The most effective way of operating a catalytic appliance is by utilizing temperature monitors. Ideally, two sensing positions will give all the information needed to tell when to engage the combustor, how well the combustor is operating, when it's time to refuel and when the combustor is no longer operational.
The upstream temperature gauge will monitor combustor inlet conditions.
The second temperature gauge should be mounted on the combustor's exhaust side, about a 1/4" off the surface and centered on the unit. This will monitor the catalytic combustion process. If only one temperature sensor is used, it should be the one that reads the exhaust temperature of the catalytic combustor.
Thermocouplings and thermometers of various designs are available for this purpose.