electric blanket project

[corporalspiffy]

i like to practice playing a recorder but there is a problem with condensation of moisture in the airway. the way to solve this problem is to keep the instrument at a warm temperature.
i have a large number of 100 ohm 1.5 watt resistors and im thinking of sewing a bunch of them into a little heater blanket that i can velcro strap around the top joint of the recorder.

the blanket will be 5 cm wide and 12 cm long while the circumference of the recorder is 8 cm which means that there is t room for about a dozen resistors.

if i use a 12 volt power supply and i want the blanket to heat up to about 25 - 30 degrees C what are the calculations i need to make to determine how the resistors should be arranged. 

[Chris C]

Before going into the calculations, I'm wondering if you might be happier with this:

https://www.sparkfun.com/products/11288

It's almost the exact size you need, and runs off 5V.  Check out the warmup profile and tutorial/video as well.

[CatalinaWOW]

You can't do the appropriate calculations with the information provided.

Your "blanket" provides some level of insulation to the outside world.  You need to know quantitatively what that is to do a calculation.

You could place one resistor in the blanket, put a known amount of power into it, and measure the temperature rise.  This would allow you to compute a thermal resistance in deg C/Watt.  That will give you enough information for your application, even though it does oversimplify the problem in many ways.

[unitedatoms]

For 12V DC source and 100 Ohm resistor, the individual resistor will dissipate P=U*U/R=1.44 Watt.

If resistor is limited to 1W max, then it is better to run it at 25..50% of intended max power.
For this put 2 resistors in series: P=U*U/(R+R)=0.72W total with 0.36W per resistor.

To estimate needed total power (how many chains of 2 resistors to put in parallel) check the power limit of power supply, available space in blanket relative to size of parts, ignition point of thermal insulating material and small print in home insurance papers.

Anything thermally insulated, even at 0.36W can ignite, since resistor has no set limit of temperature even for power lower than max. The datasheet 1W value is for open air when mounted on board.

I'd replace resistors with PTC thermistors just to avoid fire hazard.

To get familiar with thermal power of resistive heating of known power try to turn on for 1 sec and turn off 7W hands solder gun pencil and estimate heat on touch. Ballpark of power to heat say 100cm^2 aluminum sheet (about same mass as recorder part may be) to point 10C higher than ambient could be single watts. like 1-2 watt.

The USSR MLT-1 resistor stands for metal film, conformant coating tolerant, thermally tolerant. It will smell burned paint when runs at 1W because by design it should be epoxy coated after assembly. The paint on it is not tolerant to oxygen and will carbonize in open air.

[Simon]

Although I have not touched the instrument in quite some years I used to play the recorder quite a bit when I was a teenager. I don't think your problem is "condensation" it is probably spit. The other thing to consider is that the recorder is usually made of plastic so it will be quite difficult to heat the inside and you are of course blowing through it all the time. You should be regularly cleaning your recorder in any case at some not exactly lab clean moisture will get into it when you play it and unless it is cleaned regularly you will probably get mould growing in it. If you quickly do this before each playing session you should be okay. If you are having as I did quite a problem with spit whilst you are playing then simply cover the hole on the top of the recorder that sound comes from and blow into the recorder this will bloat the spirit clear of the smallest channel where it normally clogs up.

If this is a storage problem it will be easier to just store the recorder in a box with a heater and a thermostat. But that really seems to be quite overkill for such a simple problem.

[TheElectricChicken]

You might consider keeping it in a warm dry place which already exists in your home. Next to the water heater is a perfect place, as it is always hot. The pipes that come out are always hot, and about the same temperature 24 hours a day. So the way you make the storage box would determine the final temperature.

Also have you considered an old electric blanket ? you could follow and measure the heater cable and then cut a portion of it out which is proportional to the voltage you want to use to power it. For example, a 220 volt blanket cut out 1/10th and use 22v to power it and so on. It would output 1/10th the power.

[Simon]

recorders are going to be full of condensation as your blowing your moist air into them so I'm not sure if there is actually a problem.