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16/04/2014 07:20:48

Author Topic: What is the "Wattage" of a Candle?  (Read 27961 times)

chris

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  • on: 20/01/2008 12:04:28
When the average candle burns, how does one calculate how much heat energy it is producing per weight of wax, and what is the equivalent light output, in watts, from an electric lightbulb?

Chris

another_someone

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  • Reply #1 on: 20/01/2008 13:55:04
It is not the wax that burns (generates energy) but the wick.  The wax is more of a moderator to ensure it does not burn too fast.

http://en.wikipedia.org/wiki/Candlepower
Quote
The term candlepower was originally defined in England by the Metropolitan Gas Act of 1860 as the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. Spermaceti is found in the head of Sperm Whales, and once was used to make high quality candles.

At this time the French standard of light was based upon the illumination from a Carcel Burner. The unit was defined as that illumination emanating from a lamp burning pure colza oil (obtained from the seed of the plant Brassica campestris) at a defined rate. It was accepted that ten Standard Candles were about equal to one Carcel burner.

In 1909 a meeting took place to come up with an international standard. It was attended by representatives of the Laboratoire Central de l’Electricité (France), the National Physical Laboratory (UK), the Bureau of Standards (United States) and the Physikalische Technische Reichsanstalt (Germany). The majority redefined the candle in term of an electric lamp with a carbon filament. The Germans, however, dissented and decided to use a definition equal to 9⁄10ths of the output of a Hefner lamp.

In 1921, the Commission Internationale de l’Eclairage (International Commission for Illumination, commonly referred to as the CIE) redefined the international candle again in terms of a carbon filament incandescent lamp.

In 1937, the international candle was redefined again against the luminous intensity of a blackbody at the freezing point of liquid platinum which was to be 58.9 international candles per square centimeter.

Since 1948 the term candlepower was replaced by the international unit (SI) known as the candela. One old candlepower unit is about 0.981 candela. Less scientifically, modern candlepower now equates directly (1:1) to the number of candelas -- an implicit increase from its old value.

http://en.wikipedia.org/wiki/Candela
Quote
If the source emits light uniformly in all directions, the flux can be found by multiplying the intensity by 4π: a uniform 1 candela source emits 12.6 lumens.

http://en.wikipedia.org/wiki/Incandescent_light_bulb
Quote
Comparison of efficiency by wattage (120 Volt lamps)
Power (W) Output (lm) Efficiency (lm/W)
40    500    12.5
65    1000    15.0
70    1100    15.7

So, a 70W US incandescent light bulb seems to give as much light output as about 100 standard high quality candles (assuming you can still get hold of the spermaceti).
« Last Edit: 20/01/2008 13:58:40 by another_someone »

Bored chemist

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  • Reply #2 on: 20/01/2008 14:08:54
"It is not the wax that burns (generates energy) but the wick.  The wax is more of a moderator to ensure it does not burn too fast."
Nonsense, just for a start there were oil lamps with asbestos wicks.
The fact that the standard includes the rate of burning of wax shows that to be the important aspect of the matter.

another_someone

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  • Reply #3 on: 20/01/2008 15:49:26
"It is not the wax that burns (generates energy) but the wick.  The wax is more of a moderator to ensure it does not burn too fast."
Nonsense, just for a start there were oil lamps with asbestos wicks.

Oil lamps burn oil, not wax (hence why they are oil lamps, and not wax lamps).

If candles were to burn wax rather than the wick, then why, when a candle has been used, are you left with a heap of resolidified molten wax, but a wick that is no more because it has burnt away?

chris

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  • Reply #4 on: 20/01/2008 16:07:38
The candle absolutely burns the wax, which is the fuel. The heat from the flame melts the wax, which is drawn up the wick and then vapourises. The vapour mixes with oxygen and burns.

But the original question, the wattage of a candle, was sent to me by someone and I wanted some numbers and your thoughts too, to make sure I was not missing anything. For your answer, above, I'm grateful.

