Could use some help choosing thermocouple

B

Black cat

Well-Known Member
I want to put a thermocouple in my gas forge.
Ive seen a few threads on this forum about the subject but im really not sure what to get. As far as I know... A K type thermocouple and some sort of PID.
Ive been looking at the components on Auberins.com
Im trying to keep the cost low so im just looking at the basics.
Theres just so many options in K type couples. And LCD displays.

Heres what ive narrowed it down too:

http://www.auberins.com/index.php?main_page=product_info&cPath=1&products_id=14

Ok... Just the PID lol

The K types... Theres length and diameter and material... I have no idea what all of these are.

And a thermowell. Also not sure what is needed.
Im looking at some of these:

http://www.omega.com/toc_asp/subsectionSC.asp?subsection=B08&book=Temperature

Could somebody please explain some of the details of these parts and what will work with this PID controller. Or point me in the right direction thanks !
 
I've always found the best prices on ebay. Make sure that the pyrometer reads over 2000° and that the thermocouple is rated for the same thing. This will mean that it's ceramic shielded. You can rig the thermocouple to the PID but a ready made pyrometer is plug and play.

Doug
 
Doug Lester said:
I've always found the best prices on ebay. Make sure that the pyrometer reads over 2000° and that the thermocouple is rated for the same thing. This will mean that it's ceramic shielded. You can rig the thermocouple to the PID but a ready made pyrometer is plug and play.

Doug
Click to expand...

So then something like this from ebay will work ?
And it can be connected to the LCD PID in my first link ?
All I need then is a 4 inch SS thermowell ?

http://www.ebay.ca/itm/3M-Stainless...LH_DefaultDomain_2&hash=item3a8ee32aa9&_uhb=1

Thanks !
 
Wow thermowells are hundreds of dollars. Cant a person just make one using a length of stainless tube ?
Its just to protect the probe and keep the temp consistent at the probe is it not ?
 
You don't wish a SS thermowell - you want a ceramic tube to protect the T/C. By using #8 wire I suspect it will last a long time in the fire anyway. Use something like this to protect the TC if you desire: http://www.ebay.com/itm/111042480715?

Use the Auber #SYL-2362 PID - that you can use as temperature indicator only, OR - if you desire control later, you've got a PID to play around with. I use it for PID control on a toaster oven for tempering.

Ken H>
 
Black cat,
Ed Caffrey (Knife Dogs) helped me along several years ago with getting this stuff together. This stuff can get confusing quick, fast, and in a hurry.

I use a multi-meter to measure temperature I would recommend this one. Remember, it only read temp in Celsius. http://www.harborfreight.com/ac-dc-digital-multimeter-37772.html\ I run two thermocouples and two mulitmeters. It seems the PIDs and mulitmeters are made in china so why not save a few coins and use the latter version.

Here's the type of thermocouple you want: Thermo couple nomenclature at omega
Quantity Model Number KTIN-18E-12 $Tax $0.00

This is the web address for the KTIN-18E-12 http://www.omega.com/ppt/getpricesc.asp
 
One thing to note, KTIN-18E-12 is only rated to 500ºF. I think you said you wanted this in a forge where you'll expect 2,000ºF. You will need a ceramic covered, with at least #12, and preferable #8 wire to prevent rapid burnt out. These should be type "K" TC.

The DVM will work just fine for reading temperature - most that have a temperature function "should" switch between ºC and ºF. OR - put meter to milliVolt range and read mV and convert to temperature.

Ken H>
 
Thats perfect I was thinking of trying some sort of multi meter as well.
I was thinking of the ceramic tubes. My concern is accidentally breaking those thats why I was thinking of a sacrificial stainless steel tube. Or perhaps a tube to protect the ceramic tube ?
 
You're right - ceramic is easily broken. Realize also, the temperature shown by the TC is NOT the same temperature all over your forge. It will vary a few hundred ºF from spot to spot in the typical forge, depending on size of forge. Even in an oven the temperature will vary over 30ºF from spot to spot in the oven.

Note the PID Scott linked to shows a max of 400ºC, which is about right for the TC probe included. Not sure if the PID will go above or not. Some do, some don't.

Ken H>
 
Basics: A thermocouple is just 2 different metals joined together. When the junction is heated, it produces a voltage which depends on the temperature and the metal compositions used. Any metal combination can be a thermocouple.

A number of particularly useful compositions have been developed and their temperature/voltage characteristics compiled into standard tables. They have then been given letter type codes. Different types have different useful ranges.

There are several thermocouple types that could be useful to "us". The most common, and probably most useful, is type K.

The metals that form the junction in a type K are Chromel and Alumel. The tables go up to 1372 degC/2500 degF and down to -260 degC (-436 degF).

In most cases, the other materials used in the thermocouple assembly will have a maximum working temperature below the maximum working temperature of the junction.

Thermocouple wires need to be insulated from each other so that they only touch at the junction. If they touch again elsewhere, the point of contact becomes a new junction and this will louse up the reading.

