Automotive Air Conditioning Information Forum

Could someone explain charging by subcool "plateau? Thanks


I inquired about the subcool "plateau" procedure DetroitAC mentioned in this thread
https://autoacforum.com/viewtopic.php?p=27748#p27748

In that thread JohnHere suggested starting a separate thread for this question.

copy of questions in other thread

andrew vanis wrote: ↑Thu Aug 24, 2023 3:12 pm

DetroitAC wrote: ↑Tue Jun 21, 2022 2:52 pm Charge for a TXV system is determined by measuring subcool, but it's more complicated because we are looking for the "end" of a subcool "plateau".

Could you please direct me to where this subcool "plateau" process might be detailed or can you elaborate?

I'm creating a custom system so there isn't a factory capacity spec.

I have a digital gauge that automatically calculates subcool for 134a (and superheat but can't use that on this TXV system). Conveniently it also charts on a cell phone so seeing a plateau should be easy.

Fwiw, it is a VW Vanagon.

Thanks!

Hi Andrew, I can explain whatever you need to know. Explaining it correctly is another matter

I'm curious about where you live since you said 5500ft. I was born and grew up at 7250, normally aspirated engines that struggle were a fact of life.

The plateau refers to a graph of all relevant parameters on the Y axis and refrigerant charge on the X axis. If you can keep the conditions constant and add refrigerant charge in small increments, you will observe that performance is bad at the beginning and gets better as charge is added. Then there is a range of charge in which nothing changes very much with added charge if at all. This is the plateau.

At the end of the plateau things start changing again.

The plateau is a range where any added charge is just increasing the liquid level in the receiver. The end of the plateau occurs when the receiver is full.

^thanks I understand all that.

Where along that charge line is the ideal charge level?...I'm guessing right at the end of the plateau(shrug)

Does it matter/make a difference where the receiver is? Like closer to the condenser vs closer to the evaporator? I'll have about 15 feet of hose between the two and the (centers) of the evaporator will be about 4 feet above the condenser.

Are there other getting techniques other than plateau to consider for charging a custom system of unknown capacity?

I'm in Albuquerque NM. The project is a VW Vanagon with a fuel injected Subaru EJ25 2.5L engine from a 1999 Forester.

Thanks for the reply.

It does not matter where the receiver is located, anywhere between condenser out and TXV in is OK.

Optimum charge is at the end of the subcool plateau for a properly designed system. If you will Frankenstein together a unique system, the end of the plateau is still the best first guess, but non-ideal components may lead you to a different answer.

Vertical difference isn't a problem if the suction line is a downhill run, oil will tend to flow well back to the compressor. The liquid line isn't affected at all by going up in elevation.

There is also the widely used take a guess at the charge and just keep squirting it until it's cold method. Honestly, most experienced AC mechanics are doing a quick and dirty version of a precise charge determination plateau test, they take a good guess and then keep adding until it gets good and cold and discharge pressure starts to rise.

^ thank you, very helpful. I'm sure I'll have more questions.

From this three, the following is additional info the the Plateau approach - specifically the 3x ambient limit
https://www.autoacforum.com/viewtopic.p ... 997#p27997

bohica2xo wrote: ↑Mon Sep 11, 2023 7:46 pm I think I would take the Plateau approach and cautiously add some refrigerant. Monitor the pressures and vent temps with a 1500+ rpm engine speed. Add small quantities of refrigerant and wait 30 seconds to stabilize. Watch the high side pressure, and make sure you don't go past 3x the ambient temperature
For example 3x 108f = 324 psi