Balsa weights

[James Mills]

I used the balsa density calculator and it gives the option of using the weight in grams or ounces.  If I input the weight in grams the density comes out higher than if I use ounces.  Which would be more accurate?

Thanks,

James

[Bill Little]

I always use grams. 

It is just a gauge for me to determine which piece is lighter, that's all.  "Exactly" how much it weights isn't important if it generally falls in the 4lb, 5 lb, or 12 lb range.  I can tell which is "lighter', that's all.  And whether or not it is "light", "medium", or "heavy"

"Lightest" goes in the rear.

Big Bear

[Tim Wescott]

What are you using for your weight in ounces vs. your weight in grams?  1 ounce = 28.349 523 125 gram, give or take a little bit -- are your two measures consistent?

[James Mills]

What are you using for your weight in ounces vs. your weight in grams?  1 ounce = 28.349 523 125 gram, give or take a little bit -- are your two measures consistent?

The scale I have has the option to measure either both grams or ounces so I just swithced from one screen to another.

James

[Bill Little]

The scale I have has the option to measure either both grams or ounces so I just swithced from one screen to another.

James

Sorry, James, I misunderstood the question.....

You are actually asking which "type" weight measurement will result in a more accurate "density measurement"?  Being just a red neck model builder with no mechanical engineering education, I would expect it would basically rely on which "type" is most accurate on your scales.  Just how many decimal places it carries the measuring out to for each one.  .01 gram is much more accurate than .01 oz.  

Of course the sexual intellect engineers will probably dispute that!

Big Bear

[Bill Little]

So, "to me", I would expect that grams would be more accurate since you are talking smaller increments of weight measurement  with grams versus ounces.

Big bear

[Tim Wescott]

The two numbers should agree.  They don't, which means that something's wrong.  There's really only three possibilities:  You're doing something wrong, your scale is doing something wrong, or your density calculator is doing something wrong.

I don't know how it works, but if you put in some arbitrary wood size and say that it weighs an ounce, then leave the wood size alone and say that it masses 28.4gm, then the density calculator shouldn't change by very much at all (note that a 1 ounce 1/8 x 3 x 36 sheet should compute to 8 lb/cubic foot).  If changing the weight from 1 ounce to 28.4 grams causes a remotely dramatic change in the density calculation, then the calculator is in error.

I have a good idea of how your scale works, and having written plenty of embedded software in the course of my professional life, I'm all too aware of just how badly it can get screwed up without anyone knowing, and often without anyone in a position to approve enough engineering hours for a fix to give a damn.  If you have a 1 ounce weight it should mass 28.4gm or so -- if it doesn't, then the scale is in error.

If you do your double checks, and both the density calculation and the scale are correct, there's only one possibility.  If that's the case, then you need to review your technique, and figure out what you're doing wrong.

[Howard Rush]

Sorry, James, I misunderstood the question.....

You are actually asking which "type" weight measurement will result in a more accurate "density measurement"?  Being just a red neck model builder with no mechanical engineering education, I would expect it would basically rely on which "type" is most accurate on your scales.  Just how many decimal places it carries the measuring out to for each one.  .01 gram is much more accurate than .01 oz. 

Of course the sexual intellect engineers will probably dispute that!

Big Bear

That's right.  Rather than going to any actual effort to answer James's question, you made fun of the guys who were willing to help.  Thanks for not shooting us, I guess.

Assuming James refers to the balsa density calculator at http://www.nclra.org/Programs/BalsaDensity.php , he is correct.  The density comes out heavier when entered in grams.  It did for the two cases I tried anyhow.  The number came out right for the entry in ounces.  The difference probably comes from round-off error in the units conversion in the program.  I tried a few rounded-off conversions, but I couldn't figure out what was doing it.  Entering either grams or ounces gets you plenty close for balsa density, but I can see how James would wonder which of the three items Tim mentioned was amiss. 

[BillLee]

That's right.  Rather than going to any actual effort to answer James's question, you made fun of the guys who were willing to help.  Thanks for not shooting us, I guess.

Assuming James refers to the balsa density calculator at http://www.nclra.org/Programs/BalsaDensity.php , he is correct.  The density comes out heavier when entered in grams.  It did for the two cases I tried anyhow.  The number came out right for the entry in ounces.  The difference probably comes from round-off error in the units conversion in the program.  I tried a few rounded-off conversions, but I couldn't figure out what was doing it.  Entering either grams or ounces gets you plenty close for balsa density, but I can see how James would wonder which of the three items Tim mentioned was amiss.  

