New heat strips and controller

[rodlonq]

I bought some heat strips from a Chinese seller on eBay a little while back and I haven't had a chance to test them yet. I can say they look very study and have nice long leads. The link to them shows a 120V heat strip but it is available in 220/240V as well, with 40, 45 and 50 mm widths. These weren't on the site until I made an enquiry with the manufacturer to see if they could do some custom items suitable for bow making. If anyone wants to buy a pair of them, hit them up for a discount as the price is for one off sales (I believe they manufacture to customer order).

http://www.ebay.com/itm/Bow-Making-Long ... 1709798883

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Also bought a digital controller from an Aussie seller. I am going to mount the sensor as close as possible to the lower heat strip so it can sense and control the temperature. It is able to control down to 1 degree C headband.

http://www.ebay.com.au/itm/220-240V-Tem ... 417da1ed3f

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I am looking forward to testing this gear but it may be a while before I get the chance. Just thought I'd share this find as someone else might like to have a go at it. BTW I have no affiliation with either of these sellers, just hoping to help fellow bow makers buy gear at reasonable prices. I am confident this setup will work as well or better than my Bingham's Projects set up and for lower cost. Jus the aware this is 240 V so be wary of bare connections. The terminations are well shielded but it could be put in a box some sort to make it extra safe.

Cheers... Rod

[bigbob]

I've bought a couple of those controllers, Rod few years back and managed to frizzle both with my inept attempt to wire them in. Just make do with an oven temperature probe now and crack the lid as necessary.The strips look interesting and would cut oven time considerably which is always a bonus.

[Muz1970]

It will do the job I'm sure Rod. Wouldn't expect the heat trace to draw to much current. If it does use the PID controller to switch a relay and use that for the heat trace. This is what I've used on my oven.

Cheers Muz

[rodlonq]

Muz, the 40 mm heat strips are rated to output 150W @ 220V (~180W @ 240V) so the pair will draw around 1.5A. The controller is rated for 10A so it should make a lot of bows before the switching is worn out. Th wider strips have proportionately more power than the 40 mm strips. The advantage of the heat strips is that they put the heat right where you want it rather than heating a hole box and all the air in it which has probably 10 times the thermal mass of the bow and form surrounding it. It gets up to temp very fast so the cure time is cut down considerably and a lot less energy is used. I always cool the bow in the form with the pressure on though so it has no chance to warp by being taken out of the form while the glue is at high temp. Thats not an issue unless you are trying to produce more than one bow per day. The other plus is that the temperature distribution should be a lot more even than in an oven so there should be no chance of "hot spots".

Cheers... Rod

[Muz1970]

Hey Rod, so have you tested the heat strips yet?

Sounds like a bloody good excuse to build another bow.

[rodlonq]

Hehehe... better late than never. I have finally put together the temperature controller and it is doing it's maiden voyage as I write. I have to say I am more than pleased with the results of this system. Firstly I'll list the parts;

The pair of heat strips as above 150 W @ 220 V - 2 m long with 1 m insulated leads,
2 x 10 amp 240 V 3 pin male plugs
A sealed junction box - about 240 x 300 x 100 mm - from electrical wholesalers
A twin 10 amp GPO with extra switch (normally used for a light I think)
The temperature controller from above 240 V primary.
A 240 V rheostat (dashpot), can't seem to find the specs but I believe it is capable of dissipating up to 300 W
A Volt/Amp/Power meter 100 A, 80-300 V
The power meter is not necessary but because I am an information junkie (nerd ) I just couldn't resist having it for an extra $20.

3 Control Box.jpg (219.03 KiB) Viewed 4493 times

So I tested the setup with the heat strips wrapped in a blanket and decided that the rate of temperature increase may be good at around 200 W input. So I set the rheostat for 200 W. The temp controller is set to 65C cutout with a 1C dead-band, meaning it cuts back in when the temperature of the sensor falls below 64C. There is a little bit of overshoot and undershoot because the sensor is displaced from the actual heat strip but it is only 0.2C.

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It all seemed to work fine so I used it for the first layup in my new reflex reflex flatbow form.

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I have been watching it for about an hour and the switching cycle is incredibly uniform as follows;

1) The thermostat switches the power on when the sensor reading falls below 64.0C, i.e. at 63.9C;
2) The temperature of the sensor continues to fall for about 10 seconds and drops to 64.8C, after which the temperature begins to rise;
3) The thermostat switches off the power at 1:06 min when the temperature reading hits 65.0C;
4) The temperature continues to rise for a further 10 seconds to a maximum of 65.2C after which the temperature begins a steady decay until 2:45 mins from the start;
5) repeat the cycle from 1)

As an addendum, the first switching cycle started only 15 minutes after I turned it on. Now the second hour is almost up and I have to go back to the shed and turn it off. So far everything looks good (well it hasn't caught on fire or anything like that) and I am thinking it is nearly beer o'clock. Hope this may be helpful to anyone out there that may want to have a go at this system.

Cheers... Rod

Edit: 4 hours after turning the system on it is almost cooled to room temperature and ready to take out of the form to start marking out for shaping. I liking this. Shame about it is the lovely wife just reminded me the lawn has to be mowed today....

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[rodlonq]

I found a thermostat controller that may make the heat strip option more viable. I am going to order one with my set of 240V 50 mm heat strips and give it a go. The one issue I see here is that it has no power level control, it is probably only On or Off, so it will apply full power until the set temperature is reached then turn the power off. This may cause a lot of oscillation in the temperature.

HeatStripController.png (145.87 KiB) Viewed 4419 times

http://www.ebay.com/itm/281977330525

It may also make a suitable replacement or additional upgrade for anyone using the Bingham's Projects heat strip system.

Cheers... Rod

[yeoman]

I found a thermostat controller that may make the heat strip option more viable. I am going to order one with my set of 240V 50 mm heat strips and give it a go. The one issue I see here is that it has no power level control, it is probably only On or Off, so it will apply full power until the set temperature is reached then turn the power off. This may cause a lot of oscillation in the temperature.

Perhaps, and perhaps not. In my heat box the thermostat has a range of only a couple of degrees between switching off and switching on again. When the box is 'at temperature', it doesn't vary my more than three or four degrees celsius, as long as you don't open the lid.

Sounds like it could be a good bit of kit.

Dave