# Physical Units on Chart Axes

1. Oct 12, 2011

### pow216

Hello,

Just wondered what the correct way is to display physical units on chart axes.

Let's say for example thermal resistance of a heatsink extrusion is plotted as a function of length, and so the units for the y-axis are K.W-1

My 'A' Level physics teacher always insisted on (in order that the axes was dimensionless):-
/K.W-1
My previous boss insisted on round brackets
(K.W-1)
and I've seen square brackets in some scientific computing applications
[K.W-1]

Whats the consensus?

Thanks people.

2. Oct 12, 2011

### xts

I think the convention varies from country to country - I often see () in Russian texts.
I used to use [] in my texts (English, Polish, German) and never got it corrected by redactors.
I use () instead of [] if the natural language description is used rather than symbols, e.g.: (arbitrary units)

BTW: what is K.W-1??? Shouldn't it be kW-1 ?

Last edited: Oct 12, 2011
3. Oct 12, 2011

### pow216

So +1 for []

It is K.W-1 Temperature rise in Kelvin per unit power Watt

4. Oct 12, 2011

### xts

I would never use the dot, just a thinspace: $\big[{\rm K}\,\,{\rm W}^{-1}\big]$ rather than $\big[{\rm K}\cdot {\rm W}^{-1}\big]$ or $\big[{\rm KW}^{-1}\big]$

$\big[{\rm K}/{\rm W}\big]$ looks OK for me too.

5. Oct 12, 2011

### pow216

Interesting I have long been under the impression that dots were correct, have a look at #5 here:-

http://physics.nist.gov/cuu/Units/checklist.html" [Broken]

Last edited by a moderator: May 5, 2017
6. Oct 12, 2011

### xts

It says that both dots and thinspaces are equally valid.
As I noticed - the conventions may vary from country to country - the guide you cite is American one.
I learnt such conventions in Poland and Germany - here dots are rarely used, they smell for me with 'primary-school-pedantry'.

BTW - I see one more difference between conventions I used to and those recommended by NIST.
I always type $h=6.63\cdot 10^{-34}{\rm J}\,{\rm s}$, while NIST recommends to use cross: $h=6.63\times 10^{-34}{\rm J}\cdot {\rm s}$
That may be a cause while I don't like dots between units...
That would be a disaster: $h=6.63\cdot 10^{-34}{\rm kg}\cdot {\rm m}^{2}\cdot {\rm s}^{-1}$

Last edited: Oct 12, 2011
7. Oct 12, 2011

### pow216

Yes it does, agreed!

8. Oct 12, 2011

### sophiecentaur

The dimensionless option gets my vote.
The oblique ( / slash?) slash needs to be there because the label would, strictly, be W-1K but that could easily confuse a chap.
In (bog standard) algebra, one doesn't use a multiplication symbol between letters, so I should say that a dot is superfluous unless there is an actual 'dot product' involved..

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