Instrument Control; Monitoring Signals; Measurement Calculations - Thermo Scientific 49i Instruction Manual

System Description
Software

Instrument Control

Monitoring Signals

Measurement
Calculations
8-4 Model 49i Instruction Manual
Low-level embedded processors are used to control the various functions on
the boards, such as analog and digital I/O and heater control. These
processors are controlled over a serial interface with a single high-level
processor that also controls the front-panel user interface. The low-level
processors all run a common piece of firmware that is bundled with the
high-level firmware and loaded on power-up if a different version is
detected.
Each board has a specific address that is used to identify to the firmware
what functions are supported on that board. This address is also used for the
communications between the low-level processors and the high-level
processor.
Every tenth of a second the frequency counters, analog I/O, and digital I/O
are read and written to by the low-level processor. The counters are
accumulated over the past second and the analog inputs are averaged over
that second. The high-level processor polls the low-level processors once per
second to exchange the measurement and control data.
The instrument includes a sample solenoid valve and a reference solenoid
valve. The instrument can be configured to measure O
(standard cycle) or every four seconds (fast cycle). Every cycle, the two
solenoid valves switch the sample gas and reference gas streams between the
two cells. One cell contains sample gas, the other cell contains reference gas
and vice versa.
During a standard cycle, the cells are purged for seven seconds and
measurements are taken for three seconds. During a fast cycle, the cells are
purged for three seconds and measurements are taken for one second.
During every cycle, the instrument calculates the natural logarithm of the
sample gas frequency to reference gas frequency ratio in each cell and
averages these results to optimize instrument accuracy. These logarithms
provide the basis for calculating the ozone concentrations. The logarithms
are stored in a rolling stack of 30 logarithms which are averaged by the
instrument's averaging time function.
The background values for O
span and are subtracted from the ozone calculated to yield a corrected value.
are corrected for temperature, pressure, and
3
every ten seconds
3
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