Scale to weight "stuff".

[Puddlefish]

Laugh all you want. It is you that does not understand math. The 0.5 pound inaccuracy on a 30 pound weight might actually represent a greater degree of accuracy than what the 0.1 gram scale can offer. If you have a scale that measures at 0.5 pounds of inaccuracy, your scale is worthless (to me). It is easier to insure accuracy of a large object than it is of a small object. Anyone that has worked of a large object (say a ship or building) understands this. Laugh all you want. You are wrong. You might want to familiarize yourself with the concept of a percentage error. Adding a bunch of possibly inaccurate weighs only repeats an error many times. One way to insure the accuracy of the small scale is to compare the totals against an accurate large scale.

Any engineers out there want to tackle this one? I am off.....

Math major here. You are off, unless you have a horribly inaccurate small scale. If you have 100 items in your pack and each one is off exactly one gram in only one direction, you could then be off 100 grams, or 3.5 oz, or less than a 1/4 of a pound. Chances are, you don't have 100 items to weigh. Bundle your clothes in a bag, bundle your first aid/gear repair/etc together. You'll end up with less than 20 items to weigh.

Essentially you're fixating on nothing. Do you want to feed your OCD, or do you want to get an accurate weight. If you want an accurate weight, do the math. If you want to feed your OCD, go to a business with a calibrated scale, or buy and calibrate your own scale, only to find out that your tiny scale and simple addition is within a gram or two. It's a backpack with a small number of parts, not a ship with millions of parts.

Yeah, hanging scales are more fun than standing scales, just like everything tastes better on a stick. Not really, but it's fun to pretend.

[Slo-go'en]

Yeah, hanging scales are more fun than standing scales, just like everything tastes better on a stick. Not really, but it's fun to pretend.

The problem is much of our backpacking equipment is bulky and doesn't fit on the typical kitchen/postal/bathroom scale properly to get an accurate measurement. If the item overhangs the platform or is not centered well, it will not measure correctly. That's why a hanging fish scale is best for weighing the total contents of your pack, or things like sleeping bags, the empty pack, the tent, bags of clothes and so on.

[Traveler]

An inexpensive $8 hanging scale would likely be more practical than making trips to the post office, market, or pediatrician's office to weigh a pack and gear that isn't able to be weighed on a conventional platform scale. Especially at 9 pm when the wheels are turning and the itch to narrow weight hits. I would imagine calling the Postmaster to unlock the place at that hour would result in poor relationships.

[4eyedbuzzard]

Laugh all you want. It is you that does not understand math. The 0.5 pound inaccuracy on a 30 pound weight might actually represent a greater degree of accuracy than what the 0.1 gram scale can offer. If you have a scale that measures at 0.5 pounds of inaccuracy, your scale is worthless (to me). It is easier to insure accuracy of a large object than it is of a small object. Anyone that has worked of a large object (say a ship or building) understands this. Laugh all you want. You are wrong. You might want to familiarize yourself with the concept of a percentage error. Adding a bunch of possibly inaccurate weighs only repeats an error many times. One way to insure the accuracy of the small scale is to compare the totals against an accurate large scale.

Any engineers out there want to tackle this one? I am off.....

No, we do understand math. And mass and weight, and cumulative error, and . . . And even OCD.

From an engineering perspective, most digital bathroom scales have 4 significant digits. +/-.1lb is all they are capable of. Gram scales, likewise, only display 4 significant digits (three with small masses). Regardless of the scale, you are stuck with the number of significant digits. For an xx.xxlb pack, I believe one would be better off with a single measurement from a precision beam balance physicians scale than the sum of dozens of 4 figure measurements.

Math major here. You are off, unless you have a horribly inaccurate small scale. If you have 100 items in your pack and each one is off exactly one gram in only one direction, you could then be off 100 grams, or 3.5 oz, or less than a 1/4 of a pound. Chances are, you don't have 100 items to weigh. Bundle your clothes in a bag, bundle your first aid/gear repair/etc together. You'll end up with less than 20 items to weigh.

Essentially you're fixating on nothing. Do you want to feed your OCD, or do you want to get an accurate weight. If you want an accurate weight, do the math. If you want to feed your OCD, go to a business with a calibrated scale, or buy and calibrate your own scale, only to find out that your tiny scale and simple addition is within a gram or two. It's a backpack with a small number of parts, not a ship with millions of parts.

Yeah, hanging scales are more fun than standing scales, just like everything tastes better on a stick. Not really, but it's fun to pretend.

