Pyrometers

[John Page]

I've been reading through all the old posts I could find about pyrometers, but most of them are a few years old and led me to more questions than I could find answers to. I don't know much about them, but I figured that a K type thermocouple is the best for bladesmithing purposes. And that's where my knowledge ends. I came across this thermocouple, but I don't know which model to go with. The "1/8 inch(3.2mm) diameter, grounded junction, 12 inch(300mm) length" seems to be alright, but is there something I should go with instead? Also, what should I attach to it as far as pyrometers go? I haven't been able to find a decisively good product that multiple people recommend, but does that matter as much as the thermocouple? Should it connect directly with the thermocouple, or should there be some sort of intermediate wire to bring the pyrometer away from the forge?

 

Any help would be appreciated, as I am in the dark on this sort of thing. For the cost, I figured I should get my hands on one to help me with the HT and a better indication of when the forge is at welding temps.

 

Thanks for your help!

 

 

John

[justin carnecchia]

Check Thermoworks mini hand held thermocouple. It's $43 has the right temp range and thermoworks makes good products. For the sensor look for something in the temperature range you want with the length you want. I think they are around $20.

As an alternative you might want to think about a PID. They use the same thermocouple sensor and can be used as a thermometer. As a bonus sometime down the road you can hook it up to a solenoid and use it to control the temp of your forge quite accurately. They can typically be found on E-bay for less than most thermocouples. Hope this helps, Justin

[Doug Lester]

First of all, I found my pyrometer complete with thermocouple on Ebay. You can also look at kiln supply houses to get the entire thing-thermocouple, wire, and pyrometer. Go for a digital display. I find them easier to read.

 

Doug

[Greg Thomas Obach]

bought my omega pid's off of ebay.. used and bout 20 for one and almost 40 for the second

-got my k thermocouple right from omega themself... think that was about 24

 

stay away from the word " pyrometer " ... that word seems to put you into the 5-600 dollar price range.... just buy a pid

 

i've seen lots of people mention this site in the past for pid's ... never bought from them, so thats up to you

http://www.auberins.com/

 

i'd buy the thermocouple from omega.. they got the good stuff

 

 

oh .. and yes, there is a wire that connects the thermocouple to the pid

 

somethin like this has them attached

http://www.omega.ca/shop/pptsc.asp?ref=HJMTSS&Nav=tema07

 

Greg

[John Page]

Thanks for the info everyone, this helps a lot. I've never heard of a pid before, and that looks great. I'll take a look at the ones with digital displays and hopefully come to a conclusion.

Thanks again

 

John

[Dan O'Connor]

I have this one and like it.

 

Digital 2 k-type Thermocouple Thermometer Nicety

 

Has two outputs you can switch between.

[Doug Lester]

Dan, that sure beats the price on the one that I got off Ebay, which still was under $100. No doubt that there are pyrometers out there in the $500-600 price range but, as demonstrated, there are some that cost a lot less that will suit our needs. I may have to order one like you have, mine maxes out at 2000 degrees.

 

Doug

[John Page]

I have this one and like it.

Digital 2 k-type Thermocouple Thermometer Nicety

Has two outputs you can switch between.

That looks excellent! I do have one question, though. Where it says "any other K-type probe with miniature thermocouple connector can be used.," does that mean a standard K type probe will NOT work? If that is the case, where do you find the mini connector thermocouples?

 

 

John

[timgunn]

As the OP appreciates, there are 2 things needed for temperature measurement.

 

One is the thermocouple, the other is the display.

 

Realistically, you only need a simple handheld readout for checking temperatures. I use a TM902C bought off ebay. It's dirt cheap and accuracy seems good; the readings compare well with much more expensive big-name instruments. Downside is a Celsius-only readout. I'm in the UK and don't think in Farenheit, so it's not a problem for me, but it may be for some. It reads to 1300 degC, which is 2372 degF.

