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"Flammable refrigerants like R600a and R290 must always be charged by weight. Charging by volume must
be made with a refrigerant charging cylinder. The refrigerant R404A and all other refrigerants in the 400
series must always be charged as liquid. "
The refrigerating system should have a data plate on it which states the type of refrigerant and the quantity in grammes.
A refrigerator might state that it contains 250g of R600a.
So when the system is charged you need to weigh the amount if refrigerant that you put into the system, pressures are not taken into account.
Ok.....what is the difference of "charging by volume?"
Do not you use a cylinder when you charge by weight?
Simply put and as far as this Old timer sees it!
Historically CFC refrigerants could be charged into a system as vapour or liquid.
Many Modern refrigerants however need to be charged in their Liquid form.
This is because gases which make up the refrigerant "boil of at different Temperatures.
The difference between the lowest temp at which the first constituent chemical boils and the last is called the Glide.
This is why it is stated that certain refrigerants must be charged as a liquid!
Imagine a bucket of muddy water, when mixed you draw off an even amount of water to mud.
Let the bucket settle and drawing off the top results in just water with little mud within it!
The problem of a refrigerant "splitting" is made worse when you are adding small amounts from a cylinder to top up a system.
You weigh the charge in adding it in liquid form.
Charging via a charging cylinder is rare nowadays as scales are so much more accurate.
Historically on fridges ect where tiny amounts of refrigerant was needed to be added.
You could control the charge amount much more accurately using a charging cylinder.
By volume just means a gas or liquid amount.
But because different refrigerants have different volumes at the same weight.
Therefore as you cannot control accurately the amount of chemical parts which are added by volume!
Basically you need to know you refrigerant makeup is compatible with charging in gaseous form.
Check out this link!
https://www.youtube.com/watch?v=A4urHcJCR-Q
Grizzly