Teledyne 7120 - NDIR analyzer - Manual (Page 30)

Photometric Analyzer Start-up and Operation
3-7 Teledyne Analytical Instruments
temperature, and a zero-crossing switch regulates the power of the heaters
inside the sample chamber. The result is a sensor output signal that is tem-
perature independent.
The output of the preamp is fed to variable gain amplifier, which
provides automatic range switching under control of the CPU. The output
from the variable gain amplifier is sent to an 18 bit analog to digital con-
verter.
The digital concentration signal along with input from the control panel
is processed by the CPU and passed on to the 12-bit DAC, which outputs 0-
1 V dc Concentration and Range ID signals. An voltage-to-current converter
provides 4-20 mA dc concentration signal and range ID outputs.
The CPU also provides appropriate control signals to the Displays,
Alarms, and External Valve Controls, and accepts digital inputs for external
Remote Zero and Remote Span commands. It monitors the power supply
through an analog to digital converter as part of the data for the system
failure alarm it performs timing and linearizing tasks too.
The RS-232 port provides two-way serial digital communications to
and from the CPU. These, and all of the above electrical interface signals are
described in detail in chapter 3 Installation.
3.6 Linearizer
The output of the NDIR analyzer bench is non-linear (this is due to the effect
known as “Beers Law”). Figure I shows this non-linear response.
In order to produce a linear signal response, a compensation curve is
required (see Figure II). Therefore, a linearizer software routine is included
NDIR Bench Output
Figure I
Gas Concentration