Proper filling procedure for SCBAs? And also...gauge?

[VaporTrail]

So I picked up a used Omega tank, 4500psi, 75cf. I took it to the dive shop, and they told me they could only fill it to 3900PSI. Apparently there's a different compressor in the back that can fill it up to 4500, but only one person can work it. Just my luck, that person wasn't there that day. They filled it up to an indicated 3700PSI.

What worries me is that the tank was kinda warm, bordering on hot. Is that normal? I thought these tanks are supposed to be filled slowly so that temperature is kept down. Call me old fashioned, but any time a vessel containing high pressure gets hot, I start to get a little anxious.

After the tank cooled, I used it to air up my Sumatra. It's the 500cc version. I think I used it about 4-5 times. A couple of days later, I used it to air up my Winchester, which is 320cc. It filled it up once, to 200BAR. The next time, pressure topped out at 175BAR. 

Both rifles were filled from 150BAR to 200. Maybe my calculations are incorrect, but I thought I'd get more than 7 fills off of a 75cf tank. What am I missing in this calculation?

Also, how would I go about putting a gauge to measure pressure in the tank? The current gauge only shows output PSI/BAR. Can I use the gauge from my broken Chinese pump to measure tank pressure?

Thanks in advance!

[Insanity]

I know on my hand pump it cane with a dead head fitting that said only to 2000psi if you have one that could be a way to check fill pressure.

[VaporTrail]

Hmm. I was hoping for something a little more...visual? The Chinese pump supposedly goes up to 4500psi, which is why I was wondering if I could use the gauge on the tank. There seems to be a little threaded hole where something can screw into it.

[Madd Hatter]

I have used a dead head plug to check for leaks on my scba fill setup to 4500 psi with no problems plus on my hand pump up to 4000 with no problems either.
As far as the tank getting hot. 1 cubic ft of air will contain so much heat. When you compress it down to say 1 cubic inch that heat gets consentrated or densly packed  and the temperature will be hotter. Think of what happens when you release the air pressure in your air gun tank. It gets colder. When you filled that tank it will get hot but over some time it will cool back down to room temperature. Think of how to air conditioner works and you'll understand.

[redlined_b16a]

My first time filling my at44 from my 4500 psi scba I filled a little too fast and the at44 tank hot warm.
My guess is the friction from filling fast caused my heat.

Perhaps the compressor that was used to fill your tank ran for awhile and temps got hotter and transfered the heat to the air output.

Maybe a heated air dryer?

[USAFANG6799]

I was under the impression that any tank be it SCBA, SCUBA etc should be filled SLOWLY to minimize heat build-up.
I've heard of dive shops filling a SCUBA tank then letting it sit for a while to cool off then top it off to make sure it's get's filled to the maximum pressure. When I was having my SCUBA tank filled this is the procedure they used.

[VaporTrail]

Ok...I'm a dork. The dead head will allow me to see the pressure on the current gauge. Got it, got it. You can put away your tomatoes, folks. 

Rob - I have a dead head that came with the YH, so I'll put that on the whip and see how it turns out. Out of curiosity, though...what's going to happen if the pressure exceeds the rating of the dead head? The whip?

redline - any time you move air and compress it, there's going to be some heat. I've burned my arm a couple of times on the tubing of my YH.  Now that sucker gets hot pretty fast. Heat is normal when filling containers with compressed air, but since this is my first SCBA tank, I wasn't too sure of just how hot they're supposed to get. I'll speak with the shop tomorrow and ask them to be sure my tank is at 4500. I'm also probably going to try and put a gauge on it so that the shop doesn't try and pull a fast one on me. The first time they filled my tank they told me they could fill it to 3900; at the end of the fill session, the gauge was reading a pretty hot 3700PSI. I know I lost at least a couple hundred PSI after the tank cooled.

