To start, I'm going to cut straight to the part that most of you want to know. This information is based on calculations I have done personally based on typical Mastercool DOT tanks, but feel free to come to your own conclusions based on your own calculations. I prefer to stay on the safe side.

30 lb recovery tank – Fill with no more than 17lbs of R410a or 21 lbs of R22 – total tank weight will be about 35lbs for R410a and 39lbs for R22

50 lb recovery tank – Fill with no more than 32lbs of R410a or 39 lbs of R22 – total tank weight will be about 60lbs for R410a and 67lbs  for R22

Now, for the details:

First, you should look for the tare weight of the tank. It will be stamped on the top rim of the tank or handle with TW and then the weight, as shown on a common propane tank in the picture below.

Tare weight is simply the empty weight of the tank and must be factored in for whenever you are weighing the total weight of the tank.

Next, look for a stamp that says WC. This value indicates the tank's water capacity, the total weight in liquid water to fill the tank 100%.

You also need to consider a few more things before you start filling

1. You cannot fill above 80% with liquid, or you risk building up the hydrostatic pressure and exploding the tank. (That's a bad day).
2. Refrigerant does not have the same weight-to-volume ratio as water, so you must compensate based on the refrigerant type.
3. Refrigerant weight-to-volume ratio changes based on temperature, so to be safe, you would calculate the refrigerant volume at the maximum ambient temperature the tank will be exposed to in the back of your van. I figure 130°, but there is a caveat.

It has been pointed out to me by Steve Mazzoni that AHRI Guideline K 2015 states that we should use the liquid density at 77°, which means you can often fill the tank quite a bit more than what I'm suggesting here.

I am definitely not going to say AHRI is wrong, but I still feel it's better to use 130° as the number because it never hurts to be on the safer side. When we fill a tank, the tank heats up, and when we store a tank, it is often very hot in the back of the van. I feel 77° puts us in the danger zone in certain circumstances.

There are a few different ways to do the math. Some use the specific gravity of the refrigerant, but I just use cubic foot per pound at 130°F to calculate—just to be certain I am on the safe side of the range.

Water has a liquid density of 62.42 pounds per cubic foot, R22 is 66.17, and R410a 54.70. You can find other refrigerants by looking up their datasheets.

A 30lb (13.6kg) Mastercool 400 PSI (27.57 bar) recovery tank has a water capacity of 26.2 lbs. Divide that by the water density of 62.42, and you get 0.419 cubic ft of space in the tank  (25.2/62.42 = 0.419)

If you are filling the tank with R410a, you would then multiply the space in the tank (0.419 cubic ft) by the cubic feet per lb of liquid R410a at 130 degrees (54.70). You get 22.95 lbs to fill the tank completely.

However, you cannot completely fill the tank. You must only fill it to 80%, so you multiply the 100% full weight (22.95 lbs) by 0.80, which gives you 18.36 lbs—rounded down to 18 lbs—of total internal R410a weight (I go down to 17 just to be extra conservative).

If you then want to calculate the total weight of the tank + the refrigerant inside the tank, you would need to add the tare weight. For this Mastercool 30lb tank, the TW = 17.99lbs for a total tank weight of 34.99 lbs.

So, to know for sure that you are not overfilling a tank, you must have the following:

• A scale under the tank at all times
• The tare weight of the tank
• The water capacity of the tank
• Either the liquid volume per pound or the specific gravity of the refrigerant you are removing

For R22 and 410a, I came up with some quick (conservative) cheat numbers to simplify the math a bit (again, this is at 130°).

For R410a, just multiply the WC by .65 to find a safe fill weight. For R22, multiply WC by .82

You would still need to add in the tare weight to calculate the total tank weight, and if you are using a different refrigerant, you need to start the math from scratch.

When in doubt, err on the safe side—and for heaven's sake, use a scale and read the information on your tank.

—Bryan

P.S. – Tech Daniel Green made a really cool spreadsheet calculator that will help you get max fill for various refrigerants HERE.

P.P.S. – The HVAC School app has a recovery tank fill calculator that spares you from doing all the math. You can find an article that explains how to use that calculator HERE.