Kilogram Gets Controversial; Why Not Split the Difference? 520
gbrumfiel writes "As Slashdot has noted, the kilogram has a problem. The SI unit is officially defined as the weight of a 130-year-old platinum-iridium cylinder in France. But the physical object appears to be getting lighter. Scientists want to replace the cylinder with a new standard based on Planck's constant, but two experiments designed to facilitate the switch keep coming up with different results. Now one researcher is proposing a solution: just average the two diverging experiments and use that value as the official definition. Not everyone thinks that averaging the two amounts to sound research: 'Deciding to just average these two results would be perfectly proper mathematics, but it would not be science,' says Michael Hart, a physicist at the University of Manchester, UK."
Impossible (Score:5, Funny)
Physical objects interact... (Score:3)
...leaving traces. Over time, changes accumulate.
And when you are measuring something at 9 digits behind the point - a little can be a lot.
http://en.wikipedia.org/wiki/Kilogram#Stability_of_the_International_Prototype_Kilogram [wikipedia.org]
And there lies the rub... (Score:4, Insightful)
It is a definition with a physical representation. Which is obviously showing changes over time.
And it really shouldn't as it is the physical representation that is being actively used by our society - not the definition.
The point of this article is that they are trying to create a new definition based on a process that would produce an identical physical representation EVERY time the process is run - and the results of which wouldn't change over time.
And failing.
Now, as every scale in the world is NOT calibrated to that prototype kilogram, but to a copy, of a copy, of a copy... Those errors accumulate.
Until one day measurements of some toxic substance/medicinal drug/anything requiring milligram measurement start being significantly lighter/heavier than they should be in the given sample.
Cause we're not talking homeopathy here.
How it gets lighter (Score:5, Funny)
It turns out that France imposed a Mass Tax in the last few years which means the cylinder has to cough it up for the good of the state.
On the plus (or more like the non-plus) side, the people of France are now looking fit & trim.
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Re:How it gets lighter (Score:5, Funny)
So, the French govt had to run a weigh?
Re:How it gets lighter (Score:4, Interesting)
Ceci n'est pas une kg.
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Ceci n'est pas une kg.
However, that would be Belgian.
Re:Impossible (Score:4, Insightful)
What I *still* don't get is why we moved away from the ORIGINAL definition of a gram which used to be the mass of 1 cubic centimeter of water. I've heard all the "because this type of measurement was more accurate", etc. explanations but it seems that now they have no idea how to get to where they were whereas(AFAIK) the mass of 1 cubic centimeter of water hasn't really varied. Anyone able to break this down into something that actually makes sense beyond the typical responses?
Re:Impossible (Score:5, Informative)
Such a definition is ultimately circular. The volume of water depends on pressure, which itself has a mass component.
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Not dependent on pressure, temperature perhaps?
https://secure.wikimedia.org/wikipedia/en/wiki/Properties_of_water#Compressibility [wikimedia.org]
Regardless, the temp and pressure could be standardized...
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The first paragraph of your link describes how water is compressible, and not only that, how the compressibility changes with pressure.
You can't standardize pressure because to even define pressure you first have to define a kilogram. Circular.
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Better yet, from that same page: The triple point. All you need is pure H20 and you have a reference point for temperature and pressure. You could work backwards from there to the definition of mass.
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http://en.wikipedia.org/wiki/Ideal_gas_law
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yeah he's not talking about a gas though
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Pressure is measured in terms of force/area. In turn, force is a measure of mass and acceleration. Now we're back where we started and still haven't a clue.
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1 pascal (Pa) = 1 N/m^2 = 1 kg/(m*s^2). So you can't define 101.325 kPa without first defining the kilogram.
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Circular definitions aren't really a problem for anyone well-versed in algebra.
Re:Impossible (Score:4, Funny)
Then they should base it on the pound, which of course is 96 Roman drams, which of course is 96 * 32 / 25 Greek drachma, which is of course 96* 32 / 25 * 6 obols, which is of course 96 * 32 / 25 * 6 * 12 grains of barley.
Or maybe, since measurements were originally based on important items of trade, we should modernize that a bit and standardize based on a dozen iPhones.
Re:Impossible (Score:5, Funny)
And what happens to water in a vacuum?
It gets the bag wet.
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But you can't define 1 atmosphere without defining the kilogram first.
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But you can't define 1 atmosphere without defining the kilogram first.