 

If you need Farenheit, it's a safe bet that even the cheapest readouts will be at least as accurate as the thermocouple itself. It's worth spending on a good thermocouple, though.

 

I'd recommend an Omega KHXL-14G-RSC-18 thermocouple. It is a type K, mineral insulated thermocouple with an Super Omegaclad XL sheath, able to stand a working temperature of 1335 degC, 2440 degF. It has a handle and a connection cable. The diameter is 6mm, 1/4", making it rigid enough to put the tip where you want it without sagging. It has a grounded junction for fast response and is 18" long. The fast response and 18" length are a real hand-saver. In one of the old paper catalogs, there was an option of additional length. I'd suggest another 6" would be worthwhile, if it's available, to give 24".

 

http://www.omega.com/ppt/pptsc.asp?ref=KHXL_NHXL&Nav=tema06

[Clifford Brewer]

 

 

One from the late great Grant Sarver

[John Page]

Wow, great info, thanks. I'll definitely go with the K type omega thermocouple, I've heard nothing but good things about them. I have used both Fahrenheit and Celsius, so that won't really be an issue as long as I know the temps I'm going for in both of the scales. It is my understanding that the probe itself only reads on the tip, not along the entire length of the shaft, so other than the distance between the end and the wiring, are there other advantages?

 

John

[matt venier]

I use this http://www.thermoworks.com/products/handheld/TW8060.html and two type K's from omega for my heat treating rig. It's nice to know how even the temp is from the front to the back and the display is lit. I just wish you could turn the light on and leave it on for when your heat treating at night.

 

Matt

[timgunn]

Thermocouples basically measure at the junction where the 2 different materials join, so broadly speaking, at the tip. For Mineral-Insulated thermocouples, there are 3 options.

 

Exposed junctions have the outer sheath and insulating powder removed to expose the junction. This gives the fastest possible response but gives no tip protection.

 

Insulated junctions have the end of the sheath closed and the insulating powder all around the junction. Slow response, as the temperature has to equalize across the insulation, but good protection and fine for slow-changing processes. Because of the insulation around the junction, the measured value tends to be the average temperature over perhaps a couple of sheath diameters at the end of the thermocouple, so possibly not the best choice if you have steep temperature gradients.

 

Grounded junctions are like insulated junctions but with the junction itself welded to the sheath. The response is nearly as fast as the exposed junction and the mechanical protection is nearly as good as an insulated junction. They measure the temperature at the tip. The only real downside is that the junction is grounded through the sheath and this can cause interference in industrial process conditions. It's about ideal for use in a forge, but may not be the best choice in an electric furnace. That said, I use one in my homebuilt electric HT furnace.

 

Omega have a lot of very good technical information on their site and it's worth a browse. As an ancient, who predates the interweb, I tended to carry a paper copy of the Labfacility Handbook for sitework, mainly because it was much smaller than the Omega catalog. It also tells you pretty much all you need to know about thermocouples.

 

http://www.labfacility.co.uk/temperature-handbook.html

 

Sheath materials make a difference to how long the thermocouple will last and I'm not sure that all sheath materials are suitable for grounded junctions. Thermocouple life is not necessarily a pure time-at-temperature thing, cycling matters as well. So does the atmosphere it works in.

Edited April 21, 2012 by timgunn

[Dan O'Connor]

That looks excellent! I do have one question, though. Where it says "any other K-type probe with miniature thermocouple connector can be used.," does that mean a standard K type probe will NOT work? If that is the case, where do you find the mini connector thermocouples?

 

 

John

 

As far as I know it is just a standard K-type. Bought two of these for it for my charcoal kiln. Hooks up fine.

[John Page]

I see, this is all starting to make a lot more sense, thank you all for your wisdom. I'll give the handbook a read through, it looks like a wealth of information. K type, grounded is for sure the way I'll go. One more question: is it damaging to leave the thermocouple in the forge for extended periods of time, or are they meant to be left in wile you work? I'm going to be making a HT furnace one of these days, and I wouldn't want to destroy the probe right off the bat...

