How hot will a 100W single bed(150*70) electric blanket get? - Page 1
Forum section: electronics / beginners

author: shivajikobardan
author_rank: Contributor
timestamp: January 21, 2023, 03:01:23 pm
content: I want to know how much degree celsius hot will a 100W electric blanket get? Thanks for the information.

author: BillyO
author_rank: Super Contributor
timestamp: January 21, 2023, 03:10:14 pm
content: It's really not possible to answer your question as we do not (and possibly cannot) have enough information. It will depend on many things including, but not limited to, ambient temperature, humidity, thermal conductivity of the mattress, additional blankets/sheets added to the bed and their conductivity/insulation factors, the size and metabolism of the occupant, etc..All that said it will probably have a thermostat which will determine the maximum temperature.

author: pcprogrammer
author_rank: Super Contributor
timestamp: January 21, 2023, 03:26:21 pm
content: Quote from: BillyO on January 21, 2023, 03:10:14 pmOr it uses positive thermal resistance material as a self regulating system. It needs at least something because without regulation and not enough removal of heat generated by the energy it will burn up.These things often have a way to set it to keep you comfortable, like low, middle and high. This way when you feel hot you can turn it down a bit.

author: shivajikobardan
author_rank: Contributor
timestamp: January 21, 2023, 03:38:13 pm
content: >It's really not possible to answer your question as we do not (and possibly cannot) have enough information. It will depend on many things including, but not limited to, ambient temperature, humidity, thermal conductivity of the mattress, additional blankets/sheets added to the bed and their conductivity/insulation factors, the size and metabolism of the occupant, etc..ambient temperature=10 degree celsiushumidity=62%thermal conductivity of mattress=polyster mattress(probably).additional blankets not used. directly used under feet.size of occupant: 65 kgmetabolism of occupant: normal.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 21, 2023, 04:02:12 pm
content: You mean without a thermostat?Without a thermostat and with negligible losses to the environment the blanket temperature would continue to rise indefinitely limited only by the melting of the heating wire.However, there is usually a failsafe device in the blanket to limit the maximum temperature (mandatory since 2001).You can easily work out the temperature rise of a mass-Q = mc∆TQ = heat energy (Joules, J)m = mass of a substance (kg)c = specific heat (units J/kg∙K)∆T = change in temperature (Kelvins, K)You might take a guess that the blanket fabric has a c value of about 1000.1 Watt = 1 Joule per secondYou need to factor in the energy being lost (-Q) to the environment through conduction, radiation and convection. Thats the hard part, if you don't have a measure of it, its impossible to calculate Tmax.

author: pcprogrammer
author_rank: Super Contributor
timestamp: January 21, 2023, 04:19:28 pm
content: There is always the trial method. Get a thermometer and put it on top of the blanket. Turn on the power and wait until you see a max reading on the thermometer.When the blanket is meant for human use, it should not become so hot that you would burn yourself.

author: golden_labels
author_rank: Super Contributor
timestamp: January 21, 2023, 05:04:20 pm
content: Keep in mind that there is a 65 kg, 100 W heater wrapped in that blanket. That heater loses some heat through the bed-facing side, and has air cooling to keep internal temperature below 38°C, so it will not be literal 100 W added. But in practice a blanket with a living human inside will act very differently than a blanket simply exposed to 10°C air.

