Steve
I feel you are missing the point here - there is a world of difference between LPG - and refined and blended HC's -
These are not a cheap cop out they are an engineered solution to an enormous problem associated with R134a - not only from an environmental point of view - but also with performance in mind.
A 'proper' conversion from R12 to R134a is an expensive process and unless its done by the book it will never perform properly.
R134a is a very poor substitute - it lacks performance on HOT days due to the relatively low critical temp - it has a high GWP - and has issues with lubricants.
Hydrocarbon refrigerants (NOTICE NOT LPG) are a perfect solution - they do not form acids when moisture is present, they are non toxic - unlike R134a.
HC's have good critical temperature performance matching those of R12.
Typical Land Rovers will have between (+/-) 600 - 1000 g of refrigerant, with a refined, blended purpose engineered hydrocarbon only 200 - 350g is used.
R134a has a GWP of +/- 1500 and an atmospheric lifetime of > 100 years
HC's have a GWP of < 3 with an atmospheric lifetime of less than 1 year.
However I do agree with one point you made - they should not be available to everyone - they should be controlled similar to chemical refrigerants - not for the safety issue as much - but as you know - we train long and hard for our industry - and putting any high pressure gas in the hands of Joe Public is a recipie for disaster - and when it goes wrong its the product that gets the blame - not the inexperiance person doing the job.
It might be interesting to note that the 'apparent' solution that Du-Pont et al are working with - is R152 - and guess what - it has a very high percentage of HC in it - in an attempt to solve lubrication, critical temp and acidity issues !!!!
If done properly - by someone who knows what they are doing, using the correct HC - it is the perfect solution to a very large problem - worldwide.

