Do you measure metal temperature and how ?

Anyone used these S-type WRP-100 kiln probes directly in an aluminum melt?

Here's one example, though there are several more sellers. Note the fully enclosed and protected tip.

 

I have one WRP-100 ordered at AliExpress years ago, but barely used. I have two other type S thermocouples using thicker wire (0.3-0.5mm) sheathed in a similar Al2O3 tube. I used these both several times inside the furnace chamber without any problems. But I never immersed it in metal. My furnace is usually too hot for K type (1300-1400 C).

 

There's a guy on Ebay selling 80/20 and 95/5 tungsten-rhenium wire by the half metre length. It gives a non standard thermocouple similar to a type C good to at least 2329 degrees C.
The seller is selling Soviet era stuff in Ukraine and I have bought some wire off him. It's susceptible to oxidation so I was thinking of TIG welding short bits onto pure tungsten wire for the extension and those junctions should cancel out being the same metal on each side.

The best way to find the listing is to do a search for wolfram rhenium on Ebay.

 

Thanks everyone for the idea and reference. I'm understanding now why the TC type matters a bit less than what protects it from the melt.

I spent an hour or so reading about both the setup and math behind doing Seebeck cold-junction calibration. After seeing the price-tag on gear w/ accuracy of "+/-0.4%+0.2C" (IIRC), ISTM calibration would be required to go from "could be correct" to "might be correct" with a hand-made "similar to type C" junction. The article I read detailed a lab setup with an "expensive magic machine" that can maintain a 0°c cold bath indefinitely, and a probe with TWO junctions (one for the ice-bath). Measurements from that can then be used to derive a polynomial and/or lookup table for any given junction type. While fascinating, regardless, it was complex enough for me to give up before considering DIYing my own junction

At this point, I'm intending to TIG together a kaowool insulated steel box to hold the WRP-100 (with probe end sticking out), at the end of a long handle. After pre-heating, and dunking, what should happen if the melt is indeed "eating it" (i.e. will it be observable)? At $20/each, if these WRP-100's only last 10-dunks, perhaps they're worth-while. At least for the short-term, as the K-type in a carbon sheath seems like the tried/true way to go for AL dunking.

Another thought I had: Since I bought two WRP-100s, maybe for the second one I'd try coating it in a very thin layer of refractory or mortar of some sort. Just enough to "soak in" and fill up the pores of the (what I think is) aluminum-oxide (of some type). My thinking was, this coating would be easy-ish to renew as it degrades. Is this a bad/worthless/wrong idea?

Edit: Ref. Tungsten-rhenium TC wikipedia
Edit #2: Ref. "cheap" K-type build thread.

 

I have a K-type thermocouple with an inconel sheath. To date, I've only used it in aluminum a few times and the probe still looks like new. Any idea what the life expectancy would be without additional protection?

 

Fortunately the Ebay seller mentions a thermocouple made from such wire has a coefficient of 24.59 millivolts +/- 102 microvolts, unfortunately they don't mention at what temperature this voltage occurs. I had to calibrate multiple temp probes on a regular interval for experiments by having a distilled water slush made by blending distilled water ice cubes and mixing with distilled water. This gives 0 degrees C by definition and then 100 degrees C can be had by bringing distilled water to the boil: if you turn up the heat it boils faster but remains at 100 degrees as the steam carries off the excess energy. Thermocouples are linear enough that you could extrapolate the rest of the line and maybe compare with a K type at a few points.

 

I made a test thermocouple using the tungsten rhenium wires from Ebay. I used a TIG welder to join the 0.3mm wires and was able to get a decent result after a few tries. For extension wires I used 316L stainless filler wire and wrapped the thermocouple wires round the stainless a few turns and hit it with the TIG welder. I'm temporarily using screw terminals as spacers and have it plugged into a cheap K type display and it works fine. The reading are lower than a K type but it shouldn't be too hard to come up with a multiplication factor to correct the readings.









The length of the tungsten rhenium wires will have to be extended enough to keep the 316L junctions relatively cold, at present I've made them too short at 2 inches or so. If they dipped into the boiling water during testing the readings dropped suddenly so they were generating a voltage too.

