Thermocouples

MaintenanceCircleTeam

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May 7th 2007

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NEWSLETTER FOR MAINTENANCE COMMUNITY Word for the day: THERMOCOUPLES

The word THERMOCOUPLE can be split into two separate words – THERMO & COUPLE – to understand its purpose which has a history of more than 200 years. THERMO means heat and COUPLE means a pair. Hence, when pair of different metal wires is joined together at the end, it generates a voltage proportional to the heat or temperature. In 1821, T J Seebeck discovered that an electromotive force – emf for short which also means mild voltage, something similar to back emf in a motor – exists across a junction formed of two unlike metals. For example, if a circuit is formed using two dissimilar metals A & B – thermocouple – as shown in Figure 1, two junctions, p & q will be formed, which are at two different temperatures, T1 & T2. If the temperatures are same, the two voltages will be same but opposed and hence net difference is zero. However, if the temperatures are different, the voltages will not balance and a current will flow. Therefore, the net voltage generated will be a function of two materials used to form the circuit and the temperatures of the two junctions. Note that a thermocouple always requires two junctions, one of which measures the required temperatures and other one is maintained at T1 T2 known fixed temperature, known as cold or reference junction. p q Historically, the cold junction was maintained at 0°C in an ice bath. In practical wiring today, we actually only connect two wires to a controller. The cold junction will be pre-wired inside the controller B Figure 1 and solid state device adjusts the cold junction temperature accordingly to match ambient conditions. Theoretically, any two unlike materials can be used to form a thermocouple. Actually, of course, certain materials and combinations are better than others, and some have practically become standard for given temperature ranges. Materials commonly used in thermocouples may be listed as follows:

A

Copper, Iron, Platinum, Rhodium, Iridium, Constantan (60% Copper, 40% Nickel), Chromel (10% Chromium, 90% Nickel), Alumel (2% Aluminum, 90% Nickel, remaining 8% being silicon and manganese).Refer to Table 1 for features of few commonly used commercial thermocouples. Generally, these thermocouples have an average of 50µV per °C and hence need large amount of amplification. Style (Commercial Term) J K T E R S

Metal Combination Iron-Constantan Chromel-Alumel Copper-Constantan Chromel-Constantan Platinum 13%Rhodium Platinum 10% Rhodium

Minimum Temperature °F (°C) -346(-210) -454(-270) -454(-270) -454(-270) -58(-50)

Maximum Temperature °F (°C) 2193 (1200) 2501(1372) 752(400) 1832(1000) 3214(1768)

-58(-50)

3214(1768)

Sensitivity at 25°C 51.7µV / °C 40.6µV / °C 40.6µV / °C 60.9µV / °C 6µV / °C 6µV / °C

Table 1

Based on the application requirement, any of the above thermocouples can be selected. As you can see from the table for example, R & S thermocouples have more sensitivity than J & K type and hence can give accurate readings. But as sensitivity increases, the distance between sensing and input point reduces. Hence a balance must be made between the importance of application and where the controller can be located before taking a decision. One major disadvantage of these thermocouples is oxidation. As it becomes older, the tip gets oxidized and sensitivity starts reducing. Based on the working conditions, it is suggested to replace them on a regular interval – one year for example – or create a calibration plan. This will prevent costly product or process rejections and avoid errors in the process control. If you like to improvise this article or contribute or comment please mail us at: [email protected] This document contains information for reference only. We assume no responsibility for its implication.

MaintenanceCircleTeam

May 7th 2007

Page 2

Apart from these regular thermocouples, Pt100 and RTD are two most popularly used temperature measuring sensors used in the industry. Pt100 is the abbreviation for most common type of resistance temperature sensor used in the industry. It is made from pure platinum and generally has a specific resistance of 100 ohms at 0°C. It accurately follows a temperature versus resistance characteristic. Its temperature sensing range is from -200° to 850°C and hence is very commonly used in cold temperature applications. Resistance Temperature Detectors are sensors that measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is typically relatively fragile, so it is generally installed inside a sheath to protect it. The RTD element is constructed from a pure material, the resistance of which, at various temperatures, has been documented by various international standards institutes. The material has a predictable change in resistance as the temperature varies; it is this change that is used to determine temperature. RTDs are generally considered to be among the most accurate temperature sensors available. In addition to offering very good accuracy, they provide excellent stability and repeatability. RTDs also feature high immunity to electrical noise and are, therefore, well suited for applications in process and industrial automation environments, especially around motors, generators and other high voltage equipment. Table 2 gives a brief overview of various temperature sensors used in the control system. Apart from these conventional methods, Infrared, Ultraviolet, Radio frequency sensing methods are becoming very popular and are widely used in difficult to access – steel melting furnace for example – applications.

Accuracy Linearity

Thermocouples Very wide: Type T can go down below 200°C. Type W5 can approach 1800°C Generally inexpensive although type R & S use expensive platinum wire. Moderate Poor

Physical Strength

Excellent

Operating Range

Price

Change in characteristics with temperature Long term stability

Preferred Applications

Pt100

RTD

Solid State Devices

Wide: -200°C to 600°C

Narrow. Typically 40°C to 300°C

Very narrow: Typically -40°C to 125°C

Excellent Good Poor to very good Depends on probe construction

Low accuracy types very inexpensive high accuracy types more expensive than Pt100 Poor to excellent Poor Poor to very good Depends on probe construction

Moderate Very good Good to very good Depends on probe construction

Small

Reasonable

Very large

Large

Reasonable Industrial processes where temperature range or physical requirements preclude other devices.

Excellent All industrial processes within operating range where accuracy and repeatability are required.

Poor to excellent

Reasonable

Preset temperature applications. Control where narrow hysteresis is required.

Simple control applications and ambient compensating circuits.

Fairly inexpensive

Inexpensive

Table 2

If you like to improvise this article or contribute or comment please mail us at: [email protected] This document contains information for reference only. We assume no responsibility for its implication.

MaintenanceCircleTeam Page 3 May 7th 2007 In order to obtain better accuracy and consistency in temperature control, some of the precautions to be taken are listed below: z Place thermocouples as close to the process as possible. For instance, if a heater is melting plastic material, the thermocouple should be as close to heater as possible z Thermocouple cables should not have any sharp bends or too many joints z Clean the thermocouple tip on a regular basis z Avoid running thermocouple cable too long. If it necessary, install necessary amplifiers z Do not use regular electrical cables to extend the length as they will reduce the voltage levels seriously z Standard thermocouples should not run too close to other electrical cables z It is not normal to connect same thermocouple in parallel to more than two controller units – PLEASE AVOID z Replace same type of thermocouples z Proper selection, maintenance, calibration of thermocouples avoid process rejections, reduces energy consumption, avoids unnecessary temperature under and over shooting z As much as possible, standardize thermocouples in a company. This will avoid mixing up during replacements and multiple inventories Even though thermocouple is a small sensor in entire control system, if they are faulty major breakdowns can occur. Few of them are listed below to understand the seriousness which can be avoided by little preventive maintenance and Simple control applications and ambient compensating circuits.regular observation. Overheating and eventual burning of motor winding since RTD did not sense the temperature Overheating of steam resulting in explosion due to faulty thermostat z Control cabinet of a machine tripping due to failure of temperature sensor z Over freezing of room by air conditioner due to faulty thermostat z Material decomposing due to overheating, since thermocouple failed to measure required temperature And list can be endless. z z

If you like to improvise this article or contribute or comment please mail us at: [email protected] This document contains information for reference only. We assume no responsibility for its implication.