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 Post subject: Calorie converted to temperature
PostPosted: Tue Feb 11, 2014 5:35 am 
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Is there a way to convert cal/cm2 to Fahrenheit? For example, If water has a temperature of 212 degrees fahrenheit, what would this convert to in calories?


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PostPosted: Tue Feb 11, 2014 7:01 am 
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Well, the number of calories would depend on the volume of water, the altitude where you are trying to heat the water, and the temperature of the water before you began heating it.

1 calorie is the amount of energy required to raise 1 gm of water 1 degree C at standard atmospheric pressure


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PostPosted: Tue Feb 11, 2014 7:11 am 
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A calorie is the energy required to raise one gram of water one degree Celsius at one atmosphere.

Source: [url='http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/1500-wp001_-en-e.pdf']http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/1500-wp001_-en-e.pdf[/url]

°C x 9/5 + 32 = °F and (°F - 32) x 5/9 = °C

So it depends what your baseline would be. I do not know why it would be interesting but I guess you could say:

if your 1 gram of water is at room temperature, let’s say 20°C (=68°F)

Then for some reason it becomes (212°F=) 100°C

That’s an delta T of 80°C that equals 80 calories.

I do not think the source is relevant but for the statement above to be true the source has to be correct, the increase in calories and temperature has to be linear and it talks about calories and not cal/cm2.

Usually it’s more interesting to know how much damage it does on skin (i.e. how much calories per square cm reaches the skin).

I do not know how to came to the conclusion that 1 cal/cm2 = 2nd degree burn on bare skin but 1 tells us something about the energy in calories over an area of a square cm and the other says something about how much energy in calories it takes to increase 1 gram off water with 1°C.

I don’t know why you want to know how to convert it?


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PostPosted: Tue Feb 11, 2014 10:45 am 
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cal/cm^2 is a measure of energy per unit of area. Temperature is a consequence of absorption of heat into an object as well as several properties of the object itself. Thus, cal/cm^2 and degrees F/C/K have nothing at all to do with each other.

Arc flash at least right now is looking primarily at radiant heat transfer. If we know the temperatures of two objects, then the heat flux in watts would be proportional to (T1-T2)^4. There are of course a lot of geometric concerns as well as surface properties (emissivities) to take into account in this. We can then calculate the total heat transferred by integrating this over time, which for arc flash is nearly constant so we get K*(T1-T2)^4*time, and since arc temperatures are relatively constant and the temperature of the target is usually orders of magnitude less than the temperature of the arc, it becomes essentially K*t. Note: temperatures of the arc and the object being radiated effectively don't matter when arc temperatures are around 4000-6000 K and object temperatures are typically around 300 K. Because of that 4th power factor, the arc temperature pretty much drives everything. Now this calculation so far would result in the calorie part of things. Then since the goal is to normalize the arc flash over an area, it is divided by the area to arrive at energy per unit area for units.

Now to say that temperature does not play a role is not true. It certainly does. However since different materials absorb heat at different rates (aluminum reflects heat, while dark black objects tend to absorb it better), and because the amount of heat over a period of time matters, and since the impact on human skin is not linear, temperature pretty much is not the way to look at things. Instead everything is normalized to a curve that was determined by Alicia Stoll back in the 1950's by actually testing live, human subjects to determine with a given amount of heat flux (watts per area), how much time is necessary to cause a burn. Alicia Stoll also measured the same heat source against a copper calorimeter so those are what is actually used as a reference today rather than using human test subjects.

While you can't really compare cal/cm^2 to a temperature, one way to compare them is that as I understand it holding a hand 1" over the top of a cheap plastic cigarette lighter for 1 second is closely equivalent to 1.2 cal/cm^2.


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PostPosted: Mon Feb 17, 2014 5:00 am 
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The reason I am asking this question is because of a statement in our energy isolation program. It states that a liquid temperature at or below 200 degrees is not considered hazardous. In the electrical field we try to protect our employees from the risk of a second degree burn which is 1.2 cal/cm2. I would think that water at 200 degrees on your skin would cause at least a 2nd degree burn and if it does, why do we not consider it hazardous? I was just curious at the relationship between the two. Thanks for the insight.


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PostPosted: Mon Feb 17, 2014 6:21 am 
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Warm water does not radiate as much heat as plasma :P so it is not dangerous as long as there is no direct contact with skin.[/color

[color=#000000]Bit off topic: But I guess I understand what you mean, you wonder how much energy (in cals/cm2) a drop of 200 degree Fahrenheit water transfers to skin before it is either


too cold to continue to damage the skin or

how much it transfers in let’s say X seconds because that is the average reaction time to wipe it off.

I haven’t searched on how to do this, nor is it part of my base knowledge. But what I do find interesting is the use of the word liquid, I would assume that 200 F water is less dangerous than 200 F oil or even glue.


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PostPosted: Mon Feb 17, 2014 6:45 am 
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[QUOTE=Getwired]The reason I am asking this question is because of a statement in our energy isolation program. It states that a liquid temperature at or below 200 degrees is not considered hazardous. In the electrical field we try to protect our employees from the risk of a second degree burn which is 1.2 cal/cm2. I would think that water at 200 degrees on your skin would cause at least a 2nd degree burn and if it does, why do we not consider it hazardous? I was just curious at the relationship between the two. Thanks for the insight.[/QUOTE]

It all boils down to how fast the energy is delivered. Indeed, 1.2 cal/cm2 is a threshold incident energy for a 2nd degree burn when delivered in one (1) second. Only a fraction of it is required to produce same damage when delivered in shorter time interval. Read this forum thread at http://arcflashforum.brainfiller.com/threads/evaluation-of-onset-to-second-degree-burn-energy-in-arc-flash.2221/ for more information.

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PostPosted: Mon Feb 17, 2014 3:25 pm 
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The above is all correct. 200 degree water is quite hazardous but the extent of injury would depend on how long it was on the skin surface and if it cooled. The temperature isn't the issue. Is it how much surface area and how much water for how long. Many companies have protection strategies for steam but if this water is 200 degree and under high pressure or in a large vat which could pour all over a worker this could be a hazard. Hopefully they have considered the above. Never hurts to question. That is how things get fixed. You can definitely get second degree burn from 200 degree water. That's why new home water heaters have a recommended max of 140F (1 s of this exposure will scald a child and older persons) and recommend 120F for a normal setting. Most will not set above 160F. Arc temperatures are very high but normally do not involve direct plasma contact (except in voltages greater than 1000V or in direct contact with energized parts). The hot convective gases are about 50% of the energy in our arc test and the Radiant heat is the other 50% (recent work on this has yet to be published but is forthcoming from our research).


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