author: MrAl
author_rank: Super Contributor
timestamp: January 21, 2023, 09:37:15 pm
content: Quote from: shivajikobardan on January 21, 2023, 03:01:23 pmTo add to the other posts here...There is an equation called the "heat equation" that was found many years ago by Fourier if i remember right. It was a breakthrough at the time and amazingly some of the concepts developed that long ago started being used to detect underground nuclear bomb. For the problem of heat flow it gets a little involved with a partial differential equation that many dont want to get involved with. If you understand a little about electrical current flow there is a simpler way of looking at it though.It starts with the heat producing element. This can be a resistor, transistor, etc. For conduction the heat flows through a conductor just like an electrical current flow through a wire. If it encounters a resistance, just like electrical current flow it will not be able to flow as well so the temperature at the end of the heat conductor will be reduced. Then there is radiation. This is when the heat dissipates into the air round the conductor.For an electric blanket a simplified view would be the heat element, followed by a very short conductor, followed by a decent insulator, followed by conduction on one side and heat dissipation into the air on the other. If we consider the conduction side to be non dissipative, then we are left with the top side where the heat dissipates into the air.To calculate the maximum temperature you would have to know the same things you would for any other heat vs temperature problem such as a heat sink on a transistor. The main parts would be how much wattage the heat element puts out as a max (which you seem to know) and the insulation on the top side. You have a spec for the heat element, 100 watts, and next you would need a spec for the insulation, which would be a coefficient that acts to reduce the temperature effect of the heat element on the very top of the blanket. To get that, you would have to turn the control all the way up and measure the top side temperature. You could then come up with a factor that tells you how much the temperature drops from the heat element to the top surface. This would vary depending on the type of material and thickness.Once you figure all this out, you can then figure out the effective heating on the lower side, which will be hotter than the top side. That's probably the most important because you want to know how hot the animal underneath gets with a given wattage setting. You can get more information about this by looking at how a heat sink for a transistor works, then apply it to the blanket. It's a little hard to explain it all right here.Since it is hard to gather all this information, you could just stick a thermometer under the cover and turn it on full blast. You'll get a temperature reading and then you have the answer. If there is to be a person underneath then they should be there under the blanket. If it is something else like a fluid or solid then that should be under the blanket instead. That's probably the most reliable way to determine the max temperature.I can tell you from experience that they get pretty warm underneath but not really hot. You do end up turning them down. It depends on the temperature above too though as that takes some of the heat away.If you have a specific application for this you could mention that too.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 21, 2023, 10:39:47 pm
content: Quote from: Terry Bites on January 21, 2023, 04:02:12 pmReference the old joke: You must work for Microsoft, a complicated, correct and useless answer. Losses to the environment are totally non-negligible.If this is a homework problem for the OP the heat input and heat capacity part of the problem has been given here. You will need to add conduction, convection and radiation losses. For the homework problem you can assume that the blanket never gets hot enough to burn someone (on the order of 55 C) which will let you plug into the various formulas and get started on an iterative solution. A little googling will get you guesses for the blanket emissivityIf the question is simple curiosity about a product that is new to the OP there are some things to think about to bound the problem. First, and as hinted at in the prior paragraph no electric blanket will be sold that in normal operation can hurt someone, so it won't get very hot. 55 C is a very generous estimate. It is much more likely that the limit is well below this. The logic follows. All of the heat flow stuff mentioned before applies to the human being under the blanket. If the blanket reaches body temperature there will be no heat flow from the body into the blanket. And the path downward is usually very well insulated both by materials and geometry so little heat is lost in that direction. So it would be quite hot to have a 37 C blanket temperature. I would guess that the design temperature for these blankets is somewhere around the temperature at which it is comfortable to sleep without covers, somewhere in the 23-28 C range.

author: golden_labels
author_rank: Super Contributor
timestamp: January 22, 2023, 04:22:02 am
content: If we assume a flat, rectangular human of uniform density, measuring 150×70 cm2, a perfectly symmetrical blanket should keep its temperature at (36.6 °C + 10 °C) / 2 = 23.3 °C, if controlled ideally.In reality, OP should just put themselves under the blanket and check. Or see what other people with that particular blanket type experienced. And I believe the latter is the goal of this thread, and not theoretical discussion.

author: Psi
author_rank: Super Contributor
timestamp: January 22, 2023, 05:56:22 am
content: 100W is pretty high for an electric blanket, 20-45W is common.Note that some 'smart' electric blankets may have a 100W rapid-heat mode for a short time before they auto-switch back to the highest setting at like 45W.Unless the room is at -20 C I cant see how anyone could withstand being in a bed with the electric blanket running at 100W continues.Accidently leaving a typical 45W blanket on the highest setting and falling asleep results in waking up in a hot sweat at 3AM.

author: IanB
author_rank: Super Contributor
timestamp: January 22, 2023, 06:38:34 am
content: Quote from: Terry Bites on January 21, 2023, 04:02:12 pmThis answer is completely incorrect. The required calculation is not about the temperature rise of a mass, it is about the heat balance "in = out". If you turn on the blanket and leave it long enough to stabilize, it will reach a stable temperature where the heat input from electricity (100 W, say) will be equal to the heat lost to the surroundings. This is the calculation that needs to be performed.Needless to say, this calculation is realistically and practically impossible to do.The only way to find the operating temperature of such a blanket is to do an experiment where you use it in the expected way, leave it for a few hours to stabilize, and measure the temperature with a thermometer.What actually happens in practice is that the manufacturer includes appropriate safety features, such as a thermostat, a thermal overheat cutout, a self-regulating heating element, or some combination of all of these. Which means the answer to the original question is whatever temperature the manufacturer decided.The manufacturer may possibly tell you what this temperature is, but even so you need to do the experiment to confirm it.Short question:Quote from: shivajikobardan on January 21, 2023, 03:01:23 pmShort answer:It is impossible to know unless you measure it.

author: MrAl
author_rank: Super Contributor
timestamp: January 22, 2023, 06:47:28 am
content: Quote from: CatalinaWOW on January 21, 2023, 10:39:47 pmHi,I would think what he meant by that was "IF" you have no losses to the environment "THEN" (and only then) the temperature will rise indefinitely.In theory, it would rise toward infinity if there was no way it could break the walls of its containment at some point. But that's a conditional statement.Not only that, a tiny power dissipation of 0.001 watt would cause this or even 1uW if there were no losses to the environment.That's not to say that this will happen, it is just a statement about how heat works in relation to temperature. Normally we will see some loss so that the temperature eventually stabilizes.

author: IanB
author_rank: Super Contributor
timestamp: January 22, 2023, 07:14:17 am
content: Quote from: MrAl on January 22, 2023, 06:47:28 amIf you have no losses to the environment then the temperature will rise indefinitely, so calculations using heat capacity will provide no answer.If therearelosses to the environment, then the stable temperature will depend on those losses.Either way, the answer using heat capacity is useless.In reality, the whole point of an electric blanket is to have losses to the surroundings, otherwise how would the blanket heat the bed?

