Filling Your Own SCBA Tanks - How Warm is Too Warm?

[Ezana4CE]

Shooting big bores requires an ample source of air. I use a SCBA tank to fill them and I can drain it down to around 2200-2300 psi before refilling it. Once I think I drained the tank to close to 2000 psi and stopped filling around 3300-3500 psi because it felt too warm. I use an Alkin compresor to fill my 74 cuft SCBA tank and I’ve learned that it’s ok to see moisture spray out of the 1st & 2nd stage valves when I bleed them so long as there isn’t any coming from the air purifier valve on the fill side. But what I am unsure about is how to gauge when to take a break on filling a SCBA tank due to the SCBA tank heating up. I do not want to deal with moisture in my tank or my gun reservoirs. I’ve noticed that the bottom of the SCBA tank can get pretty warm during the fill process. A top off usually takes 5-7 minutes and this is my primary concern. I think that fills from zero pressure is where there is a real concern here, but I’m curious. I generally like the room temperature to be between 70F - 85F and I run a dehumidifier prior to filling my tank so the relative humidity is usually 30%-40% in this moderately humid region.

What do you use to gauge temperature when filling your SCBA tank at home? At what point do you stop to allow the tank to cool before finishing your top off or fill?

 

[Gerry52]

Dave, don't forget, these tanks were engineered with being inside burning buildings in mind. They CAN take a lot of heat! Do what you feel comfortably safe with for BOTH your tank & compressor. I know you have a mind towards safety & common sense & wouldn't do anything stupid. I stop for a rest when I feel my tank is NOTICEABLY warmer than my body heat (or close) but that's very subjective & rarely reaches that point.
Gerry

 

[rcs9250]

There will be no moisture in your tank because of the temperature rising. That is just adiabatic heating. If your molecular sieve is in good condition there will be no moisture. Well, it will be within the specs of the sieves design. In other words, very very dry.
When we filled scuba tanks (aluminum) on the dive team we would not allow any noticeable warmth while filling. That means a slow fill rate. But, we filled from banks so we could fill as slow as we wanted. With direct fill to your SCBA with your Alkin you don’t have the option of a slow fill.
You have two options. 1. Fill to 4,500psi and then stop and let cool, then top off once cooled. Or, 2. Fill and stop periodically so heat does not build up. But regardless, adiabatic heating is occurring, it’s just physics, and physics never sleeps. It’s just that with slow periodic stop/starts your cylinder is heat sinking off the warmth quicker or more often.
On the dive team we filled slow so that we could fill and go home and not have to worry about a 200 or 300 psi drop after cooling down while they sat in dive locker.
When I worked in a dive shop we filled about 400-500psi per minute then topped them off later after cooling.
You can set your SCBA in a bucket of water to aid cooling but that can be a pain.
Bottom line, if your sieve is up to snuff, you aren’t accumulating moisture in your SCBA.

 

[Gerry52]

Exactly what he said

 

[Ezana4CE]

@rcs9250 Thank you for that very detailed and articulate response.

 

[Ezana4CE]

Dave, don't forget, these tanks were engineered with being inside burning buildings in mind. They CAN take a lot of heat! Do what you feel comfortably safe with for BOTH your tank & compressor. I know you have a mind towards safety & common sense & wouldn't do anything stupid. I stop for a rest when I feel my tank is NOTICEABLY warmer than my body heat (or close) but that's very subjective & rarely reaches that point.
Gerry

@Gerry52 My main concern is moisture related to heat. I stayed in physics for about 2 days in high school and never revisited the subject - to my own detriment. So occasionally you’ll see me ask a simple question like this because there are things that I just don’t understand. As @rcs9250 mentioned about the molecular sieve, I figured the air going into the tank was dry (enough for our purposes) because when I bleed open the valve in the component that houses the molecular sieve, I don’t see or feel evidence of moisture in the exhausted air. But I’m unsure about condensation. A glass filled with cool liquid in a room where the air is warmer than the liquid forms condensation on the outside of the glass. I was wondering if a sealed container (SCBA tank) filled in a room where the ambient air is cooler than that of the air being forced into the container (or cooler than the container itself) would form any condensation on the inside. I realize that these sieves cannot remove all of the moisture from the air going into our tanks, so I thought I’d ask.

 

[Scotty1]

I usually fill multiple bottles. When filling from below 250 bar, I’ll fill all three bottles, let them cool down, and then top them off one last time. I lose too much to temp change if I don’t pause and let them cool. Water from the Alkin is unlikely with thoughtful use.

 

[rcs9250]

@rcs9250 Thank you for that very detailed and articulate response.

You are welcome. I learned a lot about HPA from the dive industry and mixed gas diving and the week long class I attended for Mako compressors in Ocala Florida a while back.

 

[brokeasajoke]

As a firefighter and someone who runs the air system and fills Scott bottles, here it is from the fire side. Speed of fill does heat the bottle and it will drastically reduce final pressure once it cools if filled too fast. I've had a pack on and the fire melting my visor on my helmet while my skin is stinging from the heat through my turnout gear sooooo, I'd say I'll slow fill if time allows. No need to beat on the tank if you dont have too but when we need air in a hurry we cram it in and go. It may be a 45min tank and you only get 10 or 15 or less (lots of factors here) but I dont want to breath hot air if I dont have to. We also fill in a closed chamber in case something was to happen to the bottle while filling. We fill off of a Stallion Air 6000psi cascade system filled with an monster electric compressor powered by a diesel generator all contained in an 16ft enclosed trailer.

 

[Trucker3573]

The more heat you generate the more pressure you will end up losing in the scba tank after it cools to ambient temp. The dive shop I go to puts the tanks in water as they fill.

 

[rcs9250]

The more heat you generate the more pressure you will end up losing in the scba tank after it cools to ambient temp. The dive shop I go to puts the tanks in water as they fill.

Please don’t take this as argumentitive.
I worked ten years at my local dive shop and was privvy to dive industry media. There was a time when filling steel and aluminum scuba tanks in water was fairly popular, especially before aluminum tanks arrived. Steels used to heat up very quickly because the walls were only about 3/16” thick and passed heat very readily.
What they found out was that lackies in some dive shops would either, let the fill whip connector hang in the water or it accidentally fell in the water tank. They did not crack the fill whip fill valve to purge the water out. So when they hooked the fill whip to the tank the small bit of water blew into the tank. Rusting and pitting would start in steels and corrosion in aluminums. After repeated fills it added up. Then, because the lackies were causing the problem the industry adopted a mandatory yearly tank inspection called Visial Inspection Plan (or Protection) VIP for short. That way they could charge the customer for the problem they created. Brilliant actually.
Water tanks slowly fell out of favor but some are still in use.
Before I bought my compressor I had a guppy sized tank. The paintball shop I went to jammed 2,000 psi into it in about 2 nanoseconds. So I used to put my tank in my chest freezer for a half hour then take it out, wrap it in a towel and then take it to the jammer. The pressure drop was minimal compared to a room temp tank that was jammed. YMMV.