How to accurately weigh something to within a gram or so?

[oldtuckunder]

Hi Folks

 

Anyone any ideas how I can accurately weigh a con rod when I can't rely on the electronic scales.

 

Problem: The recent engine demise damaged one of my forged con rods and it needs to be replaced, the manufacturers have kindly said that if I can give them the weight they will supply a single replacement thats close enough that I can rematch the full set.

 

OK so here is the problem I have two sets of electronic scales that report that they can weigh to the gram, but around the 500g mark I'm weighing they disagree by 2-3g, also if I put a steel scale weight of 8oz on they both under read by about 0.3oz. However having just taken the 8oz weight to a local Deli with expensive electronic scales (they declined to weigh the con rod but would weigh the weight!) their scales say the 8oz weight weighs 218g where as both my scales say it weighs 220g. But what the hell this proves I'm not sure.

 

Now when I'm matching weights it doesn't matter how accurate they are provided they are consistent i.e. if I'm using one set of scales and weighing a bunch of components to match them it doesn't matter if they are reading 5 or 10g +/- wrong as I'm looking for matching not exact real weight.

 

So now I have to supply a weight to someone who is going to try and match that weight on what is probably an accurate set of scales. But I havent the faintest idea how accurate the weight I'm going to supply is?

 

So I'm sure there is some clever way of using something like a measured volume of water or something added to a container (I can zero the scales with a container on them) and then adding the rod and seeing how much water + container = container + rod. But again I'm not sure how accurately I can measure that amout of water.

 

Anyone any clever idea's how I might calculate/arrive at a correct weight?

 

Alan

[john.r.davies]

Send the manufacturers one of your good rods, so they can compare weights.

 

If you're doing the matching yourself, absolute accuaracy isn't necessary, as you are comparing weights.

 

John

[oldtuckunder]

Send the manufacturers one of your good rods, so they can compare weights.

 

 

Hi John

 

Had thought of that, but had this vision of shipping one half way round the world, and then it getting lost and then having to get two rods matched!

 

Just thinking I might give the balancers a call in the morning and see if I can drive over and weigh a good one, I'm hoping they have very accurate scales. They are about the only place I can think of that might.

 

Alan

[AlanT]

It's not only the weight that matters. It's the weight distribution.

 

A heavy small end won't matter anywhere near as much as a heavy big-end.

 

So by far the best thing is to let the bloke doing the job see the rods you want to be matched.

Edited February 21, 2016 by AlanT

[TR 2100]

I think you need to plan to do this yourself with the scales you have.

 

It's not so much the actual weight, it's the comparative weights.

 

AND - it's not just of the con-rods, but assembly with pistons as well.

 

I would just accept the replacement con-rod as it is supplied, check

the assembled weights of pistons/con-rods on your own scales, and

take it from there.

 

AlanR

[Alec Pringle]

Your local Post Office should check calibrate their scales to 1 gram on a daily basis . . . . . .

 

And your local pharmacy should have scales check calibrated daily to 1/10 gram, again on a daily basis . . . .

 

Otherwise a set of proper balance scales, the kind of thing we used to weigh dope on back in our youth . . . . .

 

Cheers

 

Alec

[john.r.davies]

"Back in our youth"? eBay has FOUR PAGES of Electronic pocket scales to weigh up to 500gms, all costing less than £5, because they are now throw-away items of business.

How accurate a drug dealers scales will be, I don't know. I'll bet they under-read.

 

John

[Alec Pringle]

Hi John,

 

traditional brass weighing scales were laboratory instruments, it's not rocket science to assess with the Mk1 eyeball if the balance pans are level and the cross beam horizontal - and to then confirm that this status quo remains with equal weights in each pan . . . . . .

 

In the days of my youth it was the brass weights you had to worry about, as in were they accurate . . . . or drilled out and aluminium filled . . . . ?

 

You get what you pay for, £5 ebay scales will not read that accurately - what do you expect for a fiver ? I have three electronic scales, at 500gms they'll read 498, 500 and 502, but at 250gms it's 250, 248, 251.

 

Not a problem for comparative weights with all items present and correct, but no bloody good if you're referencing items for someone the other side of the Atlantic, and you require accuracy in absolute rather than relative terms.

 

Think chassis and engine dynos . . . . .

