Thermopile calorimeters work in a completely different manner. They seek to mix and then burn the sample gas with air. Then this air flow is controlled and monitored so as to maintain the same exhaust temperature. The airflow variations help to give an output signal which is proportional what is called the Wobbe Index. If you are wondering what that is, let us tell you. The Wobbe Index represents a value which is calculated by dividing the heating value with square root of the specific gravity.
The correct heating value measurement can be provided only if the specific gravity meter is entered. However, the Wobbe Index has one limitation. It represents the measurement of fuel mixtures which are not just natural gas. If the gaseous stream you are passing through it has heavy hydrocarbons, then there can be some problems. Thermopile calorimeters are prone to flameouts. They are also large and bulky with a number of moving parts that only increase the burden of maintenance.