Temperature URVISH SONI

Brief Overview Types of Sensors and how they work Sensor Applications Advantages and Disadvantages Sensors that will work with project

How is heat transferred? Conduction Metal coffee cup Convection Radiation 003

Temperature scales

Types of Temperature Sensors Bi Metallic Thermocouples Resistance Temperature Detectors (RTDs) Thermistors Infrared Sensors

Expansion thermometer Solid Expansion Thermometer Bimetallic thermometer Spiral Bimetal element Helix Bimetal element Liquid Expansion Thermometer- Mercury in Glass type.

Thermal Expansion Co efficient Bimetal thermometer 100 300 200 400 Two dissimilar metals, tightly bonded Forces due to thermal expansion Result Thermal Expansion Co efficient Of Metal 013

Spiral and Helical

Liquid thermometer Mercury Liquid-filled thermometer 100 Accurate over a small range Accuracy & resolution= f(length) Range limited by liquid Fragile Large thermal mass Slow Mercury 100 014

Class I-Liquid Filled Systems

Class II- Vapour Systems

Class III- Gas Filled Systems

Class V- Mercury Filled Systems

Seeback effect

Peltier an effect whereby heat is given out or absorbed when an electric current passes across a junction between two materials.

Thomson effect Thomson effect is related to the emf that develops between two parts of the single metal when they are at different temperature Thus thomson effect is the absorption or evolution of heat along a conductor when current passes through it when one end of the conductor is hot and another is cold

Thermocouples Two wires of different metal alloys. Converts thermal energy into electrical energy. Requires a temperature difference between measuring junction and reference junction. Easy to use and obtain.

Cold junction Maintaining an ice water slurry and actual cold junction is rarely feasible. Typically, the cold junction is omitted, and the potential is measured directly across the two terminal ends of the thermocouple wires at ambient temperature.

electronic cold junction compensation Simulate the potential effects that would result for a thermocouple wire pair between the terminals, at their measured temperature, and another junction at a reference temperature of 0 degrees. Measure the potential across the thermocouple wire pair in series with the simulated potential. Apply the linearizing curve to the sum, thus obtaining an estimated absolute temperature directly. This is known as cold junction compensation. Usually, the simulation is done electronically with specialized integrated circuit devices. electronic cold junction compensation

Independently measure the temperature of the cold junction Independently measure the temperature of the cold junction. Measure the thermocouple potential and apply conversion curves to determine the temperature difference across the thermocouple. Then add the known cold junction temperature to the measured temperature difference to determine the absolute temperature measurement. independent cold junction measurement

Thermowell

Thermocouple extension wires

Thermocouples selection criteria

Thermocouple Applications Plastic injection molding machinery Food processing equipment Deicing Semiconductor processing Heat treating Medical equipment Industrial heat treating Packaging equipment

Thermocouples Advantages Disadvantages Simple, Rugged High temperature operation Low cost No resistance lead wire problems Point temperature sensing Fastest response to temperature changes Least stable, least repeatable Low sensitivity to small temperature changes Extension wire must be of the same thermocouple type Wire may pick up radiated electrical noise if not shielded Lowest accuracy

Common Thermocouples Seebeck Coeff: uV/C Type Metals J K T S E N Fe-Con Ni-Cr Cu-Con Pt/Rh-Pt Ni/Cr-Con Ni/Cr/Si-Ni/Si 50 40 38 10 59 39 Microvolt output is a tough measurement Type "N" is fairly new.. more rugged and higher temp. than type K, but still cheap 037 Type J, K, T, E and N are all base-metal thermocouples. Notice their high output signals, compared to the signal from a type S (noble-metal) thermocouple. Even though the output of the base-metal thermocouples is high compared with noble metal thermocouple, the magnitude of either is still quite small. The outputs are measured in microvolts per degrees C, which tends to create serious problems in a noisy factory environment. The nicrosil/nisil thermocouple is only a few years old, but it has gained rapid acceptance in replacing type K, because it is more rugged yet still inexpensive. 037

Resistance Temperature Detectors (RTDs) Wire wound and thin film devices. Nearly linear over a wide range of temperatures. Can be made small enough to have response times of a fraction of a second. Require an electrical current to produce a voltage drop across the sensor

