IO,
You seem to like to split hairs. I don't think you would be flying your piston engined aircraft in the extremes of the Martian atmosphere or at 100,00 feet; not a good example. The point is that changes in air density caused by increased altitude effects makes little measurable difference to the fuel draw in the working environment of a piston engine carburettor. If it did then aircraft wouldn't need a mixture control. Aircraft carburettor designs became hugely complicated about fifty years ago, for just that reason.
"I don't really see a useful connection between carb icing and the mixture setting though."
This is basic stuff. A carburettor depends on the smooth passage of air through the venturi. Anything that alters or disrupts the flow has a marked effect on absolute mixture strength or on the formation of a uniform fuel/air emulsion in the manifold.
As a hobby I have worked on and modified carburettors and intakes for ground-based vehicles for many years; I used to be amazed how a seemingly minor modification to the intake, or even the filter type or position, can make a large difference in mixture strength - now I'm not at all surprised to hear how a small build-up of ice can stop an engine altogether.
........................
"ST, I see your reasons but the basic issue with partial carb heat is: The air will be warmed up but its dew point is still going to be what it was before".
A point you have perhaps missed is that there might well be liquid water (droplets) present in the ambient / intake air, especially so under a cu-nim cloud. Once the intake air becomes warmed it will contain more water than ambient. This is where the use of partial carb heat may put the air in the band for icing to occur (a small increase in the amount of water will absorb much more heat in the carb) and so the design of an effective carb heat system must allow a surprisingly large increase in temperature, to overcome the powerful cooling effects in the intake / carb combination.
As we know, the trick is to keep the intake temperature above zero degrees C so that ice cannot precipitate. The designer of an effective carb heat system will have to over-compensate for safety reasons, especially if a carb temperature gauging system is not fitted. If you look at a typical carburettor equipped engine hot air intake for a car, you might be surprised how closely the hot air shroud fits round the exhaust manifold. It looks like an over-compensation but it's like that for good reason.
The information is all out there; here is something else for you to read. This article mentions partial carburettor heat being possibly ineffective and also gives advice to lean mixture (rather than richen it) if icing is encountered:
www.casa.gov.au/fsa/2001/jul/28-31.pdf
My input now complete, I'll go back to tidying up my garden as I only fly turbines these days.