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 22, 2023, 06:29:49 pm
content: But a lot of this discussion is about a 100W blanket and the temperature will be at which temperature would the heat loss equal 100W. I would think the 100W is more than needed and a temperature control circuit is used. The wattage is only for rapid heating up then the blanket would turn off and on to keep it as a comfortable temperature.

author: IanB
author_rank: Super Contributor
timestamp: January 22, 2023, 06:36:56 pm
content: Quote from: BeBuLamar on January 22, 2023, 06:29:49 pmQuite. But then the temperature reached is the temperature set by the control circuit, and this cannot be calculated, you have to know it or measure it.

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 22, 2023, 06:48:58 pm
content: Quote from: IanB on January 22, 2023, 06:36:56 pmYes if the OP has the blanket he would have to measure it. Besides the heat loss varies depending on the room temp as well as how the blanket is used so I don't think one can design a blanket where the desired temperture is based on the heat loss only.

author: mzzj
author_rank: Super Contributor
timestamp: January 22, 2023, 07:56:32 pm
content: Human (Large male) is about 100W heater itself when sleeping and has similar surface area to OP's blanket example. so the blanket would heat roughly similarly to a large male specimen.Comfortable human skin temperature is around 33...35 degrees and that would be ballpark temperature if you cover the electric blanket with the same blanket that the 100kg male is comfortable to sleep with.

author: golden_labels
author_rank: Super Contributor
timestamp: January 22, 2023, 11:57:30 pm
content: I found the right solution. Contact Project Farm and make Todd test blankets! You will also learn, which one is best for hanging a car from it!(No, do not take that too seriously)

author: MrAl
author_rank: Super Contributor
timestamp: January 23, 2023, 05:52:51 am
content: Quote from: IanB on January 22, 2023, 07:14:17 amHi,I never said any of that was not true so not sure why you replied to my post with that list.Here is what he said that i was responding to:"Without a thermostat and with negligible losses to the environment the blanket temperature would continue to rise indefinitely limited only by the melting of the heating wire."Just about the physics behind a heat source and no losses, period. I never said anything about the specific heat capacity which i dont see would matter unless it was so large that it could absorb an infinite amount of heat energy or in the more practical case a very large amount of heat energy.Thanks.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 23, 2023, 08:22:00 pm
content: As a fun side note on the heat capacity side of the discussion, I once worked with electronics operated at cryogenic temperatures. Mounted on a very well insulated structure that had extremely low heat losses to the surroundings. Thermal impedance of hundreds of degrees per Watt. Even though the electronics consumed relatively low power they would get to extraordinarily high temperatures (well beyond silicon survival) if operated without the cryogenic system in operation. The heat capacity told how long you had to catch the problem if a newby overlooked this critical step. The answer, verified by experiment was measured in a few handfuls of seconds.

author: analityk
author_rank: Regular Contributor
timestamp: January 24, 2023, 01:02:31 am
content: Since you will not enter any parameter of your device you can go into Fourier equation of heat transportation. It is really fun partial differential equation, does mean temperature in giving point of material depend of power delivered to given (another) point of material and, in differential sense, previous temperature of this point and material properties.As long as solving PDE is tricky you can switch domain from time to 's' domain transform your solution with Fourier transform ( or if you want you can also use Laplace transform). Then you will easily solve any temperature distribution in any three dimensional system in function of power, material properties, environment and time. If this therms is new for you I strongly recommend yt channel from eigenstive. He explain this topic with occasion of PDE, ode and Fourier transform.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 24, 2023, 06:59:55 am
content: Yes. Non-negligable as I said, "without losses" the temperature will rise without limit.Please adjust your glasses. You must work for MS.

author: golden_labels
author_rank: Super Contributor
timestamp: January 24, 2023, 08:10:50 am
content: The last time I tried, around Planck temperature it all turned into unorganized mess filled with all weird radiation in the 1010quettahertz range. The structure of my blanket no longer made any sense and the microcontroller threw ENOTIME.No, I can no longer take this thread seriously: OP clearly asked about practical experience, not half-wrong babbling about theoretical physics.

author: MrAl
author_rank: Super Contributor
timestamp: January 25, 2023, 02:00:49 pm
content: Quote from: CatalinaWOW on January 23, 2023, 08:22:00 pmYes that's interesting, and me and a friend once placed a power resistor into a container of distilled water and fired it up. Water has kind of a high specific heat capacity so it takes a long time to boil the water, if it ever does due to the heat loss at the sides of the container. So in that case we would get a long time before we would have to shut down, unlike with just the bare air cooled power resistor which would get super hot even at 3/4 of its rating. With that water cooling we could exceed the power rating of the power resistor. We didnt measure the time to heat the water to a specific temperature, but you could tell it was going to take a pretty long time.I would guess that a regular electric blanket would have fairly low specific heat capacity due to the material which does not actually absorb heat too well.Maybe place a steel plate over top of itI knew someone long ago that when their heat went out they would heat up bricks in the oven and take them to bed with them for heat during the night.