Measuring the Energy Out Put of a Tea Light




Introduction: Measuring the Energy Out Put of a Tea Light

About: hgv driver but only because it pays more than I can make otherwise

As a result of a previous instructable on the flower pot and tea light heater a comment was left that the output of any candle was only 1BTU (British thermal unit). I couldn't remember at the time just what a BTU was but I knew this was wrong as candles vary greatly in size & clearly can't all produce the same output.
The intention here is to prove the heat output of a tea light is greater than 1BTU

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

Step 1: A Bit of Research

Now I was born in the UK just before it went metric and did all my education after  it did so, I didn't ever learn what a BTU was I just know it is a unit of energy but not exactly how it is deffined, it is I discoverd this; The energy required to raise one pound of water  one degree farenhite. with I note no mention a time base  so it is just a unit of energy like the international system of units(SI units)  Joule(J), and as the joule thife  appears on instructables with great regularity I'm guessing most readers have heard of those and as I was educated using SIunits that is what I am going to use and do the conversions later (or at least that was the intention but for a happy accident which allowed me to take readings for both systems simultaneously from the same experiment)
It takes 4.181J to raise the temperature of 1gram(g) of water 1 degree kelvin (actually measured in centigrade but the actual unit is the same)
1 cubic centimeter (cc) of water  has a mass of 1g 1/1000th of a litre so is known as a millilitre (ml)
This is a pretty simple experiment heat a known quantity of water with a tea light and time how long it takes to get how hot then do some maths

Step 2: Equipment

A tea light a couple of drinks cans, a can cutter, a scribe, set of scales, thermometer, water

Step 3: My Can Cutter

Reusing drinks cans in instructables isn't uncomon score a line with a knife blade and split. My design of cutter holds a stanly knife blade in possition on a pice of 1/2" wooden board with 5 tacks/ brads and holds it down with a map pin. the desired hight of cut if over 1/2" from the base or top of the tin is achived by using shims made of hardboard cork sanding blocks and food tins etc

Step 4: Design and Build the Apparatus

This was a dynamic design process ( a fancy way of saying I made it up as I went along)
I needed a container to heat the water in so I cut the top off a drinks can in hind sight I didn't need to cut he top off I could have just used an empty can.
I needed a way to hold the water container over the tea light close enough to transfer most of the heat but not so close that the flame didn't burn properly.
Logically a carefully cut and streatched base of another drinks can with inlet and exhaust holes punched in it.
I needed to know how high to cut the second can fortunately the dimple in the base of a can just contained the case of the tea lights I was using, so by stacking 2 tea lights next to a drinks can ang lighting the top one I was able to establish how high the flame reached and a margin for the joint.
The top of the dimple in the base of the can was flattened by pushing it with my thumb the top was strathched by forcing a full can a little way into it  8 equally spaced holes were punched with the scribe at the bottom edge of the can and another 8 off set to them just bellow the streached section a dry run was then carried out by placing a lit tea light  on the flattened dimple of the bottom can and placing the empty water container on top. I discovered I had badly under estimated just how much air one tea light needs to burn I ended up increasing both rows of holes to about 1/4"(6mm) diameter . Yes educated in metric but most work shop tools are still imperial.

Step 5: The Experiment

The water container was placed on the scales and 200g of water added to it, this turned out to be 7Oz  (I hadn't done the conversions in advance I just picked 200g as an easy figure to use in the maths) so I decided as my thermometer had dual scales for centigrade and farenhite I may as well record both during the experiment.the filled container was placed over the lit tea light and the start temperatures noted (14c/57f). The termometer was used to gently stir the water between reading and the temperature was recorded every 5 minutes. I stopped after 25 minutes as my wife came home and asked me what I was mucking at.

Step 6: The Maths/Results

200g=7.05Oz so 7Oz near enough start 57f end 128f: 128-57=71
0.4375x71=31.06 BTU in 25 minutes 31.06/25 1.24 BTU/min

The SI units calculation ends up with a figure in Watts(w) which I think will make more sense to many people.
Start 14c end 53c 53-14=39
39x4.181=163.059J to produce this rise in 1g of water as we had 200g
200x163.059=32611.8J in 25min.
Watt = a joule per second W=J/s so total J divided by time in seconds gives Watts
32611.8/1500= 21.7W or about the same as a car stop light

Step 7: Conclusion and Improvements

Unless there is an accepted convention that a figure in BTU is automaticaly per second or per hour, and nothing I have read suggests this is so. I have shown that a tea light (or at least the ones I was using) release much more than 1 BTU.
      As an improvements had I weighed the tea light before and after I could have calculated the out put for a full burn. Also once over room temperature the water container had a much larger area radiating heat than it did gaining heat from the tea light flame, additionally the exhaust gases still had heat in them that was lost, directing these though the water via boiler tubes  and insulating the sides of the water container would be much better. As an alternative I have designed a water container wher the exhaust is passed aroundthe out side of the container whilst I have half built this it is definitely what my wife calls mucking.
   The 21W figure is I belive low because of the above stated losses this gives the 4 tea light and flower pot heater (which gave rise to the commentthat started this) a power of 84W whilst this is clearly never going to heat your house I did only suggest it might be able to keep a small green house frost free. As I have encountered instructables for food dehydrators that use a single light bulb (60w is now the largest you can buy in the UK) as the heat source it occurs to me that a modification of this heater could be used to power such a small cabinet dehydrator and maybe alsowith wood chips be a smoker. An achivement like that would get me off the mucking charge.

Step 8: Post Script

My 'improved ' equipment was little if any more efficient than the basic set up I used for this. Not that one experiment can actually be described as scientifically valid. I was able to run it for a lot longer, and the appears to confirm that after an initial quite rapid increase the surface area of the container that isn't in contact with the heat source radiates the heat to the environment almost quicker than the tea light can heat the water, after 90 minutes 200ml of water was still only at 93c. The wax in a tea light certainly contains the energy to boil water it just doesn't release it quick enough.

Scientific Method Contest

Participated in the
Scientific Method Contest

Be the First to Share


    • Trash to Treasure Contest

      Trash to Treasure Contest
    • Rope & String Speed Challenge

      Rope & String Speed Challenge
    • Wearables Contest

      Wearables Contest

    4 Discussions

    Bill WW
    Bill WW

    5 years ago on Introduction

    Nice work. You said:

    "Unless there is an accepted convention that a figure in BTU is automaticaly per second or per hour..."

    The BTU is a unit of energy, exactly as you defined it: the amount of heat required to raise one pound of water one degree Fahrenheit. However, there is an unfortunate, unofficial, convention mainly used in advertising for heating or air conditioning systems that BTU means BTU per hour. This is incorrect. But if you see an ad stating that an air conditioner is a 500 BTU unit, they mean 500 BTU per hour.


    Reply 5 years ago on Introduction

    Thanks I had a gut feeling it might be being misrepresented like that


    5 years ago on Introduction

    1 BTU = 1054 Joules. These are units of energy. Watts are units of power, or energy per unit of time. The British unit for power was the horsepower; 1 Hp = 0.7063 BTU /sec.= 745 Watts = 745 Joules / sec. If you are measuring the RATE of heat output from a candle, this is a measure of power and would be measured in horsepower or Watts. The TOTAL energy given out by the candle over a period of time is a measure of energy and would be expressed as BTU or Joules.

    Bill WW
    Bill WW

    Reply 5 years ago on Introduction

    Who invented the term "horsepower"?

    Answer: James Watt.