Question about heating pads November 13 2003 at 3:58 AM
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I know this is an off the wall question but it is directly related to the next step I need to take for DCG processing.
Does anyone know about what temperature is the maximum for heating pads, you know the ones you use for a sore back or muscle?
I would assume it to be close and maybe just a little over the human body temperature (98.6 F or 22.7 C). This is almost exactly the temperature I want for my alcohol baths. Also, in doing some research I found there are some heating pads out there that are water proof, which would be great. What I plan to do it plact the heating pad in a developing tray or other modified holder (depending on max temperature) and then placing my sealed alcohol containers on this pad. this would be a controled way of heating the alcohol as I am sure the pads are not designed to get as hot as a hot plate and they are very inexpensive. I plan to buy a few so that I can differenciate the temperature of the differet baths as needed. With a correctly designed holder and proper insulation I could maintain a constant temperature, I believe.
If no one knows this answer but has a heating pad that they could wrap around a thermometer and let me know what temperature is achieved at maximum or each setting I would greatly appreciate it before buying one. I cannot find this specific information on the internet so if no one here can provide me with that answer I will have to call manufacturer.
I just got off the phone with SunBeam. Their model 771 thermostat, three setting, heating pad has a maximum temperature of 176 degrees F. It has three settings so I will have to do testing on low to see if it will come close to my target temperature. It sells for $17.99 US at Walgreens. This may be the answer I am looking for. I would have never thought they get that hot...go figure. Going to buy one Monday when I get back in town and test shortly after. If anyone is interested I will post my results.
I found another heating pad from HoMedics model HP-300. It has 3 settings also. I called them and they were able to give me the specific heat settings. Low is 86 - 95 degrees F., Medium is 96.8 - 104 degrees F. and high is 113 - 127.4 degrees F. This one sells for $16.99 US from www.drugstore.com. Looks like this is the one I will be buying! It is 12 inches by 24 inches. Because it is moist or dry usable she stated it should hold up to moisture but did not recommend using it under water. I am just concerned of possible spillage and didn't want to short it out with water or cause a fire by spilling alcohol on it. I may seal in somehow to prevent any contact with liquids even accidental. I may be able to combine multiple trays on one pad. I need one for Fixer, one for first rinse, and 3 to 5 for alcohols.
Great idea! You might bear in mind that the flashpoint of alcohol (the temp at which it bursts into flames) is about 85 C or about 200 F. I had a glass baking dish that I placed on a heater with a rotary control set at 3.5. One day, for some unknown reason, the temp control had been left at 8 for an hour and all the alcohol had evaporated. I quickly took the baking dish off the heater onto a cold table. The moment dish met table, dish explded into tiny shards of glass!
I have heard many things and wanted to research first hand. ( http://www.biosci.ohio-state.edu/~jsmith/MSDS/ISOPROPYL%20ALCOHOL.htm ) The boiling point of 99% alcohol is 180 degrees F. (82 C.) and the flash point is 53 degrees F. (11.6 C.) I was curious what exactly the flash point meant so I found, "The lowest temperature at which a liquid gives off enough flammable vapor to ignite and produce a flame when an ignition source is present."
So it seems we are using the alcohol above its flash point but below it's boiling point.
That is the beauty of the heating pads. Even on high at 127.4 degrees F. (53 C.) it wil be below the boiling point. Grant it, it will be warm enough to have conciderable evaporation and well above the flash point so I guess I should build and exaust hood also.
Hope you use an explosion-proof blower motor in the hood! You can also include an alcohol recovery system in the hood if you'd like. Just condense the vapors cleanly and catch the flow.
Good points, that is why I am researching before building anything. Right now I am processing 4 x 5 plates and each bath only stays open for 30 seconds or so, one at a time. But when I move to large plates, thus larger baths the hood should be designed to accomodate the concentration of alcohol in the air.
Do you have info on that motor or hood design. I don't have money to buy a clean hood for thousands. Thanks in advance,
John
Fire and Explosion Hazards:Dangerous fire and explosion hazard.Vapor can travel
distances to ignition source and flash back.Hot organic chemical vapors or mists are
susceptible to sudden spontaneous combustion when mixed with air.Ignition may occur at
temperatures below published autoignition or ignition temperatures.Ignition temperatures
decrease with increasing vapor volume and vapor /air contact time and are influenced by
pressure changes.Ignition may occur at typical elevated temperature process conditions,
especially in process equipment operating under vacuum if subjected to sudden ingress of
air,or outside process equipment operating under elevated pressure if sudden escape of
vapors or mists to the atmosphere occurs.
