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Teledyne API - Model 200EH/EM Operation Manual Theory of Operation
229
10.7.3. TEMPERATURE/PRESSURE COMPENSATION (TPC)
The software features a compensation of some temperature and pressure changes critical in the measurement
of NO and NO
X
concentration. When the TPC feature is enabled (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.
)(
)(
×
)(
(
×
)(
)(7
×
)(
)(
=_
K298
KTEMPBOX
D
Hgin29.92
HginSAMP
C
HginRCEL
Hgin
B
K323
KTEMPRCELL
AFACTORTP
(Equation 10-5)
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.
Note that, as
RCEL TEMP, BOX TEMP and SAMP pressure increase, the value of TP_FACTOR increases
and, hence, the PMTDET value decreases. Conversely, 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.
Each of the terms in the above equation is attenuated by a gain function with a numerical value based on a
preset gain parameter (shown below in CAPITALIZED ITALICS) normalized to the current value of the
parameter being attenuated. The gain functions A, B, C and D are defined as:
]__×)1
)(323
)(_
[(+1= GAINTPCRCTEMP
K
Ktemprcell
A
(Equation 10-6)
]__×)1
)("_
)("5
[(+1= GAINTPCRCPRESS
Hgpressurercell
Hg
B
(Equation 10-7)
]__×)1
)(323
)(_
[(+1= GAINTPCSPRESS
K
Ktemprcell
C
(Equation 10-8)
]__×)1
)(298
)(_
[(+1= GAINTPCBXTEMP
K
Ktempbox
D
(Equation 10-9)
The preset gain parameters are set at the factory and may vary from analyzer to analyzer. Section 6.12
describes the method for enabling/disabling the TPC feature.
04521C (DCN5731)