Okay. Lets start with the caveat that scales measure weight, not mass. Here's an explanation of the difference at https://en.wikipedia.org/wiki/Mass_versus_weight

All of the scales being discussed here measure weight (as explained in the link above, the force due to gravitational acceleration of a massive object), and are digital types relying on load cells. The typical real world accuracy of such devices is about .03%, or 3 grams over 10 kg range. Most are less accurate, with typical food scales around .05%, or 1 gram over 2.5 kg range. Virtually all work on the same principle, a voltage is applied to an electronic load cell which is deformed, which changes its resistance, which changes the voltage, which is then converted into a digital signal, which is then displayed. https://en.wikipedia.org/wiki/Strain_gauge

Regarding multiple weighings vs a single weighing:
Typically multiple weighings of items on a smaller scale with better accuracy and resolution will be more accurate than a single weighing of items combined. This is because load cells designed to withstand large loads without breaking or being permanently deformed cannot be designed to have the amount of deformation necessary to produce the high resolutions available in smaller scales. Dividing up larger loads into smaller groups for weighing purposes doesn't change their weight. It does increase the measurement accuracy. For example: Four separate weighings of items on a scale with a +/- .1 grams resolution results in a possible cumulative error of .8 grams - and it will always be more accurate than a single weighing on a scale with a +/- 1 gram resolution and a possible 2 gram error. Trust the math, not the machine.

But it's pretty much a pointless endeavor anyway, as there are so many other factors that affect measured weight to a MUCH greater degree.

Gravitational differences are one. The gravitational force acting on a mass varies by as much as 0.7% over the face of the earth. Most of this is due to latitude and centrifugal force, as both the earth and the object are moving/spinning. This force propels the object away from the earth which reduces the gravitational effect. This rotation also creates the Earth's equatorial bulge, which increases the distance of the object from the earth's center of mass. Altitude further reduces the gravitational effect due to increased distance between the object being weighed.

All told that pack that weighs 10 kg (22lbs) at the south pole weighs about 70 grams less on a mountain summit in Peru near the equator (0.7% difference). More realistic would be the 0.6% difference between places like Oslo and Mexico City. https://en.wikipedia.org/wiki/Gravity_of_Earth

Further compounding the issue are the effects of local density differences on the Earth, and even the effects of solar and lunar tidal forces.

http://www.calpoly.edu/~gthorncr/ME3...yofGravity.pdf

One must also consider the effects of moisture. Fabrics and insulation absorb and exude moisture as relative humidity changes. This would result in noticeable/measureable weight gains and losses with sleeping bags, clothing, etc. Then add the effect of actually wearing and sleeping in these items, which can really load them up with moisture. Even an ultralight sleeping bag can easily retain 100 grams or more of water weight in humid conditions.

In the final analysis, both environmental factors and gravity affect gear weight by a factor of 100 times more than scale accuracy. There really is no absolute measure of weight when it comes to gear. Weight changes due to variables like geographical location and environment. Even mass actually changes due to humidity.

And then there's the calibration issue. What standard did you use to calibrate the scale? How often is it checked? And so on . . .

What does your pack REALLY weigh? Well, where are you? And what's the weather like? Maybe it's not the answer an OCD hiker wants to hear. But it's the reality.

[Odd Man Out]

First what most are calling accuracy should really be called precision (more on that later). Second, what we are talking about is error propagation which is an application of statistics. I found a nice Web site with simplified formula and examples (link below). For the applications discussed here we can simplify even more.

First you have to get your head around the concept of uncertainty. Scientists are used to dealing with it but for "normal" people it is a challenge. Suppose you weigh your cook pot on your digital scale and it says 123 g. You must keep in mind that YOUR POT DOES NOT WEIGH 123 G! It is IMPOSSIBLE to measure the actual weight of your pot as all measurements have uncertainty. You can think of 123 g as YOUR BEST GUESS at how much it weighs, but how good is that guess?

First you need to determine if your scale is calibrated. For this you need an object whose mass is known to a high level of certainty (a standard), so you get your hands on a couple of 100 g standards. You put one on the scale and it reads 100 g. You then put the second one on and it read 200 g. Then you repeat this calibration several times and you always get the same readings over and over. The fact that you got the accepted mass of 100 and 200 tells us the scale is ACCURATE (i.e. gives the accepted value). The fact that you get the same answer every time tells us the scale is PRECISE (i.e. gives reproducible measurements).

