TEMPERATURE MEASUREMENT: ELECTRICAL TEMPERATURE SENSORS DEPARTMENT OF CHEMICAL ENGINEERING G.H. PATEL COLLEGE OF ENGINEERING & TECHNOLOGY VALLABH VIDYANAGAR – INSTRUMATION AND PROCESS CONTROL ( ) Guided By – Prof. D.V. Kapatel

SUBMITTED BY - NAME ENROLLMENT NO. AAKASH PATEL MEHUL AMRUTIYA URVESH CHAUDHARI DARSH SHAH TEJJAN DATANI HARSH MANSURIYA

CONTENTS INTRODUCTION TEMPERATURE MEASUREMENTS ELECTRICAL TEMPERATURE SENSORS THERMISTORS RESISTANCE-TEMPRATURE DETECTORS TERMOCOUPLES BIBLIOGRAPHY

INTRODUCTION Industrial temperature measurement regulation has gained importance along with the advancement in technology. The objective is to obtain values for the current conditions within the process. The components of a typical continuous measurement device are as follows: Sensors Signal Processing Transmitters

TEMPERATURE MEASUREMENT We need some sort of units to classify the measurement. It would be better to express the average thermal energy per molecule of material in terms of Joule. Measurement of the hotness or coldness of a body or fluid is commonplace in the process industries. The three dominant measurement devices used in automatic control are thermocouples, resistance thermometers, and pyrometers and are applicable over different temperature regimes.

ELECTRICAL TEMPERATURE SENSORS THERMISTORS A thermistor is a type of resistor whose resistance is dependent on temperature. Thermistors are widely used as inrush current limiter, temperature sensors, self-resetting overcurrent protectors and self-regulating heating elements. Thermistors are nonlinear temperature-dependent resistors. Normally only the materials with negative temperature coefficient of resistance (NTC type) are used. The resistance is related to temperature as: R T = R Tr * [exp ((1/T)-(1/T r ))] where Tr is a reference temperature, which is generally 298 K.

CHARACTERISTICS Sensitivity Construction Range Response Time Signal Conditioning

APPLICATIONS PTC(Positive temperature coefficient) thermistors can be used as current-limiting devices for circuit protection etc. PTC thermistors were used as heater in automotive industry to provide additional heat. PTC thermistors are used in lithium battery protection circuits. NTC(Negative temperature coefficient) thermistors are regularly used in automotive applications. NTC thermistors can be also used to monitor the temperature of an incubator. Thermistors are also commonly used in modern digital thermostats, monitor the temperature of battery packs etc.

RESISTANCE-TEMPRATURE DETECTORS RTDs are more linear than are thermocouples. They have positive temperature coefficients (PTC), Their output is extremely linear producing very accurate measurements of temperature. RTD’s are precision temperature sensors made from high-purity conducting metals such as platinum, copper or nickel. Its electrical resistance changes as a function of temperature, similar to that of the thermistor.

Contd… The more common types of RTD’s are made from platinum and are called Platinum Resistance Thermometer. The downside is that Platinum is expensive and one of the main disadvantages of this type of device is its cost. Because the RTD is a resistive device, we need to pass a current through them and monitor the resulting voltage. To avoid this, the RTD is usually connected into a Whetstone Bridge network. It has additional connecting wires for lead-compensation and/or connection to a constant current source

Contd… The advantages of platinum resistance thermometers include: High accuracy Low drift Wide operating range Suitability for precision applications Limitations Includes – At temperatures above 660 °C it becomes increasingly difficult to prevent the platinum from becoming contaminated. At low temperatures, the sensitivity of the RTD is essentially zero and therefore not useful.

There are three main categories of RTD sensors: thin film, wire- wound, and coiled elements. While these types are the ones most widely used in industry there are some places where other more exotic shapes are used, for example carbon resistors are used at ultra-low temperatures (-173 °C to -273 °C).

THERMOCOUPLES Thermocouples are voltage devices that indicate temperature by measuring a change in voltage. the thermocouple is located inside a metal or ceramic shield that protects it from exposure to a variety of environments. As temperature goes up, the output voltage of the thermocouple is rises – not necessarily linearly. Metal-sheathed thermocouples also are available with many types of outer coatings, such as Teflon, for trouble-free use in acids and strong caustic solutions.

Contd… Consist of two junctions of dissimilar metals, such as copper and constantan that are welded or crimped together. One junction is kept at a constant temperature called the reference (Cold) junction, while the other the measuring (Hot) junction. When the two junctions are at different temperatures, a voltage is developed across the junction which is used to measure the temperature.

Thermocouple Colour Codes:- Thermocouple Sensor Colour Codes Extension and Compensating Leads- Code Type Conductors (+/-)Sensitivity E Nickel Chromium / Constantan -200 to 900 o C JIron / Constantan0 to 750 o C K Nickel Chromium / Nickel Aluminium -200 to 1250 o C NNicrosil / Nisil0 to 1250 o C TCopper / Constantan-200 to 350 o C U Copper / Copper Nickel Compensating for "S" and "R" 0 to 1450 o C

ADVANTAGES Self-Powered Simple Rugged Inexpensive Wide Variety DISADVANTAGES Non Linear Reference Required Low Voltage Reference Required Least Stable Least Sensitive

BIBLIOGRAPHY Process Control Instrumentation Technology, Curtis D. Johnson, PHI Learning Private Limited, Eight Edition Perry’s Chemical Engineers’ Handbook, McGraw Hill Publication, Robert H. Perry Seventh Edition “Process Instrumentation And Control”, A. P. Kulkarni, Nirali Prakashan Thermal Sensors, Basic Types Of Thermal Sensors, basic-types-temperature-sensors, http:// basic-types-temperature-sensors Thermocouples, http://

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