Advanced Instruments, Inc.
5. Operation
Principle of Operation
The GPR-2500A oxygen transmitter incorporates a variety of advanced galvanic fuel cell type sensors.
The transmitter is a compact efficient package configured with the oxygen sensor and separate circuit
boards for signal processing and terminals for incoming power, power supply, signal output and alarm
relay contacts housed in a general purpose NEMA 4X rated enclosure.
Advanced Galvanic Sensor Technology
The sensors function on the same principle and are specific for oxygen. They measure the partial
pressure of oxygen from low ppm to 100% levels in inert gases, gaseous hydrocarbons, helium,
hydrogen, mixed gases, acid gas streams and ambient air. Oxygen, the fuel for this electrochemical
transducer, diffusing into the sensor reacts chemically at the sensing electrode to produce an electrical
current output proportional to the oxygen concentration in the gas phase. The sensor’s signal output is
linear over all ranges and remains virtually constant over its useful life. The sensor requires no
maintenance and is easily and safely replaced at the end of its useful life.
Electronics
The signal generated by the sensor is processed by state of the art low power micro-processor based
digital circuitry. The first stage amplifies the signal. The second stage eliminates the low frequency noise.
The third stage employs a high frequency filter and compensates for signal output variations caused by
ambient temperature changes. The result is a very stable signal. Sample oxygen is analyzed very
accurately. Response time of 90% of full scale is less than 10 seconds (actual experience may vary due
to the integrity of sample line connections, dead volume and flow rate selected) on all ranges under
ambient monitoring conditions. Sensitivity is typically 0.5% of full scale low range. Oxygen readings may
be recorded by an external device via the 0-1V signal output jack.
The circuit for the 4-20mA signal output and two adjustable alarms is powered by a DC/DC transformer
that requires a 12-24VDC power source and separate wiring for the signal outputs.
Caution: A loop power source is not sufficient to power this transmitter. Further, the 4-20mA signal
output does not require any external power (applying power to 4-20 mA terminal will permanently
damage the transmitter and void transmitter warranty).
Sample System
The GPR-2500A is supplied without a sample conditioning system thereby giving users the option of
adding their own or purchasing a factory designed sample conditioning system. Whatever the choice, the
sample must be properly presented to the sensor to ensure an accurate measurement. Users interested
in adding their own sample conditioning system should consult the factory. Advanced Instruments Inc.
offers a full line of sample handling, conditioning and expertise to meet your application requirements.
Pressure and Flow Consideration
All electrochemical oxygen sensors respond to partial pressure changes in oxygen. The inlet pressure
must always be higher than the pressure at the outlet vent which is normally at atmospheric pressure.
Flow-Through Configuration
The sensor is exposed to sample gas that must flow or be drawn through the sensor housing inside the
transmitter. The GPR-2500A internal sample system includes 1/8” compression tube sample inlet and
vent fittings, a Delrin or optional stainless steel sensor housing.
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