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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 .
From: Homebrew_PCBs@yahoogroups.com [mailto:Homebrew_PCBs@yahoogroups.com]
Sent: Friday, December 30, 2016 2:29 PM
To: Homebrew_PCBs@yahoogroups.com
Subject: Re: [Homebrew_PCBs] Temp upgrades of lam's NON CONTACT PASSIVE INFRARED
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:
Where do you mount this device without destroying it and melting solder on the supporting
chips in the toner work PCB heating path?
How far do you mount it from the desired location where you are pinpointing temperature measurment?
While I would agree that a Non contact passive infrared rules..........
I would like to some day compare the accuracy of the $20 passive infrared with
the K type thermocouple and a simple thermistor.
Noting that my ROYAL laminator heater control was done by a contact thermistor clamped to the aluminum heat shield
behind a roller heater.
When I was looking into thermocouple circuits and supporting chips it looked like about $15 gets that done
and
5 cents buys a thermistor....
or maybe a dollar if it has wires.
New Reprap NTC 3950 Thermistor 100K with 1 Meter wire for 3D Printer TSCAOn 12/30/2016 09:02 AM, AncelB mosaicmerc@... [Homebrew_PCBs] wrote:
Non contact passive infrared rules...
Get one of these...
https://www.sparkfun.com/products/9570