# Thermal Conductivity? Units On a Data Sheet

I've been trying to find the value of thermal resistivity for some linear disc resistors. I know that Thermal Resistivity is the reciprocal of Thermal Conductivity but the thermal conductivity on the data sheet is given in W/cm^2.C/cm which is the units for the heat transfer coefficient per cm not conductivity if the "period" is actually a multiplier and not just a period? Is this some convention I am unaware of on data sheets like this? I do have the thickness of the resistor disc in use so I think I can get the actual conductivity by multiplying the value given by the thickness squared?

They are missing parentheses, as noted by agentp, but it should be said that W/(cm^2*C)/cm is actually a relatively useful way of describing Thermal conductivity.

Let's re-format that as:

$$Conductivity =\frac{Power\ Transfer\ Rate}{ \left(\frac{Surface\ Area\ *\ Temperature\ Difference}{Thickness} \right)}=\frac{W}{ \left(\frac{{cm}^2 *\Delta °C}{cm} \right)}$$

The reason this is useful, is that it incorporates the values that you may already know about your system, in the units that you are likely to use them. Namely,

• Rate of Power Transfer through the material - ($W$)
• Surface area over which Power Transfer is occuring - ($cm^2$)
• Thickness through which Power Transfer is passing - ($cm$)
• Temperature difference between hot and cold sides of material - ($°C$)

Setting the units out as they have done means that you can intuitively input the values you have, combined with the published conductivity values, to find the Power Transfer Rate that you are most likely looking for

$$Power\ Transfer\ Rate = Conductivity*\left(\frac{Surface\ Area\ *\ Temperature\ Difference}{Thickness} \right)$$

It is clear that, for a fixed conductivity, doubling the surface area (dividing by a larger number) will mean that you will experience double the power transfer. Likewise, doubling the thickness will halve the power transfer.

This is slightly more difficult to see when using the SI units W/(m.K)

I suspect what is meant is W/(cm^2*C)/cm which reduces to W/(cm C) (I readily found examples of this by google..) Its not unusual for industry data sheets to play fast and loose with technical correctness. (Who needs those pesky parenthesis)

4 W/(m k) (more common unit) is a pretty plausible order of magnitude number for the material they describe.