Dear Stan, John and All,
I have interspersed some remarks in red.
--- On Thu, 8/27/09, STANLEY DOORE <[email protected]> wrote:
From: STANLEY DOORE <[email protected]>
Subject: [USMA:45696] Wh/mile vs J/km
To: "U.S. Metric Association" <[email protected]>
Date: Thursday, August 27, 2009, 7:55 AM
Dear Metricators:
I like the greeting!
Use J/km rather than Wh/mi as some news media are beginning to
use.
I have added some thoughts about choice of units below.
Here's a chance for USMA and others to promote and use of the SI.
I agree.
What do you think?
Stan Doore
On 2009/08/28, at 00:02 , John M. Steele wrote:
I agree, although the appropriate prefix is probably kilojoule. The
prefixes in both numerator and denominator are redundant, but may
seem "more useful" to the average person.
At the time of creating a suitable unit (note singular) it is
appropriate to consider a number of issues. I can't think of any
reason why the word 'seem' should be considered at all. Better
considerations are questions like:
1 What is the best choice of an SI unit – in this case it is joules
per metre (symbol J/m)?
2 What unit gives whole number values that will fall within the
expected normal range of values – in this case it is the joule per
metre (J/m)? See: http://www.metricationmatters.com/docs/WholeNumberRule.pdf
3 Which unit will give values that will mostly fall within the range
0 to 9999 – in this case it is the joule per metre (J/m)?
4 What is the simplest possible SI (or metric) unit for this purpose
– in this case it is the joule per metre (J/m)?
My basis for the kilojoule. Approximate specs for the Volt are 8
kWh battery (28.8 MJ) and 40 miles (64 km) on battery alone. These
may be "best case" but they yield 450 kJ/km.
Or 450 joule per metre (J/m) could be used. As nobody has yet
developed a mindset for comparing previous values, there is
absolutely no need to try to comply with supposed mindsets such as
mindsets based on kilometres (especially in the USA).
Gasoline and diesel mileage may be similarly expressed. Using DoE
typical LHV heat values, gasoline consumption of 8L/100 km (close to
current CAFE) yields 2.6 MJ/km.
Or you could express this as 2600 joules per metre. (Unless, of
course, you are frightened of big numbers for some reason. It is my
observation and belief that most folk would prefer to think of 2600
dollars rather than 2.6 kilodollars!)
(yes I know LHV heat value is redundant; it looks naked without it)
To an engineer this may be true. But, for my poor agricultural mind,
I had to look up LHV on Google – I am now no wiser – for some reason
LHV values in calories don't do much for me!
Cheers,
Pat Naughtin
Author of the ebook, Metrication Leaders Guide, that you can obtain
from http://metricationmatters.com/MetricationLeadersGuideInfo.html
PO Box 305 Belmont 3216,
Geelong, Australia
Phone: 61 3 5241 2008
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