Author Topic: UK back to "imperial" measurements ?  (Read 15256 times)

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Online IanB

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Re: UK back to "imperial" measurements ?
« Reply #150 on: June 06, 2022, 12:18:41 am »
I explained the legal meaning of a 5 lb bag of rice in a post above (reply 141).

I'm not sure that you did. You described a prescribed process for checking the accuracy of a point of sale weighing device to within a certain accepted tolerance by government inspectors.

If we look to a regulatory source, namely "NIST Handbook 130: Uniform Laws and Regulations in the areas of legal metrology and fuel quality (2022)" we find something more precise. Firstly, we find throughout the document the statement:

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When used in this law (or regulation), the term “weight” means “mass.”

This note appears following every use of the term weight in the regulations. It is supported by some explanatory text:

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The mass of an object is a measure of the object’s inertial property, or the amount of matter it contains. The weight of an object is a measure of the force exerted on the object by gravity, or the force needed to support it. The pull of gravity on the earth gives an object a downward acceleration of about 9.8 m/s2. In trade and commerce and everyday use, the term “weight” is often used as a synonym for “mass.” The “net mass” or “net weight” declared on a label indicates that the package contains a specific amount of commodity exclusive of wrapping materials.

Secondly, we find a definition of the pound mass avoirdupois in Appendix A:

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1 pound = 453.592 37 g exactly

If the amount of a pound depended on local gravity, it could not be given exactly to 8 s.f. like this.

Therefore the legal meaning of a 5 lb bag of rice is that it contains 2267.96185 g of rice, or sufficiently close to comply with the applicable regulations and measurement standards.

In careful scientific and engineering usage, the kg is an SI unit of mass, the N is an SI unit of force or weight, and the lb av is a customary unit of force or weight.

As a careful scientist and engineer, I disagree with the latter part of this. The lb (mass) is a unit of mass, and the lb (force) is a unit of force. Where the context is not clear, these are sometimes abbreviated as lbm and lbf .
 

Online coppice

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Re: UK back to "imperial" measurements ?
« Reply #151 on: June 06, 2022, 12:34:06 am »
Kg was never used as a force. Force is N (newtons)
You weren't there man. :) Before the Newton was added to the SI units, we used Kg (often written as Kgf, for Kilo equivalent of force) as a unit of force. Our physics text books changed during my time at school.
 

Offline TimFox

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Re: UK back to "imperial" measurements ?
« Reply #152 on: June 06, 2022, 01:02:24 am »
Re:  IanB
Your discussion about the difference between a "pound mass" and "pound force" is consistent with my discussion of using lb av at the grocery store to weigh food.
However, here is an example of my reasoning not to use lbm:
Imagine a spacecraft whose mass is 100 lbm.  We then apply a force of, say, 10 lbf to accelerate it in a desired direction.
Applying Newton's second law straight out of the bottle, F = m A will give the wrong result, unless we throw another factor gN, the defined standard acceleration of gravity.
In careful US usage, one should use pounds of force and slugs of mass, just as in careful metric usage one uses Newtons of force and kg of mass.

I remember in high-school physics class (11th grade), an important initial hurdle for the students was to understand the difference between mass and weight.  The instruction was in both customary and metric units, and I learned some important factors for mental calculation, such as 88 ft/sec = 60 mph (exactly).
 

Offline themadhippy

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Re: UK back to "imperial" measurements ?
« Reply #153 on: June 06, 2022, 01:19:58 am »
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If we look to a regulatory source, namely "NIST Handbook 130: Uniform Laws and Regulations in the areas of legal metrology and fuel quality (2022)"
But only applicable to america,if i was served 0.473177 liters in my liquid pint glass id be questioning the parentage of the bartender
 

Offline vk6zgo

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Re: UK back to "imperial" measurements ?
« Reply #154 on: June 06, 2022, 01:37:28 am »
Skeggy OMG. Too many wasted childhood hours. I know a bloke who lives in the middle another town of Lincoln county - Scunthorpe.

Dorset will see your Scunthorpe, & raise you "Shitterton!"
 

Offline vk6zgo

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Re: UK back to "imperial" measurements ?
« Reply #155 on: June 06, 2022, 01:42:23 am »
Kg was never used as a force. Force is N (newtons)

You have never seen a pressure gauge calibrated in kg/cm2 or kgf/cm2?
Do they only export those gauges to the US?

That’s a pressure not a force. And it should be pascals.

As it is in Oz!
 

