Temperature Measurement Sensing methods: contact and noncontact Temperature sensors generate output signals in one of two ways: 1. through a change in output voltage 2. through a change in resistance of the sensor's electrical circuit
Making the Right Selection Does the application require contact or noncontact temperature sensing? How accurate must the temperature reading be? What temperature range is involved? What's the maximum temperature the sensor will be exposed to? How fast must the sensor respond to a temperature change and deliver an accurate reading? How long should the stability and accuracy of the sensor last? What environmental restraints exist? Are protective devices such such as thermowells and protection tubes required to provide sufficient ruggedness? What's the budget?
Comparison of RTDs and Thermocouples Temperature Range Accuracy Response Time Stability Linearity Sensitivity Measurement in Air/Gas Noise Immunity Vibration/Shock Immunity RTD 200 C to 850 C ±0.001 C to 0.1 C Moderate Stable over long periods <0.1% error/5 yr. Best High sensitivity Thermocouple 190 C to 1821 C ±1 C to 10 C Fast Not as stable 1 C error/yr. Moderate Low sensitivity
Fundamentals of Thermocouples Fundamentals of Thermocouples [ ] ) ( ) ( ) ( 2 2 2 1 2 2 1 1 2 1 j j j j T T B A T T T T E dt Q Q E j j + = = α α
HARDWARE COMPENSATION CIRCUIT
Thermocouples can be divided into two groups: BASE METAL Types E, J, K, N, and T NOBLE METAL Types R, S, and B
Thermocouple Type B C E J K N R S T Materials Platinum30% Rhodium (+) Platinum 6% Rhodium (-) W5Re Tungsten 5% Rhenium (+) W26Re Tungsten 26% Rhenium (-) Chromel (+) Constantan (-) Iron (+) Constantan (-) Chromel (+) Alumel (-) Nicrosil (+) Nisil (-) Platinum 13% Rhodium (+) Platinum (-) Platinum 10% Rhodium (+) Platinum (-) Copper (+) Constantan (-) Useful Application Range 2500-3100F 1370-1700C 3000-4200F 1650-2315C 200-1650F 95-900C 200-1400F 95-760C 200-2300F 95-1260C 1200-2300F 650-1260C 1600-2640F 870-1450C 1800-2640F 980-1450C -330-660F -200-350C
Comparison of TCs & Applications Type J is useable up to 720 C. It is not very susceptible to aging up to about 540 C. It is very cost effective and is the thermocouple of choice in the plastics processing industry where temperatures rarely exceed 400 C. The iron conductor is subject to oxidation at higher temperatures and when unprotected. Type K is useable up to 1150 C in an oxidising atmosphere. Metallurgical changes can cause a calibration drift of 1 to 2 C in a few hours, increasing to 5 C over time. Type E is useable up to 820 C. It has the highest mv output of all the thermocouples and has similar calibration drift to that of Type K so the same precautions are recommended.
Type N is useable up to1260 C. It was developed to overcome several problems inherent in Type K TCs. Aging in the 300 to 600 C range is considerably less. Also Type N has also been found to be more stable than Type K in nuclear environments, where Type K has been the sensor of choice. Type T. Oxidisation of the copper limits the useable temperature to about 370 C. It has been the thermocouple of choice for applications down to 200 C. Types R and S are usable up to 1480 C. They are extremely stable but reducing atmospheres are particularly damaging. This type should be protected with a gas-tight ceramic tube and a secondary tube of porcelain, silicon carbide or metal outer tube, as conditions require. Type R delivers some 15% more mv than type S. Type B is usable up to 1700 C. Also easily contaminated, and damaged by reducing atmospheres. The same protective measures for R and S shown above apply to type B thermocouples.
most widely used TC in industrial process Type K thermocouples are the most linear of the three types J and E have better relative output than type K type K is the most popular thermocouple primarily because of its better linearity
Voltage-To-Temperature Conversion
Signal Conditioning Thermocouples generate extremely low voltages,, making them susceptible to noise. A thermocouple's temperature sensitivity is small,, requiring accurate instrumentation. A cold-junction compensation sensor is required when using thermocouples Typical thermocouple accuracy is ~1 C. Noise reduction techniques Twist wires & wrapped with a grounded foil sheath Use grounded probe Employed high CMR instrumentation amplifier
Instrumentation Amplifier @25 C: AD594 output = ( (Type J Voltage e + 16 mv) m ) * 193.4 AD595 output = (Type K Voltage + 11 m mv) * 247.3
AD594 output for J type Thermocouple @ 25ºC
Probe & Wire Typical industrial TC Thermowells Miniature thermocouples Skin Temperature Sensors
Junction Types Ungrounded Probe Grounded Probe Exposed Probe
Thermocouple Color Codes United States ASTM German DIN
True Surface Thermocouple (TST)
Standard & Continuous TC Standard Thermocouple Continuous Thermocouple Inconel Sheath Insulating Material Thermocouple Wires CT2C FTLD
Two types of Continuous Thermocouples
The SmartQ Thermocouple adapter K type Thermocouple Range: -200ºC C to 1000ºC Resolution: 1ºC1 is equipped with a micro controller by using a 40-point linearization improves the accuracy
1-Wire Thermocouple A standard thermocouple can be combined with a lithium ion monitor chip to create a smart sensor that communicates with a PC or microcontroller over a single twisted-pair cable.
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