Temperature Sensor “It is time to turn up the heat but first you must learn how to measure it” Temperature control is important for separation and reaction processes, and temperature must be maintained within limits to ensure safe and reliable operation of process equipment.   Temperature can be measured by many methods; several of the more common are described in this subsection.  You should understand the strengths and limitations of each sensor, so that you can select the best sensor for each application. PEC – UET Lahore Dr. Shahid Naveed 22. September 2018 / Dr. –Ing Naveed Ramzan 1 1

Relative Scales Absolute Scales Temperature Measurement Scales Fahrenheit (°F) Celsius (°C) Absolute Scales Rankine (°R) Kelvin (K) F = 1.8 C + 32 R = F + 460 C = (F - 32) / 1.8 K = C + 273 Temperature At the beginning of the renaissance many scientists recognized that the idea of the subjective sense of temperature needed to be quantified. They knew that the behavior, and sometimes the structure, of a body changed as it became hotter or colder. The most vivid example of the phenomenon is the behavior and structure of water as it is cooled to the freezing point. But, how does one quantify, or measure, a subjective feeling? Galileo, and others, tried to build various types of thermometers which were are successful to some extent, but they all lacked the essential quality of being able to measure temperature with any more repeatability than the common sense of touch. Along comes a German Physicist, Gabriel Fahrenheit, and the mercury thermometer. Fahrenheit's real contribution was a sealed thermometer that measured the expansion and contraction of mercury with temperature changes and it was independent of atmospheric pressure changes. The susceptibility of the non-sealed thermometers to atmospheric pressure changes later evolved into the barometer that is the basis of the next topic.Fahrenheit. He needed to calibrate his thermometer at some repeatable temperatures and so he selected the two most common and universally repeatable temperatures: the freezing point of pure water and the boiling point of pure water.Fahrenheit engraved a scale on his prototype thermometer before he calibrated it. When he measure the temperatures of ice water and boiling water, he came up with two of the most inconvenient numbers around: 32° and 212°. 2

Fahrenheit (⁰F) / Rankine (⁰R) Celsius (⁰C) / Kelvin (⁰K) Temperature Measurement Scales Imperial Fahrenheit (⁰F) / Rankine (⁰R) +/- 460 Metric Celsius (⁰C) / Kelvin (⁰K) +/- 273 100⁰C 373⁰K 273⁰K 255⁰K 0⁰C -18⁰C -273⁰C 0⁰K -460⁰F 0⁰R 0⁰F 32⁰F 460⁰R 492⁰R 672⁰R 212⁰F Fahrenheit [°F] = [°C] · 9/5 + 32 Celsius [°C] = ([°F] − 32) · 5/9 Kelvin [K] = [°C] + 273.15 Rankine [°R] = [°F] + 459.67  

Mechanical Methods Electrical Methods Methods of Temperature Measurement Mechanical Methods Electrical Methods In nearly all cases, the temperature sensor is protected from the process materials to prevent interference with proper sensing and to eliminate damage to the sensor.  Thus, some physically strong, chemically resistant barrier exists between the process and sensor; often, this barrier is termed a sheath or thermowell, especially for thermocouple sensors.  An additional advantage of such a barrier is the ability to remove, replace, and calibrate the sensor without disrupting the process operation. Thermocouples are among the easiest temperature sensors to use and obtain and are widely used in science and industry. They are based on the Seebeck effect that occurs in electrical conductors that experience a temperature gradient along their length. Thermocouples are pairs of dissimilar metal wires joined at least at one end, which generate a net thermoelectric voltage between the the open pair according to the size of the temperature difference between the ends, the relative Seebeck coefficient of the wire pair and the uniformity of the wire-pair relative Seebeck coefficient. Thermistors are special solid temperature sensors that behave like temperature-sensitive electrical resistors. A thermistor is a thermally sensitive resistor that exhibits a change in electrical resistance with a change in its temperature. The resistance is measured by passing a small, measured direct current (dc) through it and measuring the voltage drop produced. Resistance Temperature Detectors or RTDs for short, are wire wound and thin film devices that measure temperature because of the physical principle of the positive temperature coefficient of electrical resistance of metals. The hotter they become, the larger or higher the value of their electrical resistance. PEC – UET Lahore Dr. Shahid Naveed 22. September 2018 / Dr. –Ing Naveed Ramzan 4

