42/23-572 EN Rev. 5
Operator’s Manual ACF-NT, ACF-NT V0309
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FTIR Spectrometer Zero Calibration
Measuring Principle
The analyzer measures the intensity of infrared absorption of gases in the sample
cell. Absorption is determined over a spectrum or band.
Raw Spectrum
When analyzing a gas the analyzer first calculates a raw spectrum. This spectrum
is a measure of the energy reaching the infrared detector after the infrared beam
has traversed the gas. The raw spectrum contains the desired absorption
information, but this information is divided again by the zero spectrum to eliminate
any optical system changes (contamination, aging, etc.).
Zero Spectrum
The zero spectrum is determined by analyzing a sample cell filled with a “zero
gas” (a gas that does not absorb infrared radiation, such as nitrogen or air with all
infrared-active components filtered out). On a point-by-point basis, the software
divides the sample’s raw spectrum by the zero spectrum to determine the absorp-
tion spectrum. Therefore the zero spectrum is also called reference spectrum
Instrument Reaction
The absorption spectrum depends on various factors: infrared source spectrum,
optical material absorption capability, infrared detector response time. Taken to-
gether, these factors form the inherent system reaction, also known as “instrument
response”.
When should a zero
spectrum be
measured?
Since over time the instrument response can change, a new zero spectrum must
be recorded at regular intervals. This procedure is called zero calibration or “zero-
ing” the analyzer. Naturally, the analyzer must be zeroed (or referenced) before it
can be used. Since over extended periods of time the infrared source can change
and this includes the mirror in contact with the sample gas in the sample cell, we
recommend setting the analyzer zero point at least once a day.
Automatic Reference
Spectrum Recording
The analyzer system performs the zero calibration automatically twice a day every
12 hours (according to the German TÜV approval).
The FTIR controller triggers the start of this event and also controls the valves and
the flushing times. When the starting time is reached, the following sequence runs:
Phase Description
1
The “Maintenance Mode” status signal is set and the analyzer system
is switched to “Zero gas local”.
2
The measuring cell is flushed with dry air to ensure that the sample
gas is exchanged for dry air (duration 5 minutes).
3
A new reference spectrum (zero spectrum) is recorded (duration
2 minutes).
4
The analyzer system is switched back to “Sample gas”.
5
The measuring cell is flushed with sample gas (duration 3 minutes)
6
The first measurement spectrum is recorded (duration 2 minutes).
7
After the recording of first measurement spectrum is finished the
“Maintenance Mode” status signal is re-set and the measurement
results are valid.
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Summary of Contents for ACF-NT
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