Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

Thermocouples are made by welding two wires of different materials together. That's what makes the bead on the bead type. The other terminations are just methods for making rugged ways to mount and protect the bead, which is kind of fragile. The wires will stay together over the entire operating temperature range of the TC. Thermistors are usually soldered into their circuits. The temperature at which the joints will fail is a function of what type of solder was used. Depending on the type of lead material used on the thermistor, it may also possible to weld them. The RepRap glass bead thermistors that I bought have magnetic leads, telling me that they are probably weldable, and I would expect that the thermistor that comes with the cable attached is probably welded.

 

How  HIGH is what you say is high heat conditions that weaken the soldered thermocouple ?
Considering that some  are rated at over 2,000 F degrees
while others only are rated to 400 F.
 

On 12/30/2016 03:15 PM, 'keith printy' keethpr@... [Homebrew_PCBs] wrote:

 

The lead wires on thermocouples usually are soldered with some type of lead based solder. In high heat conditions this can come loose or oxidize to where it is weak . when I worked in an electronics plant and we dealt with high heat we always used thermocouples. When we used a packaged temperature controller they were usually type j . when we used microprocessor control they were type k .

 

 

+++++++++++++++++++++++++++++++++++++++ 

Which one?

RE:
 approach in favor of a purchased controller with a type K thermocouple. This cost USD 8.99 on e-bay from China. I have not yet put it into service because of the melted gears.

On 12/30/2016 01:04 PM, Harvey Altstadter hrconsult@... [Homebrew_PCBs] wrote:

 

Rob,

The Passive IR gun type thermometer is only used for making initial measurements, not for a feedback type control system. The PIR thermometers have a distance to spot ratio. The Sparkfun unit mentioned previously has a ratio of 12:1, meaning that at a distance of 12 inches, the detector is reading the temperature in a 1 inch circle. The thermistor and thermocouple approaches measure the temperature at the point of contact, a much smaller area. With the PIR thermometer, I could not read the rollers in my laminator at all because they are surrounded by the heater structure except where the paper path is. Those slots were too narrow for either of my PIR thermometers. Another issue with the PIR thermometers, usually ignored in the advertising, is that the emissivity of the surface being measured has an effect on the readings. In plain English, this means that two surfaces made with different materials could be at the same temperature, but give different readings on the PIR. Surfaces with multiple materials, if they fall in the reading area, will give a reading that integrates the two readings, rather than providing a true reading.

I tried to use the RepRap thermistor in my original control circuit, and found that while it works, the resistance vs temperature curve is very flat near 200°C, making the controller action sluggish. I abandoned the thermistor approach in favor of a purchased controller with a type K thermocouple. This cost USD 8.99 on e-bay from China. I have not yet put it into service because of the melted gears.

As far as the location of the thermistor in the Royal laminator, this is a mechanically simple location to use, and if we think about it, we understand that there is no need to measure the actual temperature of the rollers, at least in the original application of the laminator. As long as the temperature drop between the heater and the roller is known, the system can be calibrated for proper operation with the thermistor mounted on the heater structure. The Harbor Freight unit uses mechanical thermoswitches mounted on the heater structure.

In our toner transfer case, it is not clear (at least to me) that measuring at the heater is sufficient. Since we are putting single or double sided thermally conductive boards of different sizes through the rollers, I am not sure that the temperature drop is sufficiently constant that we don't have to measure the temperature at the roller. I was experimenting with this when my gears turned to mush.

Harvey

 

On 12/30/2016 8:24 AM, Rob roomberg@... [Homebrew_PCBs] wrote: