TEMPERATURE SENSORS.pptx

Sensors which Sense temperature

1. TEMPERATURE SENSORS SENSOR TECH

2. What are the temperature sensors A temperature sensor is a device, typically, a thermocouple or resistance temperature detector, that provides temperature measurement in a readable form through an electrical signal. A thermometer is the most basic form of a temperature meter that is used to measure the degree of hotness and coolness. Temperature sensor is a sensor used for measuring the temperature with or without coming in direct contact with the body.

3. APPLICATION Widely used in air conditioning and heating appliances for human comfort. In automobiles Mobile phones Smart farming Electronics cooling applications Medical devices to measure human body temperature

4. Types of temp. sensors Thermocouple RTD (Resistance temp. detector) Thermistor Semiconductor based temp. sensor Infrared temp sensor thermometers

5. What are the different types of temperature sensors? Temperature sensors are available of various types, shapes, and sizes. The two main types of temperature sensors are: Contact Type Temperature Sensors : There are a few temperature meters that measure the degree of hotness or coolness in an object by being in direct contact with it. Such temperature sensors fall under the category contact-type. They can be used to detect solids, liquids or gases over a wide range of temperatures. Non-Contact Type Temperature Sensors : These types of temperature meters are not in direct contact of the object rather, they measure the degree of hotness or coolness through the radiation emitted by the heat source. The contact and non-contact temperature sensors are further divided into:

6. Thermostats A thermostat is a contact type temperature sensor consisting of a bi-metallic strip made up of two dissimilar metals such as aluminium , copper, nickel, or tungsten. The difference in the coefficient of linear expansion of both the metals causes them to produce a mechanical bending movement when it’s subjected to heat.

7. Thermistors

8. Thermistors or thermally sensitive resistors are the ones that change their physical appearance when subjected to change in the temperature. The thermistors are made up of ceramic material such as oxides of nickel, manganese or cobalt coated in glass which allows them to deform easily. Most of the thermistors have a negative temperature coefficient (NTC) which means their resistance decreases with an increase in the temperature. But, there are a few thermistors that have a positive temperature coefficient (PTC) and, their resistance increases with a rise in the temperature.

9. Resistive Temperature Detectors (RTD)

10. RTDs are precise temperature sensors that are made up of high-purity conducting metals such as platinum, copper or nickel wound into a coil. The electrical resistance of an RTD changes similar to that of a thermistor.

11. Thermocouples

12. One of the most common temperature sensors includes thermocouples because of their wide temperature operating range, reliability, accuracy, simplicity, and sensitivity. A thermocouple usually consists of two junctions of dissimilar metals, such as copper and constantan that are welded or crimped together. One of these junctions, known as the Cold junction, is kept at a specific temperature while the other one is the measuring junction, known as the Hot junction. On being subjected to temperature, a voltage drop is developed across the junction.

13. Negative Temperature Coefficient (NTC) Thermistor

14. A thermistor is basically a sensitive temperature sensor that reacts precisely to even the minute temperature changes. It provides a huge resistance at very low temperatures. This means, as soon as the temperature starts increasing, the resistance starts dropping quickly. Due to the large resistance change per degree Celsius, even a small temperature change is displayed accurately by the Negative Temperature Coefficient (NTC) Thermistor. Because of this exponential working principle, it requires linearization. They usually work in the range of -50 to 250 °C.

15. Semiconductor-Based Sensors A semiconductor-based temperature sensor works with dual integrated circuits (ICs). They contain two similar diodes with temperature-sensitive voltage and current characteristics to measure the temperature changes effectively. However, they give a linear output but, are less accurate at 1 °C to 5 °C. They also exhibit the slowest responsiveness (5 s to 60 s) across the narrowest temperature range (-70 °C to 150 °C).