You can define 1 atmosphere, you just can't quantify 1 atmosphere. You just have to do it based on real world conditions without assigning it a number. Those real world conditions will shift far more then the reference weight though.
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We've already assigned a quantification(as opposed to definition) to an atmosphere so we use 1 atmosphere(measured however we currently measure it that accurately) with the previously mentioned method involving 1 cubic centimeter of water and then we use that to RE-define the kilo/gram. Is that so hard? We're not trying to start from scratch here, we already know the ballpark that we have to aim for, we just need a method to get there.
What's worse is that you're assuming that the kilogram that we're trying
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Read my quote above about how we already know how to measure 1 atmosphere, and we likely don't use the current "standard" to do so, so we use what we have already defined as one atmosphere with the rest of the measurements to finish the other side of this equation.. In other words, I'm proposing we work backwards from what we already have to derive a proper standard.
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The mass of a cubic centimeter of water spans a relatively huge range, depending on the composition of the water and the circumstances under which it's measured. The composition of even distilled water varies, since both hydrogen and oxygen have a variety of isotopes, the ratios of which vary from one source of water to another.
If you boil this down to a theoretical, idealized system (e.g., using a composition of water that's impossible to reproduce), you might as well base it on something more stable, like
Re:Impossible (Score:4, Informative)
The physical object cannot get lighter (less massive). By definition is 1kg no matter how much mass it has.
Actually... it can get lighter. Earth's gravitational field can get weaker as matter from earth is ejected or evaporates into space.
It can also get lighter as Earth's atmosphere gets heavier, making it more buoyant in earth's atmosphere.
That has nothing to do with how much mass the cylinder has, because MASS is not a measure of weight.
Mass and weight are independent. Weight is due to forces applied to mass inside a gravitational field; if the field weakens or other forces are applied to the mass inside the field, the weight will decrease or increase without any change of mass.
Earth's gravitational field and atmosphere is also not uniform, so there are places (or altitudes) you can bring the same object to, and it will be lighter or heavier, with its amount of mass being the same.
Re:Impossible - Local G (Score:3)
A Local G [xkcd.com] effect. Pole vaulters be aware.
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Re:Impossible (Score:5, Interesting)
I think the GP's point was that even if you chopped a sizeable chunk off it, it would still weigh precisely 1kg. It logically follows that the universe's weight, expressed in kg, would suddenly jump upwards by a very large amount.
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Technically its mass would increase, not its weight.
Sorry to be so pedantic, but that is what this entire thread is about. =P
Re:Impossible (Score:5, Informative)
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You are assuming that the physical object is unchanging. It, however, gets cleaned periodically. That has been a long-troubling aspect of the standard that has received attention before, as cleaning, no matter how careful, undoubtedly removes more than just contamination.
They are removing dark matter from it.
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Reminds me of the deer that got away (Score:5, Funny)
A physicist, engineer and a statistician are out hunting. Suddenly, a deer appears 50 yards away.
The physicist does some basic ballistic calculations, assuming a vacuum, lifts his rifle to a specific angle, and shoots. The bullet lands 5 yards short.
The engineer adds a fudge factor for air resistance, lifts his rifle slightly higher, and shoots. The bullet lands 5 yards long.
The statistician yells "We got him!"
Re:Reminds me of the deer that got away (Score:5, Funny)
Re:Reminds me of the deer that got away (Score:5, Funny)
Not necessarily - everything is relative. For example, you have to also look at it from the deers frame of reference:
A deer is wandering through the forest. Suddenly, a physicist, engineer and a statistician appears 50 yards away holding guns.
The deer looks at them carefully and thinks - a physicist, an engineer and a statistician: I'd best just stand still.
Re:Reminds me of the deer that got away (Score:4, Funny)
A deer is wandering through the forest. Suddenly, a physicist, engineer and a statistician appears 50 yards away holding guns.
The deer thinks about this carefully for a moment. The likelihood that a physicist, engineer and statistician being able to form a cohesive group is unlikely. Forming a cohesive group in a forest, even less likely, and forming a cohesive group with guns involved practically improbable, as they most likely would have killed each other arguing over some pointless aspect or theory that the deer had worked out ages ago.