Howard, I tried the same test as you using the NCLRA program and found absolutely NO difference when entering the weight in grams or ounces. But my testing was very limited.

What tests did you run that showed a difference in the calculated density when the weight was entered in grams vs. ounces?

Since I am the author of that code, I want to fix it if it's wrong.  

Bill

Edit to add: The program was using 28.34 gms == 1 oz. I changed it to use 28.349523125 gms = 1 oz. and it changes the metric density out in the second decimal place.  But in my limited testing, I never saw any change in the lbs/cu ft density.

Howard, would still like to see the tests you ran.

[Bill Little]

That's right.  Rather than going to any actual effort to answer James's question, you made fun of the guys who were willing to help.  Thanks for not shooting us, I guess.

I guess that actually depends on someone's "opinion" Howard.  I was "made fun of"..... and so far I would not even consider shooting anyone.  And who's to say I didn't "go to any actual effort"?  If I never quote a dozen "formulas", then my "opinion" could never be correct.  "Opinions" are all that is basically ever being discussed, anyway.  I have used the program on many occasions.  But then I (and several others) am supposed to take a joke but no one else is.

Assuming James refers to the balsa density calculator at http://www.nclra.org/Programs/BalsaDensity.php , he is correct.  The density comes out heavier when entered in grams.  It did for the two cases I tried anyhow.  The number came out right for the entry in ounces.  The difference probably comes from round-off error in the units conversion in the program.  I tried a few rounded-off conversions, but I couldn't figure out what was doing it.  Entering either grams or ounces gets you plenty close for balsa density, but I can see how James would wonder which of the three items Tim mentioned was amiss.  

My premise was that either grams or ounces would "gets you plenty close for balsa density", exactly like you said.  Funny that "you" would take offense, if you actually did.

Peace to you Howard.

[Larry Cunningham]

I'm told that an engineer is someone who measures with a micrometer, marks with a piece of chalk, and then cuts with an axe..

(as an engineer, I'm permitted all the "train driver" jokes I want..)

L.

My Job

It's not my job to run the train, the whistle I can't blow.
It's not for me to say how far the train's allowed to go.
I'm not allowed to blow off steam, nor even clang the bell.
But let the damn thing jump the tracks....and see who catches hell!

[Bill Little]

Hi Larry,

I actually like a whole lot of people who are engineers.

Bill

[BillLee]

Just to get this thread back on track and leaving all of the angst behind...... 

I suspect the scales that James used reads in ounces or grams. The question is: how many significant digits does the scale provide and what way does it round to the values given.

If you use the density calculator at the NCLRA website:

1 oz. 3x36x1/8 sheet of balsa is 8.00 lbs per cu. ft.

But 28 gms instead of 1 oz., it comes out 7.90 lbs per cu. ft.

And 28.3 gms comes to 7.99 lbs per cu. ft.

Since 1 oz. == 28.349523125 gms (to 9 decimals  ) a scale that reads in ounces and grams will have to make some sort of rounding for display. The classic rounding algorithm would round like

1 oz. = 28 gms, or 28.3 gms, or 28.35 gms, or 28.350 gms, or 28.3495 gms, or ......

Based on his comment "If I input the weight in grams the density comes out higher than if I use ounces." my guess is that his scales rounded the weight upward, e.g.,

1 oz. -> 28.4 or some-such.

James, can you give me the actual numbers you used?

Thanks.

Bill Lee

[Bill Little]

Hello Mr. Lee,

The question is: how many significant digits does the scale provide and what way does it round to the values given.

That is the point I unsuccessfully (evidently) tried to make.  A point that I must not have made clearly, I suppose. 

I'm truly sorry I will not take the time to work a lot of math to try and figure it all out.  With "balsa wood and model airplane building" it seems that knowing the exact weight is actually very "un"important.  We appear to use the figures given by any good density calculating program for a "comparison".  I firmly take the point of view that Billy Werwage was quoted as saying: "I just don't want to make a science project out of it."  I could, with some refreshing of my math, but I don't have enough time to do the things I really enjoy doing now.

I DO applaud, and deeply thank, those of you who DO take the time to develop these programs for our use.

Bill Little

[Tim Wescott]

I actually like a whole lot of people who are engineers.

