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...2 DESCRIPTION
2.2 Model 6548 000
Katharometer Analyser Panel – Fig. 2.2
Each panel comprises a metering valve, a drying chamber, a
thermally lagged katharometer type 6548 001 and a flowmeter.
These items are mounted on a flat panel suitable for fixing to a
vertical surface close to the sample point. The katharometers
are calibrated for the hydrogen purity measurement as well as
hydrogen in carbon dioxide and air in carbon dioxide.
Each sealed katharometer assembly incorporates a Wheatstone
Bridge made up of fine glass coated platinum filaments. One pair
of parallel arms is sealed in the reference gas and the other pair
exposed to the sample gas.
When the intrinsically safe stabilized current from the power
supply unit (Model 4234) is passed through this bridge, the
temperature of the platinum filaments rises to a point of thermal
equilibrium. Under conditions which are arranged to give
minimum radiation and convection heat transfer, the equilibrium
temperature depends on the thermal conductivity of the gas
surrounding the filament. Thus any difference between the
thermal conductivity of reference and sample gases causes an
imbalance in the bridge; this imbalance (as a millivolt signal) is
indicated by the monitor unit.
Zener diodes are connected across the input connections from
the power supply unit to the katharometer in order to limit the
maximum voltage which could be developed across the
filament bridge under external fault conditions. The current is
limited to a safe value under fault conditions by the power
supply unit.
Metering
valve
Gas
sample
inlet
Fig. 2.2 Location of Items – Model 6548 000 Katharometer Analyzer Panel
4
2.3 Model 4234 Power Supply Unit – Fig. 3.3
In order to operate a katharometer unit in the hazardous area,
one Model 4234 Power Supply Unit is required for each
katharometer. The Power Supply Unit supplies a stabilized 350
mA d.c. signal, and must be mounted in the safe area. There
are two separate versions available for either a nominal 110-
120V a.c. or 200-220/240V a.c. supply voltage. The stabilized
current output is current and voltage limited to restrict the
energy supply into the hazardous area.
The model 4234 is housed in a metal case fitted with lugs for
wall/panel mounting. Cable gland entries are provided at
opposite ends of the case for supply voltage input and
stabilized output cables to the hazardous area. The printed
circuit board assembly and diode heat sink are mounted on a
metal chassis and separate labelled terminal blocks are used
for making electrical interconnections.
The circuit is protected by a cartridge fuse. This fuse must have
a high breaking capacity (h.b.c.) rating of 4000A to comply with
the terms of the certification.
2.4 Remote Indicator/Controllers
The 6553 monitor unit has provision for retransmission values
and ancillary indicator/controllers may be connected to these
outputs, providing that they are installed in the safe area and the
installation conforms to the requirements given in Section 5.1.
Katharometer
unit case
Drying chamber
Warning. Do NOT connect mains supply to the
power supply unit with the output terminals on open
circuit. This causes premature component failure.
Caution. Ensure that the power supply unit is
correct for the mains supply voltage available. A nominal
110V unit cannot be adapted for use with a nominal 240V
supply, or the other way around.
Gas sample
outlet
Coarse zero
adjustment
Electrical
interconnections
Flow gauge
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