Mass or weight?

[jmw]

Re posting here since I got no wehere in the other forum....

This is a problem I came up with when reviewing bunkering. I've posted in Linkedin but I know I'll get the usual people chiming in about how wonderful coriolis meters are and touting fuel additives etc. so better here....

I have a problem understanding why things are done the way they are and some confusion.

I am looking at the quantity calculation during bunkering and let's assume we have measured the "observed" volume (commonly by tank dipping) - the volume of fuel at the fuel temperature which is also measured.
Typically we also have the base density as declared in the supplier's BDN and from prior lab analysis.

The calculation steps appear to be:
calculate the VCF (Volume correction factor) and convert the observed volume to standard volume (the volume at 15C).
Now multiply base density and standard volume to get the mass.


OK so far? But why that way round?

Surely it is better to:
take the base density and fuel temperature and calculate the alternative density - the density at the fuel temperature
then simply multiply observed volume and alternative density to get mass?

Same thing? not really.
The kicker is the VCF.

The VCF is effectively calculated by finding the alternative density anyway (density at fuel temperature) and using the ratio of base density and alternative density to find the VCF and then using the VCF to find the standard volume.
This involves the density in several different and unnecessary steps if we follow this sequence. And if the density is wrong, and in about 50% of cases where the commercial sample is analysed and compared to the BDN value there is a significant difference, what then?

Suppose we now measure density online.
This gives us the observed density directly (equivalent to the alternative density calculated from the base density, but measured directly and it has the advantage of being the true density value).
Technically one supposes one has to find the base density from the observed density, calculate the VCF and then the standard volume so one can again multiply the standard volume by density to get mass.
This seems to be how it has to be done since this is how the calculations are set out, including in various software solutions for bunkering which don't allow for alternative calculation procedures.

But it seems cumbersome, unnecessary and more vulnerable than simply multiplying the observed density and observed volume together to get the mass in one calculation step.

And, if we are to move increasingly to flow measurement, using observed values would seem a logical approach (assuming we use volumetric flow meters).
Bunkering is not unique in this. This procedure of converting everything to base or standard values seems quite a common way to do things......?

But suppose we have to apply the same logic to coriolis meters?
They measure "observed" mass, observed density (some of them with suitable accuracy) and the temperature.
Should they then:
Find the observed volume from the observed density and mass
Calculate the base density
Calculate the VCF
Find the standard volume
Multiply standard volume by base density to find the mass?

It is about as sensible, it seems to me, as the steps we do take.

So if it is done the way it is, there must be a good reason that I haven't understood, yes?

Oh, and it doesn't stop there.
Once we have the mass, by whatever means, we have to apply the weight correction factor to find the weight.
Why?
What is wrong with mass?
Oh, and does the coriolis meter apply the weight correction automatically or is it applied subsequently?

There are some other issues here but this is enough to be going on with I think.



JMW

http://www.ViscoAnalyser.com

 

[BigInch]

Arn't coriolis meters wonderful.

You gave me a headache with all that.

This is the procedure as I know it.
Because you are looking for the standard mass, because that's the figure you use to calculate for how much you write the cheque.

1) Calculate the VCF (Volume correction factor)
VCF is both a function of tank bulges and dents, the departure from a true geometric shape and the effect that temperature has on both the tank volume and the product within.
2) Convert the observed volume to standard volume
3) Multiply the base Standard volume by the standard density to get the standard mass.

Surely it is better to:
take the base density and fuel temperature and calculate the alternative density - the density at the fuel temperature
then simply multiply observed volume and alternative density to get mass?


No! There you don't know what the "observed volume" is because you left out the VCF function of volume. You did presumedly get the temperature correction factor part of the VCF, but that doesn't tell you the volume in the tank. I think you might believe that tanks are perfect geometric shapes that don't change their volume with temperature or the number of bulges and indentations they have. I don't know what "observed volume" is.

Your logic would seemingly work, if you were only thinking about flow measurement using meters, not volume measurement by filling tanks. The meters don't directly measure what goes in the tank, so your logic works there.




From "BigInch's Extremely simple theory of everything."

 

[jmw]

Thanks Big Inch.

Yes, the currently dominant system of tank dipping does necessarily presumably require all the steps you itemise.
But in bunkering, the Volume Correction Factor is purely the function of the temperature density relationship.

The volume at 15°C is then determined by applying the volume correction factor (VCF) given in Table 54 of ISO 91-1.

When using tank dipping there are a separate set of calculations based on the tank tables, trim and list tables and so on. This is supposed (so far as I understand it) to give an accurate observed volume.
I would imagine that the tank shape factors etc. are included in this calculation though given the tendency to simplify some things and complicate others I cannot be sure.

But then we come to this question of mass and weight.
This is where I am uncertain what is going on.
Kittiwke describes the process here[link] but not why.

