went along my uncle to get the natural gas cylinder refilled

jackson6612

Today I went along my uncle to get the natural gas cylinder refilled. He uses it at his farm. When the refilling man placed it on the weighing scale it weighed, let's say, 7 kg. During the refilling process the cylinder was placed on the scale. Once he removed the refilling pipe after the cylinder was full with 8 kg gas, the reading came to almost 7 kg. I don't know why I think that person cheated.

I could be wrong in my observation. Obvious that refilling pipe was also exerting some of its weight on the scale while connected to the cylinder. I think this is how the process went: placed the cylinder on the scale and weighed it, then connected refilling nozzle and pressed a button on the scale to get the weight value 0 kg so that the weight of only the newly refilled gas is measured, after refilling he removed the nozzle and the scale reading went down.

That would mean that the weight of refilling pipe was not included. When the gas was being refilled, wasn't it exerting an additional pressure on the cylinder? Suppose, you have a jar on the scale and holding a water pipe above it which is ejecting water at high pressure into the jar. Let's suppose the jar has been filled with some water, then once the water is stopped coming out of the pipe, the reading, whatever it was previously, would go down because there is no 'additional' pressure. Right.

NOTE: I'm not a science or physics student.

TOONCES

It sounds like he placed the container on the scale, then zeroed the scale out to ignore the weight of the container, and then added the gas. By zeroing the scale he can measure the weight of the gas he is adding and not the weight of the cylinder.

JaredJames

Agreed.

By zeroing the scale before refilling you get to see exactly how much gas is contained in it after refilling it without having to worry about the container interfering with the reading.

jackson6612

Did he cheat?

JaredJames

Why would he be cheating?

There are two ways he could do this:

a) Note the weight of the bottle whilst empty (7kg), work out how much the total should be after filling (14kg) and then add gas until the scales showed that amount.

b) Zero the scales with the bottle on it whilst empty, add gas until the scales read the required amount (7kg).

I'm curious why you think he cheated?

jackson6612

James, I would request you to read the the main posting again.

JaredJames

It's Jared.

If you paid for 8kg of gas, that is what the scale should read - assuming he zeroed it when the pipe was attached.

Allowing for basic loss whilst removing the pipe and the weight of gas held within the pipe itself, I would certainly expect there to be a drop when it is removed (aven if tiny). Should still read 8 after it though.

All depends how accurate the scales were.

I could put a lot of other factors in as well, such as when he pulled the pipe off this would have lowered the effective weight on the scales briefly (the lifting force of him pulling), if he then took it straight off you would have seen an inaccurate reading right before it was removed - perhaps the scales need to 'settle' before another reading was taken. Who knows.

If the reading went from 8.1 to 7.8, that isn't a big concern. If it went from 8.1 to 7.1 that is a concern. It depends in what that difference is.

xxChrisxx

Don't gas bottles just get filled all the way? (as in you can't exactly go in and ask for a half bottle please) The stop point is usually a cut off valve that stops at a certain pressure. So it's rather difficult to 'cheat' you out of say a kilo of gas.

gmax137

I don't think you're right - I believe that the propane is a liquid, and during the filling the big tank and your tank are at nearly the same pressure, the fill station is pumping liquid propane into your tank.

Where I get mine filled, it looks like they have the scale set up to trip when the total weight (tank plus propane) reaches the setting. This relies on the assumption that all of the tanks have the same tare weight. I've never seen them fill anything other than the usual 'barbecue' size tank, so I don't know if there's another scale, or what, for filling larger tanks. And the scale is just a beam balance thing, it doesn't cut off the propane by itself; rather the operator watches and when the beam deflects he shuts off the pump.

So, you do get charged for the propane that's in the tank when you bring it in.

To the OP - I can't explain what you saw, but I have wondered also about the effect of the fill pipe on the reading. I just figured it was a small effect (and if they accounted for it they would just charge a higher fee for a refill).

Borek

From what I understand scale was zeroed after the pipe has been connected, so mass of the pipe was corrected for. Final mass you have seen on the scale is mass of the gas MINUS mass of the disconnected pipe.

jackson6612

Is this what I'm saying below correct?

rcgldr

You didn't indicate if the scale reading increased to 8kg before the filling was stopped.

Whatever pressure is in the cylinder when the valve was shut off, that pressure remains the same, unless there's a significant change in temperature during the filling process if there's a pump involved. Regardless of the pressure and temperature issues, whatever mass was added into the cylinder when the valve was closed will remain in the cylinder.

Another example is a scuba tank. When 80 cubic feet of sea level pressure air is pressurized and added to a scuba tank (final pressure around 3000 psi), the tank increases in weight by 6 lbs, which is the weight of the 80 cubic feet of sea level pressure air added to the scuba tank.

Borek

There is grain of truth in Jackson's thinking. If you fill the tank from the hose kept above, water has momentum and has to stop, that means additional pressure on the tank bottom as long as water is poured.

JaredJames

"When the gas was being refilled, wasn't it exerting an additional pressure on the cylinder?"

I think by this the OP means that the fluid being pumped was exerting an additional force on the scales. It would be more suitable to a liquid than a gas.

When water is poured into a container on a scales, it can give a false reading. The force of the water on the bottom of the tank would increase the reading.

I don't think this applies in this scenario.

gmax137

It is a liquid. Anyway, I thought that what we were talking about was just the weight of the hose and fill nozzle, being screwed into the tank, adding to the scale reading of the tank weight. Since it looks like the tank supports the end of the hose.

pallidin

I don't know, but my experience with propane filling on a scale has been this:

1: I bring in my 40lb capacity propane tank to be filled. There is some propane in the tank.

2: They weigh the tank and make note of the weight(with NO hose connected)

3: They connect a hose to the tank and fill it until the relief valve indicates full.

4: They disconnect the hose.

5: They weigh the tank again.

6: They charge me for the difference.

Seems legit to me.

Borek

Some tanks - at least those I know from Poland - are designed to hold known mass of the gas, and they should be not filled up to the relief valve (filling should stop a little bit earlier), so the procedure calling for connecting/disconnecting the hose can be not practical. If the hose is not too rigid filling the tank with hose attached should give reasonably accurate results. What you weight during filling is mass of tank plus mass of gas plus mass of hose/valve - and the only part that changes substantially is the mass of the gas.