Jump to content

Thermocouple probe (exposed tip)


Guy Thomas
 Share

Recommended Posts

So I recently purchased a new thermocouple probe primarily for a propane heat treating forge but also for general forge applications. Meant to choose the grounded option but inadvertently ordered the exposed tip. So will the exposed tip hold up to the atmosphere in a gas forge over time or do I need to go through the hassle of returning it to Omega? I fear if I end up with any corrosive or oxidizing damage to the tip I'll have no way of knowing if I am experiencing a drift in it's accuracy.

Guy Thomas

Link to comment
Share on other sites

My experience is that if the tip goes, it pretty much stops working.

 

If it is ungrounded, but still fully sheathed, it'll hold up as well as any.  If by exposed tip, you mean the actual dissimilar wire junction is exposed, I suspect that  will die quite quickly.

 

 

-Brian

Link to comment
Share on other sites

I find that all my thermocouple  eventually get "EATEN UP" inside the forge.

I coat the exposed part of the thermocouple  with the the refractory that I line my forge with  Vesuvius  Super 3000, this makes them last considerably longer.

Link to comment
Share on other sites

This is god information Brian. Some probes have ceramic sheath which cover the entire probe. But many models don’t support them unless they can be purchased as an add on. Obviously…I have no idea!!

Gary LT

"I Never Met A Knife I Didn't Like", (Will Rogers)

Link to comment
Share on other sites

I stopped using grounded junctions a while back. I had a couple fail through thermal shock: It looked like the sheath got hot and expanded enough to break the still-cool thermocouple wire under tension. I've not really had a problem with grounded junctions: they need longer to stabilize to forge temperature, but that's not really an issue for most of us. Exposed junctions are what most of the HT kiln manufacturers use. In an electric kiln, the thermocouple needs to be the fastest-responding thing in the kiln to keep the overshoot down during the heat/cool cycles and an exposed junction is a cheap, easy way of achieving this. With forges, there is not usually a rapidly-changing temperature (unless you are using time-proportioning control to switch a gas solenoid) and ungrounded Mineral-Insulated thermocouples are fine. I'm not sure about the longevity of exposed junctions, having little experience of them.

 

The best advice I can give is to have a think about your process and make some notes, sit down with a cup of coffee, a notepad/pen and a telephone and talk to the tech sales folk at Omega. Tell them what you want to do and what your process conditions are likely to be and listen to their advice. Omega sell pretty much everything temperature-related, so they are not going to push you in an inappropriate direction simply because they don't sell a particular thing. I know a bit about temperature control through my day job: enough that I can generally get things working reasonably well, but the Omega guys do it for a living and it would be foolish not to avail yourself of that resource. 

 

Link to comment
Share on other sites

Thanks for everyone's advice, much appreciated, and yes this is a probe on which the dissimilar wire junction is completely exposed outside of the sheathing at the tip. I remember now when I was ordering I had to leave my computer for a time and came back to it as I was trying to get to bed and placed the order without double checking it.

 

Tim, you are right, a manually controlled gas heat treating forge is definitely not an application that requires split second feed back from the probe and thanks for the advice on the grounded vs ungrounded probes. 

 

Omega's website offer's this: An exposed junction is recommended for the measurement of static or flowing non-corrosive gas temperatures where fast response time is required. 

 

Pretty sure a gas forge qualifies as corrosive as far as oxidizing goes but talking with Omega's tech support does seem a good idea. Tim, I need to go back over a number of your posts on the Amal burners, I have the 1/2" propane injector (which are in stock again at their website), with the #70 jet it came with and I believe you have said it runs reasonably neutral at heat treating temperatures with that gas jet?

Guy Thomas

Link to comment
Share on other sites

For HT, I'd just go with the thermocouple you've got. Thermocouples are consumables and I'd expect the cost of Propane to be much higher than the cost of thermocouples over any given number of Heat Treats, even with an exposed junction. It's far too easy to let the desire for perfection prevent you doing something "only" very well.

 

The beauty of the Amal is that it lets you choke it down to get a rich, reducing, even potentially Carburizing, atmosphere at the correct temperature for Austenitizing. Do it outside: the Carbon Monoxide produced WILL potentially kill you if you do it indoors. At the very least, you'll feel lousy and lose valuable shop time.

 

The Propane jet is fine for HT. If the temperature is high, close the choke down. If the temperature is low, open the choke up. Maximum airflow should be where the end of the choke is level with the point on the locking screw. If you are using it in a forge and are not getting hot enough on the 70 jet, a slightly smaller jet will get a bit hotter. I suspect it's only likely to become an issue if you are trying to melt stuff or weld Wrought Iron.

 

Verhoeven mentions CO:CO2 ratio and it's probably worth reading the book to understand why it's worth running so rich for HT. 

 

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...