Teledyne T200, Руководство по эксплуатации, Страница: 331 / 425

Teledyne API
– T200 NO/NO
2
/NO
X
Analyzer Operation Manual Principles of Operation
329
13.9.1. ADAPTIVE FILTER
The T200 NO
X
analyzer software processes sample gas concentration data through a
built-in adaptive filter. Unlike other analyzers that average the output signal over a
fixed time period, the T200 averages over a defined number of samples, with samples
being about 8 seconds apart (reflecting the switching time of 4 s each for NO and NO
X
).
This technique is known as boxcar filtering. During operation, the software may
automatically switch between two different filters lengths based on the conditions at
hand.
During constant or nearly constant concentrations, the software, by default, computes an
average of the last 42 samples, or approximately 5.6 minutes. This provides smooth and
stable readings and averages out a considerable amount of random noise for an overall
less noisy concentration reading.
If the filter detects rapid changes in concentration the filter reduces the averaging to only
6 samples or about 48 seconds to allow the analyzer to respond more quickly. Two
conditions must be simultaneously met to switch to the short filter. First, the
instantaneous concentration must differ from the average in the long filter by at least 50
ppb. Second, the instantaneous concentration must differ from the average in the long
filter by at least 10% of the average in the long filter
13.9.2. TEMPERATURE/PRESSURE COMPENSATION (TPC)
The T200 software includes a feature that compensates for some temperature and
pressure changes that might affect measurement of NO and NO
X
concentrations.
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 , which is calculated
using the following four parameters:

BOX TEMP
: The temperature inside the analyzer’s case measured in K. This is
typically about 5 K higher than room temperature.

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.
As RCEL TEMP, BOX TEMP, RCELL and SAMP pressure increase, the value of
TP_FACTOR increases and, hence, the PMTDET value decreases. These adjustments
are meant to counter-act changes in the concentrations caused by these parameters.

The current value of all four of these measurements are viewable as
TEST
FUNCTIONS
through the instrument’s front panel display (see Section 4.1.1).

The preset gain parameters are set at the factory and may vary from analyzer to
analyzer. The TPC feature is enabled or disabled by setting the value of the
variable
TPC_ENABLE
(see Section 5.8).
06858E DCN7057