By the way, it seems I have been [url=http://www.eng-tips.com/viewthread.cfm?qid=184540]here before.


JMW

http://www.ViscoAnalyser.com

 

[BigInch]

"From the barge calibration tables, the observed volume can be determined" OK, that clarifies "observed volume". It corrects for the shape of the tank.

So, the real question is why mass or weight???

As noted in one of those links, density * volume does not equal mass. Density * volume = total weight in air.
Air has a certain buoyancy which reduces the weight of a given mass by the density of air * the volume of the mass.
For engineering calculations the difference is considered insignificant however, accountants, if left undisturbed will spend a week looking for 0.21 centimos. If the sales contract is mass based, the true mass must be calculated, so the density of air * the volume of the mass has to be added to extract the maximum amount of gold from the purchaser.

From "BigInch's Extremely simple theory of everything."

 

[jmw]

In other words, there is no logical reason why they should go down this route of extra and confusing calculations.
I guessed as much.

You'd think, having found the mass which doesn't change.... the difference between mass and weight being something we all learned early on at school.... that it would be fine to stop there. It makes you wonder they don't also make an adjustment for the change in gravity from one part of the planet to another, the planet being not spherical but an oblate spheroid.


JMW

http://www.ViscoAnalyser.com

 

[BigInch]

I don't see anything extra. You have to get where you want to go.
They want to get "true mass" and that's the route to it.

I'm trying to keep variations in G a secret. If that came out people would soon realize that Mount Everest is not highest peak on Earth and that could get kind of chaotic.

From "BigInch's Extremely simple theory of everything."

 

[jmw]

What I mean is, if you have density and volume, you get mass.
Mass is the same everywhere.
You then convert to weight which varies all over the place. Bunkering in deep space the fuel would be free.

JMW

http://www.ViscoAnalyser.com

 

[BigInch]

NO. If you have density and volume, you don't get mass. You get weight of the fuel in air.

No way it would be free. You'd have to find the mass of fuel added by how fast you were then orbiting the moon.

From "BigInch's Extremely simple theory of everything."

 

[jmw]

Density is mass per unit volume?
But the product of density and volume doesn't give mass but weight?

Wiki: In everyday usage, mass is often referred to as weight, the units of which are often taken to be kilograms (for instance, a person may state that their weight is 75 kg). In scientific use, however, the term weight refers to a different, yet related, property of matter. Weight is the gravitational force acting on a given body — which differs depending on the graviational pull of the opposing body (e.g. a person's weight on Earth vs on the Moon) — while mass is an intrinsic property of that body that never changes. In other words, an object's weight depends on its environment, while its mass does not. On the surface of the Earth, an object with a mass of 50 kilograms weighs 491 Newtons; on the surface of the Moon, the same object still has a mass of 50 kilograms but weighs only 81.5 Newtons. Restated in mathematical terms, on the surface of the Earth, the weight W of an object is related to its mass m by W = mg, where g is the Earth's gravitational field strength, equal to about 9.81 m s−2.

Now, if density were measured using a hydrometer I could see some potential for this to be the case but when measured in an enclosed system with a digital (vibrating element) density meter you actually get mass per unit volume not weight per unit volume, do you not?

PS I carefully said deep space and assumed stationary vessels....... but I admit there would be a cost of getting it there. Just that invoicing based on weight would mean giving the fuel supplied for free.
Some bunker suppliers include the delivery charge in the price per ton and others make a separate delivery charge.

JMW

http://www.ViscoAnalyser.com

 

[BigInch]

You only get true density if you weigh a known volume in a vacuum and, as you note, technically, continue to compensate to standard gravity. If the temperature was not standard, and the pressure not standard, you'd have to compensate for those too.

If you measure with some element given some forcing function to vibrate it at a theoretical value and you measure the frequency of the vibrations it is actually making, it can be ratioed and converted to some representation of mass, or maybe even its color, I don't know, depending on what algorithm is used to do it. It would still have to be compensated for temperature, pressure, gravity, etc. to get that density to equal "true" density, or STP density or whatever you want to call it.

The point of all of this is you have to, and only have to, calculate what you need to get to whatever units, at whatever conditions, are specified in the sales contract. Depending on what it is, it might be volume at STP, mass at STP, weight at STP, or cubits, and STP does not have to be 15[°]C and 1 Atmosphere. STP can vary and does and different sales contracts may even specify that differently depending on where the transfer takes place. Even that is temporary. Once loaded, the density of crude, for example, will continue to reduce volume, and maybe even mass too, depending on how much it offgases during transit.

Does it matter? There is a standard procedure to do the calculation, at least partly so that everyone will be accustomed to seeing it that way and to immediately pick out any errors. If you want to do it backwards, and it gets you the answer you need, I suppose you could do it, just be prepared to argue your method for the rest of your life.

From "BigInch's Extremely simple theory of everything."