Heating Blanket and Temp Controller on the Cheap(er)

[btberlin]

I recently purchased a new heating blanket and controller for a bending machine. I looked around various suppliers to see if i could do better in price than the luthier suppliers. There is a company called Omega, located in CT, that will sell one of to an end user. I purchased a 6" by 36" heater, part number SRMU020636, for $78 plus about $10 shipping. This is a 2.5 watt per square inch heater, about 500 watts total, that can reach as high as 450 degrees F in free air, if you leave it on for a long time. If you replace the "2" in the part number with a "5," you will get a 5 watt per square inch heater - same price. It will reach temperature considerably faster. The typical heaters sold by luthier suppliers is 3 watts per square inch. I believe the 2.5 watt/sq in model can be controlled by a 600watt light dimmer, and was about to do that, when i noticed that Amazon lists numerous temperature controllers for under $40. These are called PID controllers, and they use fuzzy logic to maintain a set temperature quite precisely. The one I purchased came with a Type K thermocouple, for $33. To that I added a SSR (solid state relay) and a heat sink for the latter. Later, I bought a second thermocouple probe for about $7 - it has a longer probe section than the one that came with my controller. The PID controllers are tiny, about 2.5 inches square in the face, and about 3 inches deep. They are designed for panel mounting. The one i purchased, AGPtek® K-Type SNR PID Temperature Controller, has understandable setting instructions in English. Most of the reviews I read on Amazon complained about the instructions, not the devices. But there are dozens of controllers available. The SSR that I bought is 25Amp SSR-25DA Solid State Relays for Temperature Controller, and the corresponding heat sink is Amico Aluminum Heat Sink for Solid State Relay SSR Small Type Heat Dissipation (note that the mounting holes are threaded for a 4mm machine screw, so you may want to re-tap it). Overall the setup works nicely. If you are not familiar with wiring, and wiring practices, though, get the help of someone who is, to avoid blowing any fuses or hurting yourself, possibly quite badly. The wiring diagram is on a label on the side of the controller. If you decide to buy the probe separately, here is the one I chose on Amazon - Amico 1M K Type 800 Degree Celsius Thermocouple Sensor 5mm Probe Dia. It has a 6 inch long probe section and is about 2 tenths of an inch in diameter. As a setup test, I used the heating blanket to flatten a piece of Cocobolo for a back - the piece had a potato-chip curl in it, but coco is too valuable to toss the curled piece. I used the usual sandwich of Kraft paper, sheet metal, blanket and wood, but sandwiched the entire thing between two pieces of 3/4 inch plywood which i squeezed with C clamps. Worked beautifully to permanently flatten the wood, although an enormous amount of dark resin oozed out of the wood onto the surface as a result of the heating. At first i thought i had charred the coco, but, the Kraft paper was intact, so i realized it wasn't char. The resin scraped and sanded off fine.

[tippie53]

There is one thing . I used omegas when I started and they bunt up rather quickly. I used Protherm blankets where I worked and they lasted so I swapped out . Never looked back. A 2 1/2 watt blanket can loose a lot of heat when you start. This can cause hot and cold spots. They take longer to heat when you are in actual contact with the wood. Also they are not warranted.
A good tool is an investment . Not all heat blankets are created equal. There are many ways to control the temp and I do suggest you don't use one without a timer. These don't need to be high tech just simple controls will do the trick.

[btberlin]

A few points here - first, the Omega heater has a 1 year warranty against manufacturing defects. See chat below. One year is a very generous warranty for any electric or electronic product. The heater also has a safe limit of 450 deg F, and exceeding that is sure to cause it to burn out prematurely. Using a timer alone will in no way guarantee the temperature that the blanket reaches.