 

Cheers

 

Alec

[Peter Cobbold]

Alan,

Your 500g scales are measuring to 0.5 % precision. The suppliers' scales will be similar.

Between your measurements and the suppliers' I think you'd get an accuracy no worse than 1% - isn't that good enough?

Out of balance forces are proportional to mass, but to the square of the rpm.

Peter

[oldtuckunder]

OK I have got there 510g +/- 1g which is well close enough, and if I ask them to pick one a gram or two heavier then I will have some wriggle room on reducing it. My worry was giving them an under reading as if they hit it when it came to matching I would be in removing weight from the other 5. But if I had given them an overweight target it was also possible that they would come back and say we don't have one that heavy.

 

How did I do it? OK a bit of googling tells me that 1ml of tap water at 17.7C (roomish temp) = 0.998650g if I did it out in the garage which was closer to the optimum 4C then I could have used 0.999973g but for the accuracy I was measuring they are both close enough to 1.0.

 

I have lots of things that report to measure fluid quantities, so finding three that agreed at 500ml was a good start. I also have several syringes that measure 1ml in 1/10th increments, so starting with a zeroed canister on the scales I measured in 20 syringe worth, and as each one went in the scales went up by 1g, so that gave me the confidence that I could accurately add or subtract 1g.

 

So then onto measuring my 500ml. After cross checking between each of the measurement cups a few times I could add what I believe to be 500ml of water to the zeroed canister on the scales and get 499g each time, which at a target of 499.28 @ 0.998650 makes the electronic scales remarkable accurate.

 

So now weigh zeroed canister + rod = 510g meaning scales think rod = 510.

 

Ok now take zeroed canister and add water until = 510g, then remove 11ml of water, scales drop to 499g, measure water left in canister = 500ml.

 

So 1 I think I have proved that the scales are remarkable accurate, they can weigh in 1g increments, and if adding 500ml of water that should weigh 499.28g actually gives me 499g then I think if the scales tell me that the rod is 510g that I can believe it certainly +/- the gram I was looking for.

 

Alan

[john.r.davies]

Glad you're sorted, Alan OTU!

 

Alan R mentioned the need for end-to-end balancing. I've tried to do this, without success.

 

Here's the rig I used. I built a stand, on the lines of a lab.retort stand, with a side arm at an adjustable height.

The arm was a knife edge, so that the con rod would pivot on that and not slide, abolishing friction as a bias.

The knife edge bore on a "crown cap" a disc of aluminium, crimped around the edges to fit in either big or small end, so that the pivot was as close as possible to the centre.

 

My scales were probably very similar to yer drug-dealer's, but were lifted from the kitchen of 'er indoors, honest, Guv!

 

I carefully set this up to bear either the big or small end on the same. central point of the weighing surface, as shown in the picture below ,and weighed all six rods, in series, again and again. And could never get agreement between measurements. Either my scales or my method were faulty.

 

I've seen rigs where the end on the stand is suspended by a chain, and turned discs are used to fix the suspension point on the centre of the end.

Like this: http://www.angelfire.com/ca4/CorvAIRCRAFT/RodBalance.html He seems not to use fancy scales.

I have a lathe now - would proper discs and a chain suspension allow this method to make more repeatable measurements?

 

John

Edited February 21, 2016 by john.r.davies

[Don H.]

Perhaps a brand-name food or beverage be very, very close to target weight? Say a 0.5 L bottle of mineral water, poured into a tared container (or subtracted from a separately emptied and dried identical container?

[rhino_mac]

That wouldn't work. In the UK anything with the stylised "e" mark after the weight has been filled using the average weighing system. If it doesn't have this mark, it's a minimum weight rather than an actual weight.

 

Our weights and measures law is so stupidly complicated that even the people that enforce it seem to struggle to understand it.

 

Just to complicate things further, if I were looking to weigh something to that tolerance, you would need to ensure the scales measured the same across the entire weighing surface. To calibrate our scales on a daily basis, we weigh a 50g then a 1kg certified weight (they are stamped) in each corner of the scales bed, and then the centre. It's amazing how may scales won't hold up to this test.

 

Kitchen scales are pretty rubbish (as are digital bathroom scales - stand on one of those twice and see if you weight the same!). The scales we buy are "cheap" at £250 a pop and are accurate to 0.1g over a maximum of 1500g. This is actually quite a wide range - the bigger the range the less accuracy you tend to get.

