I agree that excessive use of significant figures is inappropriate for
mental math when dealing with arcane units and trying to "see" them in
SI units. As long as the user is aware that "1 qt = 1 L" is a **rough**
rule of thumb, I have no problem with that. In fact, I use things like
that in familiarization briefings. When folks ask "How big is a liter?"
I show them one and I also say "Think quart; a liter is in that
ballpark." But I'm careful to make sure they don't take that as an
accurate conversion. As you and I know, conversions are tricky.
I share your frustration with the EIA. I wrote to them sometime ago
about their data showing railroad cars of coal, therms of gas, kilowatt
hours of electrical energy, BTUs of something else. I pointed out that
by not providing information on the diverse energy sources in SI units
they were making it impossible to compare the sectors of energy production.
Jim
Stan Jakuba wrote:
My comment - I stand by my expressing measurements that warrant barely two
significant digits with no more digits than that. In the world of energy
and power and the (ever changing) cost, two sigificant digits is enough
for such statistical data. Particularly if they are intended for public
understanding and memory-retaining, such as my table.
If you look at the EIA (Energy Information Administration) tables, you
see numbers
like 56.59 tons of CO2, 1,589.9 Billion Kilowatthours (sic), 1,364,697
million cubic feet. As another example, all liquid hydrocarbons contain
between 46 MJ/kg and 48 MJ/kg. Where 47 would obviously suffice for these
statistical purposes, there is 46.046, 46.871, etc., a far greater
precision
that there is the difference in the HHV from one shipment of the same stuff
to another. And if you think that this (1 acre = 4046.86 m² or 4046.87 m²)
is worth considering in the daily life, look at your deed; it says you
own X
ft² "more or less".
This excess of significant digits probably leads to similarly ridiculous
numbers in information published by those who use EIA data. I wrote to
EIA a
few times about the use of SI units and this digits pollution, but never
got
any replay on any subject. The only explanation I came up with - why anyone
would use 7 digits precision in numbers that have accuracy +/-10 % or
worse - is that their computers are programmed to sum up to 100 % with 7
significant digits. Or, as an example, the data collectors simply add
numbers from a kW producer with that of a GW producer not realizing that kW
is far less that the error in the GW number. Also, EIA as all American
agencies endlessly convert and thus even a common-sense-rounded number in
kWh becomes an overly precise number in Btu, kJ, MJ, etc., and vice versa.
SI prefixes allow us to express almost any value with just 3 digits. Let's
practice that advantage wherever appropriate. Unless you are a metrology
scientist or a banker, "appropriate" is most of the time.
Stan J.
----- Original Message ----- From: "James Frysinger"
<[EMAIL PROTECTED]>
To: "U.S. Metric Association" <[email protected]>
Cc: "U.S. Metric Association" <[email protected]>; "'SCC14 IEEE'"
<[EMAIL PROTECTED]>
Sent: 08 Mar 30, Sunday 13:47
Subject: [USMA:40717] Re: Electricity and Heat in SI
I see your point, Martin. It's a bit like saying a yard is a meter or a
quart is a liter. Maybe two (or three) digits would do it best. Folks
could
still round off for mental math and for a "feel" for the size of things,
but at least they would know that they are rounding off.
1 dry quart is about 1.1 L ("a bit more than a liter")
1 liquid quart is about 0.95 L ("a bit less than a liter")
1 yard is about 0.91 m
1 mile is about 1.6 km
1 BTU is about 1.1 kJ (or 1.06 kJ)
From my teaching (and other) experience I find that most folks handle
2 or
3 significant figures in decimal numbers (unless they can't handle
decimals at all), but past 3 significant figures they get nervous.
What do you think, Stan? Would that be a fair compromise between easy
mental math and avoiding the implication of absolute SI equivalence?
Would
that still suit your intended purpose for the table?
Jim
Martin Vlietstra wrote:
I did not look at the original e-mail. However looking that the e-mail
chain now, I feel that the relationship between Btu and therms and their
respective SI counterparts should be given to a few more decimal places,
otherwise the Btu and therm will appear to be “SI-friendly”.
