How Thermocouples Work - basic working principle + RTD

00:09:05
https://www.youtube.com/watch?v=v7NUi88Lxi8

摘要

TLDRIn this video, Paul from Engineering Mindset explains thermocouples and RTDs (Resistance Temperature Detectors). Thermocouples consist of two different metals joined at one end, which produce a voltage when there is a temperature difference. The most common type is the 'K' thermocouple. The video illustrates how heat causes atoms to excite, leading to electron movement and voltage generation. It then outlines RTDs, highlighting their reliance on changes in electrical resistance with temperature, often using materials like platinum. The presentation covers basic electrical concepts like Ohm’s Law and emphasizes methods for accurately measuring temperature in engineering applications.

心得

  • 🔧 Thermocouples connect two different metals for temperature measurement.
  • ⚡ Voltage indicates temperature difference created by heat.
  • 🌡️ Common thermocouple type is 'K'.
  • 🧪 RTDs use resistance change to measure temperature.
  • 🔍 Resistance increases with temperature in metals.
  • 📈 Ohm's Law relates voltage to current and resistance.
  • 📏 RTDs commonly use platinum for accurate results.
  • 🧊 Ice water provides accurate reference for thermocouples.

时间轴

  • 00:00:00 - 00:09:05

    In this video, Paul from engineeringmindset.com explains how thermocouples work, their design, and the various types available. He demonstrates the process of measuring temperature using a thermocouple, highlighting that it involves connecting two different metals which generate a voltage when a temperature difference is created. The video outlines the principle of electron movement due to heat and explains how this results in voltage differences. Paul discusses the calibration of thermocouples and introduces the concept of using ice water as a reference point for accurate temperature readings, though he notes it's not practical for most engineering applications. He concludes this section by suggesting the use of RTD (Resistance Temperature Detectors) to improve accuracy in temperature measurements, paving the way for a deeper discussion about RTDs in the following segments.

思维导图

视频问答

  • What is a thermocouple?

    A thermocouple is a device that connects two different metals to measure temperature by generating a small voltage difference.

  • How does a thermocouple work?

    It generates a voltage when two different metals are connected and exposed to a temperature difference.

  • What are common types of thermocouples?

    The most common type is the 'K' thermocouple, suitable for general-purpose use.

  • What is an RTD?

    An RTD (Resistance Temperature Detector) measures temperature based on the change in electrical resistance of a material.

  • How does temperature affect resistance in metals?

    Most metals increase in resistance as temperature rises due to increased atomic movement.

  • What is Ohm's Law?

    Ohm's Law states that voltage equals current multiplied by resistance.

  • What materials are commonly used for RTDs?

    Platinum is commonly used for RTDs due to its linear resistance-temperature relationship.

  • How can you improve the accuracy of a thermocouple reading?

    Using an RTD to measure the connection temperature and applying a correction formula can improve accuracy.