16. Model ETT-10V Vibrating Wire Temperature Sensor

17. The Encardio -rite  Model ETT-10V vibrating wire temperature meter  is used for the measurement of internal temperature in concrete structures or water. It has a resolution of better than 0.1°C and works similar to that of Thermocouple Temperature Sensors. It also has a high temperature range from -20 o to 80 o  C.

18. Model ETT-10TH Resistance Thermistor Probe

19. The Encardio -rite  Model ETT-10TH resistance temperature probe  is a low mass waterproof temperature probe for measurement of temperature between –20 to 80°C. Due to its low thermal mass, it has a fast response time. Model ETT-10TH resistance temperature probe is specially designed for the measurement of surface temperatures of steel & measurement of the surface temperature of concrete structures. ETT-10TH can be embedded in concrete for measurement of bulk temperature inside the concrete and can even work submerged underwater.

20. Model ETT-10PT RTD Temperature Probe

21. The  ETT-10PT RTD (Resistance Temperature Detector)  temperature probe consists of a ceramic resistance element (Pt. 100) with DIN IEC 751 (former DIN 43760) European curve calibration. The resistance element is housed in a closed-end robust stainless steel tubing which protects the element against moisture.

22. How Does Model ETT-10PT RTD Temperature Probe Work?

23. The resistance temperature probe works on the principle that sensor resistance is a function of the sensed temperature. The platinum RTD has very good accuracy, linearity, stability and repeatability. The model ETT-10PT resistance temperature probe is provided with a three core shielded cable. The red wire provides one connection and the two black wires together provide the other. Thus, compensation is achieved for lead resistance and temperature change in lead resistance. The resistance temperature sensor readings can be read easily using a digital RTD temperature indicator.

24. Where is the temperature sensor used? The temperature sensor’s applications include: The temperature sensors are used for verifying design assumptions that will promote safer and economical design and construction. They are used to measure the temperature rise during the process of curing concrete. They can measure rock temperatures near liquid gas storage tanks and ground freezing operations. Temperature sensors can also measure water temperatures in reservoirs and boreholes. It can be used to interpret temperature related stress and volume changes in dams. They can also be used to study the temperature effect on other installed instruments.

25. What is the difference between the temperature sensor and temperature transmitter? A temperature sensor is an instrument used to measure the degree of hotness or coolness of an object whereas, a temperature transmitter is a device that is interfaced with a temperature sensor to transmit the signals to a remote location for monitoring and control purposes. This means, a thermocouple, RTD, or a thermistor is connected to a data logger to get the data at any remote location

26. Which Is The Most Accurate Temperature Sensor? An RTD is the most accurate temperature sensor. The platinum RTD has very good accuracy, linearity, stability and repeatability as compared to thermocouples or thermistors. What is a thermocouple?

27. Principles of Temperature Measurement Temperature measurement relies on the transfer of heat energy from the process material to the measuring device. The measuring device therefore needs to be temperature dependent. There are two main industrial types of temperature sensors:- Contact - Non contact Contact Contact is the more common and widely used form of temperature measurement. The three main types are: - Thermocouples - Resistance Temperature Detectors (RTD’s) - Thermistors

28. These types of temperature devices all vary in electrical resistance for temperature change. The rate and proportion of change is different between the three types, and also different within the type classes. Another less common device relies on the expansion of fluid up a capillary tube. This is where the bulk of the fluid is exposed to the process materials temperature. Non-Contact Temperature measurement by non-contact means is more specialised and can be performed with the following technologies: - Infrared - Acoustic

29. Thermocouples Basis of Operation A Thermocouple consists of two wires of dissimilar metals, such as iron and constantan, electrically connected at one end. Applying heat to the junction of the two metals produces a voltage between the two wires. This voltage is called an emf (electro-motive force) and is proportional to temperature A thermocouple requires a reference junction, this is placed in series with the sensing junction. As the two junctions are at different temperatures a thermal emf is generated. The reference junction is used to correct the sensing junction measurement.