[Don H.]

The balances in our labs typically have the same accuracy as yours, and would indeed be cheap at £250. (In fact, they have gotten a lot cheaper over the last twenty years.)

 

The US has a similar "minimum average variation" system for measures in products in trade, but from my experience, consumer-facing products like beverages are much, much, more accurate because consumers holler pretty quickly when bottle levels appear to vary.

[oldtuckunder]

Glad you're sorted, Alan OTU!

 

Alan R mentioned the need for end-to-end balancing. I've tried to do this, without success.

 

Here's the rig I used. I built a stand, on the lines of a lab.retort stand, with a side arm at an adjustable height.

The arm was a knife edge, so that the con rod would pivot on that and not slide, abolishing friction as a bias.

The knife edge bore on a "crown cap" a disc of aluminium, crimped around the edges to fit in either big or small end, so that the pivot was as close as possible to the centre.

 

My scales were probably very similar to yer drug-dealer's,

 

Ah, who said I had a drug dealer

 

I think for end to end balancing I'm just going to give in, the forged rods were so spot on dimensionally that I think they are closer than I will ever measure.

 

When it came to balancing the set, my rods came in (with bolts) at 546g each +/- 0.5g and as 0.5g is less than the weight of deposits on some pistons after a year I'm not even worried. By carefull matching of pistons to rods I came out with 5 of the 6 on 992g total the 6th was all in extra piston weight and about 10 mins of rubbing the the bottom of the skirt on wet and dry I ended up with all 6 on 992g i.e I just needed a smigen off one piston to get a fully matched set (or as close as I think makes no odds).

 

Alan

[stillp]

Perhaps a brand-name food or beverage be very, very close to target weight? Say a 0.5 L bottle of mineral water, poured into a tared container (or subtracted from a separately emptied and dried identical container?

As mentioned, probably not, but if you have some way of accurately measuring volume, you could use some deionised or distilled water, which should was 1kg per litre.

 

Pete

[oldtuckunder]

The most useful resource I found on this issue was

 

<<http://www.simetric.co.uk/si_water.htm#tap>>

 

which actually indicates that its about 999.9g at 4C

 

Alan

[TriumphV8]

 

Anyone any ideas how I can accurately weigh a con rod when I can't rely on the electronic scales.

 

 

This time the problem is bigger than it seems on first look.

 

Not only the total weight is important but also the dividing it up

into rotating and reciprocing weight.

Why? The rotating part goes full into the counterweight calculation,

the reciprocing weight, like the piston, only half.

 

I found that mistake on several sets on TR6 conrods which all had

a nice similar weight in limits but huge tolerances when split up.

 

I would take the higher reading and let the rod make due to that

and build up my own testbench to find out both weight parts of each rod.

I made filler pieces from plastic to center small- and bigend with centering hole

to accept a bearing from rc cars sitting on a shaft to guide the rod.

The other side gets a wire pickup that sits on the scale table.

 

It is worth that extra time spent to do so and dont forget piston, pin and rings.

[TriumphV8]

 

Alan R mentioned the need for end-to-end balancing. I've tried to do this, without success.

 

 

Hi John my hint to the picture would be to add an additional center for the small end.

Another small tube with small centering hole where a little wound wire plugs in

that stands on the scale. Wire & tube must be calculated before.

 

Letting the rod drop on the scale gives crazy results because each rod drops in

a little different position on the scale.

Hopefully on the big end side is a little roller bearing?

 

You are close, with that improvement it will work nicely.

[TriumphV8]

I have a lathe now - would proper discs and a chain suspension allow this method to make more repeatable measurements?

 

 

Sorry for answering step by step, but I read during breakfast and work.

 

The problem with the shown arrangement is

1.) not perfect diametre of tubes, they must be slide fit

2.) use rc cars bearings.

3.) it is all to heavy, your approach is better only needs the small end centering

4.) dividing the weights can be done by your weight of small end and the total weight

not needed to weight the big end individually, better weight the small end 10x and take

the medium of the closest 5!

[john.r.davies]

Thank you for your advice, TRV8

As I tried to explain, but maybe the picture didn't show clearly enough, I marked the scale's weigh bed to ensure that either end contacted it on the same point every time.

