What Kind of Thermocouple Should I Use?

What kind of thermocouple should I use? Depends on several variables related to the system to be tracked, such as its media / process environment compatibility, the frequency and precision of the necessary measurements, and the regulatory climate in your sector.

Temperature measurement in many industries, from refining to pharmaceuticals to aerospace, is a key parameter in manufacturing and processing operations. Precise temperature monitoring helps to ensure safe, efficient and optimal results.

A thermocouple is invariably the measuring tool of choice for applications above 1400° F, but the selection of the ideal industrial thermocouple also requires knowledge of the process where the device will be used.

INTERFACE WITH PROCESS

First, consider whether the thermocouple is itself in direct contact with the process media or whether it is incorporated into a thermocouple assembly that includes a thermowell. Thermowells are metal, glass, or ceramic tubes that protect the thermocouple against corrosive, fast-flowing or highly hot process media. About 75% of heavy industry thermocouples use thermowell assemblies; these industries include refining, petrochemical, the pulp and paper industry, and power generation.

JUNCTION

The thermocouple junction design depends on the applications requirements for response speed and the likelihood of electrical noise being conducted through the process. A thermocouple has three variations of sensing tip (or junction): Exposed junction, where the exposed wire tips and welded bead have no covering or protection; Grounded junction, where the welded bead is in physical contact with the thermocouple's sheath; Ungrounded junction, where the tip is inside the thermocouple sheath, but is electrical (and somewhat thermally) insulated from the sheath (no sheath contact).

MATERIAL SELECTION

Material selection is the second criteria to choose. A vast majority of industrial thermocouples are made from stainless steel, but specialized alloys such as Inconel 600, Hastelloy X, Monel,  and other unique metals are required in certain applications.

MOUNTING

Next, you have to consider the mounting arrangement. You need to determine whether a more traditional industrial thermocouple/well/head design is required, or if some sort of flexible or remote thermocouple sensor is required for use in a hard to access area.

TYPE

Last, you have to decide the "type" of industrial thermocouple you need. In the case of thermocouples, "type" refers to the composition of metal wires in the instrument whose physical properties respond to changes in temperature. Different metal compositions have different temperature ranges and other properties that make them suitable for use in special applications, or inappropriate for use.

• Type J thermocouples use iron for the positive leg and copper-nickel (constantin) alloys for the negative leg. They may be used unprotected where there is an oxygen-deficient atmosphere, but a thermowell is recommended for cleanliness and generally longer life. Because the iron (positive leg) wire oxidizes rapidly at temperatures over 1000 deg.F, manufacturers recommend using larger gauge wires to extend the life of the thermocouple when temperatures approach the maximum operating temperature.
• Type K thermocouples use chromium-nickel alloys for the positive leg and copper alloys for the negative leg. They are reliable and relatively accurate over a wide temperature range. It is a good practice to protect Type K thermocouples with a suitable ceramic tube, especially in reducing atmospheres. In oxidizing atmospheres, such as electric arc furnaces, tube protection may not be necessary as long as other conditions are suitable; however, manufacturers still recommend protection for cleanliness and prevention of mechanical damage. Type K thermocouples generally outlast Type J, because the iron wire in a Type J thermocouple oxidizes rapidly at higher temperatures.
• Type N thermocouples use nickel alloys for both the positive and negative legs to achieve operation at higher temperatures, especially where sulfur compounds are present. They provide better resistance to oxidation, leading to longer service life overall.
• Type T thermocouples use copper for the positive leg and copper-nickel alloys for the negative leg. They can be used in either oxidizing or reducing atmospheres, but, again, manufacturers recommend the use of thermowells. These are good stable thermocouples for lower temperatures.
• Types S, R, and B thermocouples use noble metals for the leg wires and are able to perform at higher temperatures than the common Types J and K. They are, however, easily contaminated, and reducing atmospheres are particularly detrimental to their accuracy. Manufacturers of such thermocouples recommend gas-tight ceramic tubes, secondary porcelain protective tubes, and a silicon carbide or metal outer protective tube depending on service locations.