To understand the difference between accuracy and precision consider these two alternative outcomes. If you weigh the 100 g standard 10 times and got 110 g every time, it would mean your scale is precise but not accurate. This can be fixed by calibration. But if you weigh the 100 g standard 5 times and got readings of 98, 99, 100, 101, and 102, this indicates the scale is accurate (the average weight =100 g) but not precise. This problem is harder to deal with but can be handled by averaging multiple measurements and calculating the standard deviation to determine the uncertainty of your measurements.

Let's assume your scale is accurate and precise (the pot always weighs 123 g and the standards always give the accepted value). Because your scale only reads to the nearest gram, you should assume the weight of the pot is somewhere between 122.5 g and 123.5 g and the uncertainty in all of your measurements is +/- 0.5 g. This is usually called the error, but I like the term uncertainty better since error suggests a mistake. Since all measurements have uncertainty, you have not made an “error” but are just reporting normal uncertainty. With your device it is impossible to come up with a better estimate so you just have to live with it (or buy a better scale).

Next you weigh the other parts of your cook kit (stove, pot stand, wind screen, and pot cozy). Along with the pot you have 5 separate measurements. You add these up to calculate the total weight of your kit to be 234 g. But remember rule #1. This is NOT the weight of your kit! It is your best guess. So how good is the guess?

To calculate the uncertainty in the sum of separate measurements, you take the square root of the sum of the squares of the individual errors. For example, the number 234 g is the sum of 5 measurements that each had an uncertainty of +/- 0.5. The square of the uncertainty is 0.25. Multiply this by 5 and then take the square root to get an uncertainty of 1.1. So your cook kit weighs 234 +/- 1.1 g.

You might notice that the error has increased. As pointed out in previous posts. This is because each measurement could be 0.5 g too low so your sum is 2.5 g too low. But the chance that each one is off by the same amount (and in the same direction) is very low. But it’s also unlikely that all the errors will offset. The formula (square root of the sum of the squares) gives the statistical way calculating error propagation. However, you shouldn’t be too concerned about the fact that the error has increased. The relative error for the pot weight (0.5/123*100) and the calculated kit weight (1.1/234*100) are not all that different (0.41% vs. 0.47%).

There are two steps you can take to minimize your uncertainty which are apparent from the math. First, note that you have a lower relative error when you measure a larger mass. If your alcohol stove only weighs 15 g, that measurement has a relative error of 0.5/15*100 or 3.3% compared to 0.41% for the 123 g pot. Also, adding together fewer measurements gives less error propagation based on the sum of the squares formula. So it would be better to combine objects so you have as few weights as possible. In our example, your scale has a capacity of 200 g so you can’t weigh everything together. Weighing all parts separately and adding them up increased our relative error to 0.47%. But if you weigh the pot (123 g) and then the other 4 parts together (111 g) and add these together, you get the same 234 g, but now the error is only +/- 0.71 g and your relative error has dropped to 0.30%

http://lectureonline.cl.msu.edu/~mmp/labs/error/e2.htm

[Odd Man Out]

...But it's pretty much a pointless endeavor anyway, as there are so many other factors that affect measured weight to a MUCH greater degree....

Very good buzzard. There actually is another one you missed. The scales we use measure the NET force on the object which is the force of gravity pulling down minus the buoyant force pushing up on the object because it is immersed in a fluid (air). The buoyant force is proportional to the mass of the displaced air which is equal to the volume displaced times the density of the air. So even if the mass and volume of the object is constant, the net weight will change as the density of air changes (due to temperature, pressure and humidity). But you have to be really OCD to worry about this one and I haven't calculated how much of an effect this might have.

[4eyedbuzzard]

Very good buzzard. There actually is another one you missed. The scales we use measure the NET force on the object which is the force of gravity pulling down minus the buoyant force pushing up on the object because it is immersed in a fluid (air). The buoyant force is proportional to the mass of the displaced air which is equal to the volume displaced times the density of the air. So even if the mass and volume of the object is constant, the net weight will change as the density of air changes (due to temperature, pressure and humidity). But you have to be really OCD to worry about this one and I haven't calculated how much of an effect this might have.

Yes, I was aware of buoyancy, but chose to omit it of what became a ridiculously long explanation anyway. Good explanation of uncertainty and precision BTW. BB's head will be swimming. [Sorry BB]

[johnnybgood]

No, we do understand math. And mass and weight, and cumulative error, and . . . And even OCD.





