Servomex SERVOTOUGH LaserExact 2950, Operator'S Manual

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SERVOTOUGH LaserExact Monitor, Operator Manual 02950001A_1
OPERATION
45
5.3 Calibration of the instrument
The SERVOTOUGH LaserExact Monitor is factory calibrated using a certified gas
mixture and a calibration certificate is supplied together with the instrument. There is
therefore no need for the user to calibrate the instrument upon reception.
After some period of operation the instrument parameters may change due to ageing
of the diode laser. In this case a new calibration might be necessary. We recommend
verifying calibration of the instrument once a year using a certified test gas. Per
default the certified test gas must be a mixture of
Nitrogen with the measured gas.
There are exceptions though i.e. instruments with special tuning that require
calibration in a balance different from N2. Please refer to the calibration certificate for
the specific instrument.
Since the calibration of the instrument affects all subsequent measurements, the user
should verify the need for a new calibration. We do not recommend performing a
calibration if the instrument readings deviate less than 2 – 3 % from the nominal
value within the recommended concentration levels indicated below. This is the
typical absolute accuracy for calibration gas mixtures. One should also account for
the precision of the SERVOTOUGH LaserExact Monitor, which is about 1% of the
measured value. For O
2
SERVOTOUGH LaserExact Monitors the deviation should
not exceed 0.2 % vol. when the values for the cell pressure and temperature are
correctly set in the instrument.
The second harmonic technique used in the SERVOTOUGH Laser monitors is
intrinsically a baseline-free technique. Therefore, a calibration of the zero level is
never done. If one attempts to calibrate the zero level, the service program will
display an error message. The monitor can only be calibrated if it detects absorption
signal amplitude above a predefined level. This level is factory set and is typically 20
times the detection limit for an optical path length of 1 m.
To obtain the optimal signal-to-noise ratio the gas concentrations of Table 5-1 should
be used.
Measured gas Recommended concentration range
HCl 10 – 20 ppm
NH
3
20 – 40 ppm
O
2
ca. 1%
CO
2
, HCN, CH
4
50 – 100 ppm
H
2
S 30 – 200 ppm
CO 100 – 1000 ppm
NO 50 – 500 ppm
Table 5-1: Recommended gas concentrations for calibration.