Omega Engineering - Your one-stop source for Process Measurment & Control Support Chat
[Roy] Hello, my name is Roy, how may I help you?
[Bert] HI
[Bert] I recently purchased an srmu heater blanket. can you tell me what the warranty period is on that product, (if any)
[Roy] What is part number or order number?
[Bert] hang on a sec
[Bert] let me look up
[Bert] p/n SRMU020636
[Roy] 1 year for manufacturnig defects.
[Bert] thank you.
[Roy] Thanks for contacting us. Please do not hesitate to contact us again if we can be of further assistance.

Second, I inferred that this board was populated by a range of luthiers and luthier hopefuls from newbies to old-timer pro's. A newbie may have other criteria to select tools - for example, price - especially if s(he) is not certain that s(he) will be building 100 guitars in the future. For a few a year, the Omega is fine. And, at $78, a bargain.

Third, a 2.5 watt per square inch (not a 2.5 watt heater, which would be less than the power used by a night light bulb) is built exactly like the 5 watt heater, with resistance wire that has twice the electrical resistance per lineal inch. The internals of the heater, the heating wire, is laid out by machine or on a jig, in a uniform linear pattern, so i am not sure where you come up with the statement about cold spots.

Fourth, i did note in my post that the 2.5 watt per square inch (this is a 500 watt blanket, for 6 inches by 36 inches) takes longer to heat up. This was selected intentionally as I originally intended to use an scr 600 watt dimmer to control it. The result of that is that i wait 20 minutes instead of 10 for it to heat up to temp. Big deal. At the same price, btw, the omega heater can be purchased in 5 watt/square inch, or 1000 watts. That will heat up even faster than the 3 watt per square inch types.

Fifth, yes, there are many ways to control the temperature. Modern electronic PID controllers are one way. Interestingly, they can control temperature with a precision level of plus or minus a fraction of a degree, not that such a level of precision is needed. They also generally have two alarm outputs and can sound a warning or light a light if the set temperature is exceeded (or not reached). A timer is also a cheap way to sort of get to the set temperature, but requires some experimentation to get the appropriate time correct. Either way, the user who "sets and forgets," leaving the heated bender unattended is asking for trouble. The flashpoint of wood is over 500 degrees F, and the heating blanket would probably start smoking before the wood would. Or burn out after a few minutes, as its upper limit temperature is 450 degrees F. Nonetheless, bad things can happen relying solely on a timer if unattended. But, that would be true of ANY power tool left unattended. I noted that caution is appropriate in my previous post.

Last comment - PID timers aren't high tech any longer. They are inexpensive consumer items, found for sale on Amazon for about $30. They use "fuzzy logic" of the type used in consumer-level heating devices, such as rice cookers. Type K thermocouple probes aren't high tech either. They run about $7. My $25 multimeter, purchased a few years ago, came with a bead type thermocouple for temperature measurements. This is not high tech today. It is, however, a standard way of safely and accurately measuring temperatures in the range used on bending machines.

bert

[tippie53]

You want the heat to get into the wood asap. I use the 5 watters as I am done bending in about 3 to 4 min. They heat fast and can hold very steady. If you heat too slow the wood will set and you run a higher risk of cracks.
Yes there are many ways to make things work and to that I agree. Too much water can cause as many issues as too little.

[btberlin]

Thanks. I'll be careful to watch that the temperature doesn't cross the threshold at which wood lignin turns from a plastic to a glass, and the wood fractures. I haven't seen that yet. The 2.5 watt per square inch blanket should be pretty similar to the 3 watt per square inch ones that LMI sells. The transition, according to one wood science article i read, is about 350 deg F. I have no idea, though if it applies to all woods; IIRC the test was done in Japan on bamboo. Of course - lignin is lignin, regardless of the species. I think.

bert

[tippie53]

the main variable is the placement of the blanket.
If you use a wet paper you can place the blanket on the wood. This helps to keep the heat near the work. Also the thickness of the wood can come into play. In high figure woods I still rely on super soft ll . Also while the controllers and such work I still trust the thermometer for the temp.
The thermal mass load can pull heat away from the wood. I think you will be ok for the most part , knowing the variables and the weakness , you can use that info to help.
An interesting discussion.
there is more than one way to fry an egg

[Dave Sayers]

Interesting, I was thinking of looking for a temperature controller, but although I've only used my blanket a few times now, it gets up to temperature so quickly that I think it's easier and probably safer to simply stand with it. The blanket I have gets hot so fast I don't even trust a timer. The last job I used it for was to bend hardboard to make a holding mould for heat bent wooden binding. (Bend a few bits at once, then store.) With plenty of moisture I'd bent each piece within 8 minutes, and even then I had to keep turning the blanket off. Maybe I'll get a temperature controller just in case I get distracted.