Oh no!  Next he's going to say "some of by best friends are engineers".  You know what it means whenever someone says "I don't hate <<insert racial, religious, or professional epithet here>>,  some of my best friends are <<insert racial, religious, or professional epithet here>>" !!

Wen I was in skool my teechers said I had to learn how to spel if I wanted to be an inginere.  Now I r 1!

[Tim Wescott]

Just to get this thread back on track and leaving all of the angst behind...... 
<< cognizant discussion of weights and conversions skipped >>

If I may make any suggestion at all, as an engineer who's been down the "too many decimal places" path and has come back to tell the tale, your two best bets are to either use the most exact number for the conversion*, to just not supply too many decimal points in your answer, or both.  Fancy would be to look at the number of significant digits in the answer (i.e. take 1 and 1.000 as different numbers) and figure out the significant digits to supply in your answer.  Fancy often leads to obscure bugs in your code, but it's fun to do and makes you feel smart.

Personally, I'd figure that as temperature and humidity change, the density of a block of balsa varies by more than 0.1% anyway, which means that anything over three significant figures is just fantasy anyway.

* And if you do use the 'exact', consider that if someone's using an $80 cocaine scale** for measurement, then that scale doesn't measure the mass of the balsa, it measures the force of the balsa on the scale's little internal proof ring, and they only paid $80 for the cruddy little thing anyway.  Even if the scale were perfect, it's internal weight vs. mass calibration could, in theory, be off by as much as 0.5% (see http://en.wikipedia.org/wiki/Earth%27s_gravity).

** Uh -- precision electronic scale.  For jewels and minerals and chemistry experiments that involve turning water different colors, but nothing either explosive or mind altering, and everything altogether morally uplifting.  Of course no one would ever use it to sell cocaine -- after all, it says "not legal for trade" right on the thing!  Ehem.

[Bill Little]

Oh no!  Next he's going to say "some of by best friends are engineers".  You know what it means whenever someone says "I don't hate <<insert racial, religious, or professional epithet here>>,  some of my best friends are <<insert racial, religious, or professional epithet here>>" !!

Wen I was in skool my teechers said I had to learn how to spel if I wanted to be an inginere.  Now I r 1!

Hi Tim,

As you know,I am more prone than not to say exactly what I mean.

On my paternal side, in the last three generations before me, two had Mechanical Engineering degrees, one held several patents.  The one without was one of the leading "electronics/avionics" members of the US NAVY.  "Engineering" is actually in my genes, I have just overlooked it for personal reasons.   And intellectual capability was NOT the reason.

Bill

[Howard Rush]

Just to get this thread back on track and leaving all of the angst behind......  

I suspect the scales that James used reads in ounces or grams. The question is: how many significant digits does the scale provide and what way does it round to the values given.

If you use the density calculator at the NCLRA website:

1 oz. 3x36x1/8 sheet of balsa is 8.00 lbs per cu. ft.

But 28 gms instead of 1 oz., it comes out 7.90 lbs per cu. ft.

And 28.3 gms comes to 7.99 lbs per cu. ft.

Since 1 oz. == 28.349523125 gms (to 9 decimals   ) a scale that reads in ounces and grams will have to make some sort of rounding for display. The classic rounding algorithm would round like

1 oz. = 28 gms, or 28.3 gms, or 28.35 gms, or 28.350 gms, or 28.3495 gms, or ......

Based on his comment "If I input the weight in grams the density comes out higher than if I use ounces." my guess is that his scales rounded the weight upward, e.g.,

1 oz. -> 28.4 or some-such.

James, can you give me the actual numbers you used?

Thanks.

Bill Lee

The NCLRA calculator is just fine.  It's handier than what I've been using and gives another reason for putting an Ethernet connection in my shop.  

When I tried it last night, I got slightly different results for a 1/16 x 3 x 36 1-oz. sheet than I did for a 28.3495231-gram sheet the same size.  Ounces gave 16.00 lb./cubic ft.; grams gave 16.01.   A .00625" sheet gave a little more resolution.  It was something like 160.07 lb./cubic ft.  Another example also gave an RCH bigger number for grams than for ounces.  I went back this morning and both results were dead nuts on.  Aha!  I just saw above that you changed the conversion.  Either the before or after are plenty accurate for balsa density.  If James's scale still gives consistently higher densities for grams than for ounces, I'd suspect the scale.  

Edited to move decimal point.