NiChrome connection assistance needed for my incubator

[The_mad_scientist]

Sorry, I originally posted this on the wrong forum...

Hi, I'm very new here, so please bare with me.

I'm making a lab incubator from cheap materials. The housing is common board from Home Depot. I have an electronic thermostat unit with a 110V/10A relay I've wired to it and was trying to run a 5W tape heater with it. It didn't give enough heat. Now, I'm looking into using a NiChrome wire heating element that I construct.

The problem I have is I'm not sure if my circuit will work. I plan to use a 120V to 12V center tapped step down transformer (450mA) I got from Radio Shack to drive the element. From what I've been able to conclude, I could either use the 12V or 6V CT directly with a 6 inch 32 gage NiChrome wire to produce a heat of approximately 290 degrees Fahrenheit. Here's my concerns:
Is this approximation of heat correct?Since this is low ohms, will this damage the transformer?Will this heat excessively, resulting in burning or scorching the wood, when suspended 1 inch from the wood?
If you have alternative ideas, please let me know. I'm just not thrilled with the idea of making due with prefab heating elements because whenever I do, I seem to waste money like I did with the tape heating unit.

I've included an image of the incubator. It uploaded sideways.
...And yeah, I know I cracked the housing. I was too lazy to predrill for some of the screws.

 

[(*steve*)]

Any reason why you can't use a 100W light bulb, or even a fan heater for more power?

 

[The_mad_scientist]

I want it for my make-shift lab and suspect that some samples may be light sensitive. It's also the reason I chose to use tinted glass for the door. Besides, there's not a lot of space in there.

 

[jpanhalt]

How did you do your calculation? There are many calculators on the Internet. This one gave a different result:
http://www.jacobs-online.biz/nichrome/NichromeCalc.html



According to that source, your wire will be a little over 5 Ω. Your 450 mA supply at either voltage will not be adequate. The overloaded transformer may be a source of heat itself, if it lasts. The wire itself will be quite hot when given sufficient current.

What temperature are you trying to maintain?

Some suggestions:
1) Heat tape or pads. These are quite inexpensive at HD . You can get laboratory quality ones in a variety of shapes, wattages, and temperature ratings from commercial suppliers (higher cost).
2) Use a metal box attached to the incubator per se and put the heater in it with a small blower. We used that approach with both nichrome heating elements as well as light bulbs.
3) Use a wirewound resistor rated at least 2 to 3X the wattage you will be dissipating as your heating element.
4) Paint the light bulb black or put a metal shield over it to block light.

John

 

[The_mad_scientist]

Most of my Internet use is by my phone. Most of these calculators don't work on android. So, I estimated based on some charts I downloaded. Your screen shot was to tiny to read, so I don't know what your results were.

Yes, I was thinking that the circuit might overload the transformer.

I thought of using a resistor, but I don't want to spend too much for one. Perhaps I could use a resistor to prevent overloading the circuit, maybe without even using a transformer.

I'm not sure how many watts will be needed for my target temperature of between 250 and 300 degrees Fahrenheit (hot enough to heat quickly enough to prevent powering it all the time, but not hot enough to burn or scorch the wood). My unit interior is about 0.9 cubic feet, so I thought 2.7W would be about right (about 9 BTU's). But the 5W heat tape wasn't enough, so I don't know. The available temperature range I want inside the unit is between room temperature and at most maybe 150 degrees Fahrenheit, controlled by the thermostat.

I've tried heating tape, and just wasted money in the process.

I want to reduce the risk of cross contamination, which is possible with moving air.

Since I don't have much room in the unit and can't realistically expand the unit due to shelf space limitations, a normal light bulb can't be used. I want to incubate as many cultures in petri dishes as possible, so real estate is precious.

 

[jpanhalt]

Sounds like you are trying to culture thermophilic bacteria or doing PCR and related procedures ("PCR"). If you have a heated wire at 300°, and you want an average temp of 150° with no air circulation, then you are going to have a gradient. That is bad for an incubator -- particularly if you are doing PCR in plates.

Gentle airflow in an incubator is generally not a problem for bacteria in typical petri plates. Besides, you need moisture , and in some cases, CO2. Even when dealing with highly infectious agents, we allowed some airflow to keep uniform temperature. We just taped the plates. That step was more for our protection than to prevent cross contamination. Moreover, you can buy tape that is CO2-permeable.

The temperature range you are after is well within the range of commercial heat tapes. It seems every idea we have floated has been shot done, e.g., heating tape was a waste of money. What didn't it do? How much space does a small light bulb take compared to your coil?

Have you considered a hot-water circulator using a copper coil either inside or outside?

John

 

[The_mad_scientist]

I don't mean to be difficult. I'm working on a very tight budget, and I don't know what I will be doing with the incubator exactly. I just want to make sure it will work for most of my future tasks.