For more information about industrial thermocouples, contact Duro-Sense Corporation. Call them at 310-533-6877 or visit their web site at https://duro-sense.com.

3 Bottom Line Criteria to Help You Choose Between Thermocouples and RTDs

Both RTD and thermocouple probes monitor temperature but which one is right for your application?

The first question to ask yourself is what is the temperature range you are trying to monitor?
Generally, if the temperature is above a hundred and fifty degrees Celsius, a thermocouple would be used. For anything below a hundred and fifty degrees Celsius, an RTD would be used.

The next question to ask is what is the required sensor accuracy? 
RTDs provide more accurate readings with repeatable results, this is why RTDs are typically used when temperatures are within its monitoring range.

The last question is what is the purchase budget and how many do you need?
Thermocouples can be up to three times less expensive than RTD probes making thermocouples a good choice when purchasing a large quantity or when the budget is tight

These three criteria are VERY basic, and intended just to point you in the right direction. There are many other differences between thermocouples and RTDs that need to be understood before application.  Always consult a temperature sensor application expert prior to installing or specifying a thermocouple or RTD where failure can cause harm.

Duro-Sense Corporation
https://duro-sense.com

(310) 533-6877

RTD and Thermocouple Selection and Location for Optimal Control

Loop diagram *

A perfectly designed temperature loop would precisely balance the power required to heat the media to it's desired temperature while compensating for system losses. In the real world however, there are many external variables that upset the balance between energy input and desired temperature. To offset these external variables and ensure adequate power is available to do the work, energy calculations with liberal safety factors are coupled with temperature controllers that throttle or proportion the amount of energy added to the process.

Most temperature control loops have (5) four major components:

1) The media to be heated (e.g. metal platen, a tank of liquid, a stream of gas)
2) A energy source (e.g. electric heater, steam, hot oil, flame)
3) A temperature sensor (thermocouple, RTD)
4) A controller (e.g. electronic thermostat, PID controller)
5) Control element (e.g. control valve, SCR, SSR)

Temperature controllers provide sophisticated functions that "learn" or understand the relationship between available power and sensor temperature. They then adjust the amount of energy (heat) added, based on the current reading of the sensor and the desired temperature setpoint.  Unfortunately, temperature controllers are often relied upon to overcome the oversights and inadequacies of poor control loop design.

Lag time *

In poor control situations the controller usually takes the blame, when in actuality,  the problem lies in the system design. The controller's actions are a function of the difference between setpoint and sensor reading, the availability of energy to eliminate that error, and the sensor lag time. The further the sensor or energy source is located from the process media, the wider the swings in energy input will be, and therefore produce a more difficult loop to balance. Considering this, it is important to optimize the sensor (thermocouple or RTD) placement. Any distance or barrier between sensor and process media introduces lag which is an impediment to close control.

In the most ideal situation, temperature sensors, the energy source and process media would all be at the same physical location. Since it's virtually impossible to accomplish this, compromises have to be made to allow for the mechanical, physical, and electrical realities of the application. Here are some practical recommendations for sensor selection and placement to improve temperature loop performance:

• Thermocouples, because of their low mass generally have a response advantage over RTDs.
• Exposed junction thermocouples are the fastest responding (least lag) sensor choice, but they are also the most prone to physical and chemical damage.
• Narrow, sheathed, grounded junction magnesium oxide insulated thermocouples are nearly as fast as exposed junctions, and provide protection from the process media.
• Applications that require protection sheaths and thermowells for RTDs and thermocouples increase sensor lag time.
• An immersion length of at least 10 times the diameter of the thermowell or sensor sheath should be used to minimize heat loss along the sensor sheath or thermowell wall from tip to process connection.
• Where possible, insert the temperature sensor in a pipe elbow into the oncoming flow.
• Tapered, swaged, or stepped thermowells are  faster responding
• Always make sure the fit between sensor outer diameter and thermowell inner diameter is tight, and that the tip of the sensor is in direct contact with the thermowell. 

* Images courtesy of Tony Kuphaldt and his book "Lessons In Industrial Instrumentation"