Improved Stove Efficiency Calculator/Spreadsheet

> when you guys get your worksheets finished, you will need to get them blessed by some thermodynamicist, and a metaphysician, and a part-time hanger-on.

Ben IS a thermodynamicist. I volunteer for part-time hanger-on. Who’s a metaphysician?

> and don’t forget to post it for ALL to enjoy and use.

Ken, the link to the spreadsheet is found in the first post of this thread. Enjoy it and use it! You will probably want to make the “.7 -> .789” modification discussed above, so your tests are accurate and in line with others. If you start running your own tests, have a visit to the ASEPT thread, where many of us are posting tests (or links to tests) using this and similar spreadsheets.

ASEPT Thread

I had to use excel on another computer to enlarge the boxes on the new spreadsheet so I could use it with open office. Once the correct perimeters were set the eff% matched up. There is a difference in the amount of fuel used to boil indicated on the 2 spreadsheets. The prior ss shows 13ml and the new version shows 10.2 ml. Any idea which is more accurate as I'm using this to figure the amount of fuel to carry. Thanks everyone for working tirelessly on this!

Galen – Is it possible the prior spreadsheet is showing the fuel used in ml while the new spreadsheet is reporting it in grams?

10.2/13 yields 0.785 which is remarkably close to the density (g/ml) of alcohol.

I appreciate you numbers people!

Sorry for posting this and running. It looks like many of the questions were worked out. I'll hit a few just in case:

-The errors people are getting with my new spreadsheet is most likely do to my lazily putting in 0.7 for the density. I didn't have the correct number on hand so just threw in 0.7. My note about density being used is if you have specified fuel in "ml" or "fl oz" or energy in "btu/gal" then the spreadsheet uses the number entered for density. If specify fuel in "g" or "oz" then the density entry is ignored.

-In terms of specifying atmospheric pressure and elevation… that is not needed. The boiling temperature of water changes solely due to changes in pressure. It so happens that the average atmospheric pressure changes in a known way with elevation. When you specify elevation you are only specifying the average atmospheric pressure for that elevation. If you know the atmospheric pressure then specifying the elevation is irrelevant.

-The usability of the pull down menu is a tough one. I can make them bigger such that you can see more of the options, but it will enlarge the whole input area. I was trying to make it nice and compact, but if its not usable, its not worth it. How many options would like to see? Three or four, or more?

-Fuel energy is a good question. Everyone is using the 25 kJ/g I originally specified. It was WAG and could use some refinement. I was trying to make an additional page for this spreadsheet for people to calculate their fuel's lower heating value (Fuel Energy). I just ran out of time. I will try to put together something next week.

"If you know the atmospheric pressure then specifying the elevation is irrelevant."

So where do we get the absolute atmospheric pressure at? All I'm aware of is local barometric pressure, which is calibrated to elevation. It's not absolute, which is simply demonstrated by comparing Galens 30 inches at over 5,000 feet, to my 30 inches at under 1,000 feet.

"So where do we get the absolute atmospheric pressure at? All I'm aware of is local barometric pressure, which is calibrated to elevation. It's not absolute, which is simply demonstrated by comparing Galens 30 inches at over 5,000 feet, to my 30 inches at under 1,000 feet."

That is the absolute pressure. It is not a relative pressure. If you are both boiling water at 30 in Hg your particular elevation is irrelevant.

If I go to weatherunderground for my favorite place in MN:

http://www.wunderground.com/cgi-bin/findweather/hdfForecast?query=ely%2C+mn

click "more conditions" it shows me a pressure of 30.08 in… that is the absolute pressure for Ely, MN right now. It does not matter in terms of boiling water that Ely, MN is at 1453 ft.

What is the difference you have referenced. Is this a measured difference or a calculated difference?

And Denver shows 29.35 to me. Tampa is 30.01.

Maybe this map would better describe what I'm referring to.
(won't allow links atm)
http://www.findlocalweather.com/forecast.php?config=&forecast=pass&pass=currentwx&country=northamerica&region=&useplace=&usestate=&plot=pres&period=&dpp=0

yup, every pressure on that map is the absolute pressure. International Falls, MN and Miami, FL is 30.03 in Hg. It does not matter what there elevations are… water will boil right now at the same temperature in both locations.

At 1,400 feet, water boils at around 209 degrees. At sea level it boils at 212. This is when the barometer reads 29.92 at both locations. That's what all the calibrations I've read about tell me. It seems very wrong for me to believe that a barometer set at sea level can even operate if brought up to say, 10,000 feet. The gauge doesn't even go that low.

"It seems very wrong for me to believe that a barometer set at sea level can even operate if brought up to say, 10,000 feet. The gauge doesn't even go that low."

Glenn, if you look at the graph that I furnished, you will see that the normal air pressure at 10,000 feet is going to be along the lines of 70% of sea level pressure.

So, somehow it seems that you are confused about something.

–B.G.–

I sure am. If the air pressure changes so drastically with elevation (which it does), then someone explain to me why the barometers still read the same.

