Stupid propane tank question - tare weight

[jviss]

I found the specs for my propane tanks, which are 11 lb. capacity steel tanks made by Trident, model number 1400-0011T. The specs say the tare weight is 14 lb. Is this the value I should use for an empty tank when weighing them to determine how much gas is left?

Thanks,

jv

p.s. while the tare weight is 14, Defender, where I bought them, lists the shipping weight at 13.45 lb. How is that explained?

 

[kloudie1]

Helium filled for shipping?
just kidding.. The tare for the tank is probably correct, I'd think Defender's weight is a little off..

 

[Ross]

The tare weight should be stamped into the valve protection band but 14 seems about right. Defender may be showing the exact weight but I don't know of any carrier that charges by the fractional pound.

 

[Steve Dion]

My question is why do you care? If the tank needs to be refilled, just refill it!

Because most tank now have OPD on them, you cannot fill the up to capacity any longer.

 

[toddco]

ok, I work on scales...
Gross =Total Wt.
Tare = Empty Wt.
Net = Weight in container.

Just for Kloudie, Helium trucks weight the same empty or filled. They never let all the helium out of the tank. If they did, you would have to purge the tank before you refilled. They just let out some of the pressure.

As to the shipping weight, one of two things. Either they rounded to the next highest pound, or they included the weight of the box.

 

[tkanzler]

On the protection ring, TW is tare weight, in pounds. It's the empty weight of the cylinder with valve.

WC is water capacity, in pounds. It's how much water weight the cylinder will hold if filled right to the top, with no overfill protection device to stop it, and doesn't include tare weight.

These numbers are on the protection ring, as well as other numbers, including pressure rating and material (built into one of the other codes).

The rules say DOT type propane cylinders (like the ubiquitous 20 lb barbecue grill type, or the one on your boat) can only be filled to 80% of volume. Since propane is a hair over half the density of water (about 52%, if memory serves), if you multiply .8 x .52, you get .42. Multiply the WC number by .42 to get actual weight capacity (of propane) for your cylinder. Add tare weight to it, and that's what your cylinder should weigh when filled to max legal capacity.

The typical 20 lb barbecue cylinder has a WC number around 48 lb, so .42 x 48 lb = 20.16 lb. If it was full to the top, it would hold about 25 lb of propane. The OPD should stop filling at 20 lb, more or less, though the dealer is supposed to use weight and not rely on the OPD.

You can figure your smaller cylinder the same way, and yes, the TW is the empty weight. It gets a little dicey when filling small cylinders, like the 4 pounders on my last boat. The nursery where I get my cylinders filled didn't know what to do, since it's not a standard size (to them). I had to explain to them that 2 units at 4 lb each is 2/5 of a regular 20 lb fill, or at $1 per pound for a 20 lb fill, it's $8.

 

[jviss]

Tank weight experience report, and an additional question.

O.K., here's an update for those who care. The Trident website published specs are incorrect. The tare weight stamped on the tank is 13.1.

As for why do I care, I want to know how much gas is in teh tank, so I can evaluate whether I need to fill now, or can wait. I don't want to run out of gas unexpectedly (duh!). I guess I could fill one tank when it runs out, after switching to the other, but I'd be making the trip for filling twice as often.

At some point one tank seemed empty and I switched. Can't remember when. Checked them by 'heft' on Sunday, and they seemed to weight the same.

Weighed them at home: both 20 lb., total; but that's one pound resolution on that scale. At tare weight of 13.1, that's 6.9 lb. remaining in each, both more than half full.

Just got back from BJ's where I had them filled. One took 0.8 gallons, the other 1.3 g. That's 3.4 lb. and 5.5 lb., respectively. I asked him to re-try the first one, and he did, but it wouldn't take any more - it's an automatic shut-off filling system.

Got home and weighed them again: 23 lb. and 26 lb. This adds up. Yes, I am a nerd, I have a spreadsheet for this. The result is that tank 1 has a 10.3 lb. net, tank 2 12.4.

Question: is all of the net weight useable? At what point do they quit putting out enough pressure to run the cooktop?

Thanks,

jv

 

[jviss]

By the way, thanks all for the informative replies. For the record, BJ's Wholesale Club sells propane by the gallon, so no confusing moments or math at checkout.

 

[onecoolair]

I found the specs for my propane tanks, which are 11 lb. capacity steel tanks made by Trident, model number 1400-0011T. The specs say the tare weight is 14 lb. Is this the value I should use for an empty tank when weighing them to determine how much gas is left?

Thanks,

jv

p.s. while the tare weight is 14, Defender, where I bought them, lists the shipping weight at 13.45 lb. How is that explained?

My new word I learned today...tare weight. Thanks

 

[kirlos]

O.K., here's an update for those who care. The Trident website published specs are incorrect. The tare weight stamped on the tank is 13.1.

As for why do I care, I want to know how much gas is in teh tank, so I can evaluate whether I need to fill now, or can wait. I don't want to run out of gas unexpectedly (duh!). I guess I could fill one tank when it runs out, after switching to the other, but I'd be making the trip for filling twice as often.

At some point one tank seemed empty and I switched. Can't remember when. Checked them by 'heft' on Sunday, and they seemed to weight the same.

Weighed them at home: both 20 lb., total; but that's one pound resolution on that scale. At tare weight of 13.1, that's 6.9 lb. remaining in each, both more than half full.

Just got back from BJ's where I had them filled. One took 0.8 gallons, the other 1.3 g. That's 3.4 lb. and 5.5 lb., respectively. I asked him to re-try the first one, and he did, but it wouldn't take any more - it's an automatic shut-off filling system.

Got home and weighed them again: 23 lb. and 26 lb. This adds up. Yes, I am a nerd, I have a spreadsheet for this. The result is that tank 1 has a 10.3 lb. net, tank 2 12.4.