Measuring an RTD: 2-wire method Rlead Rx 100 + d V Rlead I ref= 5 mA Pt - R= Iref*(Rx + 2* Rlead) Error= 2 /.385= more than 5 degrees C for 1 ohm Rlead! d Self-heating: For 0.5 V signal, I= 5mA; P=.5*.005=2.5 mwatts @ 1 mW/deg C, Error = 2.5 deg C! 018 Since the RTD has a low resistance, we need to look very carefully at how we are going to measure it. Let's take a simple two-wire ohms measurement with 5 mA reference current being pumped through the unknown RTD. In the two-wire measurement, the lead resistance becomes part of the unknown. In the case of the 100 ohm platinum RTD, a total of two ohms lead resistance can result in more than five degrees C error! Let's also look at the error due to self heating. For a 5 milliamp signal, the power in this device is about 2.5 milliwatts. At 1 milliwatt per degree C, that is an error of 2.5 degrees C. The lesson that we can learn from this is to always use a 4-wire measurement, not a 2-wire measurement. If you must use a 2-wire measurement, be sure to null out the lead resistance and minimize the measurement source current. Now let's take a look at the 4-wire technique. Moral: Minimize Iref; Use 4-wire method If you must use 2-wire, NULL out the lead resistance 018

3-Wire bridge d d V 3-Wire PRTD d d Keeps bridge away from heat source 1000 100 d d Rlead 1 V 3-Wire PRTD Sense wire 1000 d Rlead 2 100 d Keeps bridge away from heat source Break DMM lead (dashed line); connect to RTD through 3rd "sense" wire If Rlead 1= Rlead 2, sense wire makes error small Series resistance of sense wire causes no error 021 You can move the RTD a good distance away from the bridge and add a third sense wire directly connected to the voltmeter. This has the advantage of keeping the bridge away from the heat source and it also has the advantage of having the lead resistances compensate for each other. However, any mismatch in lead resistances will cause an error. While this method of measurement does save one wire, it is typically not quite as accurate as a 4-wire technique. 022

d d The 4-Wire technique Rx + 100 V Rlead=1 I ref= 5 mA - R= Iref * Rx Error not a function of R in source or sense leads No error due to changes in lead R Twice as much wire Twice as many scanner channels Usually slower than 2-wire 019 With the 4-wire technique, the voltage is sensed not at the voltmeter input terminals, but at the terminals of the RTD. That means that error is no longer a function of the lead resistance. It also means that there is no error due to changes in lead resistance, as there would be in the 2-wire measurement. The disadvantage of the 4-wire technique is that is requires twice as much wire and twice as many scanner channels. It is also typically slower than a 2-wire method. 019

Bridge method 100 d 1000 d V 100 d 1000 d High resolution (DMM stays on most sensitive range) Nonlinear output Bridge resistors too close to heat source 021 You could also use a bridge to measure an RTD. The advantage of the bridge is that the voltmeter always stays on its most sensitive range. The disadvantage shown here is that the bridge resistors are too close to the heat source, and hence we will get an incorrect reading due to the temperature coefficient of the bridge resistor. This can be solved by moving the RTD away from the bridge with an extra wire. 021

RTDs Advantages Disadvantages Most stable over time Most accurate Most repeatable temperature measurement Very resistant to contamination/ corrosion of the RTD element High cost Slowest response time Low sensitivity to small temperature changes Sensitive to vibration (strains the platinum element wire) Decalibration if used beyond sensor’s temperature ratings Somewhat fragile

Thermistors A semiconductor used as a temperature sensor. Mixture of metal oxides pressed into a bead, wafer or other shape. Beads can be very small, less than 1 mm in some cases. The resistance decreases as temperature increases, negative temperature coefficient (NTC) thermistor.

Thermistors Most are seen in medical equipment markets. Thermistors are also used are for engine coolant, oil, and air temperature measurement in the transportation industry.

Thermistors High sensitivity to small temperature changes Advantages Disadvantages High sensitivity to small temperature changes Temperature measurements become more stable with use Copper or nickel extension wires can be used Limited temperature range Fragile Some initial accuracy “drift” Decalibration if used beyond the sensor’s temperature ratings Lack of standards for replacement

emissivity The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation and it may include both visible radiation (light) and infrared radiation,

Stefan–Boltzmann law Stefan–Boltzmann law, statement that the total radiant heat energy emitted from a surface is proportional to the fourth power of its absolute temperature. 5.6704 × 10−8 watt per metre2∙K4 E = σT4

Black body A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. A white body is one with a rough surface [that] reflects all incident rays completely and uniformly in all directions

List sources of error in Non-contact type thermometry Radiation pyrometer Optical pyrometer Optical Fiber Thermometry Ultrasonic thermometry Laser thermometry

Optical pyrometer

Optical Fibre Thermometry

Ultrasonic thermometry

Temperature switches

Thermostats

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