1 Btu = 1.055 kJ
1 therm = 0.1055 GJ
------------------------------------------------------------------------
*From:* [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] *On
Behalf Of *Stan Jakuba
*Sent:* 30 March 2008 14:49
*To:* U.S. Metric Association
*Cc:* SCC14 IEEE
*Subject:* [USMA:40714] Re: Electricity and Heat in SI
It is taken for granted that participants in this forum can find
innumerous sources of "equalities" at a click on their computers, and in
the printed material published for generations. I had posted some
uncommon conversion factors such as 1 c/kWh = 2.8 $/GJ or 1 billion
kWh/y
= 0.114 GW with the minimum of significant digits. And although most
participants can derived such equalities themselves, my listing them was
intended to save time and standardize the prefix in each parameter and
circumstance.
I hope I will receive interesting and practical numbers for the table
soon. Comments that are off that topic, erroneous, oddly puzzling, or
merely arcane academic wanderings will of course be ignored.
Stan J.
PS: I agree that such criteria as $/GJ in fossil fuels change.
Nevertheless, so does life. The table has its value for comparison
purposes and needs updates as the EIA and all such tables do. The units
and prefixes stay.
----- Original Message -----
*From:* Ambler Thompson <mailto:[EMAIL PROTECTED]>
*To:* [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]> ; jU.S. Metric
Association <mailto:[email protected]>
*Cc:* SCC14 IEEE <mailto:[EMAIL PROTECTED]>
*Sent:* 08 Mar 27, Thursday 17:43
*Subject:* Re: [USMA:40689] Electricity and Heat in SI
Actually the tables are in SP811.
At 11:26 AM 3/27/2008 -0500, [EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]> wrote:
Stan,
You can find the "equalities" and more (from non-SI to SI) in NIST
SP 330. That is the direction of global movement. Ignore the
reverse equalities from SI to non-SI.
Gene.
---- Original message ----
>Date: Thu, 27 Mar 2008 11:53:29 -0400
>From: "Stan Jakuba" <[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>>
>Subject: [USMA:40689] Electricity and Heat in SI >To: "U.S. Metric
Association" <[email protected]
<mailto:[email protected]>>
>Cc: "SCC14 IEEE" <[EMAIL PROTECTED]>
>
> There are several people in this group knowledgeable
> about energy and power in electricity generation and
> consumption. I am attempting to fill the spaces in
> the attached table and need help. There are wide
> ranging numbers in the literature and on the
> Internet. I am looking for "ball park" figures that
> are in the middle of those ranges. The
> numbers should reflect commercial plants. With the
> solar ones, the plants should be in operation for
> several seasons to yield the year-averaged,
> net output (not name-plate numbers and not
> projections no matter how solidly based).
> > The performance and cost numbers are commonly shown
> in a plethora of units (e.g., in EIA as kWh and Btu,
> and worse). The attached table unifies the units on
> SI. To help you get the SI values, here are several
> conversion factors:
> 1 kWh = 3.6 MJ = 3500 Btu
> 1 c/kWh = 2.8 $/GJ
> 1 billion kWh/y = 1 million MWh/y = 0.0036 EJ/y
> = 0.114 GW
> 1 quad = 1 EJ
> 1 therm = 0.1 GJ
> 1 Btu = 1 kJ
> 1 acre = 4000 m²
> 1 mi² = 2.6 km²
> 1 gallon (US, liquid) = 3.8 dm³
> 1 gallon (Imperial) = 4.6 dm³
> 1 barrel (oil only) = 0.159 m³
> 1 ft³ = 0.028 m³
> A small household's el. consumption: 20 GJ/y = 0.6
> kW = 5000 kWh/y
> A small house total consumption, moderate climate:
> 150 GJ/y = 4.6 kW = 4000 kWh/y = 140 MBtu/y.
> For reference, the US total energy consumption is
> just over 100 EJ per year and this represents
> continuous average power of 3200 GW. Of that,
> electricity amounts to 15 EJ, equiv. of 480 GW.
> Solar insolation at the surface in the US is 200
> W/m².
> > This mailing is not intended to start a discussion
> about the pros and cons of energy sources. It is
> strictly about unified units and reference numerical
> values in them.
> Stan J.
> > >________________
>Electricity in SI.doc (61k bytes)
>________________
>Electricity in SI.pdf (39k bytes)
Dr. Ambler Thompson
NIST
International Legal Metrology
100 Bureau Drive, Stop 260
Building 222/A152
Gaithersburg, MD 20899
Tel: 301-975-2333
Fax:301-975-8091
[EMAIL PROTECTED]
--
James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(H) 931.657.3107
(C) 931.212.0267
--
James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(H) 931.657.3107
(C) 931.212.0267
