# [time-nuts] Mass vs BTU Function

J. Forster jfor at quik.com
Thu Jan 27 22:33:53 UTC 2011

```If you want the thermal mass to behave close to an isothermal body,
diffusivity is very important.

For example, a large mass of still water has high heat capacity, but poor
diffusivity. Much of the heat capacity is useless.

-John

==============

> At 04:18 PM 1/27/2011, J. Forster wrote...
>>If you are considering conductivity for dynamic reasons, the correct
>>figure of merit is "Thermal Diffusivity"
>>
>>= (Specific Heat) / (Thermal Conductivity)
>
> If you want a thermal mass to help control temperature swings, the more
> heat capacity is good. Isn't more thermal conductivity also desired? It
> seems like a substance with low conductivity wouldn't gather/release
> heat well.
>
> If more of both is desired, shouldn't the figure of merit should then
> be (specific heat * thermal conductivity), since you want more of both?
>
> In answer to the original question, which asked for heat capacity per
> volume. One need only multiply the specific heat by the density. For
> the examples given, plus iron and water:
>
> (substance) (specific heat) (density) (heat capacity?)
> ( ) (kJ/kg K) (g/ml^3)(kJ/l K)
> Al 0.91 2.7 2.5
> Cu 0.39 9.96 3.9
> Pb 0.13 11.36 1.5
> Fe 0.46 7.87 3.6
> H2O 4.2 1.0 4.2
>
> So, copper is best, but iron (steel shouldn't be much different) is
> pretty close and very much cheaper. Water is better and cheaper still,
> but can be a bit messy.
>
>

```