How low (in temperature) will a typical induction heater go?
July 25, 2010 10:16 PM   Subscribe

How suitable is an induction-type hot plate for keeping a liquid stable at a low temperature (target is 70C.)?

The ones I've looked at in local shops have pre-sets with various target temperatures built in, but none of these read below 150 degrees. I called the Panasonic rep to see what they had to say, and the answer was "The lowest setting [on the particular unit I was asking about] is 75 watts." They couldn't tell me what that would work out to be in actual temperature ...

Any thoughts on this? I'm making glue sizing for treating paper, and I've been told by those in the know that letting the temperature of the mixture rise above 70 degrees will weaken it too much.
posted by woodblock100 to Home & Garden (26 answers total) 1 user marked this as a favorite
I have a Tatung induction cooker which lists the lowest temperature as 140 F. I don't know how accurate it is or how stable it maintains that temperature.
posted by wongcorgi at 10:23 PM on July 25, 2010

Is the target 70C or 70F? Your intro uses C but the rest of the question is in F.
posted by sanko at 10:27 PM on July 25, 2010

Note sure how useful this will be to you:

I also use an induction type hotplate to keep small volumes of liquid sealed in glass at ~70 C. My hotplate has an attached temperature sensor, which I keep in a piepan of water on top of the hotplate. The water moderates the on/off nature hotplate, stopping it from overheating my small volume of my samples. With a large enough volume of water and a thermometer it would be pretty easy to maintain your desired temperature, though it might be a pain.
posted by pseudonick at 10:28 PM on July 25, 2010

If the hotplate doesn't work out, you might be able to use a home sous vide controller that will turn the power to a slow cooker on and off to keep the contents at a steady temperature.
posted by sanko at 10:31 PM on July 25, 2010

more concisely: Have your glue (and a thermometer) sitting in a water bath on the hotplate and flip the hotplate power on and off to keep the water bath at the right temperature.
posted by pseudonick at 10:35 PM on July 25, 2010 [1 favorite]

Your intro uses C but the rest of the question is in F

I intended everything to be read as C ... sorry if it was confusing.
posted by woodblock100 at 10:36 PM on July 25, 2010

Was just confused as 150C doesn't seem all that low to me. In any case, a sous vide controller would take the pain out of pseudonick's suggestion. Example here
posted by sanko at 10:43 PM on July 25, 2010

Not a very good tool for the job, what you need is a temperature-controlled water bath. You could work out a safe setting if you don't mind the liquid temperature staying mostly well away from the cut-off point, but it will take some trial and error.

Even if the hot plate has a temperature sensor (which it may not, if they are giving you a power figure it probably means they are controlling current, not temperature), it is placed somewhere in the hot plate itself. The sensor temperature, which is what determines whether current flows to the heating element or not, will differ considerably from the temperature of your liquid depending on factors such as container type and material, filling height etc. This is why the rep couldn't give you a temperature: you can't really predict the liquid temperature from the hotplate setting, only that it will be lower.

Taking into account the fact that the amount of liquid in the container will probably diminish over time, either through evaporation or because you are using it, and what you have is a pretty interesting exercise in non-equilibrium thermodynamics but a pretty poor solution to your problem.

You could try filling a standard container to a specified size, and working out a cut-off point for the hotplate that will keep you within safe temperature regions, but you will need to pay constant attention to the liquid temperature and make adjustments as needed manually.
posted by Dr Dracator at 10:44 PM on July 25, 2010

Have your glue (and a thermometer) sitting in a water bath on the hotplate

This is what I'm planning to do ... but if the unit won't go low enough, I'll be flipping the switch non-stop all day, and I'll be getting wide ups and downs in the temperature.

The fact that the units I looked at have settings in degrees (rather than watts) leads me to think that they must have some kind of thermostatic sensor built in. The ideal for me is to pour my glue in, punch up 70C, and just let it run ...
posted by woodblock100 at 10:45 PM on July 25, 2010

try this: inexpensive hotplate (it's GLUE, not biological samples...relax a little...deep breaths...any hotplate will do), two pots...a big one and a small one, or a double boiler, a thermometer. ok, here's the kicker: a trivet...or the metal bracket from off a gas stove. if the glue is getting too hot in a double boiler on the hotplate's lowest setting, just get some space in between...that'll cool it down. but even a $10 hotplate should go low enough...75c is 165f...that's pretty hot.
posted by sexyrobot at 12:50 AM on July 26, 2010

posted by sexyrobot at 12:52 AM on July 26, 2010

The thing is, the thermostats used in domestic cooking / heating appliances are fairly simple affairs - they're basically on-off switches, so the amount of heating provided is either maximum, or zero. This works OK in most cases because (a) the thermal mass of the element/hotplate acts to average things out, and (b) you generally don't need to be that accurate with temperature, so any overshooting / undershooting isn't really a problem. Any scale marked with temperature is purely an averaging / approximation; even so, they tend to be highly inaccurate below about 100~150°C.

