Science & the humble bathroom scale

[MrRadio]

Does anyone know any science that says the accuracy of bathroom scales can be affected by temperature, barometric pressure, phase of the moon or other naturally occurring factors?
Perhaps this thread should be in the damn fool question forum, but from my experience these premises seem to have some validity.
MrsRadio is the queen of weight watchers, not that she has any need to be, and we see wildly fluctuating readings on her scales, all three sets, as the factors vary.

[beer4life]

Does anyone know any science that says the accuracy of bathroom scales can be affected by temperature, barometric pressure, phase of the moon or other naturally occurring factors?
Perhaps this thread should be in the damn fool question forum, but from my experience these premises seem to have some validity.
MrsRadio is the queen of weight watchers, not that she has any need to be, and we see wildly fluctuating readings on her scales, all three sets, as the factors vary.

G'Day MrsRadio,
He wouldn't take any notice of me so,
The most reliable, accurate and repeatable scales are the Beam Balance.
You've probably seen the old style platform scales in the providores aka Rural Merchants.
In fact our local Hospital has one you stand on and another that you sit in.
I usually sneer at their bathroom scale when they are trying to measure intake and exhaust.

The Spring type used domestically are notoriously inaccurate, especially with temperature variations.
There are pressure cell types used for mobile operations by the RTA etc., but have never gone into their efficiency, probably quite good in the heavy vehicle range.

Kindest Regards, " The Druid ".

[beer4life]

I've always found this an interesting subject, particularly with centre of gravity limits for aircraft.
Once you have the empty data, you can ascertain The C of G, by adding the weight of fuel, passengers and baggage at various stations from a reference point.
It should be noted that laboratory scales are Beam Balance where you could probably weigh the footprint of a gnat accurately.

Because gravity varies by over 0.5% over the surface of the earth, the is relevant for accurate calibration of scales for commercial purposes. Usually the goal is to measure the mass of the sample rather than its force due to gravity at that particular location.
Traditional mechanical balance-beam scales intrinsically measured . But ordinary electronic scales intrinsically measure the between the sample and the earth, i.e. the of the sample, which varies with location. So such a scale has to be re-calibrated after installation, for that specific location, in order to obtain an accurate indication of mass.

Kindest Regards, " The Druid ".

[MrRadio]

I learned about C of G during my pilot training, I found it highly unusual that the accepted (72 Kg?) theoretical weight of a passenger is vastly different than the actual weight of most passengers therefore leaving C of G calculations wide open to devistating error.

[trash]

Oh this is a wonderful thread.
Hold onto your hat MrRadio, because we're going for a ride of general science.
The humble bathroom scales is now the center of the universe.
Are they affected by Temperature.
They sure are. As the length of the spring changes with temperature it effects the position of the dial. (well just assume it's the old style scales).
A good set of scales uses a differential system so that as one spring lengthens, so does another pulling in the opposite direction to counteract it.
Obviously variations are only small and effect the scales than changing the actual weight of the person.

Barometric Pressure. Yes, this does make a difference. Since the atmospheric pressure is like measuring the depth of atmosphere above you. The scales are measuring you and all of the air particles above you. If you were in a vacuum the scales would read slightly less, but it's a trivial amount.

Phase of the moon. No, this does not effect your weight, but the position of the moon (and the sun) does. The effect is only very tiny, but both the moon and the sun are pulling on you. If it is midnight, the sun and the earth are both pulling you down. (E+S) But if it is midday, then the sun is pulling you up.
(E-S). The moon is the same, but the position of the moon with respect to you changes with a period of about 23 hours. It's not a monthly period (with respect to the phase of the moon) but shorter than a day because the moon is also moving as well as the earth moving you with respect to the moon.
I hope that bit isn't too confusing.
If you wanted be really picky, you could try and consider Jupiter, Venus and Mars, and even the Galaxy itself. But these values are so tiny other things like the subjects heartbeat overwhelm them.

There are some even weirder effects. The gravity of the Earth itself changes.
Both from location to location and it changes in the same place over periods in time. Again, the effect is very very small, but enough to effect the calibration of some aircraft altimeters etc.

