Temperature Measurement using sensors

Temperature sensors : 

Temperature sensors ENG. AMGAD YOUNIS 1

Topics of Discussion : 

Topics of Discussion Types of temperature sensors 1- Thermistor 2- Thermocouple 3- RTD (Resistance Temperature Detector) Advantages and disadvantages Features and applications 2

What is transducer? : 

What is transducer? Non-electrical physical quantity: temperature, sound or light TRANSDUCER Electrical signal 3

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Temperature sensors detect a change in a physical parameter such as resistance or output voltage that corresponds to a temperature change. There are two basic types of temperature sensing: ■ Contact temperature : sensing requires the sensor to be in direct physical contact with the media or object being sensed. It can be used to monitor the temperature of solids, liquids or gases over an extremely wide temperature range. ■ Non-contact measurement interprets the radiant energy of a heat source in the form of energy emitted in the infrared portion of the electromagnetic spectrum. This method can be used to monitor non-reflective solids and liquids but is not effective with gases due to their natural transparency. 4

Thermistor : 

Thermistor How it works: a thermistor uses the fact that resistance of a metal changes with temperature. Make up: Generally made up of semiconductor materials Temperature Range: About -45°C - 150°C Thermistor 5

Thermistor Advantages and Disadvantages : 

Thermistor Advantages and Disadvantages Advantages: Very sensitive (has the largest output change from input temperature) Quick response More accurate than RTD and Thermocouples Disadvantages: Output is a non-linear function Limited temperature range. Require a current source Self heating Fragile 6

Thermistor (features) : : 

Thermistor (features) : Semiconductor resistance sensors Unlike metals, thermistors respond negatively to temperature and their coefficient of resistance is of the order of 10 times higher than that of platinum or copper. (Temperature semiconductor resistance ) 7

Thermocouple : 

Thermocouple How it works: Made up of two different metals joined at one end to produce a small voltage at a given temperature. Make up: Made of up two different metals. Ex: A type J is made up of Iron and Constantan. Temperature Range Type J: 0°C to 750°C A few Thermocouples 8

Thermocouple Advantages and Disadvantages : 

Thermocouple Advantages and Disadvantages Advantages: Self Powered (does not require a current or voltage source) Rugged Inexpensive Simple Disadvantages: Extremely Low Voltage output (mV) Not very stable Needs a reference point 9

Thermocouple - Principle : 

Thermocouple - Principle Hot junCtion cold junCtion voltmeter Twisting or welding of 2 wires 10

Thermocouples : 

Thermocouples Type K : Chromel-Alumel Type J : Iron-Constantan Type E : Chromel-Constantan Type N : Nicros-Nisil Type T : Copper-Constantan It is important to note that thermocouples measure the temperature difference between two points, not absolute temperature. 11

Thermocouple - applications : 

Thermocouple - applications Thermocouples are most suitable for measuring over a large temperature range, up to 1800 K. Example: Type K: Chromel-Alumel (-190⁰C to 1260⁰C) Type J: Iron-Constantan (-190⁰C to 760⁰C) Type E: Chromel-Constantan (-100⁰C to 1260⁰C) 12

Thermocouple - applications : 

Thermocouple - applications They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such applications, thermistors and RTDs are more suitable. Or it need large difference in temp. 13

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Thermocouples temperature range, –200°C to 2315°C. (Some thermsistors have a usable range of –100 to 500°C. Bi-metal thermostats can handle temperatures from –85 to 371°C. RTDs temperatures range from 150°C to 200°C. 15

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RTD ( resistance temperature detector ) : 

RTD ( resistance temperature detector ) How it works: Utilizes the fact as thermistor that resistance of a metal changes with temperature. Make up: Traditionally made up of platinum, nickel, iron or copper wound around an insulator. Temperature range: From about -196°C to 482°C. Thin Film RTD 19

RTD Advantages and Disadvantages : 

RTD Advantages and Disadvantages Advantages: Stable Very accurate Change in resistance is linear Disadvantages: Expensive Current source required Small change in resistance Self heating Less rugged than thermocouples. 20

Resistance temperature detector (RTD) : 

Resistance temperature detector (RTD) Resistance temperature detectors (RTDs), also called resistance thermometers, are temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature Temperature Metal Resistance The resistance ideally varies linearly with temperature. 21

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Platinum Copper Tungsten Nickel Platinum: very repeatable, sensitive, expensive Nickel: not quite repeatable, more sensitive, less expensive 22

RTD - sensitivity : 

RTD - sensitivity Sensitivity : Platinum – 0.004/ °C Nickel – 0.005/ °C Thus, for a 100Ω platinum RTD, a change of only 0.4 Ω would be expected if the temperature is changed by 1°C 23

RTD – response time : 

RTD – response time Generally 0.5 to 5 seconds or more The slowness of response is due principally to the slowness of thermal conductivity in bringing the device into thermal equilibrium with its environment. 24

Example :Construction of a platinum resistance thermometer : 

Example :Construction of a platinum resistance thermometer Wire is in a coil to achieve small size and improve thermal conductivity to decrease response time Protect from the environment 25

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Application Thermostat (makes or breaks electrical connection with deflection) Generaly : 1- alarm . 2- control . 3- temp measurement . 27