When weighing the big end, a similar 'crown cap' was used in the small end . See pic below

I don't understand at all your detailed advice, "Another small tube with small centering hole where a little wound wire plugs in

that stands on the scale. Wire & tube must be calculated before." Try explaining again, please?

My thinking on the knife edge was that it would abolish any friction bias, as there would be no movement. Even a roller bearing has friction. What do you think?

And I did weigh the rods so many times that statistical testing became useful. The 'medium' means nothing, statistically. The median, mean and mode are useful, but in fact there was a high standard deviation in my numbers, indicating random bias, and that any of those 'average' numbers would not be useful.

JOhn

[TriumphV8]

Just made a snapshot for you unfortunately dont

have the whole setup availiable.

The black tube has the bearing inside, I think the

result will be okay and the rod is always in the

rectangular postion needed for good result.

 

The aluminium tube slides in the small end and

the wire is a little stand that is put on the scale.

Before i add tube and wire to the small end I put both

on the scale and press "tara".

 

The other way round I dont do the weighting but find it

only by calculation.

 

Just from the differences measuring the same rod again

you will see that it is okay, even much better than the

stock rods from what I believe they are only brought

to same weight without this devided balancing ex factory.

 

Edited February 22, 2016 by TriumphV8

[Peter Cobbold]

You guys are getting your knickers twisted over a gram or two differences between cylinders in piston-conrod masses. A couple of grams pales into utter, meaningless, insignificance when looking at the force exerted on a piston by combusting gas.

Combustion pressure varies cycle-by-cycle by around 50% .This is very well know in all si engines and is an unvoidable effect of variations in turbulent flow around the spark kerneL. For example see fig 15 here:

http://web.itu.edu.tr/~sorusbay/SI/LN04.pdf

(or any engine textbook) Now, taking a peak pressure of 50Bar and a piston crown area of 9 sq inch we get a force on the piston of 6615 lbs which is 3 million grams. And that force will vary cycle-by-cycle from ballpark 1.5 to 4.5 million grams. And the differeces between cylinders can be even greater. A gram difference on a piston is not going to smoothe that engine out when its working hard. It might spin unloaded a bit smoother. But once its working hard -- not a iota of discernable improvement from worrying about a gram here or there on a piston conrod assy.

 

The piston conrod mass does incur inertial forces as it rotates. As a torque these are smal fraction of gas pressure torque:

compare fig 2, 3 and 4

http://www.diracdelta.co.uk/science/source/e/n/engine%20excitation%20mechanisms/source.html

quote: "The total torque is found by summing these two components. Note that the torque from gas pressure dominates..."

 

Peter

Edited February 22, 2016 by Peter Cobbold

[TriumphV8]

If I had to pay for balancing I would think about if it is necessary in every case.

As this modification is just my work to invest I do the best I can, having

in mind that it makes no sense having the crank rebalanced and spoil that

with different weight of rod and piston.

 

As I prepared several engines that I drove before the overhaul I can say despite

every theory the engine runs much smoother when rebalanced if it was not

perfect before. The Rover engines were prone to be real eye shaker sometimes.

 

There was a rumour that earlier Rover engines are rebalanced if felt nasty after being

mounted together. They are running for testing and Rover had equipment to rebalance

where weights were added at the pulley. As we can not talk about bigger amounts there

because there is simply no space this in my opinion is the proof that minor imbalance

can make a driver really unhappy.

 

WIth no doubt perfect balancing becomes more important with higher revs. TR6 was

not made to meet the 7000 on the tacho and from that every help for the crank

should be given if engine is modified.

[john.r.davies]

Thank you, TRV8! The larger, black disc goes into the big end, with a rod to fit in the bearing fixed (sliding for height adjustment?) on a stand on the table?

Most nicely done! I shall have to think if I can do that too.

 

Peter,

I can't do the calculations, and do not wish to dispute you, but to ask you about a difference in few grams being accelerated from 15 m/s to zero and back again, every 12 milliseconds (25 L at 5K).

You know more than most about torsional vibration, and though I know so much less, I'm concerned to keep the variation along the crank as small as possible.

Or do I misunderstand your point?

 

John

Edited February 22, 2016 by john.r.davies