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  • 00:00:02
    [Applause]
  • 00:00:04
    hey there guys paul here from the
  • 00:00:06
    engineering mindset.com
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    in this video we're going to be looking
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    at the thermocouple
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    to understand how it works as well as
  • 00:00:14
    some of the different types this video
  • 00:00:16
    was sponsored by danfoss
  • 00:00:20
    a typical thermocouple looks something
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    like this
  • 00:00:24
    it usually comes with a handheld
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    temperature probe or even a multimeter
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    they are also built into these solid
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    cases for a much more rugged design
  • 00:00:34
    thermocouples are very quick and easy to
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    use
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    by simply plugging the probe into the
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    measurement device in this case a cheap
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    multimeter
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    and selecting the temperature setting we
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    can get an accurate temperature reading
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    in no time at all and this will work for
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    many applications
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    with a thermocouple we connect two
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    different metals together at one end
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    and the other ends connect into a
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    terminal block
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    then we use a voltmeter to read the
  • 00:01:01
    voltage difference between the two
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    the voltage here will be very very small
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    when we connect the thermocouple to a
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    multimeter and then apply heat to the
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    junction we can see it will generate a
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    voltage
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    as you can see in this example we're
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    able to generate a very small voltage
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    using a flame
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    and once we remove the heat the voltage
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    diminishes
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    thermocouples are available in different
  • 00:01:26
    temperature ranges
  • 00:01:28
    these are shown by a letter which
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    indicates their rated temperature range
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    the most common type is k this is a very
  • 00:01:35
    general purpose version
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    each letter uses a different combination
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    of materials
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    this will give us a different
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    temperature reading and allows different
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    temperature ranges
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    if we held a metal rod in our hand and
  • 00:01:51
    place the other end into a flame
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    we know the rod will heat up and this
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    heat will travel along the length of the
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    rod
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    to our hand we can see this by using a
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    thermal imaging camera
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    notice the thermal energy travels along
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    the length of the copper wire
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    away from the heat source what's
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    happening here
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    is the heat is exciting the atoms and
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    molecules which form the material
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    structure
  • 00:02:16
    the heat causes the molecules and the
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    atoms to vibrate
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    this vibration is so tiny you would not
  • 00:02:22
    be able to feel it with your hand
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    the excited atoms will allow their free
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    electrons to move more easily
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    and they will move towards the cooler
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    end of the rod
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    this only occurs because there is a
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    temperature gradient
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    a difference in temperature from one end
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    to the other
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    so at the cooler end we'll have slightly
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    more electrons than at the hotter end
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    and as electrons are negatively charged
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    we therefore get a slightly negative
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    and slightly positive charged ends of
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    the rod
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    voltage is like pressure we're measuring
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    the difference
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    or potential difference between two
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    points if you imagine a pressurized
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    water pipe
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    we can see the pressure using a pressure
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    gauge
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    the pressure reading is comparing two
  • 00:03:07
    different points also
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    the pressure inside the pipe compared to
  • 00:03:11
    the atmospheric pressure
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    outside the pipe when the tank is empty
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    the gauge will read zero because it has
  • 00:03:18
    nothing to compare
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    both are now the same pressure the same
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    with voltage
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    we are comparing the difference from one
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    point to another
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    if we read across a 1.5 volt battery
  • 00:03:30
    we get a reading of 1.5 volts
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    but if we try to measure the same side
  • 00:03:36
    we wouldn't read any voltage
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    because there is no difference we can
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    only measure the difference
  • 00:03:42
    between two different points by the way
  • 00:03:44
    we have also covered
  • 00:03:46
    how a battery works in detail in our
  • 00:03:48
    previous video
  • 00:03:49
    do check that out links can be found in
  • 00:03:51
    the video description
  • 00:03:52
    down below coming back to the
  • 00:03:54
    thermocouple
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    if we connected two wires together of
  • 00:03:58
    the same material let's say they were
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    both copper
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    and we then applied heat to the end to
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    create a temperature difference
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    then the electrons would scatter and
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    build up by the cold ends
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    however they would build up in equal
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    amounts in each wire
