N type Thermocouples

Ya, I use type S thermocouples and I feel your pain. I just bought 200ft of platinum wire for the tune of $22,000. I've started a program to convert some of our furnaces from type S to type N for control, recorder, and high limits. Of course, new leadwire is in order.

For these applications, I use N-GEO type with INCO sheath, in a ceramic protection tube. The type S are calibrated on a 6 month cycle, and I usually get about 2 years out of them before they drift too far out of spec. I work to AMS2750D, (aero-space), so once they go any more than +-4F out of spec, they must be replaced.

Once replaced with type N ( special half limits of error), I put them on a 3 month cycle, no recalibration, but replacement. So, 8 type N's to replace 1 type S. So still a considerable savings advantage.

I regularly use these control, recorder, and high limit type N T-C's to 2150F without any problems everyday, but I think that's the limit.

Several of our furnaces must go to 2300F, so I'm stuck with type S here.

I use 12,000 ft of type K, and 8,000ft of type N, refisil 20ga T-C's , for load, test, and buried applications every month. I'm moving away from the type K to the type N, ( leadwire replacement is the biggest obstacle).

Let me know how this works out for you. I have some very good vendors if you need help there.

That doesn't sound all that encouraging. The thermocouples that I want to replace go up to 2300 F. I suppose I will see how they hold up in my application, but with what you said, it doesn't sound promising.

Your in the same boat as I'm in. Type S, and Type R thermocouples are very stable and last a long time. The next step is optical sensors, and that opens up a whole new world of problems.

Let me know how it goes.

At working temperatures around 1600C (2900F) I have succecfully fitted R type with sintered aluminia sheathing and insertion pockets for in-line duct burners fitted to gas turbine exhaust gases with high turbulence and shock wave vibration.

The only problem has been care with handling the sheathing during replacemnt

I see little advantage to go to type R,

Platinum/13 %Rhodium vs. platinum (type R)
Platinum/10 %Rhodium vs. platinum (type S)

Type R actually costs more than type S, with platinum going for around $2000 oz, and rhodium for about $9000 oz.

We find that type R has better stability and costs the same or slightly cheaper than type S - probably because R type is more widely used in UK.

why don't you use K-type. It can measure upt 1400 deg C with a proper thermowell in any condition

Hello Guest,

I agree that a type K could be a candidate, but your upper range value doesn't match that found here:

This is from http://www.omega.com/thermocouples.html

The other thing is that the type N is more stable at the higher temperatures:

http://www.omega.com/temperature/Z/pdf/z214-215.pdf

Best Regards,

Mike

You could never use Type K to replace type S. I know what the chart says, but a type K, at 2300F wouldn't make it through a day, and stay in tolerance, regardless of your chosen thermowell.

I speak her through experience. I never use Type K on anything over 1800F.

However, I have to work to a very tight tolerance (AMS2750D), so everything I do is error driven.

yes, Bricktop, I never use type K above 1000C.(1800F), ran some at 1200C (2200F) but replaced with type R after 2 months due to number of failures.

Hi Bricktop,

Oh, I believe you. Nothing speaks like experience. I read guest's comment and thought "Yeah, why not use a type K?". In my subsequent research, I found some info, including how the Type Ks are not as stable as the S/Ns at the higher temps. You caught me doing my research out loud!

If you notice, I did not give anything near a conclusive recommendation. As I have found, many things are not always as simply done as they would initially appear.

Anyway, I appreciate your comments here - there's always more to learn.

Mike

We use K type in most of our furnace, but our flue thermocouple needs to reach higher ranges. A K type thermocouple would burn up in the type of temperature we have in there. As for the S type thermocouples we currently use, they are not used for fine temperature control and our tolerances are +/- 50 degrees F for the place they are. So I'm not worried much about being too far out of tolerance. We usually run our S couples to destruction. Most times when we pull them out to replace them the support for the thermocouples is gone and only parts of the wires are left.

The condition you report on removal of the type S couples leads me to ask: what is the atmosphere? What is the sheath material? Do you use a thermowell? Almost nothing is as corrosive as molten glass at 1150 C +/- 50. Would a Inconel 690 lightweight thermowell (for faster response) be a posssibility? We did use platinum pipe for air injection where erosion and corrosion were a double barreled problem, but expensive at what today would be 25K per foot. Suggest contacting mfrs for possible super alloy for sheath or well.

I'm not melting glass, I'm melting aluminum. We usually just have a ceramic tube that we put the thermocouples in to avoid having them erode away. We don't really need super accuracy or super fast reaction times. We won't be using these couples for measuring the bath temperature which is being measured by K type thermocouples. My major concern is the failure rate in the heat we will be subjecting them to. If they fail in a day, we won't really be able to use them... if it takes them 3 months on average to fail, we can work with that easily.

USE OPTICAL PYROMETERS SIMULTANEOUSLY TO CHECK AND CONFIRM THE TEMPERATURE WHETHER EQUAL.

.N-TYPE WILL GIVE SLIGHT VARIATION IN LOWER SIGHT DUE TO DISTANCE HENCE USE OPTICAL PYROMETER SYSTEM SIMULTANEOUSLY WITH DISTANCE AND ARRANGE TO FIX TOLERANCE LIMIT + -
FOR GETTING THE ACTUAL TEMPERATURE TO CONFIRM THE STANDARD TEMP IN FURNACE.
N type you can use successfully, due to slow transfer you will get slight variation lower than from standard temp insight furnace ,you can rectify this discrepancy through optical devices also.

To what standard do you calibrate the optical?

Have tested type N as alternative to type K in a glass furnace application. Problem was the 1150 C operating temp. Type N held calibration about five times longer. The mfr claimed that the problems with K prompted the invention of N, and that the silicon in the allloys delays diffusion at the junction, which kills the accuracy. Eventually had custon trees made with 10 ungrounded, I600 sheathed t/cs in an outer sheath of 3/8 inch OD. (spaced about 2 inches apart). Used two in separate I690 t/wells and changed one each three months. Well satisfied