Electrical resistance change (RTD) Pyrometers Expansion of materials Methods of Temperature Measurement Thermocouples Thermistors Electrical resistance change (RTD) Pyrometers Expansion of materials In nearly all cases, the temperature sensor is protected from the process materials to prevent interference with proper sensing and to eliminate damage to the sensor.  Thus, some physically strong, chemically resistant barrier exists between the process and sensor; often, this barrier is termed a sheath or thermowell, especially for thermocouple sensors.  An additional advantage of such a barrier is the ability to remove, replace, and calibrate the sensor without disrupting the process operation. Thermocouples are among the easiest temperature sensors to use and obtain and are widely used in science and industry. They are based on the Seebeck effect that occurs in electrical conductors that experience a temperature gradient along their length. Thermocouples are pairs of dissimilar metal wires joined at least at one end, which generate a net thermoelectric voltage between the the open pair according to the size of the temperature difference between the ends, the relative Seebeck coefficient of the wire pair and the uniformity of the wire-pair relative Seebeck coefficient. Thermistors are special solid temperature sensors that behave like temperature-sensitive electrical resistors. A thermistor is a thermally sensitive resistor that exhibits a change in electrical resistance with a change in its temperature. The resistance is measured by passing a small, measured direct current (dc) through it and measuring the voltage drop produced. Resistance Temperature Detectors or RTDs for short, are wire wound and thin film devices that measure temperature because of the physical principle of the positive temperature coefficient of electrical resistance of metals. The hotter they become, the larger or higher the value of their electrical resistance. PEC – UET Lahore Dr. Shahid Naveed 22. September 2018 / Dr. –Ing Naveed Ramzan 5

The magnitude of emf depends on the junction temperature. Thermocouples When 2 dissimilar metals are joined together to form a junction, an emf is produced which is proportional to the temperature being sensed. Seebeck Effect: The generation of current in a circuit comprising of two wires of dissimilar metals in the presence of temperature difference ---Thermocouples: When the junctions of two dissimilar metals are at different temperatures, an electromotive force (emf) is developed. The cold junction, referred to as the reference, is maintained at a known temperature, and the measuring junction  is located where the temperature is to be determined.  The temperature difference can be determined from the measured emf.  The relationship between temperature difference and emf has been determined for several commonly used combinations of metals; the mildly nonlinear relationships are available in tabular form along with polynomial equations relating emf to temperature (Omega, 1995). ----The see beck effect occurs when you take any two members of the thermoelectric series and connect wires made of them to form a circuit with two junctions. In the presence of a temperature difference between the junctions a small current flows around the circuit. Another junction is formed when the metering circuit is connected to the thermocouple The meter reads the difference between the Meas. Junc. & Ref. Junc. The magnitude of emf depends on the junction temperature. PEC – UET Lahore Dr. Shahid Naveed 22. September 2018 / Dr. –Ing Naveed Ramzan 6

The operating range can be -200°C to + 1000°C Thermistors Thermistor, a word formed by combining thermal with resistor, is a temperature-sensitive resistor fabricated from semiconducting materials. The resistance of thermistors decreases proportionally with increases in temperature. The operating range can be -200°C to + 1000°C Metallic resistance thermometers and thermistors are two types of thermometers based on the principle that the electrical resistance of materials changes as their temperature changes RTDs use metallic wires Thermistors use semiconductor materials -- Are thermally sensitive resistors that change resistance with changes in temperature They are highly-sensitive and have very reproducible resistance Vs. temperature properties. Typically used over a small temperature range, (compared to other temperature sensors) because of their non-linear characteristics PEC – UET Lahore Dr. Shahid Naveed 22. September 2018 / Dr. –Ing Naveed Ramzan 7 aaaa 7