[btberlin]

A PID controller would be just the ticket for your needs. Regardless of how fast the blanket heats up, the PID controller, available from Amazon, along with the rest of the components, would get the blanket to the right temperature, and hold it there for as long as you like. I just submitted an article to a luthier magazine, but I can send you the text privately. It gives the details of the parts i used to make the controller. Looking at my invoice from Amazon, I spent $47.38 (Prime account, so, no shipping) on the parts. The controller I purchased has alarm output(s) that can be set however you like, and could be made to signal via a buzzer or beeper if the temperature gets out of control. When you look at the pictures of the controller in Amazon, you will recognize the device as similar to the ones sold by luthier suppliers. I do caution though that you shouldn't try to construct one unless you are familiar with electrical wiring, best wiring practices, and take proper precautions to avoid electrocuting yourself or starting a fire. But, that said, if you give me your email, i'll send the text.

bert

[tippie53]

You are free to post this info here if you want. It would be good to share that info.
thanks

[btberlin]

If you are interested in building a PID temperature controller, you are in luck. The parts to do this are easily available on Amazon for under $50. From appearances and description, the controllers are functionally identical to those being sold by some luthier suppliers. A serious note of caution, though, before buying and assembling the parts. There is a complete wiring diagram on the side of the controller. If you can't make sense of it, don't try to put it together. Get the help of someone who can read the diagram and translate it into a wired, finished temperature controller. Putting it together incorrectly can cause a fire, or serious bodily harm to yourself.

That said, you'll need the following - a PID controller module. I purchased the AGPtek® K-Type SNR PID Temperature Controller from Amazon; it was chosen because the reviews indicated that the instructions were understandable. This is a major problem as they are translations, some ok, some very poor, of Chinese. Most of the controllers reviewed well from the perspective of performance. By the way – PID stands for proportional-integral-derivative. An article in Wikipedia explains how these work.

Next, you need a way to switch the current that the heater draws. The PID cannot do this itself. Instead, an SSR (solid state relay) takes the place of the old electromagnetic clicker relays; the one i purchased was the 25Amp SSR-25DA Solid State Relays for Temperature Controller. This needs a heat sink to keep it cool while it passes the current of the heater. I used an Amico Aluminum Heat Sink for Solid State Relay SSR Small Type Heat Dissipation. It's a chunk of aluminum with fins on it to dissipate the heat.

Finally, you need a type K thermocouple probe to sense the temperature of the heat blanket - wood - metal slat sandwich on the bender. The PID I listed above comes with a probe, but the tip of it is rather short. I re-formed the crimp on the probe shield, and was able to simply slip the threaded mounting collar down, leaving a nice, relatively thin probe attached to a flexible armored cable that can easily fit into the bending setup. Or, you can purchase a long-thin K thermocouple for under $10 on Amazon.

The accuracy (correctness of the temperature reading) is listed as .2%. The precision is .1 degree F. That puts the controller in strict control of your heater with almost no chance of overheating, no matter how fast it gets to temperature. So, if you set it to 350 degrees F, that's what the temperature at the probe will be, regardless of how fast it came to temperature. Nevertheless, never ever leave the heater unattended. In theory it will burn itself out at about 100 degrees F below the flash point of the wood you are bending, but bad things sometimes happen. The PID also has an alarm output that you can hook up to an external light, or buzzer or some other alerter to let you know that the temperature is going out of control.

Bert