Online IanB

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Re: UK back to "imperial" measurements ?
« Reply #156 on: June 06, 2022, 02:10:27 am »
Re:  IanB
Your discussion about the difference between a "pound mass" and "pound force" is consistent with my discussion of using lb av at the grocery store to weigh food.
However, here is an example of my reasoning not to use lbm:
Imagine a spacecraft whose mass is 100 lbm.  We then apply a force of, say, 10 lbf to accelerate it in a desired direction.
Applying Newton's second law straight out of the bottle, F = m A will give the wrong result, unless we throw another factor gN, the defined standard acceleration of gravity.
In careful US usage, one should use pounds of force and slugs of mass, just as in careful metric usage one uses Newtons of force and kg of mass.

I remember in high-school physics class (11th grade), an important initial hurdle for the students was to understand the difference between mass and weight.  The instruction was in both customary and metric units, and I learned some important factors for mental calculation, such as 88 ft/sec = 60 mph (exactly).

Indeed. Lecture 1, day 1 of my engineering degree was introducing the subject of units of measure, and the correct use thereof.

Until that point, my high school curriculum had been taught exclusively in SI units. Imagine our surprise when this conversion factor gc started appearing in all sorts of equations, and how important it was to remember it when venturing off the SI island.

Here is the famous Bernoulli equation as described in a standard industry reference:



We may observe how the mysterious factor "g" appears in some terms. And what the heck is that 144 doing there?  ???

Furthermore, what about that "foot pounds per pound" as unit of measure for head (loss)? Why don't the pounds cancel out to leave just feet? (Actually, they do, if you are a practicing engineer and you know what's going on.) The equivalent SI unit would be J/kg, and maybe that is equivalent to meters? (Yes, it is, as long as you remember the gc conversion factor when going from one to the other. See? You can't escape gc even in SI units  ;D )
 

Online IanB

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Re: UK back to "imperial" measurements ?
« Reply #157 on: June 06, 2022, 02:29:19 am »
Specifically:  when the local consumer-department inspector checks the setting on a sales scale, he places a known artifact on the scale to check the reading.  The marking on that calibration mass assumes a standard gravitational acceleration value (roughly that at sea level at 45o latitude) to know the actual weight at that standard location.  (NIST considers that acceleration value to be a defined constant, like 1 in = 2.54 cm exactly.)
However, he will use the same set of reference weights in Denver as in New York, even though a pound of steak transported to Denver weighs less at high altitude.

I realized afterwards, that this was in fact a statement designed to lead readers into a trap.

If a set of scales in New York is calibrated with a standard 1 kg weight, then when the scale reads 1 kg you will have 1 kg mass of your desired commodity on the scale.

If a different set of scales in Denver is calibrated with a standard 1 kg weight, then when the scale reads 1 kg you will also have 1 kg mass of your desired commodity on the scale. Any difference in local gravity is canceled out by the calibration.

Now, if you calibrated the scale in New York, and then transported the scale (not the steak) to Denver, then you might have a problem. But that was not the premise.

The key point here is that a 1 kg calibration weight has a mass of 1 kg, and that mass will remain the same everywhere in the world. If you use it to calibrate a scale, then the scale will read mass accurately at the location where calibrated.
 

Offline TimFox

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Re: UK back to "imperial" measurements ?
« Reply #158 on: June 06, 2022, 03:31:00 am »
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If we look to a regulatory source, namely "NIST Handbook 130: Uniform Laws and Regulations in the areas of legal metrology and fuel quality (2022)"
But only applicable to america,if i was served 0.473177 liters in my liquid pint glass id be questioning the parentage of the bartender

Why would your bartender serve you a US pint in the UK?
 

Offline themadhippy

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Re: UK back to "imperial" measurements ?
« Reply #159 on: June 06, 2022, 12:39:44 pm »
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Why would your bartender serve you a US pint in the UK?
because some one has told them the above book is regulatory  without adding "only in the usa" .
 

Offline TimFox

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Re: UK back to "imperial" measurements ?
« Reply #160 on: June 06, 2022, 01:20:49 pm »
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Why would your bartender serve you a US pint in the UK?
because some one has told them the above book is regulatory  without adding "only in the usa" .
Andhe didn't know that "NIST" is a US Government Agency?
 

Offline themadhippy

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Re: UK back to "imperial" measurements ?
« Reply #161 on: June 06, 2022, 02:04:35 pm »
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NIST" is a US Government Agency?
without searching do you know what NICEIC is?
 

Offline TimFox

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Re: UK back to "imperial" measurements ?
« Reply #162 on: June 06, 2022, 03:18:02 pm »
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NIST" is a US Government Agency?
without searching do you know what NICEIC is?
No, but I doubt it is a US agency.
 

Offline TimFox

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Re: UK back to "imperial" measurements ?
« Reply #163 on: June 06, 2022, 03:18:57 pm »
I didn't cheat, and looked it up after posting above, and, not to my surprise, it is a UK agency.
 


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