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Temperature

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Information about Temperature
Education

Published on February 14, 2008

Author: Arkwright26

Source: authorstream.com

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Temperature Measurement:  Temperature Measurement Indicators Sensors/Transducers/Switches Transmitters Temperature:  Temperature Temperature:  Temperature Temperature:  Temperature Temperature Scales & Units:  Temperature Scales & Units Slide6:  Fahrenheit [°F] = [°C] · 9/5 + 32 Celsius [°C] = ([°F] − 32) · 5/9 Kelvin [K] = [°C] + 273.15 Rankine [°R] = [°F] + 459.67   Imperial Fahrenheit (⁰F) / Rankine (⁰R) +/- 460 Metric Celsius (⁰C) / Kelvin (⁰K) +/- 273 Temperature Measurement:  Temperature Measurement Temperature devices are specified according to their construction and are classified as follows::  Temperature devices are specified according to their construction and are classified as follows: Liquid Expansion Devices:  Liquid Expansion Devices Liquid in Glass Thermometers:  Liquid in Glass Thermometers Advantages:  Advantages Disadvantages:  Disadvantages Industrial Grade Thermometers:  Industrial Grade Thermometers Filled Thermal System Filled thermometers, Gas Filled Thermometers:  Filled Thermal System Filled thermometers, Gas Filled Thermometers Closed System – no external energy required:  Closed System – no external energy required Bulb, Capillary tube, Bourdon tube filled with a liquid, vapor, or gas. Slide18:  Liquid Filled Class l A liquid-filled system completely fills with liquid. This type of system operates on the principle that liquid expands with an increase in temperature. When the liquid expands, it causes the pressure to increase, which causes the Bourdon tube to uncoil and move the needle on the scale. Typically, inert hydrocarbons such as xylene see more use because of their low coefficient of expansion. In some cases, you can even use water. Another common liquid is mercury. Mercury-filled systems are often in a separate class from liquid-filled system because they respond to temperature changes and have a higher degree of accuracy than other liquids. Slide19:  Vapor Filled Class ll A vapor system contains a volatile liquid and vapor and operates on the principle that pressure in a vessel containing only a liquid and its vapor increases with temperature and is independent of volume. With a vapor system, you measure temperature at the interface between the liquid and the vapor. For a vapor system to operate properly, the interface must remain in the bulb. Vapor has subclasses, llA, llB, llC Slide20:  Gas Filled Class lll Gas-filled systems see use in industrial applications and, in some cases, in laboratory measurements. The operation of gas-filled systems is based on the ideal gas law, and their measurement is thus an approximation at normally encountered temperatures and pressures. According to this law, the pressure of an ideal gas confined to a constant volume is proportional to absolute temperature. In a typical gas-filled system, the gas (usually nitrogen) is not perfect, so there may be a slight change in volume. However, these difference are minor and do not prevent the use of pressure measurement to indicate temperature. Bimetallic Thermometers:  Bimetallic Thermometers Bimetallic strip used as a switch:  Bimetallic strip used as a switch Bimetallic Thermometer:  Bimetallic Thermometer The bimetallic strip can be wound as a helix and will twist when heated. This twisting action can be used to drive a pointer over a calibrated temperature scale. These temperature sensors are low cost and have accuracy ranges of between 2-5 % and are mostly used for local readings. They are not suitable for providing a continuous output measurement. Change-of-State Temperature Measurement Devices:  Change-of-State Temperature Measurement Devices Radiation Temperature Sensors Optical Pyrometers:  Radiation Temperature Sensors Optical Pyrometers Non Contact Measurement:  Non Contact Measurement Used in High Temperature Applications:  Used in High Temperature Applications Called by many names:  Called by many names Electrical Temperature Sensors:  Electrical Temperature Sensors Thermistors:  Thermistors Temperature Vs Resistance Characteristics:  Temperature Vs Resistance Characteristics Positive Temperature Coefficients:  Positive Temperature Coefficients General Specifications (NTC’s):  General