Chris

Karen W.

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  • Reply #5 on: 20/01/2008 21:31:44
What is 12.6 lumens in watts?

another_someone

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  • Reply #6 on: 20/01/2008 22:29:00
What is 12.6 lumens in watts?

A lumen is a measure of perceived light falling on a given area.

Wattage is a measure of power.  In the calculation above, I looked at the amount of power required to generate the given amount of light using an incandescent light bulb radiating equally in all directions.  This makes assumptions about the inefficiencies of the incandescent light bulb (even with incandescent bulbs, higher rated bulbs were more efficient at emitting light for a given unit of power).  A different assumption for those inefficiencies (e.g. if one assumed a compact fluorescent light bulb, rather than an incandescent light bulb, and the correlation would be different).

Another assumption here is with the way that the human eye absorbs light (since it is perceived light intensity that is measured), so the amount of light for a given wattage would be perceived differently for different species of animal (and may even be slightly different for different individual human beings).

Another way of looking at the power is by looking at the power actually contained in light (this is often quoted for sunlight, since this figure is used to look at the potential power available for solar energy converters, but the figures are not usually adapted for human perception, which is not evenly distributed across the spectrum).

lyner

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  • Reply #7 on: 20/01/2008 23:00:50
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It is not the wax that burns (generates energy) but the wick.  The wax is more of a moderator to ensure it does not burn too fast.
Where did you get that idea from?
Why bother to use paraffin wax when you could just as easily use water if all you wanted to do was 'moderate' the flame'
Have you ever burned a piece of string? How many Joules of energy did it yield?
Try measuring the mass of meted residue and compare it with a fresh candle - you may notice a bit of a difference. Your assertion is rather like the Greek Philosophers', based on thought and not experiment or careful observation. That's not like you A-S.
Wax has about the same energy content as oil - it's just a convenient way to dispense it compared with a messy bottle and liquid fuel. The liquid in an oil lamp also needs to be vaporised before it can burn - just like the melted wax in the pool around the base of the wick.

another_someone

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  • Reply #8 on: 21/01/2008 00:20:48
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It is not the wax that burns (generates energy) but the wick.  The wax is more of a moderator to ensure it does not burn too fast.
Where did you get that idea from?

OK  :I :I :I I get the message.

Karen W.

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  • Reply #9 on: 21/01/2008 04:41:22
What is 12.6 lumens in watts?

A lumen is a measure of perceived light falling on a given area.

Wattage is a measure of power.  In the calculation above, I looked at the amount of power required to generate the given amount of light using an incandescent light bulb radiating equally in all directions.  This makes assumptions about the inefficiencies of the incandescent light bulb (even with incandescent bulbs, higher rated bulbs were more efficient at emitting light for a given unit of power).  A different assumption for those inefficiencies (e.g. if one assumed a compact fluorescent light bulb, rather than an incandescent light bulb, and the correlation would be different).

Another assumption here is with the way that the human eye absorbs light (since it is perceived light intensity that is measured), so the amount of light for a given wattage would be perceived differently for different species of animal (and may even be slightly different for different individual human beings).

Another way of looking at the power is by looking at the power actually contained in light (this is often quoted for sunlight, since this figure is used to look at the potential power available for solar energy converters, but the figures are not usually adapted for human perception, which is not evenly distributed across the spectrum).

Thanks George!

paul.fr

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  • Reply #10 on: 21/01/2008 08:27:31
So when a torch, for example, states that it has 1 million candle power, what exactly is that power and light output?

Bored chemist

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  • Reply #11 on: 21/01/2008 19:54:23
Here's a very crude wax candle burning quite nicely. The wick is made of fiber glass.
Any further questions?

chris

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  • Reply #12 on: 22/01/2008 11:57:13
Thanks Bored Chemist.