The heat tape barely warmed up at all. I closed as much of the gaps and holes as I can, but it won't heat more than 78 degrees Fahrenheit inside. The heat tape is about 10 inches by 3 inches. That's a lot of surface area to dissipate the heat so it makes sense that it wouldn't feel very warm to the touch, but I expected better from it. And that pitiful attempt costed me about $15. I already spent more on it then I can really afford and setbacks are just plain discouraging. So that's why I'd rather not try it again.

Will a 4W nightlight bulb work to create this amount of heat? I assume not, since the heat tape was 5W and didn't work. An incandescent fifteen Watt bulb would be a bit large. I hope to have enough room in it for 4 petri dishes per level, up to about 10 levels high. The interior measures 10.5 inches wide, 11.75 Inches deep and 11 inches tall.

I have a 12V mini laptop fan I could use, but I'm concerned it will move too much air. Think it would work anyway?

I am a nervous novice, so I don't want to risk mixing water and electricity if there's a leak.

Also, I was hoping this would give me some experience with using nichrome wire for something useful. Obviously, despite having success with using electronic resistors in circuits, I don't get some of the concepts of using nichrome. With some hands on work with nichrome, I hoped to remedy this.

 

[jpanhalt]

I don't mean to be difficult. I'm working on a very tight budget, and I don't know what I will be doing with the incubator exactly. I just want to make sure it will work for most of my future tasks.

That is an interesting problem.

Here is a short list of heating tapes/pads that easily exceed your temperature requirements:
http://www.watlow.com/products/heaters/ht_flex.cfm?gclid=CMHLooOIwL0CFa9cMgodkVEAyg
http://www.heatingtapes.com/tape.html
http://www.omega.com/prodinfo/flexibleHeaters.html

SparkFun has a very inexpensive pad that appears to go to 65°C.

As for mixing water and electricity, have you ever used an electric stove? I don't think a 12V fan inside a device with copper tubing for circulating water is much of a danger.

John

 

[duke37]

You can get metal clad resistors which should be fixed to a heat sink. These could be used bolted to an aluminium plate forming the floor of the chamber. Very little space would be used.

 

[The_mad_scientist]

Thanks for the suggestions guys. It looks like I'll be going with a wire wound resistor and a small PC fan. It looks like it's going to be the cheapest option after all.

I couldn't find a metal clad resistor, but the metal plate would've cost too much anyway.

It's too bad I couldn't find any help with the NiChrome method I originally wanted. It would've been an excellent learning experience for me.

 

[duke37]

Metal clad resistors
I do not know what exotic part of the world you inhabit that has no electronic parts distributors or computer auction sites.

We, in the UK, can get aluminium sheet from scrap yards which has been recovered from old vehicles.

 

[jpanhalt]

We, in the UK, can get aluminium sheet from scrap yards which has been recovered from old vehicles.

Here in the US, old beer cans are more available.

John

 

[daddles]

I'm not familiar with the needs of incubation, but if I was faced with this task, I'd first ask myself what things are already pretty close to what is wanted -- and that I could get cheap at a second-hand store. The first thing that popped into my head was a toaster oven. I'll bet something suitable could be found for $5 or less. You'd then wire your thermostat control to control the heating element, probably by just putting it in series with the existing control thermostat (or replace the existing stuff). If the toaster oven is too small, you can look for a bigger one, hack things up to joint two of them, or scavenge parts you can use. The point is that much of what you need is already there.

Also, don't be afraid to toss out work you've already done if a better approach comes along. Sure, you've got time invested in your wooden box already, but if you e.g. use a toaster oven instead, that box will find some use later on in some other project.

Depending on the temperature uniformity requirements, you may want to install a small fan to circulate the air.

You can estimate the heater power required by the maximum temperature you want and by modeling the heat loss through the walls. The heat flow in W is by conduction and is equal to the thermal conductivity in W/(m*K) times the cross-sectional area in square m divided by the thickness in m, then the product is multiplied by the temperature differential in K. Assume the outside of the slab (oven wall) is room temperature, the inside is the desired temperature, and you'll get the energy conducted through that wall per unit time (i.e., the power). Since you're using wood, the conductivity will most likely be a tensor, so you'll want to make sure you have the conductivity appropriate for the direction (probably across the grain). If you don't have it, I'll bet it's in Hoadley's book and I'll find it and look it up if you need it.

 

[k7sparky]

You started out asking about Ni-chrome so I would guess you have an old heating element out of a dryer or some such.
Best way to attach Ni-Chrome is spot weld. Crimp connectors don't get it.
I'm cheap and like to experiment. Screw in connectors / squeeze connectors as in ground clamps and such will work for a long time with Ni-Chrome. You start with too long a wire and keep loosening one clamp and shortening the wire to get the temp you want or get to max current of your transformer secondary.
Cheap and simplistic. Also probably violates every code ever written.
I also make low current shunts as in for meter calibration in a similar method.