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    because they are the same material so
  • 00:04:16
    both wires will conduct heat equally
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    and the temperature gradient will be the
  • 00:04:20
    same therefore our voltmeter
  • 00:04:22
    wouldn't be able to measure any
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    difference
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    however if we connected two wires which
  • 00:04:28
    were made of different materials
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    say one was made from copper and the
  • 00:04:32
    other one was made from iron
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    then the two metals will conduct heat
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    differently so the temperature gradient
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    will be different
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    that means the electron buildup but the
  • 00:04:41
    cold ends will be different
  • 00:04:43
    and so we can connect a voltmeter to
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    this and read a voltage difference
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    to make this into a useful tool we just
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    calibrate it by testing the device
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    against known temperatures
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    and marking down the voltages generated
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    then we simply use a formula
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    to calculate the temperature from the
  • 00:05:01
    voltage measured
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    for this to work best we should submerge
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    the coal junction
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    into an ice bath that way we get a
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    voltage with a reference
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    relative to zero degrees celsius
  • 00:05:13
    remember i said about the pressure in
  • 00:05:15
    the pipe and how we are comparing it
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    against the atmospheric pressure outside
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    that's because we know the pressure
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    outside the pipe is atmospheric
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    pressure so for the voltage reading to
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    be accurate
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    we need to measure against something we
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    know so we use ice water
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    because we know this temperature is a
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    constant 0 degrees celsius
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    this method is used in many science labs
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    however
  • 00:05:40
    as you can probably tell it isn't very
  • 00:05:42
    practical for most engineering
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    applications
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    so instead to improve the accuracy we
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    leave the cold connections at equal
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    ambient temperatures and then we
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    compensate for the difference
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    by measuring the temperature of the
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    connection and applying a formula
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    to offset the error to measure the
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    temperature of the connection
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    we often use an rtd temperature sensor
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    which we'll look at next
  • 00:06:09
    rtd stands for resistance temperature
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    detector
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    this is also a fairly simple design it's
  • 00:06:15
    probably easier to understand than the
  • 00:06:17
    thermocouple
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    they usually come in these different
  • 00:06:20
    designs for engineering applications
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    with a rugged casing
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    how do these work well we know that
  • 00:06:26
    electricity is the flow of electrons in
  • 00:06:28
    a circuit
  • 00:06:29
    we have covered how electricity works in
  • 00:06:31
    detail in our previous videos do check
  • 00:06:34
    that out links can be found in the video
  • 00:06:35
    description
  • 00:06:36
    down below when we pass electricity
  • 00:06:39
    through a material
  • 00:06:40
    let's say a copper wire the material
  • 00:06:43
    will have some resistance to the flow of
  • 00:06:44
    electrons
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    and we can measure this resistance with
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    a multimeter
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    different materials will have different
  • 00:06:50
    resistance levels
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    for example this one meter length of
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    copper wire
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    shows a very low resistance of just 0.2
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    ohms but this one meter length of nickel
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    chrome wire
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    shows a very high resistance of 22.1
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    ohms
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    the temperature of the material will
  • 00:07:10
    change the resistance of the material
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    most conductors will increase in
  • 00:07:15
    resistance the hotter they get
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    that's typical of metals for example
  • 00:07:20
    this copper wire
  • 00:07:21
    shows a resistance of 0.1 ohms at
  • 00:07:24
    ambient temperature
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    but when heated with a flame it
  • 00:07:28
    increases
  • 00:07:29
    up to 0.9 ohms this occurs
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    because as the atoms and the molecules
  • 00:07:35
    become excited
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    they're going to move around a lot so
  • 00:07:38
    this makes it harder for the free
  • 00:07:40
    electrons to get through without a
  • 00:07:42
    collision
  • 00:07:43
    using a formula known as ohm's law
  • 00:07:46
    voltage
  • 00:07:46
    is equal to current multiplied by
  • 00:07:48
    resistance
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    which means that as long as we keep the
  • 00:07:52
    current the same
  • 00:07:53
    a change in resistance will cause a
  • 00:07:56
    change in voltage
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    and as temperature changes the
  • 00:07:59
    resistance of a material
  • 00:08:01
    we can measure the voltage to tell the
  • 00:08:03
    temperature
  • 00:08:04
    we use a material such as platinum
  • 00:08:06
    because it has a near linear resistance
  • 00:08:09
    versus temperature gradient
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    we test the material at known
  • 00:08:13
    temperatures to obtain the graph
  • 00:08:15
    for example at 0 degrees celsius the
  • 00:08:18
    material will have a resistance of 100
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    ohms
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    and at 100 degrees celsius it has a
  • 00:08:24
    resistance
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    of 138.5 ohms there are many different
  • 00:08:28
    designs for this type
  • 00:08:30
    but typically they're either a film type
  • 00:08:32
    where the platinum is coated into a
  • 00:08:34
    ceramic plate
  • 00:08:35
    into a pattern and sealed in glass or it
  • 00:08:38
    will be a platinum wire
  • 00:08:40
    wound around a ceramic core again sealed
  • 00:08:43
    in glass for protection
  • 00:08:45
    okay that's it for this video but to
  • 00:08:47
    continue learning then check out one of
  • 00:08:48
    the videos on screen now and i'll catch
  • 00:08:50
    you there for the next lesson
  • 00:08:52
    don't forget to follow us on facebook
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  • 00:08:55
    linkedin and of course the engineering
  • 00:08:57
    mindset dot com
标签
  • Thermocouple
  • RTD
  • Temperature Sensor
  • Electrical Resistance
  • Voltage Measurement
  • Engineering Applications
  • Temperature Gradient
  • Ohm's Law
  • Platinum
  • General Purpose Thermocouple