Radiant heater question

[txvwnut]

On a radiant heater how much temperature change should you see down the tube when checking it with an infrared thermometer? For conversation lets say I measure 500f near the burner and 100f by the exhaust stack on a 30 foot tube. Is this a fair way to check the efficiency of a radiant heater?

 

[Buckgnarly]

My 20 foot tube is about 750+ at burner and closer to 100 at vent end.