Measurement principle
Manual CWD 2005
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Manual CWD2005.doc
a differential voltage (6) at the thermopile that is independent of the cooling air
temperature. Broadly speaking this is proportional to the Wobbe Index of the gas.
Airflow (3) and gas flow (7) must be regulated very accurately. The gas flow is
regulated to a constant level by an extremely precise, weighted gas pressure
regulator (8). The flow of cooling air is kept constant by using a pressure sensor (9)
to control the speed of the cooling air fan.
During heat transport from the burner to the active elements of the thermopile (4), all
parts of the system absorb heat. Heat for the thermocouples is also lost. The heat
management of this process is monitored by a number of thermal elements (10, 11,
12), and the result is included in a calculation, which ensures the Wobbe Index value
is linear. In addition, the response time of the system can be considerably reduced by
the use of a proximity equation and end point forecasting.
The output signal is not yet the Calorific Value but is proportional to Wobbe Index,
because the specific gravity of the gas is taken into account in this measurement.
For this reason, the Specific Gravity (13) of the sample gas is measured and
calculated with the Wobbe Index, in the process calculator to get calorific values in
BTU or other units.
Three output signals are generated
1. Wobbe Index as a real time measurement
2. Specific Gravity as a real time measurement
3. Calorific value as a real time product of the above measurements.