Okay. Lets start with the caveat that scales measure weight, not mass. Here's an explanation of the difference at https://en.wikipedia.org/wiki/Mass_versus_weight

All of the scales being discussed here measure weight (as explained in the link above, the force due to gravitational acceleration of a massive object), and are digital types relying on load cells. The typical real world accuracy of such devices is about .03%, or 3 grams over 10 kg range. Most are less accurate, with typical food scales around .05%, or 1 gram over 2.5 kg range. Virtually all work on the same principle, a voltage is applied to an electronic load cell which is deformed, which changes its resistance, which changes the voltage, which is then converted into a digital signal, which is then displayed. https://en.wikipedia.org/wiki/Strain_gauge

Regarding multiple weighings vs a single weighing:
Typically multiple weighings of items on a smaller scale with better accuracy and resolution will be more accurate than a single weighing of items combined. This is because load cells designed to withstand large loads without breaking or being permanently deformed cannot be designed to have the amount of deformation necessary to produce the high resolutions available in smaller scales. Dividing up larger loads into smaller groups for weighing purposes doesn't change their weight. It does increase the measurement accuracy. For example: Four separate weighings of items on a scale with a +/- .1 grams resolution results in a possible cumulative error of .8 grams - and it will always be more accurate than a single weighing on a scale with a +/- 1 gram resolution and a possible 2 gram error. Trust the math, not the machine.

But it's pretty much a pointless endeavor anyway, as there are so many other factors that affect measured weight to a MUCH greater degree.

Gravitational differences are one. The gravitational force acting on a mass varies by as much as 0.7% over the face of the earth. Most of this is due to latitude and centrifugal force, as both the earth and the object are moving/spinning. This force propels the object away from the earth which reduces the gravitational effect. This rotation also creates the Earth's equatorial bulge, which increases the distance of the object from the earth's center of mass. Altitude further reduces the gravitational effect due to increased distance between the object being weighed.

All told that pack that weighs 10 kg (22lbs) at the south pole weighs about 70 grams less on a mountain summit in Peru near the equator (0.7% difference). More realistic would be the 0.6% difference between places like Oslo and Mexico City. https://en.wikipedia.org/wiki/Gravity_of_Earth

Further compounding the issue are the effects of local density differences on the Earth, and even the effects of solar and lunar tidal forces.

http://www.calpoly.edu/~gthorncr/ME3...yofGravity.pdf

One must also consider the effects of moisture. Fabrics and insulation absorb and exude moisture as relative humidity changes. This would result in noticeable/measureable weight gains and losses with sleeping bags, clothing, etc. Then add the effect of actually wearing and sleeping in these items, which can really load them up with moisture. Even an ultralight sleeping bag can easily retain 100 grams or more of water weight in humid conditions.

In the final analysis, both environmental factors and gravity affect gear weight by a factor of 100 times more than scale accuracy. There really is no absolute measure of weight when it comes to gear. Weight changes due to variables like geographical location and environment. Even mass actually changes due to humidity.

And then there's the calibration issue. What standard did you use to calibrate the scale? How often is it checked? And so on . . .

What does your pack REALLY weigh? Well, where are you? And what's the weather like? Maybe it's not the answer an OCD hiker wants to hear. But it's the reality.

Yes, I was aware of buoyancy, but chose to omit it of what became a ridiculously long explanation anyway. Good explanation of uncertainty and precision BTW. BB's head will be swimming. [Sorry BB]

Good lawd ... my head is beginning to hurt.

[Slo-go'en]

As someone who used to design load cell calibration equipment (good to 1 part in a million) all I can say is your not weighing gold or cocaine so the run of the mill consumer scale is more then good enough. Precision is how many decimal points you take the measurement out to, accuracy is is how close to the ideal straight line those decimal points are. All kinds of things can affect accuracy. I really don't care if my pack weights 15.95435 pounds or 16.1290113 pounds, I'm going to call it 16 pounds.

[RockDoc]

Digital fish scale; inexpensive, fairly accurate, light and easy to use, lb or kg.

[cmoulder]

Order a digital fish scale on line or just go to Walmart and buy one in the sporting goods section for about 20 bucks.

^^^ What he said. Pick one.

Now, for accurately weighing smaller items and "subsystems" such as toiletry kit, repair kit, cook kit, fuel remaining in canisters etc, I use this DigiWeigh 1000. Up to 1 kg with 0.1g resolution.

[Traveler]

For the ultra advanced backpacker, there is always the issue of "how much does this shelter weigh with 8 people, their gear, and a dog wet from rain".

That'll keep a few very busy for an afternoon.....

[BirdBrain]

Let me try to simplify and then I will put on ignore. As much as I argue, I do not like to argue. It is a character flaw of mine. The only way I can cure it is to self police.

