If a person placed two bathroom scales* side by side and stood with a foot on each one, would their weight be correctly calculated by adding together the weight displayed by each scale?
It seems intuitive that they would, but I have a sneaking suspicion the physics aren’t that simple and straightforward.
Wouldn’t some of the weight be lost as a sideways load that the scales (pushing them apart) aren’t built to detect? Could some portion of your weight be duplicated between the scales (i.e. both detecting the same x pounds in, for example, your butt)?
the spring-loaded dial type that were popular before modern digital scales
The answer is yes. This is how extremely heavy people sometimes get weighed. Also, I’m pretty sure electronic scales actually have multiple sensors because the sensors are small and you’d have to balance on one in the middle.
But I guess it would be possible to arrive at an incorrect measurement with a wide stance where the feet are adding sideways pressure that’s also picked up. I don’t see how you could measure the same weight twice unless the scales are on top of each other.
No, any sideways load isn’t contributing to keeping you up. What could happen is, as iljitsch writes, that the scales might not be built to detect only purely vertical loads and will add some of the sideways load to the reading.
No. This is simple statics. The forces on your body, here gravity pulling you down and the scales keeping you from moving downwards, must sum to zero, or you’re accelerating or deforming.
Theoretically, it should work. On old spring scales, it works. Digital scales, it kind of works, but you’re not going to get a very accurate number. Both scales have trouble locking in on a number as the weight distribution shifts ever so slightly back and forth. Using a single scale, the scale can account for this variation across the surface and calculate the weight, but two scales at once can’t get their act together.
I do it when I have to, but I always note that the weight is an estimate. I much prefer a scale with a wide weighing platform for my really large patients.
You could put a board across the two scales.
Put your two scales next to each other separated by about a half inch or so. Measure the distance from edge to edge of the two scales and get a 2x8 that long.
When you need it, you put down your two scales, put the board across them, then turn on the scales (in this order so it zeros with the board on it), then the patient can step on the rig.
With it set up like this he can stand in one place and there should be less shifting around. Of course, this presents the problem of a rather large step up. Especially for someone that may already have mobility issues.
The next thing would be for you to get a large platform scale.
Great idea, and if I ever needed a truly “bariatric” sized scale on a budget, in a fixed location, this is what I would do. True bariatric scales are hundreds of dollars - way out of my budget.
In my current situation as a home health nurse, it’s not practical, nor would my company be willing to assume the liability for a jerry rigged solution.
Ooooooh…pretty! I may actually see if the boss would spring for that one. I love the non-skid surface and the nice black color to provide a visual cue for people with low vision. (Can I rant for a moment about transparent glass scales? Old people can’t see them! They step on the edge and make the whole thing flip up and scare themselves because they think it’s going to break! Also…they break! Not easily, but when I carry a scale around every day to 6-8 different locations, sooner or later I’m gonna whack it on a door jamb and then “smash!”. At least safety glass breaks smooth. It’s not too terrible to clean out of my nursing bag. :smack: )
Ding ding ding! That’s the one I have. Only I got it in pukey boob cancer pink for $17. It’s a great little scale (except for the breaking against a door jamb part; at that price I can afford to replace it, though) and my patients really do like the wider platform. Even those who aren’t large appreciate being able to keep a wider, more stable stance on it. And it fits into my nursing bag (which is actually a laptop messenger style bag) like it was made for it.
If the OP is in the market for a wide platform scale, I’d recommend that one. Just don’t whack it on a door jamb.
Short answer, already pretty much given, is yes. This is used for trucks, other massive items that need to be weighed, and for balancing the suspension on racing cars.
I was going to say, we use three scales to weigh aircraft. One for each gear leg. This allows us to compute the weight and the balance of the aircraft. This is a very useful tool for the loader of the aircraft to use when they decide where to place items in or on the aircraft. The load has to be (balanced) within the designed specifications for the aircraft to be controllable. Pilots really like their aircraft to be controllable.
We have on occasion used four scales, but that is another story for another day.
