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Principles of Operation
8.5.9.3. Thermistor Interface
8.5.10. ANALOG OUTPUTS
8.5.11. EXTERNAL DIGITAL I/O
300
This circuit provides excitation, termination and signal selection for several negative-
coefficient, thermistor temperature sensors located inside the analyzer. They are:
REACTION CELL TEMPERATURE SENSOR: A thermistor embedded in the reaction
cell manifold. This temperature is used by the CPU to control the reaction cell heating
circuit and as a parameter in the temperature/pressure compensation algorithm. This
measurement is stored in the analyzer's DAS and reported as test function RCEL TEMP.
BOX TEMPERATURE SENSOR: A thermistor is attached to the motherboard.
measures the analyzer's inside temperature. This information is stored by the CPU and
can be viewed by the user for troubleshooting purposes through the front panel display. It
is also used as part of the NO, NO
Temperature/Pressure Compensation feature is enabled. This measurement is stored in
the analyzer. Memory as the test function BOX TEMP and is viewable as a test function
(Section 4.2.1) through the analyzer's front panel.
The thermistor inside the PMT cold block as well as the thermistor located on the
preamplifier board are both converted to analog signals on the preamplifier board before
being sent to the motherboard's A/D converter.
O
SENSOR TEMPERATURE: For instruments with the oxygen sensor option installed,
2
the thermistor measuring the temperature of the heating block mounted to the sensor is
reported as test function O2 TEMP on the front panel. This temperature is maintained at
50° C.
The analyzer comes equipped with four Analog Outputs: A1, A2, A3 and a fourth that is a
spare.
A1 and A2 Outputs: The first two, A1 and A2 are normally set up to operate in parallel so
that the same data can be sent to two different recording devices. While the names imply
that one should be used for sending data to a chart recorder and the other for interfacing
with a data logger, either can be used for both applications.
Output Loop-back: All of the functioning analog outputs are connected back to the A/D
converter through a Loop-back circuit. This permits the voltage outputs to be calibrated
by the CPU without need for any additional tools or fixtures (see Section 6.13.5.4)
The external digital I/O performs two functions.
The STATUS outputs carry logic-level (5V) signals through an optically isolated 8-pin
connector on the rear panel of the analyzer. These outputs convey on/off information
about certain analyzer conditions such as CONC VALID. They can be used to interface
with certain types of programmable devices (Section 6.15.1.1).
The CONTROL inputs can be initiated by applying 5V DC power from an external
source such as a PLC or data logger (Section 6.15.1.2). Zero and span calibrations can be
initiated by contact closures on the rear panel.
Teledyne API - Model T200H/T200M Operation Manual
and NO
calculations when the instrument's
X
2
It
07270D DCN7141
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