Let me try this example. Over on Galens tests, he has reported pressures between 29.83 and 30.17. A normal range of average weather. He is at 5,297 feet of elevation and his boil temps are 202 degrees. He can only even reach 203 degrees and sustain that temperature. He reported that himself.

Now for me, at 900 feet of elevation, when MY barometer reads those same readings of ~30, my water is nowhere near ready to boil at 202 degrees (well, ok, it's close, but I reach 211 easily). Obviously it's due to my lower elevation, yet my barometer is reading the same as Galens.

So if barometers are ABSOLUTE pressures, then someone explain this phenomena to me please.

Barometric pressure as reported in weather reports and as measured by barometers are relative pressures not absolute pressures. Local atmospheric pressures are adjusted for elevation to reflect what the pressure at sea level would be. Clear as mud I’m sure.

Here’s a bit taken from Wikipedia that might give you some insight

The mean sea level pressure (MSLP) is the atmospheric pressure at sea level or (when measured at a given elevation on land) the station pressure reduced to sea level assuming that the temperature falls at a lapse rate of 6.5 K per km in the fictive layer of air between the station and sea level.

This is the atmospheric pressure normally given in weather reports on radio, television, and newspapers or on the Internet. When barometers in the home are set to match the local weather reports, they measure pressure reduced to sea level, not the actual local atmospheric pressure. See Altimeter (barometer vs. absolute).

The reduction to sea level means that the normal range of fluctuations in atmospheric pressure is the same for everyone. The pressures that are considered high pressure or low pressure do not depend on geographical location. This makes isobars on a weather map meaningful and useful tools.

Atmospheric Pressure

My electronic barometer reads absolute and cannot be adjusted to reflect sea level readings, a situation that can be a little confusing. Right now it reads 26.42 inches of Mercury. About 1/3 mile away at the same ~3100 feet of elevation above sea level there is an online weather station reporting a reading of 29.48, adjusted to reflect the much more common method of referencing to sea level. If I look at Bob's chart and another online calculator I see that 3100 feet represents about 90% of the pressure at sea level, so 0.90 X 29.48 = 26.53 which is pretty close to my barometer's reading of 26.42.

If altimeters always read 30 +/- a little bit then they would be useless as altitude indicators.

Rally cry "Let your barometers run free, unencumbered by sea level constraints, an artificial human construct!"

OK, Richard beat met to it but I will post anyway.

Which is why I've chosen to use elevation AND local (artificial) barometric readings to factor boil temperature. I've tried for 2 pages to get that point across, but have failed miserably. I guess I'd make a terrible politician lol

Glenn – you convinced me a few pages ago. I've found some ugly, but once coded up, nice equations that use Elevation and Relative Air Pressure to calculate, first Absolute Air Pressure and then Boiling Temperature.

In an Excel spreadsheet, if Elevation is in cell B1 and Relative Air Pressure is in cell B2, these two equations will get a pretty good estimate of the temperature of boiling water

Abs Air Press = B2-29.921*(1-(288.15/(288.15-0.0019812*B1))^((32.17405*28.9644)/(-0.0019812*8.9494598*10^4)))

Boiling Temperature = 49.161*LN(Abs Air Press)+44.932

Hope this helps somebody…..

Natural Log….Natural Log….it's coming back….

….ooops…it went up in smoke.

Wiki: "The natural logarithm of a number is its logarithm to the base e, where e is an irrational and transcendental constant approximately equal to 2.718281828."

…yea, that's it!

Interesting Richard.

The Antoine equation I've been using isn't perfect, and I've just done some cross checking with absolute pressures, and found it to be about a half a degree off in boil temps at higher elevations. Since I'm only using that equation to factor in mild variances, with elevation being the main player, it probably doesn't even register an error when used so minimally.

However if using absolute pressure, a more accurate formula would be a little nicer. So maybe this would fit the bill as a do-all formula. I'll have to play with those formulas when I get more time.

Glenn – I forgot to mention that those equations expect elevation in feet, relative air pressure in inHg and yield boiling temperature in Fahrenheit. These are the units I'm used to working in.

I would be interested in the latest version of the relative-barometer-adjusted spreadsheet, even if it only makes a decimal's difference for most calculations.

(I don't know of any way to obtain the absolute barometric pressure of my locale, other than to get the local pressure off a website, and then adjust it for elevation. Seems complicated.)

BTW, if y'all are looking for a local (adjusted) barometric lookup, I get mine from forecast.weather.gov

alright. You all convinced me… stupid weathermen. I'll get an updated version of the spreadsheet.

Thanks Ben!

"(I don't know of any way to obtain the absolute barometric pressure of my locale, other than to get the local pressure off a website, and then adjust it for elevation. Seems complicated.)"

Get your own barometer. Or, get an altimeter and work backwards.

–B.G.–

National Weather Service –
"STATION PRESSURE: This is the pressure that is observed at a specific elevation and is the true barometric pressure of a location. It is the pressure exerted by the atmosphere at a point as a result of gravity acting upon the "column" of air that lies directly above the point."

Edit: Wrong table was referenced. It was another "relative" data table. I removed the link

Here's a station pressure calculator
http://www.srh.noaa.gov/epz/?n=wxcalc_stationpressure