If you need to control temperature to with a few degrees, sanko is probably onto the right idea with a sous vide controller. The better ones control the temperature via some form of PWM - basically, switching the element on & off faster than a normal thermostat, and gradually ramping up the temperature rather than going full-bore "on" up to the set point. Because they know the both the current and target temperatures they can switch the heating element on for progressively shorter and shorter times as the current temperature gets closer to the target, reducing overshoot and maximising stability.

(In the industrial control world such things are often known as 'PID controllers', although in heating applications they're usually simply 'P' or 'PI' circuits/algorithms. Strictly speaking, a sous vide heater would be termed a 'P controller', as it is only responding to the present error and not past or future conditions. In fact, people have hacked industrial PID controllers to drive hotplates...)
posted by Pinback at 1:01 AM on July 26, 2010 [1 favorite]

*rolls eyes* ...sous vide cookers start around $500. trust me, a double boiler on a $10 hotplate will be just fine... it is just. glue.
posted by sexyrobot at 1:17 AM on July 26, 2010

relax a little...deep breaths

Not sure what I might have said to leave the impression there was a lot of panic happening here ...

Anyway, I wanted to make the original question as simple as possible, but I can add more detail. It's glue yes, but not 'just' glue. It's the sizing mixture for treating Japanese paper for printmaking. This is typically done by specialists (I wrote about one man here about 18 years ago), but it is now getting very difficult to get good work out of the 'last man standing' in the field, and he is now over 70, and has no apprentices. Printers like myself are going to have to learn how to do this ourselves from now on.

Proper sizing is extremely sensitive to a multitude of factors: thickness of the paper, the humidity in the air as the paper dries, the amount of astringent (alum) added to the mix, the temperature of the mix at the point the alum goes in (lower than 60C no good, over 70C no good, etc. etc.), the proper water/glue mix (varies with the requirements of the particular printing job at hand), how quickly/slowly you move your brush to flood the paper, and etc. and etc. and etc.

To help me get a handle on what I'm doing as I teach myself this process, I am going to need to maximize control and consistency, and - of course - make constant notes on all the factors. I can then compare the finished product with what I did to get there, adjust as necessary, and hopefully learn how to produce a useable finished product.

So the key point is 'control and consistency', and that's why I was looking for a heating system that will help me do that. I'm also on a pretty tight budget here, so can't spring for some industrial strength auto-control system. There is a photo in that story I linked above that shows his electrically-controlled glue tray, but considering that a local sheet metal shop just quoted me over $500 (in yen equivalent) for a bare tray alone, sans heater, it's obvious that I'm going to have to knock this thing together out of 'household' stuff.

So to see IH heaters in the applicance shop that have 'degree control' seems like a made-to-order solution. But will they actually do what I want? I can't tell ... and that's why I asked the original question.
posted by woodblock100 at 1:26 AM on July 26, 2010

I'm not sure that an induction hot plate is the best idea here - I think that induction hot plates / hobs tend to have discrete power settings (e.g. 10%, 20%, 30%......100%) and, in my experience, it can be very annoying to try to maintain a steady temperature when what you really need is 25% power - you end up switching between the 20% and 30% settings. A plain old electric hotplate with a variable temperate control might be better.

Also, I don't think you can you do the trick mentioned by sexyrobot of putting an air gap between the plate and the glue pan - since induction hobs heat using an oscillating magnetic field, I think that the hob -> plate distance would alter the heating power according to an inverse-square law - in other words, it would be incredibly sensitive to the distance.

If your target temp is 70 degrees C, you might even be better off with a hot tray - something designed for keeping food warm rather than actually cooking it.
posted by primer_dimer at 2:21 AM on July 26, 2010

Whatever the appliance is, unless it has a sensor dangling on a wire (possibly plugged into the unit with something like a headphone jack), and you actually put the sensor IN the liquid, it will not be able to hold a constant temperature where it counts, -- in the water bath. It sounds like the IH units you're looking at have internal sensors (below the hotplate surface). The temperature at a sensor inside the hotplate is only loosely related to the temperature of a water bath on top of the hotplate, which is why it can't give you the accuracy you're looking for.