Ceramic insulated assemblies are good for fixed thermocouples. They usually have sections of double-barrelled ceramic tube threaded over the wires and are not rigid enough for horizontal insertion much beyond a couple of inches or so. Putting them in a tubular ceramic sheath provides rigidity, but at the expense of response time; lots of thermal mass makes them slow to respond to changes in temperature.

Mineral Insulated thermocouples consist of the 2 junction wires embedded in compacted Magnesium Oxide powder within a metallic sheath.

By some form of near-magic, the MI cable is unrolled, the junction welded and the end of the sheath welded shut by the company that makes your thermocouple to order.

The sheath material is usually the limiting factor for the maximum working temperature of MI thermocouples. In many cases, the sheath will oxidize at high temperature and the oxide will flake off at low temperature, so the sheath gets thinner each cycle. It is pretty thin to begin with. For our purposes, type 310 stainless steel and Inconel are both considered good to 1100 degC (2012 degF). Omega have a proprietary material, Omegaclad XL, that is claimed to be good to 1335 degC (2435 degF).

MI thermocouples are reasonably rigid and can be had in a number of diameters. The most common type of junction is an insulated junction, with the junction completely surrounded by the MgO powder. This has fairly slow response. Exposed junctions just weld the measuring junction and cut back the sheath leaving the junction exposed. Response is fast, but they are fragile. Grounded junctions weld the thermocouple junction to the closed end of the sheath. This gives a reasonably fast-responding, but reasonably rugged, assembly. Not all combinations of sheath material and thermocouple type are suitable for grounded junctions.

My opinion/suggestion.

Omega KHXL-14G-RSC-24 Themocouple assembly.

It is a 1/4" diameter Omegclad XL type K with grounded junction, 24" long below a handle, fitted with a curly cable and a miniature plug.

24" long may seem excessive, but it will not seem it when you are trying to see how the temperature changes throughout your forge. 1/4" diameter is rigid enough to reach the back of most forges without sagging too much and the speed of the grounded junction is also helpful when you are trying to see how the temperature changes throughout your forge.

http://www.omega.com/pptst/KHXL_NHXL.html

Enter the part number in the search box for a price; the part no builder only gives a maximum of 18" and the extra 6" is worth it IME.

Then search ebay for a TM902C handheld readout and order one (preferably two). Price should be around 5 bucks delivered. If you only think in Farenheit, you'll need a calculator or conversion table. Alternatively you can pay several times the price for something that reads in degF.












If you need hotter, things get expensive
 
WOW Tim, that TM902C is a real deal. ºC is no big deal, and for $6! As you said order two of them.

Interesting that Omega probe is rated for 2400ºF - first I'd seen with a SS sheath at that temp. That 18" would allow probing sections of the forge to check temp. I think that would be better than the fixed probe I've got in side of forge.

Ken H>
 
I thought so. I am hugely impressed by the TM902C. I have had a few, some marked for different maximum temperatures. All have worked to 1370 degC (with a suitable thermocouple). I've had them on a calibrator and accuracy is as good as anything I've had from any of the big names at twenty times the price.

The thermocouple that comes with the TM902C is a glass-fiber insulated thing with an exposed junction, good to around 400 degC (752 degF). It is close to perfect for measuring tempering temperatures, especially if using something like a domestic oven, where it is thin and flexible enough to just close in the door seal.

Downsides are fairly slow delivery and the battery that ships with it is maybe not the best.

The Omegaclad XL seems to be a proprietary material that, on paper at least, will get into the forge-welding temperature range and just about covers the full knifemaking temperature range for reasonably sensible money (compared to Platinum-based thermocouples).
 
One thing to note, KTIN-18E-12 is only rated to 500ºF
Click to expand...

Just a bit of a correction...... the 500F rating refers to the molded transition junction on that particular thermocouple. The rated temp for use is 2300F. I've used them for several years, and in my welding forge which generally runs 2350-2400F....one will last about 4-6 months...... for me it was a compromise between cost and durability. I always place them through a hole in the forge body, with a layer of kawool between the probe body, and the exterior of the forge body....that protects the joint from melting.

For a about 2x the money, you can get this one: KQXL-14U-12 its rated for 2440F of continuous use. An important note on thermocouples that I've recently learned is that an ungrounded junction tends to last longer, and is more accurate (designated as a "U" in the part number). I now use both of the part numbers mentioned.......the 2300F model for my "general purpose" forge, which rarely sees 2000F, and the higher rated 2440F model for my welding forge.
 
Ed, as usual, you're correct about the T/C temperature. I saw the 500F rating on the top portion of the page which is for the transition and didn't scroll down the page to the sheath rating.

Ken H>
 
Ken,

No biggie! I've done the same thing MANY times. :) For a long while I had a source for those thermocouples on ebay......but I think the individual has quit ebay, now it's back to buying them from Omega. :)
 
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