In the case of a propane calibrated instrument, instrument
readings for ethanol will be high. Given that the correction
factor for ethanol in this case is 0.76; when the instrument
reads 40 percent LEL, the true concentration for ethanol is
30.4 percent LEL.
(40% LEL) X (0.76) = (30.4% LEL)
INSTRUMENT CORRECTION TRUE
READING FACTOR CONCENTRATION
When not sure what combustible gases might be
encountered, it is usually best to use a mixture of 1.1%
propane in air (50% LEL) combustible gas to calibrate the
instrument. Propane provides a sensor response which is
more typical of the wide range of combustible gases and
vapors than any other calibration mixture. Use of other
gases such as methane or hexane to calibrate the
instrument should be reserved for situations where these
are either the gases predominantly present, or where the
relative response to the calibration gas closely
approximates that of the actual gas to be measured.
2. Toxic gas alarms
The PhD
2
has three separate alarm points for toxic gases:
Ceiling, STEL, and TWA.
OSHA has assigned some, but not all, toxic substances
with a ceiling or "Peak" exposure level. This is the highest
concentration of a toxic substance to which an
unprotected worker should ever be exposed, even for a
very short time. Never enter an environment even
momentarily when concentrations of toxic substances
exceed the ceiling level.
The Time Weighted Average ( or TWA) is the maximum
average concentration to which an unprotected worker
may be exposed over an eight hour working day. During
this time, STEL and ceiling concentration limits may not
be exceeded.
OSHA has assigned some, but not all, toxic substances
with a Short Term Exposure Limit. The STEL is the
maximum average concentration to which an unprotected
worker may be exposed in any fifteen minute interval
during the day. During this time, neither the eight hour
TWA or the ceiling concentration may be exceeded. Any
fifteen minute periods in which the average STEL
concentration exceeds the permissible eight hour TWA
must be separated from each other by at least one hour.
A maximum of four of these periods are allowed per eight
hour shift.
The table below shows the highest levels at which these
alarms should be set. If OSHA has not determined a
ceiling value, for greatest safety the PhD
2
ceiling alarm
should be set at the same value as the STEL alarm. If
OSHA has not determined a STEL value, the PhD
2
STEL
alarm should be set at the same value as the TWA.
4. U. S. Department of Labor, Occupational Safety
and Health Administration (OSHA) 1989 Permissible
Exposure Limits for select toxic gases:
In the following table "NA" indicates no value has been
assigned by OSHA.
Note: Customers should be aware that OSHA
permissible exposure limits may be subject to
change. Recent court decisions have affected the
enforcement of permissible exposure limits published
or modified since the initial enactment of OSHA in
1971. The following table shows the OSHA
permissible exposure limits as published in the 1989
edition of the Code of Federal Regulations (29 CFR
1910.1000). It is recommended that customers verify
that the values given below are correct and current
before using them to update their alarm set-points.
Gas Ceiling TWA STEL
CO 200 PPM 35 PPM NA
H2S NA 10 PPM 15 PPM
SO2 NA 2.0 PPM 5.0 PPM
Cl2 NA 0.5 PPM 1.0 PPM
HCN NA NA 4.7 PPM
NH3 NA NA 35 PPM
NO NA 25 PPM NA
HCl 5 PPM NA NA
NO2 NA NA 1.0 PPM