The deer comes to the realization that the only way that this situation could be real is if some other being had concocted it as a piece of fiction in some alternate universe. The deer sighs and holds perfectly still as a shot goes long, and another goes short, and the third guy never shoots his gun, claiming the dear has been shot. The deer shakes its head sadly for the poor being who created this temporary reality, and for itself as he will have to endure this ritual for as many times as other beings invoke it. Eventually the deer will be allowed to return the ethereal pool of creation, where perhaps its next incarnation will be something more interesting, perhaps as the man from Nantucket or the woman from New Zealand.
Black Cows in Scotland. (Score:3, Funny)
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Re:Reminds me of the deer that got away (Score:4, Informative)
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If a bullet hit the ground five yards past a deer, it went under his legs, not through him.
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(emoticon used to prevent pesky "insightful" mods)
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What if it was a very fat deer? : )
The physicist would be worried. If the deer were fat and made of neutron the bullet could take a hyperbolic course and come back and hit one of the party. The engineer would be worried - if they shot it how on earth would they drag it back to the car. The statistician would be looking around to see if he could find the very skinny deer that maintained the statistical average.
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Does it matter? (Score:3, Interesting)
The differences are so minimal that I can hardly believe it matters. The only issue is if the difference between the new definition and previous measurements is statistically significant. If you can't show that that would be the case, then pick whatever number between the two measurements that is easiest to work with mathematically, perhaps one with the most zeros (in decimal, since the metric system is designed to work well with powers of 10).
Re:Does it matter? (Score:5, Interesting)
Speaking as an experimental physiscist
ahem. 175parts per billion is 1.75e-7. For metrology that is a huge discrepancy. What is worse is that the measurements themself are a factor of 5 better, leaving no room for error.
For experiments where the physicists believe they understand them this is unacceptable, because it actually means the pysics of at least one method of both is not well enough understood, i.e. you have a systematic error. If the physics is not well understood then you don't know if the systematic error will be constant.
If the measurement will not be constant then the average will also not be constant. So an metrology institute where a reference weight should be define will need both methods and still not get any stable definition.
If you already need to afford both methods, then you can create reference weights and at the same time check if the difference between both methods is the right one and constant at your place.
Important rule in experimental physics: NEVER average over systematic mistakes. Average over random results. On systematic mistakes, the word average makes no sense
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Besides, if someone learns why, well, that's more knowledge in our bag o' tricks, eh? And that is what science is really all about.
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And that's the crux of the issue. Both results should be the same within the margin of error. The fact that they're not either indicates that the methodology is off or we simply don't unders
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Maybe there's something more to it. We already know that depending gravity isn't a constant in the universe and can be effected by outside forces. It may be that the cylinder's weight is a true constant, but do to whatever be it something is causing mass to weigh less compared to 130yrs ago or even 10yrs ago.
Personally I'd say it warrants more investigation beyond 'well lets weigh the average'.
Bread not working? (Score:4, Funny)
Let them eat pounds!
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Bread price in the UK can vary by shop you insensitive clod!
not the first time (Score:2)
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Well, duh. (Score:5, Funny)
Why don't they just take the weight of a gram and multiply it by 1024?
Re:Well, duh. (Score:5, Informative)
I think that would be a kibigram.
Re:Well, duh. (Score:5, Funny)
I think that would be a kibigram.
Don't let the industry fool you. They introduced that distinction so they can put less in a box and still sell it to you as 1kg of Mac and Cheese.
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"...sell it to you as 1kg of Mac and Cheese."
I prefer Linux and cheese. It's the cheesiest.
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I like to cut out the middleman and have a cheese GNUburger.
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Or take the weight of the Library of Congress and divide it by the air speed velocity of an unladen swallow...
Oh, right I forgot. An African swallow.
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I think it'd be a spiral.
finally (Score:2)
How much... (Score:2)
Is a kilogram in terms of fractions of an elephant please?
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African or Indian?
This is why science is so hard (Score:5, Interesting)
In this case the background is that the standard for mass, unlike time or distance, cannot independently be constructed in the lab. This means that science and industry are susceptible to two issues. The first is degradation of a physical standard, in this case a hunk of metal in France. The second is that one is dependent on other to create proxies of the standard, and as a result have no true assurance of the accuracy of the standard. A suitable lab with suitable personal can masure time and distance without the need of a proxy manufactured by others, and no dependence on a fixed physical object.. There is a desire for the same to be true for mass.
