Air Conditioning Pressure Formula

[Lynn Larsen]

Everyone,

While cleaning out my office (about 4 years over due- I found my second windscreen though!!- does that give you any indication of how overdue it was /ubbthreads/images/graemlins/blush.gif), I came across a formula that VintageAir had given me with which to check the amount of freon (134a) in your system. When setting up the system, I was told to put x pounds of freon in. This amount was given to me before I had told them much beyond the fact that I was using a Sanden 505 for 134a. When I balked, mentioning that I had a mid engine car with at least 12 ft. of lines between the compressor, condensor and evaporator and saying that there had to be a more scientific way to determine if the proper amount of freon was in the system, they gave me this formula:

Run the test at 1500 rpm. Place as strong a fan as you can infront of the condensor coil. Close doors and windows and put A/C blower on medium.

High Side Pressure should = 2.35 X ambient temp in ºF (160-250psi)
Low Side Pressure should be between 6psi and 12psi in a steady state (once system stabilizes.)
Center Duct Temp should be 36ºF - 46ºF (This will vary depending on the distance between the evaporator coil and the outlet where the temperature reading is taken and how well insulated, if at all, the ducting is.)

I am attaching the VintageAir Troubleshooting guide (M$ Word doc).

Regards,
Lynn

 

[Ron Earp]

Hey Lynn,

Not sure about Vintage Air stuff but I needed to fix two AC systems recently. I've got a gauge tree and the right stuff to measure them, but I had no pressures. What I did was take three working cars that had been running fine and measured their low-side pressures to figure out how much R134a to put in.

Once I did that I found that with a BMW 540, Ford Lightning, and Plymouth Van the low sides were all around 25-30psi or so and all blew air around 35F out of the register when checked with a meat thermo probe about 6" in the vent and they were moving at 45mph or so. This on a 88F day.

So, for the two non-working AC cars I simply charged while watching the low-side pressure until I got to about 25-30psi and it seemed to work fine. Both cooled well and are still cooling well. Not very scientific but it worked out okay.

I have put too much R134a in systems, which is why I bought the gauges, and if you do that they will not cool worth snot.

R

 

[Lynn Larsen]

Ron,

Yeah, I bought a set of gauges also. I had charged plenty of R12 systems using the sight glass and a thermometer, but I had heard that R134a was very sensitive to having the proper charge. So, I figured that, since I had popped for the gauges, I wanted a more precise method for measuring the proper charge in the system.

I thought I had lost the formula they gave me, so I wanted to record it here. But then I found out that the instructions were also in VintageAir's troubleshooting doc which is attached to my first post. They also provide the pressures and temps for R12, just in case anyone still has this refrigerant in their A/C and is rich enough to buy some (assuming they can find it at all.)

The only departure from their method that I might take has to do with the car windows and doors. I had always been told by auto A/C guys to charge a system with the doors and windows wide open and with the A/C set to NOT recirculate inside air (this is the normal A/C setting- as opposed to Max- for American cars without a recirc switch) and the A/C fan on high. This helps assure complete phase change in the evaporator coils and a maximum low side pressure. If any of the refrigerant remains liquid or doesn't absorb enough heat to expand fully, you could end up putting in too much freon trying to get the proper pressures on the low side.

Otherwise, it makes sense to check temps in a scenario that mimics the normal operational mode as much as possible. And this is what they are trying to achieve with their setup instructions.

Regards,
Lynn

BTW: I have used the term "freon" in a very generic sense.