The thermistors can be in the shape of a rod, bead or disc. Manufactured from oxides of nickel, manganese, iron, cobalt, magnesium, titanium and other metals. PEC – UET Lahore Dr. Shahid Naveed 22. September 2018 / Dr. –Ing Naveed Ramzan 8

Small sizes and fast response Low cost Suitability for narrow spans Thermistors Advantages: Small sizes and fast response Low cost Suitability for narrow spans Disadvantages: More susceptible to permanent decalibration at high temperatures. Use is limited to a few hundred degrees Celsius. Respond quickly to temperature changes, thus, especially susceptible to self-heating errors. Very fragile PEC – UET Lahore Dr. Shahid Naveed 22. September 2018 / Dr. –Ing Naveed Ramzan 9

Resistance Temperature Detector- RTD Electrical Resistance Change (RTD) Resistance Temperature Detector- RTD RTD (Resistance Temperature Detector) is a temperature sensitive resistor. It is a positive temperature coefficient device, which means that the resistance increases with temperature. The resistive property of the metal is called its resistivity. -- This type of sensors is based on the observation that different materials can have different resistive profiles at different temperatures. These properties are mainly electrical in nature. -- Industrial RTDs are very accurate: the accuracy can be as high as ±0.1°C. The ultra high accurate version of RTD is known as Standard Platinum Resistance Thermometers (SPRTs) having accuracy at ±0.0001°C. The industry standard is the platinum wire RTD (Pt100) whose base resistance is exactly 100.00 ohms at 0.0 °C. PEC – UET Lahore Dr. Shahid Naveed 22. September 2018 / Dr. –Ing Naveed Ramzan 10 10

Electrical Resistance Change (RTD) Platinum Wire RTDs (PRTs) PRTs have established themselves as the de-facto industry standard for temperature measurement, and for many reasons: linear temperature sensors Resistance vs temperature characteristics are stable and reproducible linear positive temperature coefficient (-200 to 800 °C) very accurate and suitable for use as a secondary standard -- Industrial RTDs are very accurate: the accuracy can be as high as ±0.1°C. The ultra high accurate version of RTD is known as Standard Platinum Resistance Thermometers (SPRTs) having accuracy at ±0.0001°C. ---The wire is cut, coiled and housed in a protective overcoat (thermowell) ---Each RTD is standardized to provide a specific resistance per degree ---The temperature can be determined by using a R-T table. PEC – UET Lahore Dr. Shahid Naveed 22. September 2018 / Dr. –Ing Naveed Ramzan 11 11

Radiation pyrometers ( measurement of radiant energy) Pyrometry is a technique for measuring temperature without physical contact An apparatus for measuring high temperatures that uses the radiation emitted by a hot body as a basis for measurement. Radiation pyrometers ( measurement of radiant energy) Optical Pyrometers (comparison of the intensities ) PEC – UET Lahore Dr. Shahid Naveed 22. September 2018 / Dr. –Ing Naveed Ramzan 12

Radiation Pyrometer Radiation pyrometers -- Electromagnetic waves propogated through he space with common velocity of 299.774 km/s. -- Here concern is with UV radiation and IR Radiation. PEC – UET Lahore Dr. Shahid Naveed 22. September 2018 / Dr. –Ing Naveed Ramzan 13 aaaa 13

Bimetallic Thermometer (Expansion of solids) Expansion Thermometers Bimetallic Thermometer (Expansion of solids) Effect of unequal expansion of a bimetallic strip Different metals have difference coefficient. Configured as spiral or helix for compactness - Can be used with a pointer to make an inexpensive compact rugged thermometer. Bimetallic thermometer PEC – UET Lahore Dr. Shahid Naveed 22. September 2018 / Dr. –Ing Naveed Ramzan 14 14

Bimetallic Thermometer (Expansion of solids) Expansion Thermometers Bimetallic Thermometer (Expansion of solids) PEC – UET Lahore Dr. Shahid Naveed 22. September 2018 / Dr. –Ing Naveed Ramzan 15 15