Thanks again

 

John

[Doug Lester]

The thermocouple on my heat treating forge is inserted through the wall of the forge and it stays on through the process to give constant readings.

 

Doug

[timgunn]

It all depends on what you are doing.

 

If you are setting the fuel:air mix on a forge, it's usually best to take it out once things are stable, just to save it getting damaged when you stick stuff in and out.

 

Heat will tend to soak back along the thrmocouple, so you need to be careful of overheating the "cold" end. Handheld probes usually have a plastic handle. Mostly it's just common sense.

 

Type K thermocouples tend to suffer "drift" above about 1000 degC. Type N was developed to overcome this, but all the really cheap readouts, like the TM902C, only take type K inputs. From that point of view, it's best to take them out when they're not doing anything useful. To keep it in perspective, the drift only really tends to show over days or weeks under continuous use in industrial processes; 3 hours a week for a year works out at the equivalent of about a week of continuos use, so it's probably not too big an issue for most small knifemakers.

 

The sheaths tend to Oxidize and the oxide layer prevents further oxidation. If they get hot, then cold, the oxide layer can flake off, losing a little of the sheath on each cycle. Different sheath materials suffer from this to different degrees; 316L Stainless suffers when it cycles above about 850 degC, 310 Stainless above about 1100 degC. Nicrobel about 1250 degC, etc. From this point of view, leaving them in is better.

 

At the end of the day, it's best to view thermocouples as consumables and just do whatever fits best with your way of working.

 

If you are going to build your own HT furnace, it's worth getting a good ramp/soak controller. I have built five so far. After testing the first one, I have been buying Solo controllers from AutomationDirect, despite having a vitually unlimited supply of free non-ramp/soak controllers. The improvement in performance, especially when tempering, is huge. I use the 1/4 Din Solo SL9696VRE or 1/16 Din SL4848VR. I prefer the bigger 1/4 Din version because I have fat fingers and middle-aged eyesight.

 

http://www.automationdirect.com/adc/Shopping/Catalog/Process_Control_-a-_Measurement/Temperature_-z-_Process_Controllers/1-z-4_DIN_Size_%28SL9696_Series%29/SL9696-VRE

 

It's the cheapest thing I've found that will do the job well. It's worth keeping an eye on what is available though, as, like most electronics, controllers are getting better and cheaper all the time.

 

I'm not 100% sure, but I think Omega sell the same controller with their own branding.

 

http://www.omega.com/ppt/pptsc.asp?ref=CN7200_Series&Nav=temp06

[John Page]

Outstanding! Technology is such an amazing thing, I'm amazed by it every day. The precision of these controllers is fantastic, much better than the old 'look into the fire and see how hot it looks' method. With something like this, does the heat source have to be electric, or do they operate with gas? How does the connection from the controller to the heat source actually work? I suppose I'm not seeing how it actually does the controlling..

 

John

[Doug Lester]

These units go far beyond measuring/monitoring the temperature of a forge or oven. They can also be used to control it. I would probably use one of these with an electric oven to regulate the temperature. If all I was wanting to do was measure the temperature, there are cheaper ways to go. These things can also be rigged to a solenoid on the gas line and an ignition system in the burner to regulate a gas oven or forge but I would be cautious. Electricity isn't normally explosive, propane is. These things still need a thermocouple connected to them to work. They also need to be wired in to a power line.

 

Doug

Edited April 22, 2012 by Doug Lester

[timgunn]

There are several ways of doing the heating part, but the easiest, most accurate and often cheapest is to use electric heating coils.

 

The Solo controller I linked to has a low-voltage DC output to switch a Solid State Relay. The SSR switches the mains power to the heating coils. The SSR is switched on and off on a cycle. I use a 2-second output cycle on my electric HT oven/furnace. For half power, it is on for 1 second, off for 1 second. For one quarter power, on for half a second, off for one-and-a-half seconds. You get the idea.