Question: is all of the net weight useable? At what point do they quit putting out enough pressure to run the cooktop?

Thanks,

jv

Basically all of the weight is usable, as long as there is liquid propane left the pressure will remain constant (for a given temperature)

 

[tkanzler]

Basically all of the weight is usable, as long as there is liquid propane left the pressure will remain constant (for a given temperature)

This is true, but sometimes the evaporation rate will slow down due to poor heat transfer from the outside, and the resulting temperature drop within the cylinder. But then, that means the temperature is no longer the same (like a freon horn when it's used), so the pressure drops.

With less liquid in the cylinder, it's not as easy for heat to get into the tank as the gas is being used, so you can help by keeping it warm, usually by letting the sun shine on it, and/or reducing the rate of gas use, and/or making sure it's not insulated against heat getting to it (within normal limits), maybe by opening the door of the propane locker (assuming the cylinder is in a locker, outside the boat cabin) to let some heat in.

The faster the gas is used, the faster it has to flash to gas, so the faster heat has to transfer into the cylinder from the outside to keep the pressure up. That's probably why some folks attribute reduced gas pressure to low liquid level, and mistakenly think that you can't use all that's in the cylinder.

 

[kloudie1]

Toddco..?? If I put 500# of liquid helium in a tank it will weigh the same as when it was empty?? Kool but that is not my experience

Edit: I agree that the "tare" weight is the same but the weight of the system on the road is not.

 

[jviss]

This is true, but sometimes the evaporation rate will slow down due to poor heat transfer from the outside, and the resulting temperature drop within the cylinder. But then, that means the temperature is no longer the same (like a freon horn when it's used), so the pressure drops.

With less liquid in the cylinder, it's not as easy for heat to get into the tank as the gas is being used, so you can help by keeping it warm, usually by letting the sun shine on it, and/or reducing the rate of gas use, and/or making sure it's not insulated against heat getting to it (within normal limits), maybe by opening the door of the propane locker (assuming the cylinder is in a locker, outside the boat cabin) to let some heat in.

The faster the gas is used, the faster it has to flash to gas, so the faster heat has to transfer into the cylinder from the outside to keep the pressure up. That's probably why some folks attribute reduced gas pressure to low liquid level, and mistakenly think that you can't use all that's in the cylinder.

Interesting. At one point we switched tanks because we assumed the first was empty; I don't recall the event, but I imagine it was because the range burners wouldn't stay lit. It turns out there it was probably half full. Could this be because of the effect you describe? If we had warmed the tank a bit it might have worked?

Thanks,

jv

 

[tkanzler]

I suppose that's possible. Just like sweat cooling your bod via evaporative cooling (and for that matter, the processes of hermetic air conditioning, or even evaporative 'swamp coolers'), evaporating propane takes heat. If your cylinder couldn't get heat into it as fast as the propane was asked to evaporate, then the temperature would have dropped to where the internal pressure was too low.

I've seen automotive A/C techs putting Freon bottles into warm water to speed up the process, or just to make the stuff move, when refilling car systems. Same thing.

If the gas starts out fine, then slows down (increasingly low flame), it's probably a heat transfer problem. If giving it some time with the burners off makes the flame(s) high again, it's a heat transfer problem

Liquid conducts heat better than gas, so when the liquid is low, there's not as much contact with the cylinder's skin, which is where all the heat comes in, so heat transfer into the cylinder drops off, and the temperature drops to where the pressure is unacceptable. Of course, at some point, it will hit equilibrium - the slower heat transfer lets the temperature drop to where the gas flow is low, and the demands on the system balance. Problem is, the resulting gas flow rate is too low.

Next time that happens, put your hand on the side of the cylinder, down where the liquid is, and see how cold it is. You may even see condensation there, like with those little computer liquified 'air' bottles, for cleaning computer guts. They get bloody cold when used for more than a few seconds.

 

[jviss]

I suppose that's possible. Just like sweat cooling your bod via evaporative cooling (and for that matter, the processes of hermetic air conditioning, or even evaporative 'swamp coolers'), evaporating propane takes heat. If your cylinder couldn't get heat into it as fast as the propane was asked to evaporate, then the temperature would have dropped to where the internal pressure was too low.

I've seen automotive A/C techs putting Freon bottles into warm water to speed up the process, or just to make the stuff move, when refilling car systems. Same thing.

If the gas starts out fine, then slows down (increasingly low flame), it's probably a heat transfer problem. If giving it some time with the burners off makes the flame(s) high again, it's a heat transfer problem

Liquid conducts heat better than gas, so when the liquid is low, there's not as much contact with the cylinder's skin, which is where all the heat comes in, so heat transfer into the cylinder drops off, and the temperature drops to where the pressure is unacceptable. Of course, at some point, it will hit equilibrium - the slower heat transfer lets the temperature drop to where the gas flow is low, and the demands on the system balance. Problem is, the resulting gas flow rate is too low.

Next time that happens, put your hand on the side of the cylinder, down where the liquid is, and see how cold it is. You may even see condensation there, like with those little computer liquified 'air' bottles, for cleaning computer guts. They get bloody cold when used for more than a few seconds.

Thanks. I will give it a try, though the tank locker is somewhat inconvenient to get to.

 

[Ross]

If the tamk locker is difficult to access, how do you open and close the tank valve?

 

[jviss]

If the tamk locker is difficult to access, how do you open and close the tank valve?

It's not super hard to get to, it's just that I have to open the lazarette, open four latches to open the lid of the air-tight locker, lift the lid and put it somewhere, etc.

I don't close the tank valve daily, or even weekly, I use the solenoid to control the gas. I test the solenoid regularly, but shutting it off while a burner is lit and seeing that the burner goes out.