USAFANG - that was my impression as well. I've even read that some shops will place the tank in water to help keep the temps down. I don't think they do that here. I also have my doubts about the fill speed here, but I won't speculate for now. Tomorrow I'll be going to the dive shop to get it filled, hopefully to 4500.

[AlanMcD]

My experience with dive shops is that if you want a true fill fill, they will ask you to leave the tank and come back and pick it up later.  They have to fill it twice (sometimes three times for practical reasons) and let it cool between fills.

Most are used to filling primarily 3000 psi and 35000 psi dive tanks.  They typically have a "bank" of tanks that they fill in bulk when they run their compressor, and then use that stored compressed air to fill tanks as needed. When you show up, the bank might only be at 3800 psi.  So they fill off that, but then need to go direct to the compressor for the rest.  Lots of cool down time involved.  I once showed up when they had not filled the tank the second time, and they chose to throw it on a booster and filled it up to about 4700 so it would cool down to the right level.  Made me nervous, but it worked OK.

Now I have a Shoebox and don't worry about all that - but I did use dive shops for several years.

[VaporTrail]

Ok...so that's exactly what they did Friday. I dropped it off in the morning, and they told me they'll give me a call when it's ready. They called around 3pm. I wasn't able to pick it up that day, but when I stopped by on Saturday, they said it was filled up to 4500. The guy filled it up and then let it cool and then topped it off.

I haven't filled any guns with it yet, as I've been a little caught up with my son's final exams and work. Maybe this weekend I'll get a chance to use it.

[Mark Davis]

Compressing gas produces heat.
Expanding gas produces cold.
 
When I got my air tank, the fire dept gave me the first fill. when he handed it to me the gage read 4,500 PSI, and the tank was hot.

Later that day, filling the rifle, the tank was air temp, and the pressure had dropped enough I was looking for a leak.

Now just topping of from 3000 to 4500, quite a bit less heat, less pressure drop when the tank cools.

[ShakySarge]

It is perfectly normal for a tank to get hot when filling up, especially with the higher pressures. There are however, ways to reduce the amount of heat and in the end, pressure lost. I like to place my tanks in a cooler filled with ice around the tanks and then fill. It doesn’t reduce the heat of the air coming in, but, it quickly cools it, without any internal condensation so that it maintains it’s pressure afterwards.

Otherwise, expect a drop in pressure of anywhere between 200-500psi once it cools depending on how fast the compressor or cascade system fills it.

[VaporTrail]

Compressing gas produces heat.
Expanding gas produces cold.
 
When I got my air tank, the fire dept gave me the first fill. when he handed it to me the gage read 4,500 PSI, and the tank was hot.

Later that day, filling the rifle, the tank was air temp, and the pressure had dropped enough I was looking for a leak.

Now just topping of from 3000 to 4500, quite a bit less heat, less pressure drop when the tank cools.

Well, now that the tank will have some pressure in it (the first fill was from zero), I'm thinking it should heat up a little less. Maybe this weekend I'll have a chance to take it out on the field.

My little Spark will still be coming with me though. 

[VaporTrail]

It is perfectly normal for a tank to get hot when filling up, especially with the higher pressures. There are however, ways to reduce the amount of heat and in the end, pressure lost. I like to place my tanks in a cooler filled with ice around the tanks and then fill. It doesn’t reduce the heat of the air coming in, but, it quickly cools it, without any internal condensation so that it maintains it’s pressure afterwards.

Otherwise, expect a drop in pressure of anywhere between 200-500psi once it cools depending on how fast the compressor or cascade system fills it.

Hmm...I don't think they have anything like that when they fill. I think with the first fill the tank got really hot because they filled from zero and it took about 4-5 minutes. I'd really like to take a look and see how they fill it with their compressor.

[ShakySarge]

Most places don’t have a cooling method. Just something I learned along the journey. Going from zero was the biggest factor. Now that it has air, it won’t get as hot.

[VaporTrail]

Aah...got it. Yeah, I figured that fill from zero was big contributor.

Hopefully things go smoothly moving forward.