Teledyne 2120 - Trace Nitrogen in Argon Analyzer - Manual (Page 25)

Trace Nitrogen in Argon Analyzer Operational Theory
Operational Theory
2.1 The Analyzer
The Model 2120 Trace Nitrogen in Argon Gas Analyzer is a rack
mountable self-contained unit for measuring trace amounts of nitrogen
in an argon gas stream.
A customer supplied sample system directs a stream of refined
argon to the instrument at a pressure between 2 and 7 psig (14-48 kPa).
The process stream enters at a constant flow rate and passes through a
glass analytical cell to which metal electrodes are attached. The
electrodes are connected across a high-voltage transformer that provides
a silent electrical discharge to ionize the gas in the cell, causing it to
glow. A high-voltage transformer takes the signal from a 255 Hz power
supply and steps it up to the high-voltage ionization level at 255 Hz. The
gas ionizes on both halves of the voltage cycle producing an output
frequency of 510 Hz. If nitrogen is present, it adds to the spectrum of
light generated. This light is optically filtered so that only a single
emission line in the nitrogen spectrum is visible. The resultant light is
detected by a photomuitiplier. The intensity of the emitted light is
proportional to the nitrogen content in the argon gas stream. Figure 2-1
is a block diagram illustrating the general operational principle.
A Hamamatsu high-voltage socket is used for the photomultiplier
tube (PMT). It contains the resistor ladder for the intermediate dynodes
in the PMT and a 1.22 VDC reference voltage generator. The socket
works with a potentiometer, labeled "1 P28 Plate Adjust" on the back
of the analyzer, to provide proper plate voltage to the PMT. See Figure
1-2.
The signal from the photomultiplier tube is processed in the
amplifier board. The amplifier compensates for the selected
measurement range. It filters out line frequency interference and
demodulates the signal then converts it to a 0-10 mV output to drive the
digital panel meter and the 4 - 20 mA output module.
Teledyne Analytical Instruments
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