Inconsistency between volume, density, and mass

steve_miller691

Hi Forum,

I have a part whose volume, density, and mass calculation appear inconsistent.

See screenshot below.  I would expect Volume*Density = Mass
However, 0.001m^3 * 1.738e3 kg/m^3 does not equal 2.569kg

I am new to Onshape and am likely missing something.  
Any suggestions?

Steve Miller

Jason_S

Hey Steve,

What is happening is we are rounding 0.00147...m^3 to .001 m^3. For most things where you are using meters to define volume, going to 3 significant figures is "good enough" for volume. If you want a more accurate result change your length units to centimeters. 

You can change this for all documents or change individual documents:

 1. Click on your name in the top right, and go to "Manage account." From your account settings, go to "preferences" on the left list. Here you can select your default units.

 2. You can change this from the Document menu (three bar menu next to the "Onshape" button in the top left) then go to workspace units and edit your units there for an individual document.

Let me know if you need any further help!

Jason

Jason_S

Hey Steve,

What is happening is we are rounding 0.00147...m^3 to .001 m^3. For most things where you are using meters to define volume, going to 3 significant figures is "good enough" for volume. If you want a more accurate result change your length units to centimeters. 

You can change this for all documents or change individual documents:

 1. Click on your name in the top right, and go to "Manage account." From your account settings, go to "preferences" on the left list. Here you can select your default units.

 2. You can change this from the Document menu (three bar menu next to the "Onshape" button in the top left) then go to workspace units and edit your units there for an individual document.

Let me know if you need any further help!

Jason

3dcad

I would like to able to set different unit for volume and dimensions. I use mm for dimension but for volume I would like to set m3 and m2 for area.

OpenR2

Jason, I think Steve has identified a bug in the UI panel.

You are not actually using significant digits for the Volume are you? You are just rounding? If you were using significant figures wouldn't the panel say 1.74e-3 m^3? 0.001 m^3 only has 1 significant digit?

Also aren't you using 4 significant digits in all these panels and not 3? Doesn't 1.738e+3 have 4 significant digits?

If you do the calculation with the numbers in the panel you have 0.001m^3 * 1.738e3 kg/m^3 = 1.738Kg? This is 33% off from the correct answer.
If you had significant figures in the panel you would have something like 1.478e-3 m^3 in the dialog.
If you do the calculation with the numbers in the panel you have 1.478e-3m^3 * 1.738e3 kg/m^3 = 2.569 Kg? Which is .... the correct answer?

OpenR2

I think there is more wrong with this UI panel than just the volume.
Everything that does'nt have an e+x or e-x looks potentially problematic to using these number downstream.

Can you post the actual numbers for Surface Area, Y, and Z to 7 decimal places?
Unless these are exactly 0.126000000, 0.003000000, and 0.0340000000, I think there is a problem. In how you are attempting to fallback to non scientific display.

Is there a reason that you are not printing everything out in scientific notation.significant figures?

3dcad

@OpenR2
Why do they need to match exactly? Why should there be scientific notation?

I think this (don't mind about numbers) is easy to read and even non-tech person (almost) understand what it says.

• Mass: 4.429 kg
• Volume: 51498736.937 mm3
• Surface area: 9000975.079 mm2

If you need higher precision, just change units. 3 decimals should be enough precision but as I mentioned earlier I would like to set each unit separately to get needed information in correct unit straight out.

billy2

I use these numbers a lot, haven't seen any issues.

OpenR2

Steve asked a question about why when he used the numbers in the panel the math didn't add up.

Jason answered his question but I'm not sure his answer was 100% correct. At least not 100% clear.I believe he was mixing up concept precision and the concept of significant figures in his explanation.

I answered Steve's question, I believe more correctly.

The issue is when multiplying two numbers that are magnitudes of order apart from each other. Significant figures is how this issue is addressed. CAD is put math and It's how just how the math works.

Whether you need the numbers in the panel to match is a different matter. I was not addressing that question.

The panel has both proper scientific notation with sig figs and 3 decimal place non sig fig mixed together. And that mixture seems to lead to Steve's question. 

It it looks like Onshape are going with sig figs in the panel as soon as they can make the change.

steve_miller691

Thanks for all the help.  I figured out the trick of switching the units to get the exact numbers, and just now came back to the thread.  There are a lot of useful comments in this thread about significant digits and the option of specifying units by measured quantity, and I have the impression Onshape is considering them.