BTW, many larger airplanes have built in scales. These usually measure the amount of bend on the axles of the landing gear. Once again, a very useful tool for the load-master. He/she has less math to do with this system. Less math means fewer mistakes. Fewer mistakes means safer flying, which I think is a good thing.
Scale platforms are often designed with three or more measuring points.
On most scales, these points are transducers called load cells.
The reason you may need more than one cell comes from the engineering reality that the platform needs to remain parallel with the (presumably normal to gravity) surface beneath the platform. The load cells themselves are typically a small fraction of the size of the platform above them.
As a rough rule, single load cell platforms will have two parallel plates with a load cell in between while three or more cell systems sit on the floor or another stable support structure.
If you have a large platform with a single load cell bolted to the middle (an I-beam-shape in profile), off center loads will twist the load cell and warp the plates that make up the parallel surfaces and make for inaccurate weighing relative to the same load applied in a single point immediately above the load cell. You can use use thicker plates to a certain point. You can then go to a truss, arm or other structure instead of a plate but this starts to get complicated and expensive and adds height.
Imagine trying to support a roof from a single point.
Up to a certain size, it will work ok like a patio umbrella.
However, as the roof surface and/or load increases, it will soon becomes clear that a three-or-more posted structure is easier and a more secure design.
As you can see, there’s a series of performance tradeoffs to be made between capacity, platform dimensions and profile height.
If you have a low capacity kitchen scale, the weights are small enough that a pair of plastic plates will work well enough.
Increase the load capacity and the platform size to something what you see on the deli counter for cold cuts and potato salad. This might have a stainless pan which sits on four posts which are the top of a tallish structure beneath but it is still a single cell. This is alright since you sort of expect a deli scale with a keypad and generous display and a printer to be somewhat tall.
Now imagine a $30 digital bathroom scale. This will probably have a load cell in each corner since people are relatively heavy, ensuring the surface the owner steps on remains parallel is difficult and the height to step onto the thing is expected to be really low.
With something like a pallet or truck scale, you can assume that the load will never be centered, a large platform (4’X4’ for starters) is desirable and the capacity’s at least a couple tons and maybe up to 100 tons. Looking back on the patio umbrella umbrella analogy, it is clear that multiple cell platforms are the way to go.
Lay a level slab, install the scale platform and you’re on your weigh.
You can mentally test the OP’s question by assuming that your scales are balance bars. The force on one side of the fulcrum would equal the force on the other side, regardless of whether there is one or multiple bars connecting them.
Any sideways force on one scale would have to be balanced by an equal and opposite sideways force on the other scale. This is simple statics. If they didn’t balance, you would be moving in one sideways direction or the other. Which you aren’t.
Bottom line, you can safely ignore any sideways forces, and simply sum the (downward) forces reflected on the two scales.
As a matter of engineering for the simple case of weighing a standing obese human whose legs will be within a few degrees of vertical and where the accuracy required is on the order of plus/minus half a percent, the fraction of sideways force captured by the load cells or whatever can be (judgment call) ignored. Precision will be well-preserved across the reasonable range of sideways forces possible without toppling the person being weighed.
I’m not certain the OP was talking about weighing obese people, but along the way the thread morphed pretty far in that direction.
aside:
Thanks to **jnglmassiv **& **48Willys **for some expert input.
What makes me wonder is my childhood memory of the reading on a bathroom scale changing if I crouched down, stood on my tiptoes, rocked forward our backward on my heels, or leaned heavily towards one side of the scale.
If a person is standing with on foot on each scale, they’re most likely going to be putting all* their weight on one side of the scale (the section closest to the other scale).
Oldfashioned bathroom scales were based on mechanics and a single spring. They were often only accurate if you were fairly evenly balanced in the middle of the scale, and leaning to any side would effect the mechanics so your true weight wasn’t transfered to the spring.
It’s a great little scale (except for the breaking against a door jamb part; at that price I can afford to replace it, though) and my patients really do like the wider platform. Even those who aren’t large appreciate being able to keep a wider, more stable stance on it. And it fits into my nursing bag (which is actually a laptop messenger style bag) like it was made for it.