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I Forge Iron

Pyrometers


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I am starting this new thread on pyrometers as I believe that there is much interest in accurate temperature measurement in heat treating.
I have a thermocouple pyrometer which reads to 2000 degrees F. which I have used in a brick pile forge. I have very successfully used this pyrometer in heat treating some pipe notching dies which I machined out of O6. I heated the dies in molten salt (common rock salt). I inserted the thermocouple probe into the molten salt along with the suspended die and when the temperature rose to the desired hardening point I reduced the flame and let the steed die soal a little toallow the die to come up to the temperature of the salt bath. Just make sure that you preheat your steel as the salt bath will heat your steel very quickly. The advantage is that no part of your steel can get no hotter than the temp. of the salt bath. Great for thin edges and safer than molten lead. Be ready to hear a pop when you quench as there is a skin of selt on the steel.
There are other pyrometers that work on an optical principal which I know little about but would like to hear more as they will read higher temps.
If you think that this is a worthwhile topic please add what you know about temperature measurement.
Warren

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There are other higher temperature reading pyrometers out there including many hand held units like Multimeters for electrical work. While I don't often need to be that accurate in my work, we use pyrometers for the kilns in both the pottery and glass work that my arts group does. All of them read way over 2000 F.

Heating objects in a molten liquid is a great way to transfer heat, much better than hot air in the standard forge fire. There is one danger if you overheat the liquid salt of course, you release chlorine gas, forming dilute hydrochloric acid and sodium. I've worked on wood fired salt kilns for glazing pottery, the sodium being the chemical we wanted and chlorine just the bi product. Chlorine of course is pretty toxic and the sodium can be reactive.

Correct me If I'm wrong.

Edited by frogvalley
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Thank you for the comments. Isn't this a great site? I am sure learning here. So calcium chloride is a quite common salt but I am learning hear can break down the salt molecule, therefor any salt that is chlorine based is dangerous, I guess that would include calcium chloride, still, I suppose it would be OK if done outside and you don't get your nose too close as I have done with NaCl. There is calcium carbonate which has carbonation properties and may add carbon to the surface. (?) How about calcium nitrate? Calcium sulfate and calcium phosphate could/would be damaging to steel. I think that some of this stuff won't melt. Of course there are other salts beside sodium and calcium based salts which I know little about. There many risks in life that we are exposed to daily. One just needs to understand the risks and take appropriate precautions.
Warren

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I have noticed a lot of cheap thermocouple meters which claim to measure up to 2400F. If you use these over 1600F, the probes will stop working. Unless the manufacturer is reputable, it will not be disclosed that the cheap insulation in the probes degrades at a much lower temperature than the meter range specified. I have a type K thermocouple with ceramic bead insulation. It is useful for heat treating red hard steels. But I don't like to subject it to over 2000F, because even though the ceramic insulation can take it, the higher ranges are only for the thick (14 ga) wire thermocouples. The thin ones are inherently limited by corrosion at the high temperatures, regardless of the range of the meter or the bead insulation.

I have also been curious about the use of simple optical measurements to measure then higher end of the forge temperature scale. There is a plan for a disappearing filament pyrometer floating around the Internet. Has anyone made one of these and had success with it? How about an absolute measuring single color photodiode pyrometer? I hear that these things put out about 4 volts into near-infinite impedance at 2500F.

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  • 4 years later...

I realise this is an old thread but I wonder if anyone else has been using an optical pyrometer since this thread has been posted and if there is anything to watch out for while using one ?

 

I managed to obtained access to an  (>40 years) old PYRO optical pyrometer which uses a disappearing filament method.

post-39957-0-21545000-1367393296_thumb.j

The pyrohead, meter, photo tripod adapter and a range of focal length lenses all come in this handy in a lab padded suitcase.

 

The only thing missing is the original power supply. 

Fortunately I used the same pyro at university nearly 40 years ago so I am familiar with how it works and know that it uses a variable ~3V DC power supply (PS). It will even function on 2AA batteries but it can only be  accurately calibrated using a variable PC. I don't have a 3V DC power supply so I adapted a 5 - 12 V PS and a large variable resistor.

 

Here's how it all looks when it's setup

post-39957-0-34868800-1367393465_thumb.j

The range of lenses means it can be used over distances from about 5" to infinity from the item being measured. This is of course necessary as the unit should not be too close to the forge. 

 

Here's the scale

post-39957-0-71887500-1367393521_thumb.j

- plenty of range and sensitivity but I doubt I will be needing any more than the first range (650 to 1400ºC). My experience is that in practice the object needed to be at ~750ºC before it could be measured reliably but that is still a useful starting temp.  It even has a couple of calibration points on the scale whereby it has to be drawing so many mA at specific points in the scale. These points can be checked with a mA meter and the PS V tickled up and down til it shows the right current.

 

My forge is awaiting a gas line to my shop so I can't play with it yet.

I have a a thermocouple and IR scope type thermometer that I can cross check with.

I will report back when I make a few measurements

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There is one danger if you overheat the liquid salt of course, you release chlorine gas, forming dilute hydrochloric acid and sodium. I've worked on wood fired salt kilns for glazing pottery, the sodium being the chemical we wanted and chlorine just the bi product. Chlorine of course is pretty toxic and the sodium can be reactive.

Correct me If I'm wrong.

Sodium chloride boils at higher temperatures than we use. When it boils it makes sodium chloride gas, not chlorine and sodium. You need electrolysis to make sodium gas with salt-water. Heating won't do it anymore than heating water would make oxygen and hydrogen.

 

That said, extremely hot molten liquid salt is still a potential hazard. Get moisture or oil in it and you've got a makeshift cannon.

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Sodium chloride boils at higher temperatures than we use. When it boils it makes sodium chloride gas, not chlorine and sodium. You need electrolysis to make sodium gas with salt-water. Heating won't do it anymore than heating water would make oxygen and hydrogen.

Electrolysis of a water NaCl solution will make Cl and H gas but it cannot make Na gas, so the Na stays dissolved in the water.

When solid NaCl is molten it no longer exists as a crystal of NaCl but liquid Na+ and Cl- ions. In this state (ie no water) electrolysis can extract liquid Na, and Cl gas.

If the salt is held molten at near boiling temperature (1413ºC) it can release small amounts of Cl gas. The amount is small but can be enough to "irritate eyes" but this is a minor risk compared to other problems with using molten salts.

Like you said heat treatment won't be working at these temperatures

Despite the above, NaCl is used in fire extinguishers to fight light (magnesium, sodium and potassium) metal fires. The salt is encased in a fire retardant plastic which immediately melts releasing the salt which cools the fire and together with the plastic smothers the fire.
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