Thermo Scientific MIRAN 205B Series Instruction Manual page 87

Chapter 7 Configuration and Setup
Reference Compensation Summary
Since the SapphIRe analyzer is a single beam infrared spectrophotometer, any changes to
the analyzer which may cause drift are associated as changes to the detector signal
compared to the zero signal. As the analyzer warms up, changes to the electronics, the
infrared filter, and the gas cell affect the data. To overcome these subtle changes, rezero
the analyzer more often.
Another way to mitigate these effects and to increase the stability of the data is to employ
reference compensation. Reference compensation repeatedly compares the detector
signal at an non-absorbing wavelength to the signal at the analytical wavelength. The
processor then compares these two signals to the associated signal during the zero cycle.
The analytical wavelength signal is then adjusted, if necessary, to compensate for any
shift.
Choosing the correct reference wavelength is very important. All the calibrations in the
fixed library have a reference wavelength of 4.004 microns associated with the
calibration. By enabling reference compensation, the analyzer uses 4.004 microns as the
wavelength. This wavelength filter was chosen because there are virtually no gases that
absorb at this wavelength. The problem with this wavelength is that it is not completely
similar to other analytical wavelengths. However, using this reference wavelength
provides suitable compensation for most applications. If this compensation is inadequate,
it may be necessary to use a different reference wavelength. Ideally, the wavelength
should be close to the analytical wavelength. For example, if ETO is measured at 11.6
microns, then a wavelength in the 10 to 12 micron range should be chosen. However, the
wavelength that is chosen must be a non-absorbing wavelength for the primary gas and
any interfering gases. Choosing a wavelength that is affected by another gas can cause
significant errors in the displayed data if that gas is present. The goal is to factor out
wavelength neutral changes due to the analyzer. If the signal for the reference
wavelength is increased by a gas, the correction by the processor will most likely be
overstated.
Before choosing a different wavelength, the parameters in the Fixed Library must be
transferred to a User Library entry. This is the only way to edit any parameter in fixed
library calibrations.
The Reference Compensation Summary allows reviewing the reference compensation
data for the application (see Reference Compensation Summary Display, below). To
accept this data, press . The display jumps to the Humidity Compensation
ENTER
Display. To change the data in the Reference Compensation Summary, press . The
1
Reference Compensation Mode Display is presented.
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