Specific Heat Capacity - Thermistor?

Hi, long story short, I need to find the specific heat capacity of a thermistor to sub into the equation cmΔT, does anyone know what it is?

I'm aware different thermistors have different values but I just need a ball park. I've tried everywhere to find the value but it only gives me 'how to find the value' which is past my level of understanding atm.

Posted from TSR Mobile

Reply 1

Stonebridge

Original post by munchen102

You need to know what that thermistor is made of. There are a number of possible materials it could be. (Eg silicon, but not necessarily.)

Then search on Google for the sp. ht. capacity of that particular material.

If all else fails just look up the value for silicon.

Reply 2

uberteknik

Original post by munchen102

Manufacturers will not tell a value unless you ask them directly and even then you probably won't get an answer.

The normally quoted specification is the thermal time constant (TC) measured in seconds which is a direct ratio of the thermal capacity and inverse ratio to the dissipation coefficient.

This is the time taken for the thermistor to change it's body temperature by 63.2% over a given temperature span when subjected to a step change (abrupt change) in temperature with measurements made under zero power conditions in a thermally stable environment.

But you will need to consult manufacturers specifications again as the range of this value is huge: from fractions of a second up to minutes.

The % TC figure is exponential following the curve (1-(e^-1))

The factors affecting the thermal time constant are:

The mass and the thermal mass of the thermistor itself;
Thermal coupling agents / assemblies between the thermistor and the body being measured;
Mounting config: probe assembly or surface mounting.
Thermal conductivity of the materials used to assemble the thermistor
in probe housings.
The environment the thermistor will operate in.

This is a good site for all things thermistor including a full explanation and large range of devices and their specifications.

http://www.cantherm.com/products/thermistors/choosing_ntc.html

Reply 3

munchen102

I have found the value and it says

'Heat capacity - 22.8 mJ/K'

Can anyone tell me what the difference between mJ/K and J/kg/*C ?

Posted from TSR Mobile

Reply 4

Joinedup

Original post by munchen102

I have found the value and it says

'Heat capacity - 22.8 mJ/K'

Can anyone tell me what the difference between mJ/K and J/kg/*C ?

Posted from TSR Mobile

a difference of 1°C is exactly the same as a difference of 1K

I expect they supply the heat capacity of one component rather than of 1kg of components.

Reply 5

munchen102

Original post by Joinedup

a difference of 1°C is exactly the same as a difference of 1K

I expect they supply the heat capacity of one component rather than of 1kg of components.

So, for example the mass of the thermistor is 0.12g, would I just keep it in grams and not convert into kg?

Posted from TSR Mobile

Reply 6

Stonebridge

Original post by munchen102

So, for example the mass of the thermistor is 0.12g, would I just keep it in grams and not convert into kg?

Posted from TSR Mobile

You don't need it. You have been given the heat capacity of the device, not specific heat capacity. So you already know how much energy is required to raise its temperature by 1 deg.

Reply 7

munchen102

So my c in the equation is 22.8?

E= 22.8*0.12*0.36
= 0.985 J

(0.12 = mass in grams, 0.36=ΔT)

Or is there somewhere I'm going wrong?

Then subbed this value into E=Pt
P=E/t
= 0.985/60
= 0.0164 W
(60s is the time)
I'm trying to find the power of a laser btw and this doesn't match the value at all.

Posted from TSR Mobile

Reply 8

Stonebridge

You don't need the mass.
Just multiply the heat capacity by the rise in temperature.
Note that the heat capacity is in milli joule per deg rise, not joule.
It's 22.8 x 10 ^-3j