Teledyne LGA-3500 - Laser Gas Analysis system - Manual (Page 85)

Laser Gas Analyzer Maintenance and Calibration
Teledyne Analytical Instruments 71
Always bear in mind that the measurement accuracy of the system
is determined by the accuracy of its calibration. With this in mind,
determine first if the system needs calibration. When it is indeed
necessary, make sure it is performed correctly. We suggest that you use
the calibration tube that comes with the system from TAI. Figure 6.1 is
the schematic of the calibration setup. The calibration is performed
either by operating the keyboard of the LCD front panel on the central
processing unit or through a PC connected to the central processing unit
through the RS232 port.
The concentration of the calibration gas depends on both the
system’s measurement range and the environment temperature. If the
concentration is too high, it can saturate the measurement signal. If the
concentration is too low, the calibration accuracy would be affected
because the calibration tube and the connection tubes absorb some of the
gas, and the background noise would be large with respect to the signal.
Calibration gas should be a mixture of nitrogen and the gas under
test, and the concentration of the gas under test is set according to
the calibration requirement.
When calibrating some highly absorptive gases (such as H
2
O, HCl,
HF and NH
3
), especially if the measurement range of the system is
small, special care should be taken as the calibration tube and the
connection tubes could absorb a significant amount of the gas and
severely affect the calibration accuracy. When calibrating these gas
species, it is strongly recommended to:
• Use short tubes for connection, especially those used to connect
the standard gas container to the calibration tube.
• Make sure calibration devices and connection tubes are dry.
Purge them with dry clean nitrogen a few times before injecting
the standard calibration gas.
• Before starting the calibration process, purge the calibration tube
a few times with the standard calibration gas.
• Use relatively large flow rate, such as 5 L/min.
• Connect the calibration system with Teflon tubes.
• Wait till the measured gas concentration value is stable.
Observe whether the measured gas concentration value changes
much when the flow rate is increased. If it doesn’t, then the gas