This tech tip was written by Eric Kaiser from TruTech Tools. Thanks, Eric!

Filling refrigerant recovery cylinders is a task HVACR technicians perform regularly, but figuring out how much refrigerant to safely put into a cylinder can still be confusing. The challenge almost always comes back to refrigerant density or specific gravity.

Specific gravity, also known as relative density, is a ratio of the density of a substance to the density of water. In other words, water = 1.0, and anything that has a different density is assigned a higher or lower value. Lower values are lighter than water and will float, and higher values are heavier and will sink.

To get the amount of refrigerant allowed in a cylinder correct, we use calculations from AHRI Guideline K (most recently updated in 2024). This guideline outlines how to calculate a safe fill amount using three key values:

• WC (Water Capacity or the total weight in liquid water required to fill the cylinder 100%)
• TW (Tare Weight or the empty weight of the cylinder)
• SG (Specific Gravity at 77°F or the density of a liquid refrigerant at a fixed temperature)

The WC and TW are both stamped on the collar of every recovery cylinder, but the SG is not, and refrigerant does not weigh the same as water. That means we need to adjust for the refrigerant’s specific gravity because a gallon of refrigerant doesn’t weigh the same as a gallon of water, and a pound of refrigerant doesn’t take up the same volume as a pound of water. So, we can’t simply fill a tank based on its water capacity.

This is where things can go wrong. If a technician does not perform a calculation and fills a cylinder completely with liquid refrigerant, leaving no space for expansion, pressure can build dangerously. This can lead to a catastrophic rupture of the tank. To prevent this, AHRI Guideline K sets a safety limit: only 80% of the internal volume of a recovery cylinder should be filled with liquid. The remaining 20% is called headspace, which allows room for the liquid to expand as a result of temperature increases.

The real challenge comes with obtaining the specific gravity at 77°F values. Unfortunately, it’s not consistently published or easy to find, which makes calculating safe fill weights difficult. As technicians, we have enough things to be worried about without trying to find obscure values buried deep in technical literature.

To solve this problem, Roman Baugh of HVAC School proposed a practical solution one day: create refrigerant-specific multipliers by combining AHRI’s formula with verified specific gravity data. Working with refrigerant manufacturer Chemours, accurate specific gravity values were obtained, and a simplified fill calculator tool was developed.

With this tool, you no longer need to run full calculations every time. Instead, you just multiply the tank’s WC by the refrigerant’s multiplier to find the safe fill weight and add the tank TW to get the maximum gross cylinder weight. It’s fast, easy, and most importantly, safe.

One word of caution: new refrigerants in the market—R-32 and R-454B—are significantly lighter than many common refrigerants technicians are used to. These lower specific gravity values mean their multipliers are smaller. 

Don’t rely on experience alone—always do the math before filling tanks with any refrigerant. You can use the TruTech Tools simplified calculation tool to help.

—Eric Kaiser

P.S. — You can find the calculation tool and some other great resources on the TruTech Tools website. TruTech Tools has a resource center, and you can download all sorts of free guides at https://trutechtools.com/ttt-free-downloads.