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Thermocouple Information from Back Issues of The Salamander News

What is an FRK? December, 1997 issue
What is an Interchangeable Thermocouple? December, 1997 issue
Just Say No -To Potted In Thermocouples. December, 1998 issue
Thermocouple Options. October, 1999 issue
Thermocouple Wells now provided as a requested option. November, 1999 issue

December, 1997 issue:

What is an FRK?

The newest type of thermocouple available to our customers for use with our ceramic elements is commonly called the "FRK". The FRK, or Fast Response type K thermocouple, was designed by our engineers to offer an alternative to our "BOLD" replaceable thermocouple and also for our distributors who are quoting against other element manufacturers. The FRK is comparable to the industry standard for cast-in thermocouples, while the "BOLD" replaceable thermocouple is our unique design developed to be used with any Salamander element at any time. 

The FRK thermocouple is a cast-in type of sensor that is embedded within the element as it is cast. Its position in the body of the element allows the FRK to offer a fast response to changes in the temperature of the heater. This fast response however does not accurately reflect the actual surface temperature of the element. This is due to the fact that the sensor is placed in close proximity to the embedded coil, therefore reflecting the coil temperature as higher than the true emission temperature. This is typical of thermocouples throughout the industry. Once this differential temperature is realized, it can be compensated by adjustments in controls.

The FRK is only available in type K because of limitations in manufacturing. A type J sensor would not be able to withstand the exposure to the high heat resulting from kiln firing. Options available for customers needing type J thermocouples would be to use the replaceable thermocouple or the outdated method of "potting" the thermocouple following manufacturing. "Potted" thermocouples, though inexpensive, are inaccurate and inconsistent in their measurement, and often physically unstable. The potting can become loosened over time due to general usage or vibration, eventually dislodging the thermocouple.

The availability of elements with the FRK thermocouple is the same as for other made to order elements, delivery would normally depend on the quantity ordered. There are certain elements that are stocked with the FRK thermocouple at Mor Electric Heating but it is best to call our engineers for up to date information regarding availability and lead time. To order an element with an FRK, simply add FRK to the part number. For example: FTE-650FRK 240V. Our sales staff has been trained to discourage customers from ordering potted in thermocouples, so each order not indicating an FRK will be questioned. We encourage our distributors to do the same by advising users of the advantages the FRK has to offer.

What is an Interchangeable Thermocouple?

The spring-form, interchangeable thermocouple was first introduced in The Salamander News in December 1994. At that time, the design was unique to Mor Electric Heating Assoc., Inc. and, to date, we are the still the only company to offer anything like it. It was designed for use with our Salamander BOLD elements. The BOLD concept introduced a design change in our total line of ceramic elements to include a larger color indicating decal, one piece mounting clip, and thermocouple wells cast in as standard, for use with the interchangeable thermocouple.

Once the concept was understood, the product was widely accepted, most commonly in the United States where ceramic elements in general were often considered new technology. It is our hope that other countries, with longer histories of using ceramics, will also begin to incorporate the use of the interchangeable thermocouple and realize the advantages this unique concept has to offer.

The BOLD thermocouple is a replaceable type of sensor, available in both type J and type K that is very flexible and easy to use. It is independent of the emitter and can be inserted into the standard drilled thermowell of any Salamander element at any time. Because of the thermocouples exact and consistent position in the body of the element, it offers a very stable and accurate temperature response, though slower than that of the FRK The unparalleled flexibility of the BOLD thermocouple makes them easy to sell and allows for ease of installation, maintenance, and zoning.

Originally designed with 12 inches of lead length, the thermocouples now come standard with 24 inches of leads. We have a large stock of both the type J and type K and are available for immediate delivery with your element order.

Part Numbers: J-24-A (Type J), K-24-A (Type K)

December, 1998 issue:

A customer called the other day wanting to know the materials to use to pot in a thermocouple that had fallen out. This was not a surprise to us, we had potted in the thermocouple, at the customers request, and the potting lasted about one week before the thermocouple fell out.