As a fine craftsperson myself, I'm inclined to think that the old guys specializing in your craft probably have not traditionally used a lot of electronic gadgetry to maintain perfectly constant temps. They may not use thermometers much at all. More likely, they are sensitive to changes in the consistency or smell or behavior of the glue, and they frequently tweak the temp according to what they observe. You'll probably learn to do the same. On the other hand, I know the learning curve can be especially steep when there are many variables and you don't have a master there in the room with you, pointing out what you need to pay attention to and what you're doing wrong. A little technology might well be worthwhile.

Maybe you could jury-rig something using an immersion thermostat (from an industrial or HVAC supplier) to control a cheap hotplate? Your requirements are pretty specialized, so an inexpensive off-the-shelf solution seems unlikely.
posted by jon1270 at 2:23 AM on July 26, 2010 [1 favorite]

I'm inclined to think that the old guys specializing in your craft probably have not traditionally used a lot of electronic gadgetry to maintain perfectly constant temps.

This is absolutely the case. It's the same with my own printing work - there are no measurements; it's all a matter of feel and experience. I'm certainly not intending to try and 'automate' this to a point where I could 'push a button' and it will all work properly. I'm just trying to control some variables as closely as I can, to help me get up the learning curve, as you mentioned.

A plain old electric hotplate with a variable temperate control might be better

Perhaps so ... or perhaps even simpler - just a sheet metal tray for the liquid, with an immersion heater in it. It shouldn't be too hard to find one with some kind of thermostat, I think.

In any case, thanks to everybody for all the thoughtful replies. It does seem like the original thought of using an IH heater may not be the best approach to take, so I'll now concentrate on another solution ...
posted by woodblock100 at 3:20 AM on July 26, 2010

Rereading your original post, if your primary worry is that letting the temperature of the mixture rise above 70 degrees will weaken it too much, then perhaps get one of those IH hotplates AND ALSO a digital meat thermometer - the kind that has a probe on a long wire, separate from the display. Stick the probe in the water bath and set the meat thermometer to some threshold a few degrees below your danger level. That way you'll have a beeping alarm to alert you to serious trouble, so you can focus your attention on the glue and paper without being distracted by worry about the temp.
posted by jon1270 at 3:44 AM on July 26, 2010

I'm working on a wax melter with a target temperature of +130F to replace a friend's frying pan approach, for safety reasons. (Also, I am recently finished with a milk pasteurizer with a target temp of 161F.)

Components in my wax design are an immersed thermocouple, a ring heater, and a cheap PID controller from eBay. These things are cheap, accurate, tunable. I'll share details if you are nice. (and I am sure you are!)

In experiments with water, I can maintain a 5 degree F range at the sensor using the self-tuning PID feature of the unit. (I had to augment the output of the PID controller with a solid-state relay (SSR). ) The particular controller I am using modulates the output to provide 0..100% drive to the SSR, and is not just an on-off control. The whole thing works rather well, but I am still dealing with safety issues.

To get consistent temperature in any medium, you will have to homogenize and destratify the medium, meaning an agitator may be required. I don't know how important this is to your process, but remember, you are controlling temperature AT THE SENSOR, and have no knowledge of the conditions elsewhere unless you measure them.

I don't think that the induction heater part is a show stopper. You can use whatever you want for process heat delivery. Depending on what you want to accomplish and what your process specs are, there are many, many alternatives to achieve a temperature band of whatever precision you wish.

I do this sort of thing a lot. Feel free to email if you run into a wall and want any advice.
posted by FauxScot at 3:56 AM on July 26, 2010 [1 favorite]

Incidentally, there isn't a conversion for Watts to temperature.

A Quanity of material will change temperature in relation to the power delivered (Watts) over time and is a function of its volume, mass, specific heat, and radiation/convection losses.

The model you have to achieve to make a mass stable is to balance heat losses with heat delivery.

It is made simpler by reducing heat loss. Larger masses are usually more stable, but if the material is a liquid, it will stratify, how much being dependent on the viscosity of the material and some other factors. There will always be a gradient in temperature between the heating element and the sensor, too.

