40d:Specific heat

The SPEC_HEAT token

The SPEC_HEAT token controls a material's specific heat capacity.

Specific heat is not a temperature. It is, more or less, the amount of energy it takes to raise the temperature of the material from one temperature to another. Gold, for instance, has a very low specific heat, so it takes very little energy to raise its temperature by one degree. Water, with a very high specific heat, takes 32 times as much energy to raise its temperature by one degree. For more information, see [1] [2] [3].

SPEC_HEAT appears to be in units of J Kg^-1 K^-1 at STP; i.e. 1000 times the normally reported c_p in Joules per gram per degree Kelvin, or exactly the more rarely reported c_p in Joules per kilogram per degree Kelvin.

It is not understood why the game uses the °C or °K scale here, while using the °DF = °F + 9968 scale elsewhere.

Sample specific heats as used by Dwarf Fortress

Material	cp in J Kg-1 K-1	DF uses
Adamantine	n/a	7500
Aluminum	897	900
Copper	385	385
Carbon (Graphite) [*]	710	409
Carbon (Diamond) [*]	519	409
Gold	129.1	129
Iron, cast	≈500	450
Iron, pig	>500	500
Iron, pure	443	450
Iron, wrought	480-500	450
Nickel	444	444
Zinc	388	390
Alcohol, ethyl, anhydrous	2440	not in raws
Alcohol, ethyl, 100 proof	3856	not in raws
Beef, carcass, not frozen	2850	not in raws
Charcoal	≈ 1000	not in raws
Cheese, Cheddar, aged 60 weeks	3012	not in raws
Coke	850	not in raws
Glass, crystal	≈ 500	not in raws
Glass, silica	790	not in raws
Sand	835	not in raws
Silk (silkworm?)	≈ 1380	not in raws
Silk (hornet)	≈ 1500	not in raws
Silk (spider, viscid)	≈ 1400	not in raws
Water, liquid, at STP	4218	not in raws
Water, solid, at STP	2114	not in raws
Wood [†]	1700 to 2900	not in raws

[*]  Note the inconsistency in the Carbon values.
[†]  The specific heat of wood varies somewhat between different species. However, the variance is dominated by the moisture content of the wood. Moreover, wet wood behaves very differently below freezing. See [4]

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The default value for SPEC_HEAT is 409. However, default values are not established for the various creature_product_SPEC_HEAT tokens.

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[5] and [6] have tables with other materials’ specific heats.

References may report specific heat in Joules per gram per degree Celsius. Because Celcius and Kelvin use the same scale, differing only in their zero points, this is exactly the same as reporting in Joules per gram per degree Kelvin.  J/g°C is equal to J/g°K

Sometimes references will give specific heat in calories per gram Kelvin, or calories per kilogram Kelvin. To convert calories per gram Kelvin to DF units, multiply by 4184. To convert calories per kilogram Kelvin to DF units, multiply by 4.184.

Older references may give specific heat in B.T.U. per pound per degree Fahrenheit. 1 B.T.U. per pound per degree Fahrenheit is by definition equal to 1 calorie per gram per degree Kelvin. Btu/lb°F is equal to cal/g°K. So just multiply by 4184 to convert to DF units.

Sometimes references will report specific heat of liquids or gases in Joules (or calories) per mole per degree Kelvin. The symbol for this is C^p (with a capital C). This is less useful for our purposes, as you will need to know the molecular weight (in grams per mole) of the compound to convert it.

An example: the molar heat capacity of ethyl alcohol is 113 Joules per mole per degree Centigrade. The molecular weight of ethyl alcohol is 46.07 grams per mole. Divide 113 J/mol°C by 46.07 g/mol, resulting in 2.45 J/g°C. Multiply by 1000 g/Kg, resulting in 2450 J/Kg°C. That is our desired result in DF units.

Note that specific latent heat is a different concept; do not use those values for SPEC_HEAT.