Page 338 - Instrumentation Reference Book 3E

Absorption and reflection techniques 321

The absorption cell is a thick-walled tube with amplifier is applied to a gain-controlled amplifier
heavy flanges. Standard construction is in mild which produces an output signal of 3 V peak-to-
steel, nickel plated, but type 3 16 stainless steel peak mean. Thus the mean value of I, + I, is
construction is available where required. The maintained constant. The signal is demodulated
windows are of calcium fluoride, sealed with and smoothed to obtain the 6 Hz envelope wave-
Viton O-rings and retaining rings. A heater wire form. A sample-and-hold circuit samples the
is wound on the cell, and the sample gas passes signal level near the end of each half-cycie of the
through a tube in thermal contact along the envelope, and this produces a square wave whose
length of the cell before entering it at the end. amplitude is related to I, - I,,,. Since Ir + I, is
Provision is made for rodding out tubes and held constant, the amplitude is actually propor-
entries in case of blockage. A thermistor tional to (Ir - Im)/(Zr + I,) which is the required
embedded in the cell wall detects the cell tempera- function to give a linearized output in terms of
ture which is controlled by a circuit in the elec- sample concentration. This signal is amplified
tronics unit. The cell is thermally insulated and and passed to a phase-sensitive detector, consist-
sealed inside a plastic bellows. The enclosed ing of a pair of gating transistors which select the
space is coupled to the purge system. The two positive and negative half-cycles and route them
end units each have a sealing window so these is to the inverting and non-inverting inputs of a
a double seal between the cell and the interior of differential amplifier. The output of this amplifier
the detector and source units. Since the source is provides the 0-5 V output signal.
inside a further sealed module, there is minimal The synchronizing signals for the sample-hold
danger of the hot source being exposed to leakage and phase-sensitive detector circuits are derived
from the sample cell. The gaps between the three from the torsional oscillator drive circuit via
units are normally sealed with neoprene gaskets, appropriate time delays. The instrument span is
and the whole device is sufficiently well sealed to governed by selection of feedback resistors in the
maintain a positive purge pressure of at least 2 cm low frequency amplifier, and a fine trim is
water gauge with a purge gas consumption of achieved by adjusting the signal level at the
8.3 cm3/s. For use with highly flammable sample gain-controlled amplifier. This is a preset adjust-
gases, the sealing gaskets at either end of the ment-no operator adjustment of span is consid-
absorption cell may be replaced by vented gas- ered necessary 01- desirable. A front panel zero
kets. In this case a relatively large purge flow may adjustment is provided. This adds an electrical
be maintained around the cell, escaping to atmos- offset signal at the phase-sensitive detector. The
phere across the windows. Thus, any leak at the system is normally optically balanced (Le.;
windows can be flushed out. I, = I,) at some specified concentration of the
To facilitate servicing on site, the source, detec- measured variable (usually zero).
tor, torsional vibrator, lamps, pre-amplifier, and The current output and alarm circuits are
source .voltage control are all removable without located on a separate printed circuit board. The
the use of a soldering iron. Since the single-beam voltage output is applied to an operational ampPi-
system is tolerant to window obscuration and the fier with selected feedback and offset signals to
internal walls of the absorption cell are not produce O-lOmA? 5-20mA, or 10-50 mA output.
polished, cell cleaning will not be required fre- The required output is obtained by soldered selec-
quently, and in many cases adequate cleaning tor links. The output current is unaffected by load
may be achieved in situ by passing solvent or resistances up to 1 kR at 50mA; or 5 kR at
detergent through the measuring cell. There is 10mA.
no need to switch the instrument off while doing A front panel alarm-setting potentiometer pro-
this. If it becomes necessary the cell can be very vides a preset signal which is compared with the
quickly removed and disassembled. analyzer output voltage in a differential amplifier.
The electronics unit contains the power sup- The output of this opens a relay if the analyzer
plies together with signal processing circuits, tem- output exceeds a preset value, which may be
perature control circuits? output and function either a low or a high analyzer output as required.
check meter operating controls and signal lamps. The alarm condition is indicated by two signal
The housing is of cast-aluminum alloy, designed lamps on the panel, and the system can be
for fiush panel mounting. The circuitry is mostly arranged to operate external alarms, or shut-
on plug-in printed circuit boards. The indicating down circuits.
meter, controls, and signal lamps are accessible The power to the cell heater and the detector
through a window in the door. The unit is semi- cooler is controlled from a bridge circuit contain-
sealed, and a purge flow may be connected if ing thermistors which detect the temperatures of
sealed glands are used at the cable entry. The the absorption cell and detector.
signal processing circuits are contained on The indicating meter on the front panel has a
printed circuit boards. Output from the pre- calibrated output scale, and is used in conjunction