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Model 4688-IR / 4679-IR
Operation & Maintenance Manual
Manual #087-0007G, 10/2003
Appendix 3
Instrument Cross-Sensitivity
In some applications the Model 4688-IR may be used to detect more than one
combustible gas present in the atmosphere. Cross-sensitivity is defined as the
reading produced by a one gas when the instrument is set to detect another
(different) gas. Under these circumstances it is
EXTREMELY IMPORTANT TO
ADHERE TO THE FOLLOWING GUIDELINES:
1.
The instrument can only detect those gases available in its software.
To determine which gases the instrument is capable of detecting, observe
the instrument's SR.GN values and refer to the USER Menu.
2.
The instrument MUST be set to detect the gas which, using the
appropriate chart below, is determined to be the least visible
(absorbed) by the instrument.
Failure to do so could result in the
undetected presence of a combustible at or above its 100% LEL !
Cross-Sensitivity Programming Examples
Cross-Sensitivity Programming Examples
Cross-Sensitivity Programming Examples
Cross-Sensitivity Programming Examples
Cross-Sensitivity Programming Examples - These
examples only apply to units with a sensor type 2. This
can be viewed by SR.tY.
Suppose the instrument may be exposed to either METHANE/AIR or ETHANE/
AIR separately, or in a mixture. We are free to set the instrument to monitor any
one of the gases so long as we adjust the alarm set points safely.
In this case,
we want to trigger ALARM 1 when the least sensitive gas reaches 30%
LEL, and ALARM 2 when the least sensitive gas reaches 50% LEL.
When the instrument is set to detect METHANE
METHANE
METHANE
METHANE
METHANE
(((((SR.GN=1):
• 30% LEL of METHANE (1.50%V/V) produces a METHANE reading of
30% LEL
• 30% LEL of ETHANE (0.90%V/V) produces a METHANE reading of
117% LEL,
which is an approximate value.
• 50% LEL of METHANE (2.50%V/V) produces a METHANE reading of
50% LEL
• 50% LEL of ETHANE (1.50%V/V) produces a METHANE reading of
303% LEL,
which is an approximate value.
Solution:
Solution:
Solution:
Solution:
Solution:
> Set alarm 1 (A1.SP) to the lowest value, 30 %LEL.
> Set alarm 2 (A2.SP) to the lowest value, 50 %LEL.
When the instrument is set to detect ETHANE (
ETHANE (
ETHANE (
ETHANE (
ETHANE (SR.GN=2):
• 30% LEL of METHANE (1.50%V/V) produces an ETHANE reading of
14% LEL,
which is an approximate value.
• 30% LEL of ETHANE (0.90%V/V) produces an ETHANE reading of
30% LEL
• 50% LEL of METHANE (2.50%V/V) produces an ETHANE reading of
19% LEL,
which is an approximate value.
• 50% LEL of ETHANE (1.50%V/V) produces an ETHANE reading of
50% LEL
Alarm 1
Alarm 2
Alarm 1
Alarm 2
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