Temperature Sensors

Objectives Names and different methods to measure the temperature Resistance temperature detectors (RTD) Thermister NTC and PTC Thermocouples Pyrometers Smart temperature sensors Applications

Thermocouples Thermocouples (T/C) are formed when two dissimilar metals are joined together to form a junction. Joining together the other ends of the dissimilar metals to form a second junction completes an electrical circuit. A current will flow in the circuit if the two junctions are at different temperatures. The voltage difference between the two junctions is measured, and this difference is proportional to the temperature difference between the two junctions. Three effects are associated with thermocouples. Seebeck effect: It states that the voltage produced in a thermocouple is proportional to the temperature between the two junctions. Peltier effect: It states that if a current flows through thermocouple one junction is heated (puts out energy) and the other junction is cooled (absorbs energy

EMF generated by thermocouples The emf generated can be approximately expressed by the relationship: where T1 and T2 are hot and cold junction temperatures in K. C1 and C2 are constants depending upon the materials. For Copper/ Constantan thermocouple, C1=62.1 and C2=0.045 . Thermocouples are extensively used for measurement of temperature in industrial situations. The major reasons behind their popularity are: (i) they are rugged and readings are consistent, (ii) they can measure over a wide range of temperature, and (iii) their characteristics are almost linear with an accuracy of about 0.05%.

Law of thermocouple we have three laws of thermoelectric circuits that provide us useful practical tips for measurement of temperature . law of homogeneous circuit (i): The first law can be explained using figure 1. It says that the net thermo-emf generated is dependent on the materials and the temperatures of two junctions only, not on any intermediate temperature. law of intermediate metals: If a third material is introduced at any point (thus forming two additional junctions) it will not have any effect, if these two additional junctions remain at the same temperatures . This law makes it possible to insert a measuring device without altering the thermo-emf. law of intermediate temperatures :The third law is related to the calibration of the thermocouple. It says, if a thermocouple produces emf e1, when its junctions are at T1 and T2, and e2 when its junctions are at T2 and T3; then it will generate emf e1+e2 when the junction temperatures are at T1 and T3

Applications Advantages In the process control of chemical reactions, temperature control is of major importance, since chemical reactions are temperature dependent. In an average household , at least a dozen temperature sensors can be found in various places, raging from coffee machine to heating system to a car. Low cost Small size Robust Wide range of operation Reasonably stable Accurate for large temperature changes Provide fast response Advantages

Resistance Temperature Detectors (RTD's)

construction RTD’s are built from selected metals (typically Platinum), which change resistance with temperature change. The resistance temperature detector (RTD) measures the electrical conductivity as it varies with temperature. The electrical resistance generally increases with temperature, and the device is defined as having a positive temperature coefficient. The magnitude of the temperature coefficient determines the sensitivity of the RTD. Apart from Platinum, other metals are used for RTD’s such as Copper and Nickel. Platinum is the most common and has the best linear characteristics of the three, although Nickel has a higher temperature coefficient giving it greater sensitivity.

Temperature coefficient The temperature coefficient defines how much the resistance will change for a change in temperature, and has units of ohms/oC. The greater the temperature coefficient, the more the resistance will change for a given change in temperature. This ultimately defines how sensitive the device is. RTD’s are generally quite linear, however the temperature coefficient does vary over the range of operation. As an indication, the temperature coefficient for Platinum is averaged at 0.00385 over the range from 0oC to 100oC, but varies by about 2% over this range.

Resistance Temperature Detectors (RTD's) Advantages - Good sensitivity - Uses standard copper wire - Copper RTD’s minimize thermocouple effect Disadvantages - Bulky in size and fragile - Slow thermal response time due to bulk - Self heating problems - More susceptible to electrical noise - More expensive to test and diagnose

Thermisters

Basic of Thermisters A thermistor is a semiconductor device formed from metal oxides. The principle of temperature measurement with a thermistor is that its resistance changes with temperature. Most thermistors differ from normal resistors in that they have a negative coefficient of resistance, this means that the resistance decreases with an increase in temperature. Negative (NTC) thermistors are the more common although positive (PTC) are also available.

Types of Thermisters Types of thermistors vary in a number of ways and one change is their response to temperature changes. Thermistors are not linear, and their response curves vary for the different types. Some thermistors have a near linear temperature resistance relationship, others are available with a sharp change in slope (sensitivity) at a particular characteristic temperature.

Advantages and disadvantages - Small size - Fast response - Very high sensitivity (Select range) - No cold junction compensation - Inexpensive - Polarity insensitive - Wide selection of sensors Disadvantages - Not easily interchangeable - Non linear - Narrow span - Fragile - High resistance, noise problems

Pyrometers

Pyrometers Pyrometric methods of temperature measurement use the electromagnetic radiation that is emitted from a material. The emitted radiation is proportional to the temperature. Any object with a temperature above absolute zero will radiate electromagnetic energy. Infrared pyrometers measure the amount of energy radiated from an object in order to determine its temperature. There are a number of different types of infrared pyrometers: - Total radiation - Single wavelength - Dual wavelength

Advantages and disadvantages - Non contact measurement - High temperature sensing - Remote sensing - Fast response and can sense objects in motion - Sense small or area targets Disadvantages - Expensive - Non linear response - Subject to emissivity of material - Require wide range of operation