I'd still like to get some numbers on my original question though. We can work out how much light is coming from the candle, but what's its efficiency? How much heat does it produce versus the amount of light?

Chris

Bored chemist

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  • Reply #13 on: 22/01/2008 20:58:08
Since it's not very hot it will be less efficient than a tungsten lamp.
If you really want to know then find the rate at which the original "standard" candle burned spermacetti ie 12o grains per hour and convert that into kilograms per second
2.15996367 × 10-6 kg / s (How did people get by without Google?)
Then you need to know how much heat is generated by burning a kilogram of spermacetti.
Now that's a more tricky question but with my chemist's hat on I can tell you it will be pretty similar to the heat of combustion of a typical fat or oil.
A quick look at a bottle of cooking oil gives 3389KJ/100 ml and 91.6g of fat/100 ml so that's 37.0 KJ/g or 37MJ/Kg
Burning a gram of oil a second would give 37KW (yes, thirty seven kilowatts) of power but we are only using 2.15996367 × 10-6 kg / sec so it's about 80 Watts  of heat.

An 80W tungsten lamp would produce something like a hundred times as much light and even that would only be about 5% efficient, so a candle is about 0.05% efficient at producing light.
It's just as well that eyes are good.

daveshorts

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  • Reply #14 on: 23/01/2008 16:02:20
The IR camera I made recently sheds some light on this question

A candle in the visible

The same candle in the near IR, look at how much brighter it is than the room (which is lit by fluorescents) and hence how much energy is being wasted.
« Last Edit: 23/01/2008 16:05:38 by daveshorts »

techmind

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  • Reply #15 on: 23/01/2008 19:23:57
When the average candle burns, how does one calculate how much heat energy it is producing per weight of wax, and what is the equivalent light output, in watts, from an electric lightbulb?

Chris
As other posters have said, you can't really measure perceived light in "watts". Light is measured in lumens, which takes into account the spectral sensitivity of the eye.
A pure monochromatic green light at 555nm wavelength has an efficacy of 683lm/W, from the definition of the lumen.
A "good" white fluorescent tube has an efficacy of 100lm/W
A "compact fluorescent" is only about half as efficient, at 50lm/W
Ordinary household lamps rate at around 12-15lm/W

You could weigh a candle, burn it for an hour, then re-weigh it to find how "fast" it is burning. I'd go with bored chemists' suggestion of then looking up typical calorific value of wax (but be careful with food-technologists' factor of 1000 in their calories!). I believe Kaye and Laby (data book) is now online...

I'm quite certain the efficacy of a candle will be less than the 12lm/W of a tungsten bulb.

Apparently one "candlepower" (as in human-useful light output, not heat) is defined as 4xPI lumens, ie 12.56lm.
« Last Edit: 23/01/2008 19:25:38 by techmind »

chris

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  • Reply #16 on: 31/01/2008 21:51:02
Dear Bored Chemist and techmind,

thank you for the elegant answer to the question. We'll include it in the podcast.

opus

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  • Reply #17 on: 31/01/2008 22:07:43
So it is wax gas that burns in a candle, as well as the wick....?

chris

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  • Reply #18 on: 04/02/2008 08:38:14
Correct. The heat from the flame initially melts the wax allowing it to be drawn up the wick. Closer to the flame the higher temperature vapourises the wax, which mixes with oxygen and burns. The "space" left by the liquid leaving the wick as vapour allows fresh molten wax to move in to replace it. In some respects it's very similar to the way in which a tree pulls water from it's roots to the leaves where it evaporates.

Chris

opus

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  • Reply #19 on: 04/02/2008 19:14:47
Thanks Chris- so you could say you have both physical change (melting-reversible) and chemical change (burning-irreversible) in the same process.......?

Pumblechook

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  • Reply #20 on: 07/07/2008 20:52:25
I would think a typical candle is of the order the light ouput of a one Watt filiament bulb judging by how much it illuminates a room.....  Matter of milliwatts of light.   

 

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