I cannot find any specific number on actual autoignition or ignition temperature but the flash point is so low it is exceeded at room temperature.
I probably was using "flash point" inaccurately. I heat the alcohol to about 60C in a baking dish and just cover with window glass. By flash point I meant that at about 80 or so, the fumes travel far enough to get in contact with any nearby electrical outlets, not to mention the heating source itself. The alcohol seemingly spontaneously ignites because the flame might track back to the source. At room temp, the SVP of alcohol is pretty low, you can place a glass plate above it and you won't collect any alcohol. At 60, however, the plate glass (held approx an inch above the surface of the alcohol) has enough alcohol to fill a shot glass in about a minute!
A hood is a good idea and you can probably get a hood for cookers for a few hundred.
Dinesh mentioned in his post that we built our fume hood from a kitchen stove model. I actually bought a new one for about $80 at Home Depot, which had the full hood enclosure with fan, ready to plug into AC. But you can also get the fan alone there for about $10 plus any ducting bits you might want to use for pretty cheap part by part.
Note that if you use a ducting tube, put the open end in your room and put the fan at the OTHER end - outside the room. You still enjoy the exhaust draw, while leaving the "ignitable source" farther away, where the alcohol may even have cooled and condensed before it gets that far.
sigh It's funny how the mind works, or doesn't. I hadn't even thought about using a kitchen hood for such a purpose but it seems obvious now that you mention it.
How far from the work surface did you mount the hood?
I had thought about that but I am leary about the fan therein. I was thinking more about building my own hood and using a fan that is set up like a water pump. The motor is outside the fan area and connected by a shaft. I think it would be something like the blower fans in a forced air heating system where the air that is being moved is comes only into contact with the fan housing and blades. Something like this one but smaller.
Also, if you even seen the hoods at a lunch shop, you may notice they have grates that collect the grease before the air is exausted to outside. That would serve well to capture most of the alcohol and not send it outside in the air. I have to research the effects of alcohol on aluminum.
I use both of these products to control the temperatures of my chemistries for silver halide. I simply put the heater under the work area and place the temperature controller sensor into one of the rinse baths. This allows my to do holography when the studio is at 50F like it is today.
This is exactly what I am looking for. The model this link takes you to is the design but I would need to call the manufacture and ask about specifics on Isopropyl venting.
I wonder if this should be re-thought. Perhaps it is better to blow accross the table rather than sucking. This way there is less need for an explosion proof system. At least for us hobbiests anyhow. Any thoughts?
You still need to exhaust the fumes somewhere though.
How about a dual blower/sucker system that blows hepa-filtered air across the table and into a hood that is linked to a hose with a sucking fan at the outside end?
You'd get a cooling effect from the blower and the added benefit of hepa filtered air and be able to use the table when you need a dust-free environment as well as when you need noxious fume removal.
I was thinking of a glove box with a positive pressure from a Hepa filter and a vent to outside. Then there is no need for explosion proof motors and there is no risk to the holographer.
It is very important to have air flow be Laminar if you are really looking for a truely dust free envrionment. The problem is moving explosive gasses through a fan. Perhaps a blower for a flue? They would have to be designed for explosive gasses. There are such things, I don't know any prices.
I don't have the link handy, but it seems to me (not being an explosions expert) that one of the recently mentioned fans would do the trick. I don't have the link handy but one of them had the motor external to the air path with a shaft running to the duct.
I would guess there is the further concern of making sure that when the bearings wore out that there is no metal contact to make a spark. Perhaps they somply use a alluminum blade or something.
The blower I posted that I am responding to is a flue exaust blower. I would mount it at exit point so that negative flow is created thorough a duct to the top of the hood. This unit is designed not to inniate a spark and is used for venting possibly flamable gasses.
As far as clean room, If the blower is mounted as I stated above and the duct attaches to the hood then you are creating negative pressure. Completely enclose and seal the work area with the hood at the top with only one entry point for the air and that would be through a heppa filter. This would require two heavy gauge gloves to be built into the front of the unit through a clear plexyglass front. You would put all you material in the enclosure and work through the gloves. Clean room environment would thus be had. My coating room is not clean thus this would be my only option, I am in a cellar.