Though we will continue to do what the customer requests, we will also continue to discourage the use of potted in thermocouples. Aside for the fact that they will soon become loose and eventually fall out, if its temperature sensing accuracy you are in need of, a potted in thermocouple offers the least amount of accuracy, not only due to placement, but the sensitivity it loses as it loosens in the well.

We realize the reasons behind the request for potting. Ceramic elements with cast-in thermocouples are rarely stock items and can take anywhere from 2 to 3 weeks to manufacture. It was for this very reason that we developed the spring-formed interchangeable thermocouple and put wells as standard into every element we manufacture.

Specifications: Type J and K ungrounded thermocouples 1/16" dia., 316 stainless steel sheath, with 24" (305 mm) leads
Part #'s:   J-24-A  (Type J Thermocouple)
                K-24-A  (Type K Thermocouple)

We also realize that spring-formed thermocouples cost a lot more than having one potted in, but remember, its reusable, interchangeable, higher in accuracy, immediately available and not going to fall out. It’s an issue where price must be measured against cost effectiveness. If you’re thinking you can’t afford a spring-formed thermocouple, can you afford not to?

October, 1999 Issue:

Thermocouple usage with ceramic heating elements continues to be a question of world wide concern. Having two manufacturing locations and selling our product to over 40 countries around the world, gives us an insight into our customer’s thermocouple knowledge, preference and usage. One thing that everyone seems to have in common is "confusion" over what they need, what is available, and what they will get. To help in clarifying this, I have put together a chart to not only help in thermocouple selection, but also help in matching need with the price and performance of what is offered. This understanding will also be of help in evaluating the changes and improvements that are inevitable with the continuing development of the thermocouple.

Keep in mind that a thermocouple, as used in the ceramic heater industry, is a basic tool used to measure as closely as possible, the actual temperature reached inside the heat source. This temperature is then used as a guide to control or maintain the actual process temperature. A "thermoelectric couple" gets its name from using two conductors of different metals joined at the ends which produce a thermoelectric current when there is a difference in temperature between the ends. The industry has shortened the name to "thermocouple".

Thermocouples may be inserted into any heat source in different ways. Their placement, position, and method of securement, all have an effect on their accuracy and performance. Due to the solidity of ceramics and high temperature kiln firing needed to get it to that state, thermocouple usage has always been a challenge.

The initial approach, and still the most elementary, was to drill a hole (thermowell) into the tower of the element after it was completed, insert the thermocouple, and "pot" it in using a "potting" or ceramic material. The difference is, that this potting is not kiln fired. It merely dries, giving a loose placement to the thermocouple which may later crumble and cause the thermocouple to fall out. Placement is also difficult and often inconsistent. It is impossible to know just how close it comes to the surface of the element.

Several ceramic heater manufacturers decided to improve on this system by actually "casting in" the thermocouple at the same time they are manufacturing the element. This method assured secure positioning and accurate placement. The problem comes in the tolerance of the wire to the two, sometimes three, intense kiln firings. High heat can cause the fragile thermocouple wire to become brittle and break off easily, creating a high failure rate in manufacturing and a limited lifespan in the field.

We are, to date, the only manufacturer to offer a third possibility in our creation of the springform, interchangeable, thermocouple. This concept relies on the use of the drilled in thermocouple well, but provides a secure fitting and the ability to remove or replace the thermocouple as needed. Placing wells in all of our elements, gives the potential for all Salamander elements to have a temperature sensor installed.

Unfortunately, none of these types are perfect. We are still in the process of researching a more tolerant wire for the cast in, fast response thermocouple, but even when that problem is resolved, the FRK will never offer the accessibility that the interchangeable thermocouple can offer.

The chart below is devised to help you discover the current thermocouple option that is best for each of your specific applications. This chart should be used for each temperature sensing need and no one option should ever be selected as "standard".

November, 1999 issue:

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