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LAN_403

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Wed Sept 30 2009, 05:06PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

We have an instrument at work (that costs enough even government labs do a double take) that had 3 pyrometer detectors coupled into a single fiber probe, and by looking at the ratio of the signal from each detector it could determine the emissivity of the sample. I am not sure how it would apply to things it wasn't designed to measure (eppi films), but it is possible.

In any case, it seems that it would be more useful to focus on a practical design for such a dispensing head before getting too bent on measuring the temperature of the metal down to the nearest degree.

It seems to me the biggest issue you would have is preventing freezing of the metal, since an induction heater will do its best to only heat the metal that is inside the coil--but to make anything useful a tip will need to extend down some distance from the coil. When you shut the machine down all of the metal is going to freeze, so the only solution I can think of is to make sure the metal can't went the tip (depending on the allow, ceramic would be nice or possibly water putty for a more realistic alternative), and having the passage from the melting zone to the tip angled slightly outward--such that you can run the wire feed motor while the system is still cold and break the frozen blob loose, then engage the heater and melt it off, and purge the tip with liquid metal to heat it to operating temp.

However it seems like after all of the effort, using a simple resistive heater as was mentioned earlier would be a heck of a lot easier. If you can live with Al as the hottest metal you want (~660c) an Incoloy heater cartridge would work fine, they are rated to 750-800c. If you use strait nichrome wire around a ceramic tip you could run about 1000c (possibly more, but lifetime would be reduced), which should almost get you brass/copper/silver melting points. Also if you just have a heater core with embedded nichrome in it you heat the whole tip up, and avoid freezing issues.

Also with a resistive heater application you wouldn't have any problem embedding a thermocouple in the assembly to regulate temperature of the tip

doctor electrons

Wed Sept 30 2009, 09:20PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Steve McConner wrote ...

Can you explain your claim that a thermal imaging camera isn't affected by emissivity?

Sure!
Since there is no such thing as a Black Body, the Infrared Radiation of normal objects will appear to be less than the Contact Temperature. The rate (percentage) of emission of Infrared Radiation will thus be a fraction of the true Contact Temperature. This fraction is called Emissivity.
This is a quote. "To make a temperature measurement of an object, the thermographer will refer to the emissivity table to choose the emissivity value of the object which is then entered into the camera. The camera's algorithm will correct the temperature by referring to the emissivity percent and calculate a temperature that would more closely match the actual Contact Temperature of the object.
If possible the thermographer would try to test the emissivity of the object in question. This would be more accurate than attempting to determine the emissivity of the object via a table. The usual method of testing the emissivity is to place a material of known, high emissivity, in contact with the surface of the object. The material of known emissivity can be as complex as industrial emissivity spray which is produced specifically for this purpose or it can be as simple as standard black insulation tape, emissivity 0.97. A temperature reading can then be taken of the object with the emissivity level on the imager set to the value of the test material. This will give an accurate value of the temperature of the object. The temperature can then be read on a part of the object not covered with the test material. If the temperature reading is different, the emissivity level on the imager can be adjusted until the object reads the same temperature. This will give the thermographer a much more accurate emissivity reading. There are times however when an emissivity test is not possible due to dangerous or inaccessible conditions. In these situations the thermographer must rely on tables."
Thermal cameras have the ability to calculate out emissivity, all others do not. Sensors and lasers have big trouble with emissivity. And Harry is correct that the more high end laser thermometers do in fact have the adjustment for it. However they are very inaccurate.
MinorityCarrier made a statement about having to factor in emissivity that is also correct, it is a must. Thermal cameras are simply better at doing this than any other technology known. Thermal cameras measure temperature in the entire field of view, sensors and ir thermometers measure in a very narrow spot size. Ambient as well as a gradient of temperatures from the target to the ambient surroundings are measured simultaneously. This gives a greater and more reliable picture of the objects actual core to surface temperatures.

Conundrum

Fri Oct 02 2009, 09:45PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

i have several thermocouples here as my soldering iron keeps blowing elements :( used up three so far.

seems that the replacement elements are *not* as reliable as the ones supplied with new irons. Â£10 and a lot of time wasted.

Proud Mary

Sat Oct 03 2009, 12:00AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Conundrum wrote ...

i have several thermocouples here as my soldering iron keeps blowing elements :( used up three so far.

seems that the replacement elements are *not* as reliable as the ones supplied with new irons. Â£10 and a lot of time wasted.

I made the mistake I'm always warning others about and dumped a mere 20J into a triggered gap I've been working on, and my soldering station about two feet away died at once, but I can't honestly say I was ever that taken with temperature readouts and all that. I still have Antex and other irons cover the range 15W - 180W so shan't be spending any more money on fancy soldering stations with parts that are hard to get. I do everything with Ersin multicore Pb solder, so there's no need for fancy adjustments to accomodate novelty lead-free solders and all that jazz.

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