Different charging methods

I have only ever worked on environmental chambers. Some are single stage (R404a etc) some are cascade (R23). Some are liquid nitrogen/liquid CO2.

I have never worked on a commercial refrigeration unit of any description or an A/C system. So I have never used any of the charging methods I see guys using on YouTube. Well, apart from charging to the sight glass sometimes.

The designed superheat and / or subcooling figures are never supplied by the manufacturers of environmental chambers. So even if I was to measure these I wouldn't have a comparison to evaluate them.

Anyway, back to the point. I worked on a chamber recently where all the gas had leaked out. I repaired the leak. I checked the manufacturers data plate to see if a charge weight was provided. I checked the maintenance history for a quantity of gas but there was nothing.

I remembered a method my college tutor told me about.

If you get a brand new cylinder of refrigerant and it has 9kg inside. Calculate the volume of the cylinder. The manufacturers will have allowed for expansion in the cylinder so you can calculate a ratio of weight to volume. You will have a constant.... K.

Then measure the volume of the liquid receiver on your system and use the constant (k) to calculate the charge weight.

This method actually works. But obviously, it will not work every time depending on circumstances.

I have validated it by doing a blind test on other chambers. Some have a label on the side with the charge weight. So I didn't look at the label, did the calculation and compared it. Mostly it was the same plus or minus a small difference.

It is a useful method if you are in a situation where you don't know how much gas a unit will need.

The theory is that if the system is pumped down, there's no way the liquid receiver can be too full.

Another method I saw was a guy who had a thermal imaging camera. He pumped the unit down and got an image of the end of the receiver. There was a line where the liquid level was. And it looked like it was about 80% full.

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