Solar Flares which cause a change in the earth's magnetosphere and again effects on material on Earth in tiny ways.

The Earth's rotation, which speeds up and slows down as well as the centrifugal force of it's rotation will always cause tiny variations in the measurements from place to place.

All of the above are trivial. They do not effect the subject's weight in any appreciable way. Validity... NO.

There are changes in the person themselves. The most notable factor is water. Drink a litre of water and you have put on 1kg. A day without a few drinks and a little bit of sweating on a hot day or a good piss and it's very easy to loose that kilogram. If I was to be accountable for a person's weight, I'd weigh them every 12 hours without exception.

Another factor is what we immediately eat and dump. Drop the kids off at the pool and there goes half a kilo or more if you were at an all you can eat buffet last night.

Clothes are another big contributing factor. Different cloths or shoes can make a noticeable difference.

OK... now I'm going to venture onto very dangerous ground.
Occams Theorem. The simplest answer is often the most correct (and the one which people least like to hear.)
I know you have said MrsRadio is not a fatty, so I'll just use the word generically with reference to my own family.

I have a few fatties in my family and I have come to the conclusion that there is a gene for delusional thinking. My sister has it, and I can see she got it from my parents. Apparently it's recessive in me, or that might be a self negating statement

Ok.. fatties are THE WORLDS BEST LIERS.
They are so good at it they have everybody convinced of their lies and at the top of that list is themselves.

Here is the alpha statement you will hear every fatty say;
"I don't know why I can't loose any weight ?"
Answer: Because the laws of thermodynamics don't apply to you.

My sister looses a kilo every week. Over the past year she has lost 52kg.
Problem is, it's the same kilogram each week
Get any of my family members near my set of scales and any weight loss magically disappears and suddenly that week my scales are wrong.

Human error or delusion at the biggest contributing factors.
If you have a couple of scales, set them all the same and keep them together. Then each day you can compare them with the same target to see if there is a variation. Control the scales.
If you can, try and tamper detect them. Somebody may be adjusting them without your knowledge.

If three scales are agreement with each other, then the scales are not suspect. If one scale is out from the other two, then it becomes the subject of interest.

[beer4life]

G'day Cobber,
I dips me lid.

ob·fus·cate , ob·fus·cat·ed, ob·fus·cat·ing, ob·fus·cates 1. To make so confused or opaque as to be difficult to perceive or understand: "A great effort was made . . . to obscure or obfuscate the truth" (Robert Conquest).

You never cease to amaze me.
Kindest Regards, " The Druid ".

[allover]

Was watching Aircrash Investigators the other night, the plane a Beechcraft (i think) appro 20 seater fell out of the sky, one reason was it was tail heavy. The pilots used the magical figure of 72 kg per person to calculate a total all up weight plus baggage and fuel. Total weight 1720 kg max load was 1770kg. When they did the investigation they found the ave weight for an American was determined before the second world war. now they reckon the ave American is 87 kg. Plus people now carry heavier baggage. The plane was more than 300 kg over weight
Now if you want to see where Aussies are headed just go to the mid west as idon't think the ave scales are going to be strong enoughhttp://www.austech.info/images/icons/icon10.gif

[Mgoldbe1]

Nothing short of Genius Trash...and well worded to boot...

[trash]

I'm taking the hang glider out tomorrow. I haven't flown it for a while.
87kg. Shit ! I'm fat and I'm going to lawn dart into the sand !

I can't be tail heavy, it don't got a tail !
With my harness and in flight snacks I figure I weigh in about 110kg.
The aircraft itself weighs less than 20kg !

I always figured the heavies were just so big that approximate positions for passengers were near enough. After all, If you think of just one hosty with a trolley moving from the center of gravity out 25 metres to the passengers in first class. That's a big leverage.
But then, with air rushing over the horizontal stabilisers at 800kph, it might only need a slight trim

[beer4life]

G'Day Cobbers,
Just to clarify one extremely important criteria.
All aircraft have a critical fore and aft limit for their C of G.
What this amounts to is that even though the total all up weight can be within limits, or even over slightly, if the C of G is outside the above limits, then you have a tail or nose heavy situation which is outside the control of the pilots, and if not just embarrassing, but disastrous.
My apologies for digressing, but if your misses is grossly overweight, make sure that she gets a seat approximately at one third of the Wing Chord, that is the nominal C of G. Warn the pilot before, if she goes aft to the ablutions.