Specifications (NTC’s) Thermistor Bridge Circuit:  Thermistor Bridge Circuit http://www.amwei.com/views.asp :  http://www.amwei.com/views.asp Resistance Temperature Detectors RTD’s:  Resistance Temperature Detectors RTD’s RTD Materials:  RTD Materials RTD Construction:  RTD Construction Wire:  Wire Classical construction, excellent interchangeability Coil:  Coil Good stability, rugged Hollow Annulus:  Hollow Annulus Fastest response, most expensive Film:  Film Newer design, easier to make, interchangeability is not the best Temperature – Resistance Characteristics:  Temperature – Resistance Characteristics Platinum (Pt) Pt100 (100 Ω @ 0 ⁰ C) 0.4 ohms/C Pt1000 (1000 Ω @ 0⁰ C) 4 ohms/C Nickel Ni120 (120 Ω @ 0⁰ C) Copper Cu10 (10 Ω @ 0⁰ C) RTD Temperature vs Resistance :  RTD Temperature vs Resistance Temperature Coefficient:  Temperature Coefficient RTD Resistance vs Temperature Table (alpha = 0.00385):  RTD Resistance vs Temperature Table (alpha = 0.00385) Wiring Configuration:  Wiring Configuration Wiring Configuration:  Wiring Configuration The RTD may be located hundred’s of feet away. Resistance in the leads will be added to the RTD’s causing an error in the measured temperature. Effects of lead wire resistance:  Effects of lead wire resistance 100 ft of 30 gauge wire can introduce a lead resistance of about 10 ohms per lead 3 – Wire RTD’s:  3 – Wire RTD’s The 2 leads connected together will be the same colour. (eg. Red,Red) Most industrial RTD’s will be 3 – wire types Some RTD’s have 4 wires??? RTD Spec’s:  RTD Spec’s Example of Manufacturers Spec’s:  Example of Manufacturers Spec’s Protective Wells:  Protective Wells Spring Loaded RTD in Thermowell:  Spring Loaded RTD in Thermowell RTD Colour Coding:  RTD Colour Coding RTD Pros & Cons':  RTD Pros & Cons' Thermistor / RTD Comparison:  Thermistor / RTD Comparison Thermocouples (TC’s):  Thermocouples (TC’s) Seebeck Effect produces a mV:  Seebeck Effect produces a mV The mV per degree will depend on the combinations of metals used. Manufacturers produce a variety of combinations specified as “types” example - Type T, J, K, E Thermocouple Types:  Thermocouple Types Manufacturers have perfected a variety of metal combinations and specify them as “types” example - Type T, J, K, E ….. Each type produces its own specific mV per degree, these values are published in the TC tables Each type has a different temperature range (Type T can only measure up to 400 ‘ C, Type K 1300 ‘C) The Types are colour coded to make it easy to identify them in the field. Thermocouple Types :  Thermocouple Types TC - C/mV Comparison:  TC - C/mV Comparison TC Tables:  TC Tables Each TC type has a corresponding ThermoElectric Voltage (mV) table standardized according to NIST (National Institute of Standards & Trades) That table will list the mV produced by each TC type for any given temperature. (Type T table is shown below) Reference Junction is 0⁰ C Measuring Temperatures - Example:  Measuring Temperatures - Example According to the Type J tables the meter should read 5.269 mV when the TC is measuring a temperature of 100⁰ C But it reads 4.25 mV instead Because the meter leads form another junction which produces another emf equal to room temperature. The mV at the reference junction temperature must be added to the meter Measuring & Reference Junction:  Measuring & Reference Junction 100 ⁰C 20 ⁰C Meter mV = MJmV – RJmV 4.25 = MJmV - 1.019 MJmV = 4.25 + 1.019 = 5.269 mV 4.25 mV Type J (Iron/Constantan) Reference Junction Compensation:  Reference Junction Compensation Ice Bath Compensation:  Ice Bath Compensation mV Thermocouple Extension Wire & Transmitters:  Thermocouple Extension Wire & Transmitters Extension wires must be matched to the thermocouple type.:  Extension wires must be matched to the thermocouple type. Thermocouple Welds:  Thermocouple Welds TC Pro’s & Con’s:  TC Pro’s & Con’s RTD vs TC vs Thermistor:  RTD vs TC vs Thermistor RTD vs TC vs Thermistor:  RTD vs TC vs Thermistor Response Time – Time Constant:  Response Time – Time Constant Need to Know Summary:  Need to Know Summary Temperature Scales & Units:  Temperature Scales & Units Liquid Expansion, Bimetallic, Optical Thermometers:  Liquid Expansion, Bimetallic, Optical Thermometers Electrical Temperature Sensors:  Electrical Temperature Sensors RTD’s:  RTD’s Thermocouples:  Thermocouples Relative comparison between sensors:  Relative comparison between sensors

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