30. The voltage across the thermocouple increases as the temperature rises and a suitably calibrated instrument, capable of measuring small voltages, can be used to measure the change. The process temperature is obtained from the voltage, either by reading from a graph or by using thermocouple tables. Thermocouple tables list the voltages corresponding to each temperature. A table is required for each thermocouple type.

31. The relationship between millivolts and temperature is not linear. In microprocessor based equipment, the conversion is done based on the data stored in the device. The sensing, or hot junction is inserted into the area where the temperature is to be measured. The reference, or cold junction is normally connected to the measuring instrument and held at 0 oC.

32. For accurate temperature measurement, the reference junction temperature must remain constant or suitable compensation provided if it should change. To reduce inaccuracies, most thermocouples are now installed with instruments that provide automatic reference compensation

33. One of the most accurate ways of compensating for temperature change is to maintain the reference junction at 0 oC. This however is not that practical, and some form of compensation needs to be used. The technique of cold junction compensation measures the actual temperature and applies a correction to the thermocouple reading. The correction is made by adjusting the voltage by an amount equal to the difference between the actual temperature and 0 oC.

34. Another method of providing this compensation is to pass current through a temperature responsive resistor, which measures the variation in reference temperature and automatically provides the necessary correction by means of a voltage drop across the resistor.

35. The Seebeck Effect: A thermocouple works on the Seebeck Effect. This is where (as previously mentioned) two wires of dissimilar metals are electrically connected at one end. When the junction is heated or cooled, a voltage is produced which is proportional to the temperature. The Peltier Effect: The reverse of the Seebeck Effect is possible and can be useful. By applying a voltage and causing a current to flow between two wires of dissimilar metals, it is possible to generate a temperature difference. Because of the different electrothermal transport properties of the metals, it is found that one of the junctions will be heated and the other cooled. This process is referred to as the Peltier Effect

36. Practical applications include cooling small electronic parts, or even to provide a 0oC reference junction for a thermocouple. Other applications using this principle are becoming increasingly popular for heating and refrigeration.

37. Thermocouple materials The following materials are used to manufacture different types of thermocouples TYPE B Positive material –Platinum Rhodium 30% Negative material –Platinum Rhodium 6% Temp. range -0 to 1800 C TYPE C Positive material –Tungsten Rhenium 5% Negative material- Tunsten Rhenium 26% Temp. range 0 to 2300 C

38. Construction Thermocouples are fusion-welded to form a pure joint, which maintains the integrity of the circuit and also provides high accuracy. Grounded junctions provide good thermal contact with protection from the environment. Ungrounded and isolated junctions provide electrical isolation from the sensor sheath. Thermocouples are usually encased in a protective metal sheath. The sheath material can be stainless steel which is good for temperatures up to 870 oC. For temperatures up to 1150 oC Inconel is used.

39. E-type are the most sensitive thermocouple available, and have the highest change in emf per temperature change, but they tend to drift more. They can be used in oxidising atmospheres.

40. Advantages - Low cost - Small size - Robust - Wide range of operation - Reasonably stable - Accurate for large temperature changes - Provide fast response

41. Disadvantages - Very weak output, millivolts - Limited accuracy for small variations in temperature - Sensitive to electrical noise - Nonlinear - Complicated conversion from emf to temperature

42. Application Limitations Small temperature changes give a very small change in voltage. A platinum thermocouple, for example, will give a change of about 10microvolts for a 1 Oc change in temperature. It is because of the weak output signal from thermocouples that they are susceptible to electrical noise and are limited to applications requiring the measurement of large changes in temperature. Thermocouples are not linear and the conversion from the generated emf to temperature is involved. The calibration of thermocouples does change over time, and this is due to

43. contamination, composition changes (possibly due to internal oxidation). Rapid changes in temperature may have an effect, but high temperatures definitely can affect the stability of the device. It is proven that when a K-type thermocouple is cycled to 1100°C it can vary by as much as 10%. The integrity of the conductivity of a thermocouple has to be maintained and as such cannot be used exposed in conductive fluids.