Kindest Regards, " The Druid ".

[beer4life]

I'm taking the hang glider out tomorrow. I haven't flown it for a while.
87kg. Shit ! I'm fat and I'm going to lawn dart into the sand !

I can't be tail heavy, it don't got a tail !
With my harness and in flight snacks I figure I weigh in about 110kg.
The aircraft itself weighs less than 20kg !

I always figured the heavies were just so big that approximate positions for passengers were near enough. After all, If you think of just one hosty with a trolley moving from the center of gravity out 25 metres to the passengers in first class. That's a big leverage.
But then, with air rushing over the horizontal stabilisers at 800kph, it might only need a slight trim

Never underestimate the Laws of Aerodynamics.
As load increases on a wing, you must increase the angle of attack of the wing. There is a limit at approximately 14 degrees where the wing stalls and Bye Bye Birdie. This also applies to the increase load in maneuvers other than straight and level.

[gordon_s1942]

Another point you all seeming have missed is the position on the Bathroom Scales of the readout and your bending forward to look down at said reading inversly proportional to the visual obstruction of the so called 'Overhang' situated at the mid point.
Simply put when you stand on the scales and you try to read the scale, you tend to lean forward and the futher the overhang, the more your changing your Centre of gravity which on spring scales can be quite noticable.
I cheated the other day and used the electronic scales at the Vets, bugger said I havent dropped any.
Beyond as B4Life says the old Railway or Produce store balance beam type scales which are hard to find these days, I try to use the same coin in the slot machine at a local Pharmacy because hopefully they are tested regularly for accuracy.

[trash]

Never underestimate the Laws of Aerodynamics.

I never underestimate them. Occasionally I might forget about them, but that usually only ever happens once.

This weekend the wind too cross to provide usable lift along the ridge I was intending to fly. The rain has also moved in and that causes an even more interesting problem.
When the hang glider gets wet the little beads of water sitting on the leading edge and the top surface break up the laminar air flow. That dramatically reduces the lift and while it can still fly, it sinks like a stone.

Conclusion; I'm now wet and fat

[ssrattus]

what about those glass bathroom scales that are spring less and use strain gauges instead?

[Colin 2905]

... or a good piss and it's very easy to loose that kilogram.

Reminded me of that story from last year. Plenty of links when I did a Google search, but this one will do....

[trash]

The electronic strain gauges should also be effected, though I'm not exactly sure in what way. The electronics scales you will notice are zeroed each time you turn them on and before use. This is the first correction.
They generally only produce a current like any piezo device while they are changing shape. As you step on and off the scales, not as you stand on them. A simple bit of calculus measures how much area is under the current curve, di/dt and this is representative of the weight. A second different piezo device which would be expected to produce a different absolute current should give a proportional result confirming the correct weight.
If the temperature changes the piezeo device slightly, then this will be proportional on both sensors and can be factored out.

Of course the devices can be used in reverse. Place a static weight on a scale and measure any tiny differences. Now you have a gravitometer.

[Godzilla]

Tell Mrs Radio to exhale when shes weighting herself.

[hughdman]

One thing to note, I was once a daily weigher and noticed by weight could be up to 1.5kg different depending on the time of day.

My belt is now my scales.

[MrRadio]

Tell Mrs Radio to exhale when shes weighting herself.

She does .... It's called a sigh.

[trash]

Tell Mrs Radio to exhale when shes weighting herself.

She does .... It's called a sigh.

Mffffffft heheheheh.

I got busted by my 5th grade teacher for ruining her science experiment.
She showed us that air had weight by putting a balloon on a beam balance with a weight at the other end. The class was in awe until I asked her to let the air out of the balloon and repeat the experiment. I demonstrated that all she was weighing was the balloon.
The class laughed and I was on an express ticket to the principals office.

Fat bitch she was too. I bet she still thinks the laws of thermodynamics don't apply to her.