Also, the issue of response time is involved. How fast do you want to recover from a thermal perturbation?
posted by FauxScot at 4:04 AM on July 26, 2010

Components in my wax design are an immersed thermocouple, a ring heater, and a cheap PID controller from eBay. These things are cheap, accurate, tunable. I'll share details if you are nice.

Well, one quick question right off the bat. I did a quick eBay search for those units, and there do indeed seem to be many available. (In my case, as I am just a short train journey away from Akihabara in Tokyo, I would try looking there first, though). But one thing I don't quite understand - the ones that seemed to be most common (like this one), are controlling an output of only about 3A. I'm not sure what would be necessary to make a unit like this control the cycling of an electric heating unit (which would be running at around 1200watts or so, I guess ...) Or is this where your SSR relay comes in?

How fast do you want to recover from a thermal perturbation?

Temperature seems to be critical mostly at the point of mixing, to get the ingredients to 'mix' properly. After that, I just need it to remain 'generally' stable. I will be dipping the brush in constantly, which should provide good mixing and homogenity. I will have to be aware of evaporation, adding more water to compensate. And I will be using this thing all year round - in an unheated Japanese workshop. So there is certainly going to be no single 'setting' that will do everything; I will have to learn how to 'play' this thing. But as I mentioned above, the more help I get with the basics - general temperature control - the easier my life will be.
posted by woodblock100 at 5:15 AM on July 26, 2010

The unit you referenced on eBay appears to be similar to the unit I bought for my wax melter. The PID self-tuning works pretty well. You can tweak the PID parameters afterwards if you want, or you can completely use a bang-bang method of control. Very flexible little machine.

Note that the thermocouple that they show appears to be a 'well' type, but when you see it, you'll quickly determine that it's not. It isn't leak-tight. You'll have to make mounting arrangements that accommodate the sensor. (I drilled and tapped a short steel bolt, into which I inserted the probe. I stuffed it full of thermal grease before inserting the probe, to assure fast heat transfer, but the bolt adds mass and thus reduced sensor response time. You can come up with something different, of course, and I would certainly point you to for a huge variety of sensor types. I finally settled on an immersion probe, which gives me comforting feelings that I am measuring the wax temp, and not the enclosure temp. Downside is I have to make sure there is always enough material in the pot, or I won't read a wax temperature. I'll read the air temperature.

Omega has some ring heaters that are cool. If you order the mounting clamps, order more than one. Their price is for 1 pc.

The 3 amp rating of the controller is for the relay output. The SSR output depends on you supplying an external SSR. The controller only drives it. You can choose whatever one you want, of course. The current rating needed is the Watts of your heater divided by the voltage... which I presume in Japan is 230 Volts? Here is one that will work.

If you can't find one in Japan, I'd be surprised. I think I have an extra one here abouts that you can have, if you want.

Hope that helps. Good luck.
posted by FauxScot at 5:51 AM on July 26, 2010

I've used a combination of a cheap industrial temperature controller ($15 on ebay), large crock pot, and a bowl floating in it as a water bath and it maintains +/- 2C pretty well in that range.

The crock pot is good because it's designed to provide heat right in that 150F range so the oven is on probably 80% of the time and it's nicely insulated. There's a lot less up and down than there would be with a hot plate.
posted by pjaust at 5:58 AM on July 26, 2010

I picked up a cheap thermal immersion circulator on eBay for ~$50, have a look there. You could also buy a controller and supply the rice cooker yourself which are $150.
posted by wongcorgi at 7:31 AM on July 26, 2010

So the key point is 'control and consistency',
wow. (you are doing 'art' wrong). in a nutshell, there is no such thing as 'control' (we live in a giant explosion, everything is chaos, etc etc)...the temp. guage on the stove will be innacurate, the thermometer will be inaccurate, the weather will have no control over these things and a desire for control will only lead to suffering. keep it simple. now, consistency i can help you with...that water bath for the glue is of vital acts as your buffer (all heating elements are going to be doing some degree of cycling on and off) the bigger the bath, the more stable the temperature. also, it will take longer to heat up and get started. tradeoff as necessary.
remember: it is the inconsistencies that make art beautiful.
posted by sexyrobot at 10:09 AM on July 26, 2010 [1 favorite]

you are doing 'art' wrong ... remember: it is the inconsistencies that make art beautiful

Sorry, but this simply demonstrates your total and utter lack of knowledge of what I am doing. But thanks for trying ...
posted by woodblock100 at 2:21 PM on July 26, 2010 [3 favorites]

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