 

The problem here is that the W/Re wires will oxidize strongly above 1000 C. That is the reason that these TCs are barely used industrially. Even the steel industry uses discardable type S thermocouples (like the WRP-100 mounted on a cardboard cylinder) dipped once in the liquid steel. W/Re would be much cheaper, so I think that is the reason they don't use it.

 

I agree that it's attacked by both oxygen and water vapour even more, but it should be possible to manage the effects with design long enough to get decent use from such a thermocouple:

The Ebay tungsten rhenium seller is selling 0.3mm wire which is way thicker than most platinum type thermocouple wires, I think that in a suitable ceramic sheath it'll last long enough for me to dial in a new furnace's combustion rate. For dipping into molten metal, a graphite sheath should work as the graphite would react with any oxygen chemically and there's bugger all oxygen in a melt unless something went wrong. From what I could read, the main issue is keeping tungsten rhenium away from high temperature water vapour ( from combustion) which then attacks the tungsten component in a repeating cycle of oxidation.

I've ordered some aluminium oxide tube with two holes up the middle, good to about 1400 degrees C. That should slow down water vapour a bit and give some mechanical support to the wires if they become embrittled. Aluminium oxide melts at over 2000 degrees C, but these products are sintered at 1400 degrees C. I'm not sure what the material will do in between, hopefully develop a glaze barrier.

As far as a readout goes, I found a datasheet for the OP07 op-amp that has a decent thermocouple amplifier circuit (page 12) with cold junction compensation. Connect that to a millivoltmeter and you'd be good to go and should even be able to set the gain to match the voltage for direct readings. I'm going to build the circuit and report back on trying to calibrate it.

 

Indeed, Al2O3 tubes works flawlessly. My S type TC is also sheathed in such a tube and is rather resistant to thermal shock (I can pull it out of a 1400 C furnace). BTW, my type S has 0.3mm thick wires which is more robust than 0.1mm wires as the cheap WRP-100 have. There is also a 0.5mm WRP-100 but it costs $150 for a 6" tube length.

 

I also use the cheap display meter, a K type thermocouple in a graphic rod for Aluminum. For Cast iron I wait for the sparks to start flying, and then give it another 5 minutes. I dont have a good plan for br0nze. I guess the one I have for aluminium would work there as well.
Cheers;
Mark

 

A different method is using an infrared pyrometer, but it cannot be too close (1 ft or closer) to a very hot (> 1000 C) object, because its face is made of plastic, which will be damaged by the radiant heat.
I am thinking about making a mask of aluminum sheet with cavities in it for the sensor and the laser heads. And then putting a layer of Kaowool or other insulation between the 'face mask' of the pyrometer and the plastic face as the aluminum mask still gets hot due to the radiant heat.

Any ideas on this ?

 

You could go "old school" and buy a disappearing filament optical pyrometer used in foundries and steel mills to measure high temperatures. There's one on Ebay right now for US$70, the trouble is the postage to my country is $267.

 

I have one I bought on ebay. It does OK for Cas Iron but it kind of subjective to where the operator decides is the vanishing point of the filament. Seems to vary about 100 degrees F. Also, a little bit of a hassle keeping track of which scale you are on.

 

Found these relics on ebay, same place I bought my original used ones about twenty years ago for the school foundry. Dual band, so supposedly pretty accurate, and a narrow enough FOV that we were able to aim it down the vent hole of the gas furnace from up on the fume hood nine feet above and hit the middle of the crucible. Came with enough cable to mount the display on the wall. Never tested accuracy to precise limits, but it matched the handheld contact ones well enough, and didn't get broken constantly like high school kids were likely to do with the handheld ones.

 

These devices with 'disappearing filament' are indeed very oldschool. Modern devices are the handheld infrared pyrometer guns. I have two such ones eligible for up to 1600 C, from ebay and Amazon. The problem is the plastic face when it is too close (e.g. above the vent hole of a propane furnace to peek into the crucible) to the heat source. I tried mounting an aluminum sheet onto it with a hole in it to the sensor.