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.
1)Carb Air Temp gauges don't really indicate the temperature of the air flowing through the carb---they instead more closely indicate the temperature of the metal surface of the carb just below the throttle butterfly. The momentary temperature of the air itself can be considerably warmer or considerably colder. And, the temperature of the throttle butterfly is likley colder than temp indicated by the gauge, depending on the pressure drop past the butterfly. So the gauge is better described as simply Carb Temp Gauge. Applying full carb heat may raise carb air temp by 50-70 deg F, but may only raise the carb temp by 5-10 deg F.
2) The temperature drop past the throttle butterfly in most cases--during cruise settings is about 9 deg F. So, if the Carb Temp Gauge is reading 41 deg F or higher, it is likely that the throttle butterfly temperature is above 32 deg F---and ice would not form or stick to the plate. However, if the ambient air is cold enough (below 12 deg F) raising the temperature of the carb using carb heat could be counterproductive as it could result in raising the air temperature to a point where liquid condenses and ice forms on the throttle plate. So for preventing carb ice during some cruise conditions, the Carb Temp Gauge can be useful for indicating the effective use of carb heat.
3) Temperature drop from ambient, past the venturi and the throttle can be be 50 deg F or more. The drop in temp due to fuel vaporization and other factors is why carb ice is possible, even at ambients as high as 80 deg F.
4) 32 deg F is the melting point of water, but water can be supercooled and maintain a liquid state below 32 deg F.
5) There is some research that engine performance is enhanced, and lead deposits are reduced by using carb heat and raising carb temperature when ambient air is below 32 deg F.
But despite all the limitations and complexity, I am convinced the additional data provided by the gauge is valuable. It provides one more data point for decisions regarding the use of carb heat--especially during cruise.
So, I submitted an MRA to Cubcrafters (my plane is a S-LSA) for authorization to install a carb temp gauge.
Last edited by stroutmail; 11-27-2019 at 10:32 AM.
I submitted an MRA to Cubcrafters (my plane is a S-LSA) for authorization to install a carb temp gauge. (see previous post)
There are several options.
Each contains essentially three components: A Gauge, Wiring, and a Probe.
I decided that a Thermistor probe was better than a Thermocouple for this temperature range-more accurate and more stable.
I did not want to eliminate any of the gauges now in the panel. So my only choice would be to mount gauge above the glare shield or below the panel. (If I did, a complete engine data management 3 1/8 gauge from Electronics International would have been my choice, despite that they use a Type K thermocouple probe. I also would have considered one of UMA's 3 in 1 gauges.)
UMA Instruments offered a 1.25" diameter guage (slightly larger face than the pocket thermometer I use to check AC duct temps..about same size as wristwatch face.) that could be easily mounted under the panel without interfering with my knees.
UMA Instruments offers a thermistor probe.
With or without the Carb Temp Gauge, I will continue to use the same procedures: 1) "Apply Carb Heat on Downwind, before dropping RPM below 2000, Remove Carb Heat on Short Final"; 2) "Apply Carb Heat before Takeoff when conditions merit..remove carb heat during takeoff"; and 3) Monitor OAT and Dew Point Spread AT CRUISE ALTITUDE; and 4) Closely monitor cruise RPM for signs of carb ice: and 5) Without carb temp gauge..periodically pull carb heat during cruise. AND during long descent..leaving it on for at least 15-20 seconds and leaning the mixture while on. (This activity during cruise would be less necessary or not at all with the gauge.) And, even with a Carb Temp Gauge, only consider using partial carb heat during steady state cruise--all other times "Apply full carb heat when risk of carb ice is high".
Last edited by stroutmail; 12-01-2019 at 06:52 AM.
Always amazing to me, the creative energy of the experimental aircraft world! (I prefer a more sophisticated "factory" specifically engineered product.)
One of the questions I had about the install was the probe---his homemade probe made it seem quite simple.
He gives some flying experience with the gauge which I found quite useful.
Last edited by stroutmail; 12-02-2019 at 07:32 AM.
Another interesting article. A government research project in 1982 tested carb temp gauge in a 0-200 engine. Full pdf report available by searching Google with term "Cavage 0-200A carb temp". Cavage is author of report.
Essentially carb ice forms on throttle plate. It can happen quickly during cruise and can very quickly lead to engine shutdown. The carb temp gauge will help give early warning or allow prevention at cruise rpm, but gauge is not helpful at below 1400 rpm. In other words, since gauge is below throttle, at low rpm, the temp drop from the throttle plate vs the venturi is not measured by the gauge..so during low rpm, just follow normal POH procedure and pull carb heat.
But during cruise..the gauge is very useful. Note at 59 F, throttle plate is below freezing, carb temp gauge reading about 8-10 F higher at 1800 RPM and above. So using carb heat to raise carb temp gauge to 45F or above is useful. See graph. Note that throttle plate below freezing does not always lead to carb ice, but at dew point spread less than 20 F conditions I often fly in, it is highly probable..especially at 50% throttle (2200-2250 rpm) Screenshot_20200119-200145_Drive.jpg
I am moving ahead with install of UMA instrument carb air temp gauge. After waiting a month for MRA, decided to move to E-LSA which was just completed. Many benefits of moving to E-LSA, with only disadvantage I see being that plane cannot be rented, used to tow banners/gliders or used to train pilots other than owner.
Last edited by stroutmail; 01-19-2020 at 06:21 PM.
Here is pic of left side of panel with Carb Air Temp, EGT/CHT. The Carb Air Temp gauge is the small 1.25" diameter one from UMA mounted with custom bracket under the standard panel. Gauge and bracket angle/orientation set for best pilot view for the 30-50 F range. 15 degree up tilt and gauge rotated 90 degrees so 30-50 F range is 9 to 11 o'clock. Considered the 2 1/4" -8A "combo" unit with up to 8 temps from EI, but I preferred the simpler 3 1/8" unit for EGT/CHT and wanted a thermistor rather than thermocouple for Carb Temp. I did not want to have to switch channels or use a scan--too complicated for me. Plus, UMA gave me a custom painted yellow/green range.
Happy to report that flying with the new carb air temp gauge is great. No more anxiety about whether carb ice is forming. Easy to know when to pull carb heat in cruise.
The carb heat system on the Sport Cub is excellent, increases carb temp rapidly. Raises carb temp even at 1200 RPM descending base to final.
Using partial carb heat to maintain a certain fixed temp did not work as expected on a cold day...probably the air entering from inside cowl was too cold or ram air too strong, as temp did not rise until 3/4 or more carb heat input. In cruise, when carb temp dropped to 40-45 F, I just pulled full carb heat and temp very quickly rose to 70-80 F, then dropped very gradually over time back to 40-45 F after turning heat off. Probably the partial carb heat will work when ambient temps are closer to 60 F and humidity is high.
Keep in mind that carb temp gauge is measuring air and carb body below throttle plate, so probably reading 10 degrees higher than above throttle at 2100-2200, so keeping above 40-45 F on the gauge is probably keeping the throttle plate above 30-35 F.
FYI.above info.flying in ambient 48 F. Dewpoint 32 F.
Last edited by stroutmail; 03-07-2020 at 05:55 PM.
Re: Carb Ice with the 0-200A
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