Here is info from MidContinent that is very usesful..
"The engine should be run up to cruise RPM, at which time the gauge should indicate atemperature drop in the carburetor of approximately 15° C or 26° F. This may vary withdifferent configurations of intake systems and the amount of manifold pressure that inturn controls the rate of expansion of the gas-air mixture in the carburetor."
"This means that standard practice actually amounts to carryingapproximately 5° C (9° F) of heat above freezing measured at the throttle valve, which isthe most critical point. In practice we find it sufficient to carry 5° C of indicated heat abovefreezing except when the outside air temperature would be subject to suddenly extremevariations, or extreme icing conditions. Even under these circumstances, the pilot mustremain alert and be able to add more carburetor heat. Constant monitoring of the gauge isrequired during possible icing conditions."
The temp probe is not really measuring carb AIR temp, but rather something closer to the temperature of the carburetor just below the throttle plate. (This is why when full carb heat is applied, the temperature rise is lower than some expect.) And, it is not measuring the temp of the throttle plate itself. So the gauge is really improving your information, it is not telling you everything you need to know to know for sure whether there is carb ice forming or not.
Last edited by stroutmail; 11-25-2019 at 08:52 AM.