The stock AC drier has a binary switch connected via a valve on the top of the unit. The switch senses pressure, and disables power to the compressor clutch if pressure drops below a specific point, to protect the compressor. A trinary switch adds a third function - an additional wire to feed to an electric fan (on/off trigger.) Ferraridoc's mod doesn't make much sense to me; I'm not sure why he found it easier to plug the drier, then relocate the switch elsewhere. Details would be good to know.
My standard set up had a brass T-piece screwed into one port of the dryer, in line with the A/C line, and a low pressure switch in the other port. The T-piece had a temperature bulb screwed into it which led, via capillary tube, to a switch mounted on the pedal box - the system inferred high pressure from high temperature. When I re-did the whole system, it was neater and simpler to screw the A/C line straight onto the dryer, and attach a trinary switch to the other spare port. Not wanting to push it too far, I didn't use the mid pressure fan switch until I was happy with the system function. I still haven't hooked it up, but I will one day... The advantage of the mid pressure switch is it lessens the electrical load, as the start current from the A/C clutch isn't at exactly the same time as the radiator fans, and if you're blatting along the highway with good air flow across the condenser, the fans won't cut in at all.
Great thread! I fear the day that I have to DYI my a/c, but I see it coming. R12 is legal in California. I have been getting recharges in my 87 BMW and 83 Ferrari for years. Its just hard to find shops willing to put up with the hassle of finding/buying the stuff.
Even though a newbie, I gotta dispel some rumors, etc. R12 isn't illegal but you do need an EPA refrigerant handling license if you try to buy it....which is better left to the "pros" at a shop. R134a is NOT a direct drop-in replacement for R12. If you do end up putting 134a in a 12 system, you're gonna foul the system. The expansion valve is set for R12, the components are designed for R12 pressures, and the oil in the compressor is completely incompatible with R134a. It's best to just replace the system (Vintage Air, perhaps?). Propane (R290) works great and is widely used in Europe, etc. but only because their laws governing these things dictate that the system be ABSOLUTELY tight with no leakage. Systems in the U.S. (be it auto or home) are allowed by the EPA to leak by a bit which is fine for basic refrigerants....but not a flammable gas. If you let propane leak out under a hood, you're just inviting disaster, no matter where you live on the globe. The term Car-B-Que comes to mind. When replacing your filter/drier, you only have about 20 minutes of working time before that thing is spent and useless. Get it put in place as quick as you can once you pull the end caps off. The descicant inside will immediately begin to absorb moisture in the air and unless you live in Arizona or the Sahara, you will have wasted your money. Just leave the caps in place until youre ready to do the deed. And lastly, don't leak check your system with a vacuum pump. Test it with dry Nitrogen (like 50 psi). Nitrogen will not expand or contract with the ambient temp so a steady pressure will remain all day long if your system is tight. If you still have a leak in the system, a vacuum pump will just pull in more contaminates. Also, a basic gauge set will not show a true vacuum. You won't be able to tell if the system is holding at 5 microns or 30 microns. Pulling a vacuum is for charging the system....not testing the integrity of the repairs. Sorry for the long post. There's just SO much to keep in mind when doing an A/C system. Lots to be mindful of. Matt
Nitrogen behaves as an ideal gas and follows the ideal gas law, PV = RT. Since V (volume) of the system is basically constant, as is the gas constant R, if temperate goes up or down, pressure goes up or down. But yes, the system should be purged with dry nitrogen as it will draw moisture out of the system.
It's not the nitrogen is non-expansile, it's the "dry" bit. It's the water vapour in air that causes most of the pressure change (and most of the climate change, but that's for P&R). That's why you put N2 in the tyres of a race car (or in your road car if you're a wanker), where 1/2psi difference can throw the balance out.
FWIW, I just discovered that Hydrocarbon refrigerant R441a , containing Ethane, Propane, Isobutane, (similar - maybe the same as Duracool and other existing hydrocarbon refrigerants), has been approved in the US to replace R134 in vehicles. So the US has recognized such refrigerants as safe for whatever application. I always thought that the concept that a couple of pounds of propane (hydrocarbon)- based refrigerant in a system is dangerous compared to 15+ gallons of gasoline sloshing around and possibly leaking from a dozen different fittings seems a bit ludicrous. In any case, I'm glad to see it is approved because it definitely works well as my experience with my 328 showed. As has been noted, a new expansion valve must be fitted. I used a new R12 (OEM) expansion valve for Duracool when I converted my 328 from the previous R134 conversion to Duracool because 134/12 have very similar cooling characteristics but it is possible that some research might find a better valve. However, I believe that no matter WHAT you do to an OEM system on these cars, the evaporator capacity is the weak link. Re nitrogen.. nothing wrong with using it if you want to go through the time/trouble/expense but pulling a vacuum draws the moisture out of the system; that's one of the primary reasons/results of pulling a vacuum.
BAD TYPO IN MY PREVIOUS POST!!... I wrote, "...because 134/12 have very similar cooling characteristics..." I MEANT TO SAY "Duracool/R12 have similar cooling characteristics." When I realized it/went back to change the post I was too late to be able to edit it.
Actually, I think it's more about repeatability that sensitivity. Sensitivity of pressure to temperature change is greater for N2 than for dry air and generally more sensitive to temperature changes than humid air. The problem with humid air is air with 60% humidity has a different sensitivity than air at 30% humidity, for example.
