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Teledyne API - Model T200H/T200M Operation Manual
8.8.2. CALIBRATION - SLOPE AND OFFSET
8.8.3. TEMPERATURE/PRESSURE COMPENSATION (TPC)
07270D DCN7141
Aside from the hardware calibration of the preamplifier board (Section 7.6.4) upon
factory checkout, calibration of the analyzer is usually performed in software. During
instrument calibration (Section 5) the user enters expected values for span gas
concentration through the front panel keypad and supplies the instrument with sample gas
of know NO and NO
X
values and the software computes values for the new instrument slope and offset for both
NO and NO
response. These values are stored in memory for use in calculating the NO,
X
NO
and NO
concentration of the sample gas. By default, the DAS stores 200 software
X
2
calibration settings for documentation, review and data analysis.
Instrument slope and offset values recorded during the last calibration can be viewed on
the front panel. NO SLOPE, NOX SLOPE, NO OFFS and NOX OFFS are four of the
test parameters accessible through the <TST TST> buttons.
The software features a compensation of some temperature and pressure changes critical
in the measurement of NO and NO
(default setting), the analyzer divides the value of the PMT output signal (PMTDET) by
a value called TP_FACTOR. TP_FACTOR is calculated according to the following
equation.
RCELL
=
TP
_
FACTOR
A
Where A, B, C, D are gain functions. The four parameters used to compute TP_FACTOR
are:
•
RCELL TEMP: The temperature of the reaction cell, measured in K.
•
RCEL: The pressure of the gas in the vacuum manifold, measured in in-Hg-A.
•
SAMP: The pressure of the sample gas before it reaches the reaction cell, measured
in in-Hg-A. This measurement is ~1 in-Hg-A lower than atmospheric pressure.
•
BOX TEMP: The temperature inside the analyzer's case measured in K. This is
typically about 5 K higher than room temperature.
The current value of all four of these measurements are viewable as TEST FUNCTIONS
through the instrument's front panel display.
that, as RCEL TEMP,
TP_FACTOR increases and, hence, the PMTDET value decreases.
increases in the reaction cell pressure (RCEL) decrease TP_FACTOR and, hence
increase the PMTDET value. These adjustments are meant to counter-act changes in the
concentrations caused by these parameters.
concentrations. The readings are then compared to the expected
concentration. When the TPC feature is enabled
X
TEMP
(
K
)
( 7
in
×
B
323
(
K
)
RCEL
BOX TEMP and SAMP pressure increase, the value of
Principles of Operation
Hg
)
SAMP
(
in
Hg
×
×
C
(
in
Hg
)
29.92
(
in
Hg
)
BOX
TEMP
(
K
)
D
298
(
K
)
(Equation 9-5)
Conversely,
305
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