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
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 TSCA
On 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