 

When powerful heating elements are used at low temperatures (think tempering temperatures in a furnace that can reach Austenitizing temperatures for stainless steels), the rate at which the temperature rises whilst they are powered is very high. To avoid overshooting the temperature, it's necessary to have a short cycle time, hence my 2 seconds. I've also tried one second, but can't see any improvement, whilst 5 seconds gives measurably poorer control.

 

For gas heating, the process needs to have enough thermal inertia to smooth out the temperature, through the much longer on/off cycle time needed for switching the gas solenoid valve. It's great for a salt-pot setup with lots of thermal mass, but much less good for a well insulated small oven/furnace. It actually works very well on a Don Fogg style oil-drum HT forge.

 

Controllers are also available with analog outputs. These can run either modulating gas valves or phase-angle switching SSRs, but this is probably more of a specialist thing and doesn't offer anything "we" can't do with a switched output for less effort and money.

[John Page]

This is such a wealth of information, thank you both. This is definitely turning into one of those the more you know, the more you realize you don't know situations. I was originally thinking of making a gas HT oven, but now that I am a little less in the dark, I'll start looking at the electric instead. I'll take a look at some of the HT oven builds and see if that's in the realm of feasibility of things I can do.

[Alan Longmire]

Thanks for all the tech info, Tim! This stuff is still in the "one of these days" realm for me, but I'm going to pin the topic so it'll be easy to find for everyone.

[John Page]

Given all the positive feedback about Omega, I decided to order the thermocouple from them on Saturday, and it came in the mail today. Couldn't be happier with their customer service and condition of the package that arrived. Although I don't have the PID to attach it to yet, I don't anticipate any troubles.

Once I get the rest of the equipment, I'll start the HT oven build (still undecided between how to go about it..I have a few thoughts for a propane furnace, but I'll do some testing first to determine if it is plausible).

Cheers!

 

John

[John Page]

Got the last of the pieces in the mail today, and despite all the info from Tim et. al, I still managed to screw this up

After a little bit of tinkering, I managed to get everything working in the end. I took a few pictures of what is going on here, as what I was able to find was not the most descriptive thing in the world. Hopefully, this will help those out there who are less informed on the subject (myself first among them).

 

First, and the source of most of the trouble, is that there are 2 types of K type input/output. There is the Flat Pin (mini K type) and Round Pin (regular (?) K type. Now, on almost all the packaging and detailed description outside Omega's catalogue, K type means mini-K type, even if it says "fits all K-type thermocouples." This will not fit the Round Pin thermocouples. If in doubt, look at the input, and if it is two wide, thin holes, that is the flat pin. If it is two circular holes, where one terminal (the negative) is larger than the other (positive), then it is the Round Pin K-type thermocouple.

 

Second, and fortunately for obvious reasons, these two junctions are compatible in a manner of speaking. What that means is because they are both K-type (and it might apply to the others as well, but I have no idea), they can be spliced together. The temp readout suggested by Dan came with two probes, although they only go up to a fraction of what I need, and one of them was dysfunctional (lucky for me). I cut off the end of it and spliced it into the probe I ordered from Omega. After fiddling with it for a few minutes and aligning those blasted microscopic holes, I was in business.

 

Third, the female K-type junctions (from Omega) work with both Round and Flat Pin K-type thermocouples. To avoid confusion, they are not the same holes, but there are two sets, one for the flat and one for the round. In my case, it is somewhat irrelevant, but might be useful none the less.

 

I'll put some pictures below, so they are not cluttered up and confusing. Thanks again to those of you who helped me understand this, I couldn't have done it without you! Hopefully, I'll start the HT Oven build soon and put this all to good use.

 

 

John

[John Page]

Here's the display, with the thermocouple attached and working. You can sort of see the other input on the right (same as the one it is plugged into; there are two inputs). This is the Flat Pin K-type input, which is more visible in the next picture.