Second, no one knows if the hunk of metal is shrinking, and if it is how much it is shrinking by. If the experts knew it was shrinking, then they could figure out how to at least partially correct it. The hunk of metal might not be charging at all, or it could be accreating matter. Without an independent standard, which does not apparently exists, as everything is based on the hunk of metal, all there is is guesswork.
The third is the idea that Planck's Constant is being used to create the standard. In fact Planck's constant is one two approaches. The other is to create a sphere from a silicon and use Avagadro's Constant to define the mass. The problem is that these two approaches do no lead to consistant results, with an error about an order of magnitude large than the expected error.
The issue with averaging is that while one does average within a result, and even results that are taken from similar procedures, it is unclear that averages in this case is suitable. It seems to me that the results point to an interesting area of research, and rather than just averaging, more work should be done understanding the inconsistency. If it is not random error, and not an artifact, then something really fascinating might be going on.
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How do the determine the mass? (Score:2)
How do they determine the mass of their 1Kg reference?
Is it simply by measuring the force it exerts when influenced by a gravitational force of 1G?
If so, how do they measure to ensure that 1G is still the same acceleration that it was when the standard was introduced?
Do they also allow for the fact that it is displacing a certain amount of air -- and therefore is subject to the forces of buoyancy that will tend to make it lighter, depending on air density, humidity, etc?
While the predominant factor is the m
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Each national bureau of standards carts its standard kilogram over to France and compares it directly to the international standard kilogram with a balance. G is not involved.
Unfortunately, each time they do this they get slightly different results. The difference between the international standard and the average of the national standards is increasing.
UK? (Score:2)
The Curious Incident of the Dog in the Night-Time (Score:2)
Count it (Score:2)
Why don't they just count how many atoms are in it, and define the kilogram as the sum of the counts of each of the types of atoms making up the alloy?
Time is defined in a similar way, and don't tell me we don't have the technology, IBM has been shoving individual atoms around for decades now. They could do it again, and this time it would be for a cause more useful than making tiny IBM logos.
I'm not sure what the composition is, but I don't think there can be more than 6 * 10^24 atoms in it. Should be to
easy (Score:2)
Here's an easy reference: Planck units.
Just define everything in terms of Planck units and nothing will ever change.
Main problem (Score:2)
Metric System (Score:2, Funny)
I've always said this whole metric system has been a farce from the get go. First the unit of length is based on a fraction of the circumference of the earth, only measured WRONG, and now we get that the reference mass has been changing with time putting the amount of kippers in a kilogram in doubt.
I say we just scrap the whole thing and go to a more humanistic system based on things like the length of a man's stride etc. since obviously getting something accurate is just right out.
Re:Metric System (Score:5, Informative)
I'm not at all sure you are serious, but enough people seriously hold this opinion that it is worth responding.
A good system of units needs:
1) Base units which are well defined and independently reconstructible (i.e. a suitably equipped lab can calibrate their equipment purely from the definition of the units.)
2) Logically constructed compound units (e.g. units of force are derived from the units of mass, time and distance.)
3) Logically constructed convenience units (e.g. kilometres for use for distances which would be an inconveniently large number of metres.)
4) To be widely used.
The initial choice of your base units is largely arbitrary - whether it was a from a not-very-accurate measure of a king's foot size or from a not-very-accurate measure of the Earth's circumference. Item (1) can be satisfied equally well (or, in the case of mass, badly) by the metric or imperial systems. The definition of the metre has long since changed from the size of the Earth to quantities measurable in a lab (as has the definition of the foot.)
The SI system (based on metric measures) beats the imperial system hands down on items 2 and 3, and because of this now has a large advantage also on item 4.
Item 2: In Imperial you might measure (heat) energy in BTU and mechanical energy in some mixture of foot-pounds-seconds, but then you need a conversion factor to compare the two. Such conversion factors are never needed in SI.
Item 3: Imperial also messes up the convenience units by having lots of weird conversion factors (e.g. an acre is (I think) a furlong by a chain. How many square feet is that? How many ounces in a ton?*) Metric uses convenience units constructed from base units via consistently named factors of 10 or 1000.
You can't use the current problems with the kilogram as a reason to prefer imperial to metric, as imperial will be just as prone to exactly the same problems. The (UK) Imperial pound is similarly defined by the mass of a unique artifact. In the US, it is defined relative to the kilogram. Mass is the last base unit which doesn't satisfy requirement (1), and the efforts to fix this are what has triggered this entire debate.