0.5 lbs off at 30 lbs is a 1/60th possible error. You could easily have double that error with a gram scale and not notice. We comfort ourselves with a the small error and ignore the possible percentage error. We see the large error (0.5lb) and ignore the percentage of that error.

I say again, weighing the whole is more accurate that weighing the parts. 0.5lbs deviation at 30lbs is likely more accurate than the gram scale. Adding errors produce larger errors. If you do not grasp that, do yourself a favor and never be the planner for building a large object. Large objects are measured in process as they are built. The most advanced ships on the planet are accurate to 0.5" at 500'. This accuracy is accomplished by measuring in process and tuning accuracy as it gets larger. The designer, engineer , and building understands this. Seems like some think I am stupid. My trouble is I was a math major that went off to build things.

No... my head is not swimming. I am insulted by the debate.... and that is my failing that I freely admit.

[BirdBrain]

There are two steps you can take to minimize your uncertainty which are apparent from the math. First, note that you have a lower relative error when you measure a larger mass. If your alcohol stove only weighs 15 g, that measurement has a relative error of 0.5/15*100 or 3.3% compared to 0.41% for the 123 g pot. Also, adding together fewer measurements gives less error propagation based on the sum of the squares formula. So it would be better to combine objects so you have as few weights as possible. In our example, your scale has a capacity of 200 g so you can’t weigh everything together. Weighing all parts separately and adding them up increased our relative error to 0.47%. But if you weigh the pot (123 g) and then the other 4 parts together (111 g) and add these together, you get the same 234 g, but now the error is only +/- 0.71 g and your relative error has dropped to 0.30%

http://lectureonline.cl.msu.edu/~mmp/labs/error/e2.htm

I would suggest some read rather than insult and pretend to understand. OMO is saying what I am saying (adding many factors that I did not want to. I defined my environment to discount many variables. Something a high school physics student should grasp). The larger object is measured more accurately than the sum of the inaccurately measured parts.

Math. Learn it before you insult.

[BirdBrain]

Double post.

Edited out.

Have fun guys.

To the OP. You are on the right path. Find a accurate hanging scale. It will produce better results for what you are trying to accomplish without the long drawn out exercise in futility to produce a total that is likely to be less accurate than a bathroom scale.

[jefals]

This won't work for the person that wants to have a scale to weigh a cloth at the store, but I took my loaded pack to REI and weighed on their hanging scale before my last adventure. I figured if anybody had one that I could trust, it would be REI.

[Gambit McCrae]

Don't think it was mentioned but walmart has a fishing scale with a hook on it. 5 bucks and works great. My pack is more or less the same weight every trip, so if its a lbs this way or that it doesn't bother me.

[Puddlefish]

I would suggest some read rather than insult and pretend to understand. OMO is saying what I am saying (adding many factors that I did not want to. I defined my environment to discount many variables. Something a high school physics student should grasp). The larger object is measured more accurately than the sum of the inaccurately measured parts.

Math. Learn it before you insult.

I didn't read a single insulting post in this thread. One could argue, that you telling people to learn math is insulting, if one were easily insulted. The math is pretty much fixed. Other than that, one has to have givens, and assumptions, and choose between certain engineering principles to decide which ones are applicable for the given purpose. Just because an engineering principle exists, it does not mean that it's pertinent to every application.

I'll put up my level of accuracy with an $11 factory calibrated Walmart oz. scale and the addition method against the accuracy of a far more expensive hanging scale of unknown accuracy. I am confident that my pack base will be somewhere around 15 lbs. I guess I could buy a $40 hanging scale and find out that my pack base will be somewhere around 15 lbs. If I had the $300 to spend on a high precision hanging scale, I'd still be better served putting that towards lighter/more efficient/effective gear/food/postage/etc.

The OP states they have OCD. It's not a kindness to enable OCD behavior, without at least suggesting why an alternative/cheaper method might be more suitable towards their needs.

[Heliotrope]

Ahhh gone are the old days when I had at best a rough estimate of my gear weights. Then I loaded in lots of food and often 2 gallons of water (no water filters). On top of all this went large format photography equipment and in my hand was a tripod. I stepped on a bathroom scale before I left home and hoped for the heaviest pack weight. Heavier was something to be proud of. Ultralight hikers were those weirdos that cut off their toothbrush handles.


Sent from my iPhone using Tapatalk

[ChuckT]

Perhaps some clarification is in order.
Saying OCD I am laughing at _my_ seemingly endless pursuit of what I'm toting and what it weights. There is no intention what so ever on my part to cause offense to anyone.
Apologies to everyone😯!