Nichrome wire: Watts to Celcius

[HerbertMunch]

Can anyone tell me if there is a nice formula for converting watts to degrees c, or is it not that easy ?

Many thanks,
chris.

 

[Nigel Goodwin]

What are you trying to work out?, the two aren't anything similar.

 

[HerbertMunch]

Oh, i thought (foolishly) that watts represented power dissipated as heat.

Im trying to work out the relationship between current and heat through a length of nichrome.

Im making a small heat blower (for no apparent reason, other than to experiment), and wish to know how much current i would need to put through the nichrome in order to heat the air by a few celcius.

thanks.

 

[Nigel Goodwin]

I would suggest you stick some power through it and measure the rise, there are far too many variables to mess about with - such as how fast the wire loses the heat.

For a blower heater it's even worse, as the blower makes it lose heat MUCH, MUCH, faster.

 

[mabauti]

try wit a temperature sensor? (lm35, lm335 , or Thermistor or DS1820)

 

[HerbertMunch]

Ok thanks very much.

 

[Pommie]

The fact that the air is forced makes the calculation much simpler. Once up to temperature the energy must heat the air so, if you now the amount of air flowing the temperature rise will be,

K = 1.2 * W / V

Where,

W = Watts
V = Liters/Second air flow. (assumed at 20°C)

So for a flow rate of 2L/S and a 50W element the rise will be 30°K and the outgoing air will be at 50°C.

This comes from the fact that it takes 1.2kJ to heat 1m³ of air 1°K.

Mike.

 

[HerbertMunch]

The fact that the air is forced makes the calculation much simpler. Once up to temperature the energy must heat the air so, if you now the amount of air flowing the temperature rise will be,

K = 1.2 * W / V

Where,

W = Watts
V = Liters/Second air flow. (assumed at 20°C)

So for a flow rate of 2L/S and a 50W element the rise will be 30°K and the outgoing air will be at 50°C.

This comes from the fact that it takes 1.2kJ to heat 1m³ of air 1°K.

Mike.

Nice one mike, thats really helpful, thanks

Any idea how i measure the flow of air?

 

[HerbertMunch]

Im using a 12v dc case fan, measuring 91mm². Is there any way of working out the flow mathmatically?

 

[Pommie]

Im using a 12v dc case fan, measuring 91mm². Is there any way of working out the flow mathmatically?

There probably is but I don't know how. They normally state it in the spec. If you find a similar fan on the internet you should get a ball park figure. Alternatively, stick your heater in the flow, measure the temp and calculate the airflow.

Mike.

 

[stevez]

I would take a little effort but you might also be able to make do with materials on hand or otherwise scavenged.

The volume of air per unit time (liter/second, cubic feet/minute) can be measured in a tube or duct by measuring velocity of air - and with the velocity of air and a known cross sectional area the flow rate can be calculated.

A manometer can be made from some clear plastic tubing - water with some food coloring can be added for improved visibility. A probe, made from relatively thin tubing (connected to the manometer) is inserted into the airstream of interest so that the end faces exactly in to the wind, so to speak. The water in the manometer is displaced by an amount that represents the velocity pressure. A measurement of velocity pressure allows direct calculation of velocity.

My explanation isn't a tutorial or instruction on how to do it. You might google and find what you need - or ask some more questions. All of this might be more trouble that it is worth to you.

I'd be inclined to arrange a test fixture so that an amount of air is blowing thru a tube or duct - with a test coil of nichrome. Apply varying amounts of current and note the temperature rise. The temperature rise allows you to do the math to determine the mass flow rate of air or the volumetric flow. Quite a few instruments that are sold to measure air velocity work this way - air flows past a heated wire and the temperature rise indicates air flow rate.