One could go a step further, and define your fundamental units in terms of fundamental physical constants (i.e. the Plank mass, Plank time and Plank distance, charge on an electron, etc.) In such a system of units, the speed of light is 1, the formula for the energy of a photon doesn't need a constant in it etc. In practice, we can't use such a system, because we can't measure (in particular) the universal gravitational constant G with sufficient accuracy. Every time we got a better measure of G, our entire system of units would need to be updated. (I.e. with current technology, this system can't satisfy requirement (1) above.)
* And how many different sorts of ounces and tons are there? It is quite a few.
Spacetime (Score:4, Interesting)
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The whole point of changing a definition is to base it on something that's easier to measure to very high accuracy. We can't even measure the curvature caused by the Earth that accurately and even if we could, we don't know the mass of the Earth that accurately either.
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You're aware of how impractical that is, right? The first experiment to prove that had to be on the astronomical scale. Einstein's experiment was to examine how the light from stars was bent when viewed close to the Sun, and was only possible to perform during a total eclipse. To define something that requires those kinds of distances and masses to observe when the precision required is so high seems unwise.
The quality of instruments you'd need to determine how far a 1kg weight would bend a beam of photo
They call that math? (Score:5, Funny)
Math is: When there's this room... with only one person in it... and then two people leave that room... now you have to wait until another person goes back in before it's actually empty.
Kilo (Score:3)
Mathematically I was under the impression that one kilogram is what exactly one liter of water weighs.
Do not believe that the French developed the metric system for it is based on an ancient system of weights and measures based upon the time for Venus to move (transit) a particular distance across the sky. In those days a circle was divided into 366 degrees rather than 360 which matches the number of days in a year. The ancient clock system used then was more accurate than what we use today as well as the calender. Their system avoided the "leap year"
This technique developed thousands of years ago combines both the avoirdupois pound and the metric system and is based on what is referred to as a "Megalithic inch".
There is much substantiated already that ancient monuments such as Stonehenge were measured with an accuracy of 1/10000 of a millimetre.
For further information check Amazon for "Civilization One" by Christopher Knight and Alan Butler. A very interesting book, I am about half through this very enlightening book.
See what a pint, gallon, or bushel really is and how it was developed.
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Why does it sound like that researcher was looking for a quick answer just so he could get to the pub?
He probably wants to get started on measuring pints.
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This is NOT dimensional analysis, sorry.
See:
http://en.wikipedia.org/wiki/Buckingham_%CF%80_theorem [wikipedia.org]
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Re:Can't the kilogram be derived from other SI uni (Score:5, Insightful)
There are a ton of posts above arguing over that, and you can't use that to define mass because it is affected by pressure. Pressure has a mass component so it ultimately becomes circular.
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> Clearly I'm missing something here.
Yes. According to theory the experiments should agree. They don't. This means that either the theory is wrong (indicating exciting new physics) or that somebody is making a mistake ("systematic error"). In either case they can't tell which experiment is correct until the discrepancy is resolved.
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That's essentially what they are doing in one of the methods (except for using silicon instead of oxygen). Problem is that should, according to theory, give the same result as the watt balance method. It doesn't.
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The problem is that "how many atoms is a mol" is defined in terms of the kilogram.
One of the solutions proposed is to exactly figure out how many atoms are needed to weigh 1kg*atomic weight, thus defining the mol.
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How about working out the number of grams from one mol of Oxygen(?) atoms? If they all have an atomic mass of ~16 you know they're about 16 grams. :o
Not controllable enough.
How you make sure that all atoms are of the "standard isotope" type and you have the correct number of atoms? What "environ conditions" (Buoyancy? gas adsorbtion?) How do know the number of atoms in your "definition" sample doesn't vary over time? Even if pushing to the extreme: what temperature should it have/what predominant electronic state? (higher inner energy, higher mass).
This is where the definition based on the Watt balance [wikipedia.org] is, in my opinion, easier to control/reproduce.
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Averaging the results of two experimental measurements is not Science, whereas averaging the results of many hundreds of measurements to determine global temperature anomaly is.
Averaging the results of different experiments that consistently give different answers is not correct. Averaging multiple measurements of the same experiment is.
How about an analogy?
You have two rulers, which do not agree. If you measure something with both and then average the result, you get a wrong result - unless the two rulers happen to be out by the same amount in different directions. If one ruler is correct, you've just broken your result.
If you use one ruler then you can average the measureme