Analytical Industries GPR-7500 AIS Trace PPM Hydrogen Sulfide Analyzer - Manual (Page 10)

Advanced Instruments, Inc.
10
Advanced Electrochemical Sensor
Technology
All electrochemical sensors driven by a bias potential (three electrode configuration) function on the same
principle and are specific to a certain gas. They measure
the partial pressure of the target gas from low PPM to up to
1% levels in air, inert gases and gaseous hydrocarbons.
The target gas, in this case, Hydrogen Sulfide, diffuses into
the sensor through a diffusion limiting membrane, reacts
electrochemically at the sensing electrode and produces an
electrical current output proportional to the
H
2
S concentration in the gas phase. The sensor’s signal
output is linear over all measuring 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.
Proprietary advancements in the design and chemistry of
the sensor’s components add significant advantages to this
extremely versatile H
2
S sensing technology. The sensor maintains its sensitivity to with +/-5% of its span (it
does not sleep during continuous use; a typical symptom seen with conventional electrochemical H
2
S
sensors)) over a 1-3 months period. To maintain accuracy over the useful life of the sensor, calibrate the
sensor every 1-3 months. Under normal use, the sensor is expected lost from 18-24 months.
The H
2
S sensor recovers from an upset condition (exposure to very high H
2
S) to low PPM level in a matter
of few minutes.
Electronic
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. The result is a very stable signal that reflects H
2
S concentration in the
sample very accurately. Response time of 90% of full scale is less than 60 seconds on all ranges (actual
experience may vary due to the integrity of sample line connections, dead volume and flow rate selected)
under ambient analysis conditions. Sensitivity is typically 0.5% of full scale of the low range. The display has
the resolution of 0.1 PPM H
2
S (on 0-20 PPM Full scale range). The analog output signal may be recorded
by on external device via the 4-20 mA or optional 1-5V signal.
Sample System
The standard GPR-7500AIS is supplied without a sample conditioning system thereby giving users the
option of adding their own or purchasing a factory designed sample conditioning system, see section 2 QC
Certification for optional equipment ordered. Whatever the choice, the sample must be properly conditioned
before introducing it to the analyzer sampling system to ensure accurate measurements.
The GPR-7500AIS is generally supplied with a minimum of a sample flow control valve and a flow meter. A
coalescing filter (installed at the sample inlet) and an H2S scrubber (installed at the sample vent line to
remove H2S from sample before venting) are most common optional components. A pressure regulator,
with or without a pressure gauge is also available as an option. Users interested in adding their own sample
conditioning system should consult the factory. Advanced Instruments Inc. offers a full range of sample
handling, conditioning and expertise to meet your application requirements. Contact us at 909-392-6900 or
e-mail us at
[email protected] for recommendation.
Calibration and Accuracy Overview
Single Point Calibration: As previously described, the electrochemical H
2
S sensor generates an
electrical current proportional to the H
2
S concentration in the sample gas. In the absence of H
2
S the sensor
exhibits an absolute zero, e.g. the sensor does not generate a current output in the absence of H
2
S. Given
these linearity and absolute zero properties, single point calibration is possible.
Sample Pressure:
Since the sensor is sensitive to the partial pressure of H
2
S in the sample gas, the
output is a function of the number of molecules of H
2
S per unit volume. When sample is vented to the
atmosphere the sensor essentially remains at atmospheric pressure. However, a positive or negative