January 2005Analytical Instrumentation & Environmental Monitoring
The EXA-based GD402 Hydrogen Purity Analyser enables high performance measurement without reliance on thermal conductivity properties.
Hydrogen cooling
The EXA GD402 monitors the concentration of H2 in air; typically in the range is 85-100%. A low H2 alarm alerts the operator when corrective action is required.
Purging
GD402 also serves as a purge gas monitor. When the generator is down for maintenance, the analyser can be switched from 'H2 Purity' mode to 'H2 in CO2' (0-100%). As the H2 is displaced by the CO2, the analyser accurately tracks the concentration on the display and its second 4 to 20 mA output. Once the H2 has been displaced, the GD402 is switched to measure 'Air in CO2' (0-100%), completing the purge cycle. The second 4-20 mA output is automatically switched to track the air in CO2. This purge cycle is reversed as the generator is prepared to go back online.
Principle
EXA GD402 is a density-measuring device employing the principle that the resonant frequency of a thin-walled cylinder will vary according to the density of the gas surrounding it. Concentration values are computed directly from the density data measured. The key to GD402's accuracy, long-term stability and maintenance-free operation is its multimode oscillation system, whereby the cylinder resonator is forced to resonate simultaneously at two different frequencies. The resultant ratio measured eliminates errors caused by moisture or dust build-up, thus significantly increasing the instruments accuracy and reliability.
The unique, rugged sensor offers significant advantages over other sensing technologies:
* One sensor for 'H2 in Air', 'H2 in CO2' and 'CO2 in Air'.
* High accuracy ±0,1% linearity.
* Linear response of sensor means simple calibration.
* Low pressure drop.
* No warm-up time.
* Fast response time to 90% within 5 seconds.
* Fast calibration time.
* High resistance to moisture and dust sensor can be cleaned if 'slugged' with oil/moisture.
* Does not require reference gas for operation.
* Immune to external vibrations - (shock-mounted sensor).
* Thermal stability - unaffected by ambient temperature fluctuations.
* Familiar EXA interface with simple 'Yes/No' setup.
* Two outputs for two different ranges.
* Low cost of ownership.
Calibration
The entire calibration process is simplified with the EXA GD402 analyser. Only two calibration gases are required and these are already on users' sites. The GD402 is zeroed using 100% hydrogen, and is spanned using 100% CO2. There is no need to maintain dangerous mixtures of H2 in air. Calibration is performed using the simple Yes/No prompts for which the Yokogawa EXA Analysers are well known. There is no need to calculate responses or response curves. The EXA GD402 does it for the user. Typical calibration time is less than 30 minutes.
Maintaining hydrogen purity at the correct level is critical for proper, safe operation. Yokogawa has designed the EXA GD402 to be reliable under a wide range of harsh process conditions. The overall result is a system designed with users applications in mind.
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