Temp limits to a lightbulb warmed fridge fermenter

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Temp limits to a lightbulb warmed fridge fermenter

Postby carfreefamily on Thu May 26, 2016 9:31 am

I built a fermentation chamber out of a dorm fridge, using a 100 watt lightbulb mounted on an upright base at the bottom of the fridge and a YATC5 temperature controller.

I used it last night to make yogurt, and it worked well.

My wife wanted to use it to make some herbal salves, but the temperature needs to be 150 degrees for four hours.

I was warming it up, but when I checked it at around 135, I noticed the plastic of the refrigerator behind the bulb was blistering.

I'm wondering many things. Can I just shield the black plastic interior of the fridge behind the bulb with some aluminum foil and still heat it up to 150? Should I never, ever try to heat the interior of a fridge up to 150 using a 100 watt lightbulb? Should I use a lower wattage? Would a ceramic terrarium heater be more gentle on the interior than a light bulb? Should I just stick to the lower temperatures needed for fermentation and send my wife elsewhere for salve-making?

Well, that pretty much covers my questions.
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Re: Temp limits to a lightbulb warmed fridge fermenter

Postby Gutted on Sat Jun 04, 2016 6:54 pm

carfreefamily wrote:I built a fermentation chamber out of a dorm fridge, using a 100 watt lightbulb mounted on an upright base at the bottom of the fridge and a YATC5 temperature controller.

I used it last night to make yogurt, and it worked well.

My wife wanted to use it to make some herbal salves, but the temperature needs to be 150 degrees for four hours.

I was warming it up, but when I checked it at around 135, I noticed the plastic of the refrigerator behind the bulb was blistering.

Please be aware that most of the planet now use Celsius rather than Fahrenheit and therefore these numbers will not mean much. I will encourage you to be international to maximise the chances of replies. I try to remember to provide converted figures so that everyone has a reference point. I recommend Das Unit Converter (freeware) which can convert between hundreds of different units very accurately except for maybe the currency conversion which is a lot less reliable. Other conversion programs may be available but I prefer this one and it's free to use.
135F = 57.2C
150F = 65.6C

Where are these temperatures taken from? They seem very high! 65.6C will cook most bacteria so I doubt this is the medium temperature. Is this the fridge inside air temperature? Around 38C/100.4F is more ideal for most bacteria and some like lower than that 30C/86F. These would be the temperature of the fermenting medium and not air temperature which is likely to be higher.

My yoghurt maker was getting to 60C measured on the bottom of the yoghurt maker using a K Type sensor which was producing very poor results. Many cheap yoghurt makers have no temperature control and just heat up and the ambient temperature can affect the fermenting temperature which in the case of my yoghurt maker was getting far too high. This is still a lot lower than the temperature that you have given.

I'm wondering many things. Can I just shield the black plastic interior of the fridge behind the bulb with some aluminum foil and still heat it up to 150? Should I never, ever try to heat the interior of a fridge up to 150 using a 100 watt lightbulb? Should I use a lower wattage? Would a ceramic terrarium heater be more gentle on the interior than a light bulb? Should I just stick to the lower temperatures needed for fermentation and send my wife elsewhere for salve-making?

Well, that pretty much covers my questions.

That is the problem with plastics, they do not tolerate high heat very well. The radiant heat from the light bulb is the problem. Have you tried measuring the temperature near the bulb close to the plastic to see what temperature it gets to? Some form of thermal insulation would help but I doubt that aluminium foil would be sufficient. You have the fridge so it seems silly to not make use of it. Being a bit resourceful to find a way to make it work. I do suggest that you find out the temperature near to the bulb.

Some of the thermal insulation, a thin piece of 25mm/0.98", maybe even smaller would do the job as the stuff provides very good insulation from heat. That would stop enough of the heat from the light bulb to stop the damage to the plastic.

Either that or move the light bulb. Maybe extend it or the fitting to move it away from the plastic. Have a bracket fixed to suitable location in the fridge with the lightbulb socket which should lower the temperature. Presumably you are using an old filament lightbulb. A different form of heater would probably be a better idea as it seems wasteful using so much energy producing unnecessary light. It's the heat you need and not the light after all.

A problem I can see is that you have to raise the temperature without getting the heat near to the plastic or insulate sufficiently to prevent it from affecting the plastic. I think that a different form of heater would be preferable but I am not familiar or have any experience with the one that you mentioned and therefore cannot comment any further.

I am sure that you can keep the wife happy by being a bit resourceful after all it helps to keep her happy for peace and quiet if nothing else.

I hope that I have provided some ideas but after 10 days you might of already sorted it out. Let us know how it goes ;)
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Re: Temp limits to a lightbulb warmed fridge fermenter

Postby carfreefamily on Mon Jun 06, 2016 8:08 am

Thanks for the reply. The high temperature is not for culturing, but for creating salves with oil and herbs. My wife switched to using the temperature controller in a crock pot water bath for that.

At the lower temperatures needed for yogurt, I do not believe the bulb has been as much of a problem. (I do have a piece of aluminum foil behind it and may add some of the aluminized bubble wrap insulation, since I have some on hand.) I just checked, and the plastic may be a little more blistered, but it is hard to tell if it is recent or from the time I had the temperature so high.

The setup, in general, is working great.
carfreefamily
 
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Re: Temp limits to a lightbulb warmed fridge fermenter

Postby Christopher Weeks on Tue Jun 07, 2016 10:09 am

Gutted wrote:...it seems wasteful using so much energy producing unnecessary light.


It's a little off-topic, but since the original problem sounds sorted, I thought it'd be OK. I think the above is wrong. The only energy that's escaping the system as light is the light that shines out when the fridge door is open. Otherwise, the bulb acts as a point heat-source and also radiates energy in the form of light that strikes the box's interior and is at that point converted into heat. So a lightbulb should be providing a more even heating experience by casting the light around the inside of the chamber, and not wasting energy.

The heat created by the bulb is normally considered waste when it's being used for lighting, and the light would be waste when using the bulb for heating, if the box were transparent to visible light, but this is a different situation.
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