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Replacing and Upgrading my Thermocouple

Type N Thermocouple Upgrade

So recently (at the start of 2024) my thermocouple packed up on me and as karma or the universe willed it I had some blades I needed to harden some CPM MagnaCut for a client (read my guide). Not great timing.

I’ve had the furnace close on six years by this stage and while I bought it second-hand… Needless to say it was about time to replace the thermocouple anyway. I’ve replaced the element twice already for various reasons, but that’s a story for another time.

CN-40 PID showing a sensor error

If the error code wasn’t another, a test with a voltmeter confirmed the thermocouple was dead. When I removed it from the furnace for a visual inspection, I saw that the bead had cracked.

TIP: You can check a thermocouple with a standard volt meter set to the Ohm’s / Continuity function by checking across the positive and negative leads, to determine if the probe circuit is open or not. If finding that the thermocouple does not read, the problem is most likely a bad connection, broken wiring, or open internally within the thermocouple probe circuit.

I went to sdknives.co.za for a replacement as it is where I get furnace spares, but they were out of stock on this occasion. After phoning and emailing around I eventually found a local supplier (Graecor Industries) who could supply me with a new thermocouple at a reasonable price.

Before this project, I had built a brewing controller for my other hobby (craft beer) and researched different types of thermocouples etc. The most common are the “Base Metal” thermocouples known as Types J, K, T, E and N. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple.

So imagine my surprise that they offered a Type N thermocouple.

  • Type K (Nickel/Chromium alloy leg and a negative Nickel/Aluminium leg) is the most common type of thermocouple. It’s inexpensive, accurate, reliable, and has a wide temperature range. Type K thermocouples are recommended for continuous oxidizing or neutral atmospheres and are mostly used above 538°C [1000°F]. Maximum continuous temperature is around 1,100°C.
  • Type N (Nicrosil (Nickel/Chromium/Silicon) leg and a negative Nisil (Nickel/Silicon) leg) shares the same accuracy and temperature limits as Type K. It is slightly more expensive but has better repeatability between 572F to 932°F (300°C to 500°C) compared to type K. The addition of the specific quantities of Silicon reduces the ‘ageing’ effect, meaning Type N thermocouples can be used in applications where Type K elements have a shorter life and stability problems (high temperatures) due to oxidation and sulfur (‘green rot’).

As I largely work with stainless steels (austenitising temperatures around or higher than 1100°C), I thought the better accuracy at higher temperatures was worth a little more money at the end of the day.

Thermo Well

The other upgrade I completed was to fit a ceramic thermowell. This will further protect the thermocouple during operation, whether that be from contacting a blade or just the environment within the furnace. I’m hoping to get many years of reliable service from this one.

I have noted that the thermowell insulates the thermocouple a fair amount when starting up the furnace, but as far as I can tell the furnace holds the set temperature well, once up to temp.

Thermocouples n k side by side | Topham Knife Co
Top to bottom: Thermowell, Type N and the old Type K thermocouples

Fitment & Installation

You’ll note that the newer Type N (10mm) thermocouple is thicker than the old Type K (6mm). But the thermowell (15mm OD) is wider still. This meant some alternations in the form of a larger hole that would accept the thermowell needed to be made.

A step-drill was excellent for drilling through the sheet metal. I found that running a 16mm drill bit in reverse (true story), gave me a clean hole through the bricks (which are a bit old and fragile). All in all, it was not the nightmare I envisioned. You’ll note that I took measurements beforehand (the black lines) to ensure the wider thermowell wouldn’t hit or encroach on an element. I also vacuumed the furnace to remove any bits of brick or dust that could potentially damage the element.

NB! If the Leads on your thermocouple are reversed (positive & negative legs), the temperature measured will appear to be varying in the opposite direction relative to ambient temperature. It will be fairly obvious as your temperature reading will be decreasing in value rather than increasing. The Red wire on a thermocouple is generally the negative (-) wire as per ANSI standards but each type has a different colour code.

Testing: If in doubt which wire is negative or positive, you can test the wires with a magnet (Type K). The negative lead will usually show a magnetic character and positive leads are non-magnetic. A multimeter can also be used; select the millivolt signal and position the probes (leads red + and black – ). If the displayed value is positive (eg: 13.6mv) the multimeter to thermocouple connections are correct.

The next step was to update the PID (CN-40) settings to Type N and match the output readings. If you don’t do this step, the PID will think there is still a Type K thermocouple in place and your temperature readings will be off). If you consult the user manual you’ll find the “INP” setting (set to 05) in the Control Parameter Display. This menu can be accessed by pushing and holding the up and down arrows together for 3 seconds.

I did run the auto-tune function on the furnace following installation so that the PID can set all the relevant values. This helps minimise any over-shooting when coming up to temperature. In real life though the last few degrees take a bit longer to hit.

And that’s it. The new Thermocouple and Thermowell are installed and the furnace is back up and running. I did some test coupons that were tested by Bohler, to confirm everything is working as expected.

For those who make use of my heat-treating services, rest assured that you’re getting the best and most accurate heat treatment I can offer.

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