It should be. He states it so often.WBahn,
Is the NISTs opening line going to be a tag in your signature line?
The value of a quantity is its magnitude expressed as the product of a number and a unit, and the number multiplying the unit is the numerical value of the quantity expressed in that unit.
If I ever set up a sig, it will almost certainly be something like this. After being a member of the forum for just a few weeks, I was really thinking I should have made by user name "UnitsNazi".WBahn,
Is the NISTs opening line going to be a tag in your signature line?
The value of a quantity is its magnitude expressed as the product of a number and a unit, and the number multiplying the unit is the numerical value of the quantity expressed in that unit.
Shouldn't that be 8.93 kW?...
P = 38A * 235V = 8930W = 8.39kW
That would be my call. I was taught in electronics exam calcs if no precision is specified to always use 3 decimal places as a classroom standard.Ramussons said:...
Maybe the answer is wrong because of Decimal Place Accracy
... Should have been 8.93 KW instead of 8.9 KW
Sure. Hopefully after my eye surgery on Thursday I will be able to actually see what I've typed!Shouldn't that be 8.93 kW?
If you are going to be the forum "units nazi" can I be the "dyslexia nazi"?
The general rule is to use three sig figs if the proper number of sig figs can't be readily determined. You should never use two or less or more than four unless you have good reason. For reporting purposes, a common rule when I was starting out was to not count a leading 1 as one of the three sig figs. Arguably, a leading 2 or even 3 shouldn't be counted, but I'm not aware of anyone asserting that.That would be my call. I was taught in electronics exam calcs if no precision is specified to always use 3 decimal places as a classroom standard.
If specifically asked to report the answer in a given unit, then technically the units have been provided and the answer would just be the numerical portion, as providing the units in the answer would effectively square the units. But I would personally accept the answer if the units were provided and would argue with an examiner that didn't that they were being unreasonable.If the examiner specifically asked for the answer in kW, the two correct answers would be 8.930 and 8.930kW.
Yes....
But using "3 decimal places" doesn't work. Take this problem, for instance. Would someone have to put
P = 8390.000 W
if reporting the answer in watts?
Again, yes. Results to 3 decimal places were required in exams....Or put
P = 0.008 MW
if reporting the answer in megawatts?
I think you are wrong there. English semantics allows for some redundancy where it would be logical or "common sense"....
If specifically asked to report the answer in a given unit, then technically the units have been provided and the answer would just be the numerical portion, as providing the units in the answer would effectively square the units.
...
Nice work, thanks! I haven't seen that in many years....
The proper way -- but seldom worth the effort -- is to do a propagation of errors.
...
Then that makes it actually pretty meaningless. I think we can agree that the level to which a quantity is known is independent of the prefix one attaches to the unit when the quantity is reported.Again, yes. Results to 3 decimal places were required in exams.
I never said that it was "3 decimal places" or nothing. That's a false dichotemy. In fact I specifically recommended three significant figures when lacking a good reason to do otherwise, and I think that the three sig fig rule is far superior to a blind 3 decimal places rule. The primary thing is that it maintains the level of reported accuracy independent of the prefix or even units that are used.General rules are not always perfect. "3 decimal places" is a perfectly good standard for many cases, and is better than no standard.
Three points:Take for example this situation;
R = 2v / 3A = 0.666 (repeating) ohms
Going with a standard of 3 decimal places gives a very reasonable 0.667 ohm(assuming rounding was done).
Going with the initial precision of <1 significant digit gives you two fairly unreasonable answers of 0 ohm or 1 ohm.
The boundaries between scientific rigor and prose semantics is blurry, to be sure. I don't think there is a concrete right or wrong answer on this one. Which is why I indicated that I would not only accept either, but I would take issue with someone that didn't.I think you are wrong there. English semantics allows for some redundancy where it would be logical or "common sense".
It would be nice to know.It will be interesting to see what the "error" in the O.P's question actually was, but I'm pretty sure we already found it.
Remember a number of years ago the Mar's Climate Orbiter crashed because "....the flight system software on the Mars Climate Orbiter was written to take thrust instructions using the metric unit newtons (N), while the software on the ground that generated those instructions used the Imperial measure pound-force (lbf). This error has since been known as the "metric mixup" and has been carefully avoided in all missions since by NASA..." (from Wikipedia).Consider this analogy:
.......................
Me personally? I'm very happy with the answer of 459.029 mm....
Something is measured as being 0.502 yards. So one person reports that as being 1.506 feet, another person reports it as being 18.072 inches, and another person reports it as being 459.029 millimeters.
Now, you only see the 459.029 millimaters. Are you going to really believe that this measurement of something, whatever it is, that is nearly half a meter long is really known to within a micron? Or are you going to question the legitimacy of reporting a result to that many sig figs?
...
I think the point he's trying to make is that the significant digits imply the precision of the measurement. I can take my my yard stick, which has graduations of 1/4" and take a rough measurement of a piece of wood to be 1.5ft. Let's say this piece of wood is from a mock-up of something that my team and I are planning to build out of metal, but is still a work in progress. I provide the measurement to someone else who casually asks for it, and they convert it to 457.200mm and provide figure that to the person who does the CAD work. So the CAD guy sees 457.200mm, which implies a high degree of accuracy from a precision tool, and he specifies in a tolerance to the machinists of 457.200mm ± .05mm, which is a very tight tolerance and hard to machine. They waste a bunch of time and money machining this super tight tolerance, and in the end, it's off by a full 1/8" because of the precision of the original measuring device.Me personally? I'm very happy with the answer of 459.029 mm.
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