It's the other way around. P = M*R*T/V. Take the partial derivative of P with respect to T and you get M*R/V. This implies the change in pressure with respect to temperature is linearly dependent on the gas constant with M and V constant. So the larger the gas constant the greater the change in pressure with respect to temperature. Dry air has an atomic weight of 28.95 . The atomic weight of nitrogen is 28.02. Therefore nitrogen is slightly less sensitive to temperature changes than air.
You are right. Nitrogen has the larger gas constant, 55.15. Gas constant for air is 53.34. Therefore nitrogen is slightly more sensitive to temperature changes. I looked at the atomic weights. But gas constant is universal gas constant divided by atomic weight giving nitrogen the slightly larger gas constant. Getting old and careless.......
Actually, it's more than that. The gas law is actually PV = NRuT, where P is pressure, V volume, N number of moles of gas contained in V, Ru is the Universal gas constant and T temperature. Now, N = m/M were m is the mass of gas in the volume and M is the molar mass. Thus PV = m (Ru/M) T and Ru/M is the gas constant for the particular gas. As you agree, R is bigger for N2 than for air. But let's not get ahead of out selves. What about a gas mixture (like air)? How do we get to R? We start by noting that in a mixture each gas exerts a pressure as if the other gases aren't there at all. Thus the pressure in a volume containing a gas mixture is the sum of the pressures from each gas P = p1 + p2 + p3 +.... + pi But each pressure is pi = Ni Ru T/V where Ni is the number of moles of gas "i" Thus, for a 2 component gas P = N1 Ru T /V + N2 Ru T/V = (N1 + N2) Ru T / V = Nm Ru T / V where Nm is the total number of moles of gas in the mix. we also know that the total mass of the gas is m = N1M1 + N2M2 = NmMm from which we get the molar mass of the mixture as m/Nm = Mm. But compare this to P = NRuT/V for a single species gas. Ru is a constant. If V is the same and T is the same, then to have the same pressure N = Nm. That is, it doesn't matter how many different gases are present in the mix. To achieve a given pressure in a given volume at a given temperate you must have a total of N mole of gas, whether it be a pure gas like dry nitrogen or a mixture, like dry air, or even air with some water vapor in it. So it doesn't make any difference what you fill your tires with. Once filled and sealed, there is some number of moles of gas , N in them and they all have the same P sensitivity to temperature variation. Take air at 80*F, 50% relative humidity. You have 11.07 grams of water vapor for every 1000 grams of air. So for every mole of dry air there is 0.0178 moles of water vapor. Or, 1 mole of air at 80*F, 50% humidity has 0.9825 moles of dry air and 0.0175 moles of water vapor. If this air were trapped in a volume the relative humidity would change as the temperature changes, but the mass of dry air and the mass of water vapor will remain constant as would their mole fractions and the total number of moles of gas in the volume. The point is that it doesn't matter what the composition of the gas is. All that matters is the total number of moles of gas in the volume. So why N2? I've heard a few other explanations. One it that N2 molecules are bigger than O2 so the N2 doesn't weep through the tire as easily. The same would apply to water vapor since water vapor molecules are even smaller. So maybe. And there is the problem that if the tire cooled too much the water vapor could condense and that would reduce the number of moles of gas in the volume, hence the pressure would drop more than just by the temperature effect. But eliminating that possibility I can't see any difference in temperature sensitivity between dry air, N2 , or any other gas mixture. Note that we can also write V = NRT/P. If we let N =1 mole then we can find the volume one mole of gas, any gas, or any gas mixture, occupies at a given temperature and pressure. Assuming standard pressure (1 atmosphere) and temperature (273K), 1 mole of any gas (or any gas mixture) has a volume of 22.4L. So the temperature sensitivity thing is off the table or I'm going senile. (I am going to be 71 in 9 days.)
After thoroughly botching it up here there is an easy way to look at it. For a closed system the density of the gas, whether nitrogen or air, is calculated from the ideal gas law and it equals P/(R*T) where R is the gas constant specific to the gas in question. The derivative of pressure with respect to temperature is the product of density and specific gas constant. In each case density remains constant with heating/cooling because the mass and volume do not change. So plugging into dP/dT = density *R =( P/(R*T)) *R = P/T. In each case P/T is the same. Therefore dP/dT is the same and each pressure changes by the same amount with temperature.
The bottom line is that it's not about air vs N2 or any other gas. It's about wet vs dry. Simple example, Air at 80F 70% humidity becomes 100% humidity if the temperature drops to 70 and if the temp drops further some of the water vapor condenses to liquid water.
Uh...Ya think maybe we're over-analyzing this a bit? I'm mighty impressed with the knowledge of physics here; I got lost at the first "PV = NRuT!" Heck, it's just an auto AC system essentially no different technically than they were in the 1950's when they started showing up as a common option. Any AC tech from 1960 would have no trouble using the gauge set and vacuum pump he had then to service/maintain these AC units.
That is the easy part. The "hard" part, for lack of a better word, is dealing with a conversion from 12 to 134. It is NOT suck out and refill simple but many here think that is all it takes and nothing could be further from the truth.
Right right right... how many moles this, what volume that, etc. Where does one find Dry Nitrogen for the DIYer?
At Costco tire service, or any tire shop that offers nitrogen fills! (Which, as this PV=nRT discussion has highlighted, are bogus and offer zero advantage over air in tires for street use)
I still say R12 is the best way to go, especially for someone who is technical enough to tear down and rebuild an AC system. The handling of Freon is not particularly technical, just requires the right equipment and procedures and of course certification (can obtain online). -F
If I had to teardown/rebuild my AC system, I would absolutely convert to 134 (my car's PO already did it.) R12 is simply too much of a hassle to source and utilize, plus far more expensive... even if it's technically legal in your area.
