Automotive Air Conditioning Information Forum

I have pieced together a custom system consisting of a 1970 evaporator TXV and POA (adjusted for r134a) with a newer compressor, parallel flow condenser, aftermarker drier, and custom hoses. I can not seem to find a thread or online post on how to determine an appropriate charge amount for a custom system. I was thinking there must be a way to add refrigerant incrementally and monitor pressures or temperatures and look for some kind of plateau or spike.

Can anyone point me in the right direction?

This thread has some interesting information: https://www.autoacforum.com/messageview ... adid=11759
1) It describes how an OE might set the charge amount using high ambient conditions in a wind tunnel and adding charge until they reach the desired subcooling in the condenser
2) It describes a more practical method of charging by adding incremental amounts of refrigerant until the pressure changes little with additional refrigerant. This indicates the receiver is starting to fill with liquid. Then once the pressure begins to rise again (indicating the receiver is full of liquid), stop adding charge.

I came across another version of the charging-by-increments and monitoring pressure. It was embedded in this thread and here is a quote:

bohica2xo wrote:You can add small amounts of refrigerant while looking at the pressures to sneak up on a full charge with a TXV system like that.
On a custom system where the charge level is only a guess I handle it like this:

Watching the high side with it idling & the cabin fan on low, add a half ounce of refrigerant to the low side & wait for a minute. Did the high side stabilize at the same pressure? Or 1 psi higher? That means the refrigerant is being stored in the Receiver as liquid, where it takes up very little space.

If you add 0.5 ounces + 0.5 ounces + 0.5 ounces, and see little or no high side rise, it is still stacking up in the receiver & the line from the condenser.

If you see a 5 psi rise with only a half ounce of refrigerant, it is filling the condenser itself. This is the "spike" One should recover the charge at that point & weigh it. Then deduct an ounce & use that for a charge spec.

The problem is you only know what "enough" is, by going too far...

Add a little refrigerant, monitor pressures & the vent temps. Remember to let the system stbilize for a minute after adding. Log the resuls & post them for us.

These is one part I don't understand about the quoted method described by bohica2xo. He states that post-pressure spike is "too far" and that you'd have to deduct an oz after finding this point. At this point the condenser has just started to fill with liquid. I thought the condenser should have some saturated liquid refrigerant in the bottom so it can begin to subcool before it goes back to the receiver. In this thread where DetroitAC described the increment-pressure method the OEs used, it sounds like they would stop incrementing after they have ~20F subcool coming out of the condenser. But of course they are also looking at maximum load conditions in a controlled climatic wind tunnel with the car running 25-30mph; the method described by bohica2xo is at idle with low blower speed.

I am still looking for a good description of charging by sight glass and a comparison with the incremental pressure method. I'm also curious how ambient conditions affect the approach. i.e. if it's 80 deg outside what do you do differently from if it is 90deg outside.

Well, since most of us don't have a wind tunnel, we find other ways.

The physics of a refrigerant gas make it predictable at a given temperature. As long as the system is above 50f there will be both liquid & gas. The P/T relationship will affect the gas pressure, but the liquid volume change over a temperture range is small.

Modern parallel flow condensers have a lot of area, with small internal volume. Reciever / TXV systems hold most of the liquid in the receiver, with the last tube or two moving liquid. Adding a small overcharge can result in filling the condenser to twice the liquid level it was designed for.

Charging by sight glass is something you will find described in detail in an older publication of something like the Motors Manual - on the shelf at your local library.

The sight glass is in the receiver discharge line. The receiver has a dip tube that runs to the bottom of the vessel on the discharge line. In general the process for sight glass charging, refrigerant is added untill the sight glass is "clear" - showing 100% liquid flow - then 2 to 4 ounces more refrigerant is added to fill the receiver up the rest of the way.