Product Description (Details)
GM32 · Technical information · 8011923 · V1.4 · © SICK AG 13
Subject to change without notice
2.5.2 Check cycle (function)
The following influencing factors are the main cause of drifts in concentration measure-
ment:
● Wavelength scale drift
● Spectral resolution drift
● Absorption drift
Methods to monitor these influencing factors are described in the following.
2.5.2.1 Wavelength scale monitoring
The grating spectrometer used in the GM32 shows, due to the design, wavelength scale
drifts caused by mechanical effects (e.g. length changes caused by temperature fluctua-
tions). Compensation is realized through continuous determination of this drift during
measuring operation and calculation back to the original state through numeric interpola-
tion.
Wavelength drift is monitored in two stages during the check cycle:
1 Determining the position of an NO absorption line ensures correct functioning of wave-
length drift compensation.
To do this, a spectrum is examined with the NO cell swiveled in and the line position
deviation calculated as compared against the original device state.
A maintenance request is signaled when this deviation exceeds a limit value (normally
0.046 nm)
2 The overall wavelength drift is checked relative to the original device state. A mainte-
nance request is also signaled here when a limit value (normally 1 mm) is exceeded.
These measures ensure wavelength drift is compensated correctly and therefore has no
influence on measured values.
2.5.2.2 Resolution drift monitoring
Spectral resolution of the grating spectrometer used can change during operation due to
mechanical changes or contamination (scattered light). Resolution deterioration normally
leads to a reduction in sensitivity and the device displays values that are too low.
This is why the spectral resolution is examined during the check cycle.
To do this, the half-width of an NO absorption line is calculated from the NO cell spectrum
(see above).
A maintenance request is signaled when this deviation exceeds a limit value (normally
0.035 nm) as compared against the original state.
This check ensures resolution changes are detected before device sensitivity changes sig-
nificantly.
2.5.2.3 Absorption drift monitoring
Measured values output by the device have, to a large extent, a linear association with the
measured absorption. The consequence is that errors during absorption measurement
have a direct effect on measured values. Possible error sources for absorption determina-
tion are fluctuating scattered light intensities in the device as well as erroneous determina-
tion of the dark current of the detector array.
Two grating filters that serve as standard are used to check absorption determination. The
absorption for each of the two filters is determined during the check cycle and compared
against the values in original state. The deviations for both filters are averaged and this
value checked against a limit value (normally 2% of the measured value). A maintenance
request is signaled when the limit value is exceeded.
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