MUNI User’s Guide
13
5.4.1 Recommended Span Gases
5.4.1.1 Span Gas Selection
mPower recommends a mixture of 60 ppm CO / 15 ppm H
2
S / 2.5% CH
4
(50% LEL) / 18% O
2
,
(balance N
2
) to allow calibration of 4 standard sensors at the same time. Other calibration gas
options are listed in the table below.
Sensor
Resolution-Range
Calibration Gas
LEL/O
2
/CO/H
2
S 4-Gas Mix
See sensors below
50%LEL/18% O
2
/60 ppm CO/15 ppm H
2
S
Oxygen (O
2
)
0.1-30%Vol
18 %Vol O
2
and/or 100% N
2
to set zero
Combustibles P
ellistor
(LEL%)
1-100%LEL
50 %LEL (2.5 %Vol Methane, balance air)
Hydrocarbons NDIR (LEL%)
1-100%LEL
50 %LEL (2.5 %Vol Methane)
CO (Carbon Monoxide)
1-1000 ppm
60 or 100 ppm CO
H
2
S (Hydrogen Sulfide)
0.1-100 ppm
15 or 25 ppm H
2
S
HCN (Hydrogen Cyanide)
0.1-50 ppm
10 ppm HCN
SO
2
(Sulfur Dioxide)
0.1-20 ppm
5 ppm SO
2
Note:
Use balance gas of air if available, otherwise use nitrogen balance gas (except pellistor LEL must have oxygen)
5.4.1.2 Alternative Calibration Compounds for LEL Sensors
Because LEL sensors are broadband detectors, they can be calibrated with many possible gases.
The type of calibration gas is selected from a list of of over 50 compounds in mPower Suite
(Section 6), typically methane, but propane and pentane are also commonly used. The
measurement gas is also selected in mPower Suite. Correction Factors (CFs) are calculated and
automatically applied to make the display read in equivalents of the Measurement Gas. Note that
CFs are very different for NDIR sensors than for pellistor sensors. Thus far, CFs are only
implemented for the pellistor LEL sensors, while for the NDIR sensor only methane can be used.
TA Note 7 gives comprehensive lists of factors for both pellistor and NDIR LEL sensors along
with slightly adjusted values for EMEA countries. Contact mPower for help in implementing any
of these alternative Correction Factors.
5.4.2 Calibration Procedures
5.4.2.1 Air Calibration
Air calibration sets the zero baseline for toxic and combustible gas sensors and 20.9% for
oxygen. It is done in normal fresh air or other clean air source. It is always best to zero the
MUNI before Span Calibration, rather than after.
•
Enter Configuration Mode and the CAL? sub-menu, where the first item is Air Calibration:
Config Mode
→
CAL?
→
AIR CAL?
•
Press Left [+/OK] and a 30-second count-down starts. When finished, “PASS” or “FAIL” is
displayed. To Abort, press Right [ /↓] at any time during the count-down.
•
The oxygen sensor performs a Span (set to 20.9%) during Air Calibration and can be Zeroed
(set to 0.0%) using nitrogen in the Single Span menu.
